Network Working Group                                        M. Nystroem
Request for Comments: 4758                                  RSA Security
Category: Informational                                    November 2006


       Cryptographic Token Key Initialization Protocol (CT-KIP)
                         Version 1.0 Revision 1

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The IETF Trust (2006).

Abstract

   This document constitutes Revision 1 of Cryptographic Token Key
   Initialization Protocol (CT-KIP) Version 1.0 from RSA Laboratories'
   One-Time Password Specifications (OTPS) series.  The body of this
   document, except for the intellectual property considerations
   section, is taken from the CT-KIP Version 1.0 document, but comments
   received during the IETF review are reflected; hence, the status of a
   revised version.  As no "bits-on-the-wire" have changed, the protocol
   specified herein is compatible with CT-KIP Version 1.0.

   CT-KIP is a client-server protocol for initialization (and
   configuration) of cryptographic tokens.  The protocol requires
   neither private-key capabilities in the cryptographic tokens, nor an
   established public-key infrastructure.  Provisioned (or generated)
   secrets will only be available to the server and the cryptographic
   token itself.

Table of Contents

   1. Introduction ....................................................4
      1.1. Scope ......................................................4
      1.2. Background .................................................4
      1.3. Document Organization ......................................5
   2. Acronyms and Notation ...........................................5
      2.1. Acronyms ...................................................5
      2.2. Notation ...................................................5
   3. CT-KIP ..........................................................6
      3.1. Overview ...................................................6
      3.2. Entities ...................................................7
      3.3. Principles of Operation ....................................7
      3.4. The CT-KIP One-Way Pseudorandom Function, CT-KIP-PRF ......10
           3.4.1. Introduction .......................................10
           3.4.2. Declaration ........................................11
      3.5. Generation of Cryptographic Keys for Tokens ...............11
      3.6. Encryption of Pseudorandom Nonces Sent from the
           CT-KIP Client .............................................12
      3.7. CT-KIP Schema Basics ......................................13
           3.7.1. Introduction .......................................13
           3.7.2. General XML Schema Requirements ....................13
           3.7.3. The AbstractRequestType Type .......................13
           3.7.4. The AbstractResponseType type ......................14
           3.7.5. The StatusCode Type ................................14
           3.7.6. The IdentifierType Type ............................16
           3.7.7. The NonceType Type .................................16
           3.7.8. The ExtensionsType and the
                  AbstractExtensionType Types ........................17
      3.8. CT-KIP Messages ...........................................17
           3.8.1. Introduction .......................................17
           3.8.2. CT-KIP Initialization ..............................17
           3.8.3. The CT-KIP Client's Initial PDU ....................18
           3.8.4. The CT-KIP server's initial PDU ....................20
           3.8.5. The CT-KIP Client's Second PDU .....................23
           3.8.6. The CT-KIP Server's Final PDU ......................24
      3.9. Protocol Extensions .......................................27
           3.9.1. The ClientInfoType Type ............................27
           3.9.2. The ServerInfoType Type ............................28
           3.9.3. The OTPKeyConfigurationDataType Type ...............28
   4. Protocol Bindings ..............................................29
      4.1. General Requirement .......................................29
      4.2. HTTP/1.1 binding for CT-KIP ...............................29
           4.2.1. Introduction .......................................29
           4.2.2. Identification of CT-KIP Messages ..................29
           4.2.3. HTTP Headers .......................................29
           4.2.4. HTTP Operations ....................................30
           4.2.5. HTTP Status Codes ..................................30

           4.2.6. HTTP Authentication ................................31
           4.2.7. Initialization of CT-KIP ...........................31
           4.2.8. Example Messages ...................................31
   5. Security considerations ........................................32
      5.1. General ...................................................32
      5.2. Active Attacks ............................................32
           5.2.1. Introduction .......................................32
           5.2.2. Message Modifications ..............................32
           5.2.3. Message Deletion ...................................34
           5.2.4. Message Insertion ..................................34
           5.2.5. Message Replay .....................................34
           5.2.6. Message Reordering .................................35
           5.2.7. Man in the Middle ..................................35
      5.3. Passive Attacks ...........................................35
      5.4. Cryptographic Attacks .....................................35
      5.5. Attacks on the Interaction between CT-KIP and User
           Authentication ............................................36
   6. Intellectual Property Considerations ...........................36
   7. References .....................................................37
      7.1. Normative References ......................................37
      7.2. Informative References ....................................37
   Appendix A. CT-KIP Schema .........................................39
   Appendix B. Examples of CT-KIP Messages ...........................46
      B.1. Introduction ..............................................46
      B.2. Example of a CT-KIP Initialization (Trigger) Message ......46
      B.3. Example of a <ClientHello> Message ........................46
      B.4. Example of a <ServerHello> Message ........................47
      B.5. Example of a <ClientNonce> Message ........................47
      B.6. Example of a <ServerFinished> Message .....................48
   Appendix C. Integration with PKCS #11 .............................48
   Appendix D. Example CT-KIP-PRF Realizations .......................48
      D.1. Introduction ..............................................48
      D.2. CT-KIP-PRF-AES ............................................48
           D.2.1. Identification .....................................48
           D.2.2. Definition .........................................49
           D.2.3. Example ............................................50
      D.3. CT-KIP-PRF-SHA256 .........................................50
           D.3.1. Identification .....................................50
           D.3.2. Definition .........................................51
           D.3.3. Example ............................................52
   Appendix E. About OTPS ............................................53

1.  Introduction

   Note: This document is Revision 1 of CT-KIP Version 1.0 [12] from RSA
   Laboratories' OTPS series.

1.1.  Scope

   This document describes a client-server protocol for initialization
   (and configuration) of cryptographic tokens.  The protocol requires
   neither private-key capabilities in the cryptographic tokens, nor an
   established public-key infrastructure.

   The objectives of this protocol are:

   o  To provide a secure method of initializing cryptographic tokens
      with secret keys without exposing generated, secret material to
      any other entities than the server and the cryptographic token
      itself,

   o  To avoid, as much as possible, any impact on existing
      cryptographic token manufacturing processes,

   o  To provide a solution that is easy to administer and scales well.

   The mechanism is intended for general use within computer and
   communications systems employing connected cryptographic tokens (or
   software emulations thereof).

1.2.  Background

   A cryptographic token may be a handheld hardware device, a hardware
   device connected to a personal computer through an electronic
   interface such as USB, or a software module resident on a personal
   computer, which offers cryptographic functionality that may be used,
   e.g., to authenticate a user towards some service.  Increasingly,
   these tokens work in a connected fashion, enabling their programmatic
   initialization as well as programmatic retrieval of their output
   values.  This document intends to meet the need for an open and
   interoperable mechanism to programmatically initialize and configure
   connected cryptographic tokens.  A companion document entitled "A
   PKCS #11 Mechanism for the Cryptographic Token Key Initialization
   Protocol" [2] describes an application-programming interface suitable
   for use with this mechanism.

1.3.  Document Organization

   The organization of this document is as follows:

   o  Section 1 is an introduction.

   o  Section 2 defines some notation used in this document.

   o  Section 3 defines the protocol mechanism in detail.

   o  Section 4 defines a binding of the protocol to transports.

   o  Section 5 provides security considerations.

   o  Appendix A defines the XML schema for the protocol mechanism,
      Appendix B gives example messages, and Appendix C discusses
      integration with PKCS #11 [3].

   o  Appendix D provides example realizations of an abstract
      pseudorandom function defined in Section 3.

   o  Appendix E provides general information about the One-Time
      Password Specifications.

2.  Acronyms and Notation

2.1.  Acronyms

   MAC      Message Authentication Code

   PDU      Protocol Data Unit

   PRF      Pseudo-Random Function

   CT-KIP   Cryptographic Token Key Initialization Protocol (the
            protocol mechanism described herein)

2.2.  Notation

   ||       String concatenation

   [x]      Optional element x

   A ^ B    Exclusive-or operation on strings A and B (A and B of equal
            length)

   K_AUTH   Secret key used for authentication purposes

   K_TOKEN  Secret key used for token computations, generated in CT-KIP

   K_SERVER Public key of CT-KIP server

   K_SHARED Secret key shared between the cryptographic token and the
            CT-KIP server

   K        Key used to encrypt R_C (either K_SERVER or K_SHARED)

   R        Pseudorandom value chosen by the cryptographic token and
            used for MAC computations

   R_C      Pseudorandom value chosen by the cryptographic token

   R_S      Pseudorandom value chosen by the CT-KIP server

   The following typographical convention is used in the body of the
   text: <XMLElement>.

3.  CT-KIP

3.1.  Overview

   The CT-KIP is a client-server protocol for the secure initialization
   of cryptographic tokens.  The protocol is meant to provide high
   assurance for both the server and the client (cryptographic token)
   that generated keys have been correctly and randomly generated and
   not exposed to other entities.  The protocol does not require the
   existence of a public-key infrastructure.

   +---------------+                            +---------------+
   |               |                            |               |
   | CT-KIP client |                            | CT-KIP server |
   |               |                            |               |
   +---------------+                            +---------------+
           |                                            |
           |        [ <---- CT-KIP trigger ---- ]       |
           |                                            |
           |        ------- Client Hello ------->       |
           |                                            |
           |        <------ Server Hello --------       |
           |                                            |
           |        ------- Client Nonce ------->       |
           |                                            |
           |        <----- Server Finished ------       |

   Figure 1: The 4-pass CT-KIP protocol (with optional preceding
   trigger)

3.2.  Entities

   In principle, the protocol involves a CT-KIP client and a CT-KIP
   server.

   It is assumed that a desktop/laptop or a wireless device (e.g., a
   mobile phone or a PDA) will host an application communicating with
   the CT-KIP server as well as the cryptographic token, and
   collectively, the cryptographic token and the communicating
   application form the CT-KIP client.  When there is a need to point
   out if an action is to be performed by the communicating application
   or by the token the text will make this explicit.

   The manner in which the communicating application will transfer CT-
   KIP protocol elements to and from the cryptographic token is
   transparent to the CT-KIP server.  One method for this transfer is
   described in [2].

3.3.  Principles of Operation

   To initiate a CT-KIP session, a user may use a browser to connect to
   a web server running on some host.  The user may then identify (and
   authenticate) herself (through some means that essentially are out of
   scope for this document) and possibly indicate how the CT-KIP client
   shall contact the CT-KIP server.  There are also other alternatives
   for CT-KIP session initiation, such as the CT-KIP client being pre-
   configured to contact a certain CT-KIP server, or the user being
   informed out-of-band about the location of the CT-KIP server.  In any
   event, once the location of the CT-KIP server is known, the CT-KIP
   client and the CT-KIP server engage in a 4-pass protocol in which:

   a.  The CT-KIP client provides information to the CT-KIP server about
       the cryptographic token's identity, supported CT-KIP versions,
       cryptographic algorithms supported by the token and for which
       keys may be generated using this protocol, and encryption and MAC
       algorithms supported by the cryptographic token for the purposes
       of this protocol.

   b.  Based on this information, the CT-KIP server provides a random
       nonce, R_S, to the CT-KIP client, along with information about
       the type of key to generate, the encryption algorithm chosen to
       protect sensitive data sent in the protocol.  In addition, it
       provides either information about a shared secret key to use for
       encrypting the cryptographic token's random nonce (see below), or
       its own public key.  The length of the nonce R_S may depend on
       the selected key type.

   c.  The cryptographic token generates a random nonce R_C and encrypts
       it using the selected encryption algorithm and with a key K that
       is either the CT-KIP server's public key K_SERVER, or a shared
       secret key K_SHARED as indicated by the CT-KIP server.  The
       length of the nonce R_C may depend on the selected key type.  The
       CT-KIP client then sends the encrypted random nonce to the CT-KIP
       server.  The token also calculates a cryptographic key K_TOKEN of
       the selected type from the combination of the two random nonces
       R_S and R_C, the encryption key K, and possibly some other data,
       using the CT-KIP-PRF function defined herein.

   d.  The CT-KIP server decrypts R_C, calculates K_TOKEN from the
       combination of the two random nonces R_S and R_C, the encryption
       key K, and possibly some other data, using the CT-KIP-PRF
       function defined herein.  The server then associates K_TOKEN with
       the cryptographic token in a server-side data store.  The intent
       is that the data store later on will be used by some service that
       needs to verify or decrypt data produced by the cryptographic
       token and the key.

   e.  Once the association has been made, the CT-KIP server sends a
       confirmation message to the CT-KIP client.  The confirmation
       message includes an identifier for the generated key and may also
       contain additional configuration information, e.g., the identity
       of the CT-KIP server.

   f.  Upon receipt of the CT-KIP server's confirmation message, the
       cryptographic token associates the provided key identifier with
       the generated key K_TOKEN, and stores the provided configuration
       data, if any.

   Note: Conceptually, although R_C is one pseudorandom string, it may
   be viewed as consisting of two components, R_C1 and R_C2, where R_C1
   is generated during the protocol run, and R_C2 can be generated at
   the cryptographic token manufacturing time and stored in the
   cryptographic token.  In that case, the latter string, R_C2, should
   be unique for each cryptographic token for a given manufacturer.

   +----------------------+    +-------+     +----------------------+
   |    +------------+    |    |       |     |                      |
   |    | Server key |    |    |       |     |                      |
   | +<-|  Public    |------>------------->-------------+---------+ |
   | |  |  Private   |    |    |       |     |          |         | |
   | |  +------------+    |    |       |     |          |         | |
   | |        |           |    |       |     |          |         | |
   | V        V           |    |       |     |          V         V |
   | |   +---------+      |    |       |     |        +---------+ | |
   | |   | Decrypt |<-------<-------------<-----------| Encrypt | | |
   | |   +---------+      |    |       |     |        +---------+ | |
   | |      |  +--------+ |    |       |     |            ^       | |
   | |      |  | Server | |    |       |     |            |       | |
   | |      |  | Random |--->------------->------+  +----------+  | |
   | |      |  +--------+ |    |       |     |   |  | Client   |  | |
   | |      |      |      |    |       |     |   |  | Random   |  | |
   | |      |      |      |    |       |     |   |  +----------+  | |
   | |      |      |      |    |       |     |   |        |       | |
   | |      V      V      |    |       |     |   V        V       | |
   | |   +------------+   |    |       |     | +------------+     | |
   | +-->| CT-KIP PRF |   |    |       |     | | CT-KIP PRF |<----+ |
   |     +------------+   |    |       |     | +------------+       |
   |           |          |    |       |     |       |              |
   |           V          |    |       |     |       V              |
   |       +-------+      |    |       |     |   +-------+          |
   |       |  Key  |      |    |       |     |   |  Key  |          |
   |       +-------+      |    |       |     |   +-------+          |
   |       +-------+      |    |       |     |   +-------+          |
   |       |Key Id |-------->------------->------|Key Id |          |
   |       +-------+      |    |       |     |   +-------+          |
   +----------------------+    +-------+     +----------------------+
        CT-KIP Server        CT-KIP Client     CT-KIP Client (Token)
                               (PC Host)

   Figure 2: Principal data flow for CT-KIP key generation - using
   public server key

   The inclusion of the two random nonces R_S and R_C in the key
   generation provides assurance to both sides (the token and the CT-KIP
   server) that they have contributed to the key's randomness and that
   the key is unique.  The inclusion of the encryption key K ensures
   that no man-in-the-middle may be present, or else the cryptographic
   token will end up with a key different from the one stored by the
   legitimate CT-KIP server.

   Note: A man-in-the middle (in the form of corrupt client software or
   a mistakenly contacted server) may present his own public key to the
   token.  This will enable the attacker to learn the client's version

   of K_TOKEN.  However, the attacker is not able to persuade the
   legitimate server to derive the same value for K_TOKEN, since K_TOKEN
   is a function of the public key involved, and the attacker's public
   key must be different than the correct server's (or else the attacker
   would not be able to decrypt the information received from the
   client).  Therefore, once the attacker is no longer "in the middle",
   the client and server will detect that they are "out of synch" when
   they try to use their keys.  Therefore, in the case of encrypting R_C
   with K_SERVER, it is important to verify that K_SERVER really is the
   legitimate server's key.  One way to do this is to independently
   validate a newly generated K_TOKEN against some validation service at
   the server (e.g., by using a connection independent from the one used
   for the key generation).

   The CT-KIP server may couple an initial user authentication to the
   CT-KIP execution in several ways to ensure that a generated K_TOKEN
   ends up associated with the correct token and user.  One way is to
   provide a one-time value to the user or CT-KIP client after
   successful user authentication and require this value to be used when
   contacting the CT-KIP service (in effect coupling the user
   authentication with the subsequent CT-KIP protocol run).  This value
   could, for example, be placed in a <TriggerNonce> element of the CT-
   KIP initialization trigger (if triggers are used; see Section 4.2.7).
   Another way is for the user to provide a token identifier which will
   later be used in the CT-KIP protocol to the server during the
   authentication phase.  The server may then include this token
   identifier in the CT-KIP initialization trigger.  It is also
   legitimate for a CT-KIP client to initiate a CT-KIP protocol run
   without having received an initialization message from a server, but
   in this case any provided token identifier shall not be accepted by
   the server unless the server has access to a unique token key for the
   identified token and that key will be used in the protocol.  Whatever
   the method, the CT-KIP server must ensure that a generated key is
   associated with the correct token and, if applicable, the correct
   user.  For a further discussion of this and threats related to man-
   in-the-middle attacks in this context, see Section 5.5.

3.4.  The CT-KIP One-Way Pseudorandom Function, CT-KIP-PRF

3.4.1.  Introduction

   The general requirements on CT-KIP-PRF are the same as on keyed hash
   functions: It shall take an arbitrary length input, and be one-way
   and collision-free (for a definition of these terms, see, e.g., [4]).
   Further, the CT-KIP-PRF function shall be capable of generating a
   variable-length output, and its output shall be unpredictable even if
   other outputs for the same key are known.

   It is assumed that any realization of CT-KIP-PRF takes three input
   parameters: A secret key k, some combination of variable data, and
   the desired length of the output.  Examples of the variable data
   include, but are not limited to, a current token counter value, the
   current token time, and a challenge.  The combination of variable
   data can, without loss of generalization, be considered as a salt
   value (see PKCS #5 Version 2.0 [5], Section 4), and this
   characterization of CT-KIP-PRF should fit all actual PRF algorithms
   implemented by tokens.  From the point of view of this specification,
   CT-KIP-PRF is a "black-box" function that, given the inputs,
   generates a pseudorandom value.

   Separate specifications may define the implementation of CT-KIP-PRF
   for various types of cryptographic tokens.  Appendix D contains two
   example realizations of CT-KIP-PRF.

3.4.2.  Declaration

   CT-KIP-PRF (k, s, dsLen)

   Input:

   k     secret key in octet string format

   s     octet string of varying length consisting of variable data
         distinguishing the particular string being derived

   dsLen desired length of the output

   Output:

   DS    pseudorandom string, dsLen-octets long

   For the purposes of this document, the secret key k shall be 16
   octets long.

3.5.  Generation of Cryptographic Keys for Tokens

   In CT-KIP, keys are generated using the CT-KIP-PRF function, a secret
   random value R_C chosen by the CT-KIP client, a random value R_S
   chosen by the CT-KIP server, and the key k used to encrypt R_C.  The
   input parameter s of CT-KIP-PRF is set to the concatenation of the
   (ASCII) string "Key generation", k, and R_S, and the input parameter
   dsLen is set to the desired length of the key, K_TOKEN (the length of
   K_TOKEN is given by the key's type):

   dsLen = (desired length of K_TOKEN)

   K_TOKEN = CT-KIP-PRF (R_C, "Key generation" || k || R_S, dsLen)

   When computing K_TOKEN above, the output of CT-KIP-PRF may be subject
   to an algorithm-dependent transform before being adopted as a key of
   the selected type.  One example of this is the need for parity in DES
   keys.

3.6.  Encryption of Pseudorandom Nonces Sent from the CT-KIP Client

   CT-KIP client random nonce(s) are either encrypted with the public
   key provided by the CT-KIP server or by a shared secret key.  For
   example, in the case of a public RSA key, an RSA encryption scheme
   from PKCS #1 [6] may be used.

   In the case of a shared secret key, to avoid dependence on other
   algorithms, the CT-KIP client may use the CT-KIP-PRF function
   described herein with the shared secret key K_SHARED as input
   parameter k (in this case, K_SHARED should be used solely for this
   purpose), the concatenation of the (ASCII) string "Encryption" and
   the server's nonce R_S as input parameter s, and dsLen set to the
   length of R_C:

   dsLen = len(R_C)

   DS = CT-KIP-PRF(K_SHARED, "Encryption" || R_S, dsLen)

   This will produce a pseudorandom string DS of length equal to R_C.
   Encryption of R_C may then be achieved by XOR-ing DS with R_C:

   Enc-R_C = DS ^ R_C

   The CT-KIP server will then perform the reverse operation to extract
   R_C from Enc-R_C.

   Note: It may appear that an attacker, who learns a previous value of
   R_C, may be able to replay the corresponding R_S and, hence, learn a
   new R_C as well.  However, this attack is mitigated by the
   requirement for a server to show knowledge of K_AUTH (see below) in
   order to successfully complete a key re-generation.

3.7.  CT-KIP Schema Basics

3.7.1.  Introduction

   Core parts of the XML schema for CT-KIP, found in Appendix A, are
   explained in this section.  Specific protocol message elements are
   defined in Section 3.8.  Examples can be found in Appendix B.

   The XML format for CT-KIP messages have been designed to be
   extensible.  However, it is possible that the use of extensions will
   harm interoperability; therefore, any use of extensions should be
   carefully considered.  For example, if a particular implementation
   relies on the presence of a proprietary extension, then it may not be
   able to interoperate with independent implementations that have no
   knowledge of this extension.

   XML types defined in this sub-section are not CT-KIP messages; rather
   they provide building blocks that are used by CT-KIP messages.

3.7.2.  General XML Schema Requirements

   Some CT-KIP elements rely on the parties being able to compare
   received values with stored values.  Unless otherwise noted, all
   elements in this document that have the XML Schema "xs:string" type,
   or a type derived from it, must be compared using an exact binary
   comparison.  In particular, CT-KIP implementations must not depend on
   case-insensitive string comparisons, normalization or trimming of
   white space, or conversion of locale-specific formats such as
   numbers.

   Implementations that compare values that are represented using
   different character encodings must use a comparison method that
   returns the same result as converting both values to the Unicode
   character encoding, Normalization Form C [1], and then performing an
   exact binary comparison.

   No collation or sorting order for attributes or element values is
   defined.  Therefore, CT-KIP implementations must not depend on
   specific sorting orders for values.

3.7.3.  The AbstractRequestType Type

   All CT-KIP requests are defined as extensions to the abstract
   AbstractRequestType type.  The elements of the AbstractRequestType,
   therefore, apply to all CT-KIP requests.  All CT-KIP requests must
   contain a Version attribute.  For this version of this specification,
   Version shall be set to "1.0".

   <xs:complexType name="AbstractRequestType" abstract="true">
     <xs:attribute name="Version" type="VersionType"
      use="required"/>
   </xs:complexType>

3.7.4.  The AbstractResponseType type

   All CT-KIP responses are defined as extensions to the abstract
   AbstractResponseType type.  The elements of the AbstractResponseType,
   therefore, apply to all CT-KIP responses.  All CT-KIP responses
   contain a Version attribute indicating the version that was used.  A
   Status attribute, which indicates whether the preceding request was
   successful or not must also be present.  Finally, all responses may
   contain a SessionID attribute identifying the particular CT-KIP
   session.  The SessionID attribute needs only be present if more than
   one roundtrip is required for a successful protocol run (this is the
   case with the protocol version described herein).

   <xs:complexType name="AbstractResponseType" abstract="true">
     <xs:attribute name="Version" type="VersionType" use="required"/>
     <xs:attribute name="SessionID" type="IdentifierType"/>
     <xs:attribute name="Status" type="StatusCode" use="required"/>
   </xs:complexType>

3.7.5.  The StatusCode Type

   The StatusCode type enumerates all possible return codes:

   <xs:simpleType name="StatusCode">
     <xs:restriction base="xs:string">
       <xs:enumeration value="Continue"/>
       <xs:enumeration value="Success"/>
       <xs:enumeration value="Abort"/>
       <xs:enumeration value="AccessDenied"/>
       <xs:enumeration value="MalformedRequest"/>
       <xs:enumeration value="UnknownRequest"/>
       <xs:enumeration value="UnknownCriticalExtension"/>
       <xs:enumeration value="UnsupportedVersion"/>
       <xs:enumeration value="NoSupportedKeyTypes"/>
       <xs:enumeration value="NoSupportedEncryptionAlgorithms"/>
       <xs:enumeration value="NoSupportedMACAlgorithms"/>
       <xs:enumeration value="InitializationFailed"/>
     </xs:restriction>
   </xs:simpleType>

   Upon transmission or receipt of a message for which the Status
   attribute's value is not "Success" or "Continue", the default
   behavior, unless explicitly stated otherwise below, is that both the

   CT-KIP server and the CT-KIP client shall immediately terminate the
   CT-KIP session.  CT-KIP servers and CT-KIP clients must delete any
   secret values generated as a result of failed runs of the CT-KIP
   protocol.  Session identifiers may be retained from successful or
   failed protocol runs for replay detection purposes, but such retained
   identifiers shall not be reused for subsequent runs of the protocol.

   When possible, the CT-KIP client should present an appropriate error
   message to the user.

   These status codes are valid in all CT-KIP-Response messages unless
   explicitly stated otherwise.

   o  "Continue" indicates that the CT-KIP server is ready for a
      subsequent request from the CT-KIP client.  It cannot be sent in
      the server's final message.

   o  "Success" indicates successful completion of the CT-KIP session.
      It can only be sent in the server's final message.

   o  "Abort" indicates that the CT-KIP server rejected the CT-KIP
      client's request for unspecified reasons.

   o  "AccessDenied" indicates that the CT-KIP client is not authorized
      to contact this CT-KIP server.

   o  "MalformedRequest" indicates that the CT-KIP server failed to
      parse the CT-KIP client's request.

   o  "UnknownRequest" indicates that the CT-KIP client made a request
      that is unknown to the CT-KIP server.

   o  "UnknownCriticalExtension" indicates that a critical CT-KIP
      extension (see below) used by the CT-KIP client was not supported
      or recognized by the CT-KIP server.

   o  "UnsupportedVersion" indicates that the CT-KIP client used a CT-
      KIP protocol version not supported by the CT-KIP server.  This
      error is only valid in the CT-KIP server's first response message.

   o  "NoSupportedKeyTypes" indicates that the CT-KIP client only
      suggested key types that are not supported by the CT-KIP server.
      This error is only valid in the CT-KIP server's first response
      message.  Note that the error will only occur if the CT-KIP server
      does not support any of the CT-KIP client's suggested key types.

   o  "NoSupportedEncryptionAlgorithms" indicates that the CT-KIP client
      only suggested encryption algorithms that are not supported by the
      CT-KIP server.  This error is only valid in the CT-KIP server's
      first response message.  Note that the error will only occur if
      the CT-KIP server does not support any of the CT-KIP client's
      suggested encryption algorithms.

   o  "NoSupportedMACAlgorithms" indicates that the CT-KIP client only
      suggested MAC algorithms that are not supported by the CT-KIP
      server.  This error is only valid in the CT-KIP server's first
      response message.  Note that the error will only occur if the CT-
      KIP server does not support any of the CT-KIP client's suggested
      MAC algorithms.

   o  "InitializationFailed" indicates that the CT-KIP server could not
      generate a valid key given the provided data.  When this status
      code is received, the CT-KIP client should try to restart CT-KIP,
      as it is possible that a new run will succeed.

3.7.6.  The IdentifierType Type

   The IdentifierType type is used to identify various CT-KIP elements,
   such as sessions, users, and services.  Identifiers must not be
   longer than 128 octets.

   <xs:simpleType name="IdentifierType">
     <xs:restriction base="xs:string">
       <xs:maxLength value="128"/>
     </xs:restriction>
   </xs:simpleType>

3.7.7.  The NonceType Type

   The NonceType type is used to carry pseudorandom values in CT-KIP
   messages.  A nonce, as the name implies, must be used only once.  For
   each CT-KIP message that requires a nonce element to be sent, a fresh
   nonce shall be generated each time.  Nonce values must be at least 16
   octets long.

   <xs:simpleType name="NonceType">
     <xs:restriction base="xs:base64Binary">
       <xs:minLength value="16"/>
     </xs:restriction>
   </xs:simpleType>

3.7.8.  The ExtensionsType and the AbstractExtensionType Types

   The ExtensionsType type is a list of type-value pairs that define
   optional CT-KIP features supported by a CT-KIP client or server.
   Extensions may be sent with any CT-KIP message.  Please see the
   description of individual CT-KIP messages in Section 3.8 of this
   document for applicable extensions.  Unless an extension is marked as
   Critical, a receiving party need not be able to interpret it.  A
   receiving party is always free to disregard any (non-critical)
   extensions.

   <xs:complexType name="AbstractExtensionsType">
     <xs:sequence maxOccurs="unbounded">
       <xs:element name="Extension" type="AbstractExtensionType"/>
     </xs:sequence>
   </xs:complexType>

   <xs:complexType name="AbstractExtensionType" abstract="true">
     <xs:attribute name="Critical" type="xs:boolean"/>
   </xs:complexType>

3.8.  CT-KIP Messages

3.8.1.  Introduction

   In this section, CT-KIP messages, including their parameters,
   encodings and semantics are defined.

3.8.2.  CT-KIP Initialization

   The CT-KIP server may initialize the CT-KIP protocol by sending a
   <CT-KIPTrigger> message.  This message may, e.g., be sent in response
   to a user requesting token initialization in a browsing session.

   <xs:complexType name="InitializationTriggerType">
     <xs:sequence>
       <xs:element name="TokenID" type="xs:base64Binary" minOccurs="0"/>
       <xs:element name="KeyID" type="xs:base64Binary" minOccurs="0"/>
       <xs:element name="TokenPlatformInfo"
         type="TokenPlatformInfoType" minOccurs="0"/>
       <xs:element name="TriggerNonce" type="NonceType"/>
       <xs:element name="CT-KIPURL" type="xs:anyURI" minOccurs="0"/>
       <xs:any namespace="##other" processContents="strict"
         minOccurs="0"/>
     </xs:sequence>
     <xs:attribute name="id" type="xs:ID"/>
   </xs:complexType>

   <xs:element name="CT-KIPTrigger" type="CT-KIPTriggerType"/>

   <xs:complexType name="CT-KIPTriggerType">
     <xs:annotation>
       <xs:documentation xml:lang="en">
          Message used to trigger the device to initiate a
          CT-KIP run.
       </xs:documentation>
     </xs:annotation>
     <xs:sequence>
       <xs:choice>
         <xs:element name="InitializationTrigger"
           type="InitializationTriggerType"/>
         <xs:any nameSpace="##other" processContents="strict"/>
       </xs:choice>
     </xs:sequence>
     <xs:attribute name="Version" type="ct-kip:VersionType"/>
   </xs:complexType>

   The <CT-KIPTrigger> element is intended for the CT-KIP client and may
   inform the CT-KIP client about the identifier for the token that is
   to be initialized, and, optionally, of the identifier for the key on
   that token.  The latter would apply when re-seeding.  The trigger
   always contains a nonce to allow the server to couple the trigger
   with a later CT-KIP <ClientHello> request.  Finally, the trigger may
   contain a URL to use when contacting the CT-KIP server.  The <xs:any>
   elements are for future extensibility.  Any provided <TokenID> or
   <KeyID> values shall be used by the CT-KIP client in the subsequent
   <ClientHello> request.  The optional <TokenPlatformInfo> element
   informs the CT-KIP client about the characteristics of the intended
   token platform, and applies in the public-key variant of CT-KIP in
   situations when the client potentially needs to decide which one of
   several tokens to initialize.

   The Version attribute shall be set to "1.0" for this version of CT-
   KIP.

3.8.3.  The CT-KIP Client's Initial PDU

   This message is the initial message sent from the CT-KIP client to
   the CT-KIP server.

   <xs:element name="ClientHello" type="ClientHelloPDU"/>

   <xs:complexType name="ClientHelloPDU">
     <xs:annotation>
       <xs:documentation xml:lang="en">
          Message sent from CT-KIP client to CT-KIP server to

          initiate a CT-KIP session.
       </xs:documentation>
     </xs:annotation>
     <xs:complexContent>
       <xs:extension base="AbstractRequestType">
         <xs:sequence>
           <xs:element name="TokenID"
             type="xs:base64Binary" minOccurs="0"/>
           <xs:element name="KeyID"
             type="xs:base64Binary" minOccurs="0"/>
           <xs:element name="ClientNonce"
             type="NonceType" minOccurs="0"/>
           <xs:element name= "TriggerNonce"
             type="NonceType" minOccurs="0"/>
           <xs:element name="SupportedKeyTypes"
             type="AlgorithmsType"/>
           <xs:element name="SupportedEncryptionAlgorithms"
             type="AlgorithmsType"/>
           <xs:element name="SupportedMACAlgorithms"
             type="AlgorithmsType"/>
           <xs:element name="Extensions"
             type="ExtensionsType" minOccurs="0"/>
         </xs:sequence>
       </xs:extension>
     </xs:complexContent>
   </xs:complexType>

   The components of this message have the following meaning:

   o  Version: (attribute inherited from the AbstractRequestType type)
      The highest version of this protocol the client supports.  Only
      version one ("1.0") is currently specified.

   o  <TokenID>: An identifier for the cryptographic token (allows the
      server to find, e.g., a correct shared secret for MACing
      purposes).  The identifier shall only be present if such shared
      secrets exist or if the identifier was provided by the server in a
      <CT-KIPTrigger> element (see Section 4.2.7 below).  In the latter
      case, it must have the same value as the identifier provided in
      that element.

   o  <KeyID>: An identifier for the key that will be overwritten if the
      protocol run is successful.  The identifier shall only be present
      if the key exists or was provided by the server in a
      <CT-KIPTrigger> element (see Section 4.2.7 below).  In the latter
      case, it must have the same value as the identifier provided in
      that element.

   o  <ClientNonce>: This is the nonce R, which, when present, shall be
      used by the server when calculating MAC values (see below).  It is
      recommended that clients include this element whenever the <KeyID>
      element is present.

   o  <TriggerNonce>: This optional element shall be present if and only
      if the CT-KIP run was initialized with a <CT-KIPTrigger> message
      (see Section 4.2.7 below), and shall, in that case, have the same
      value as the <TriggerNonce> child of that message.  A server using
      nonces in this way must verify that the nonce is valid and that
      any token or key identifier values provided in the <CT-KIPTrigger>
      message match the corresponding identifier values in the
      <ClientHello> message.

   o  <SupportedKeyTypes>: A sequence of URIs indicating the key types
      for which the token is willing to generate keys through CT-KIP.

   o  <SupportedEncryptionAlgorithms>: A sequence of URIs indicating the
      encryption algorithms supported by the cryptographic token for the
      purposes of CT-KIP.  The CT-KIP client may indicate the same
      algorithm both as a supported key type and as an encryption
      algorithm.

   o  <SupportedMACAlgorithms>: A sequence of URIs indicating the MAC
      algorithms supported by the cryptographic token for the purposes
      of CT-KIP.  The CT-KIP client may indicate the same algorithm both
      as an encryption algorithm and as a MAC algorithm (e.g., http://
      www.rsasecurity.com/rsalabs/otps/schemas/2005/12/
      ct-kip#ct-kip-prf-aes defined in Appendix D)

   o  <Extensions>: A sequence of extensions.  One extension is defined
      for this message in this version of CT-KIP: the ClientInfoType
      (see Section 3.9.1).

3.8.4.  The CT-KIP server's initial PDU

   This message is the first message sent from the CT-KIP server to the
   CT-KIP client (assuming a trigger message has not been sent to
   initiate the protocol, in which case, this message is the second
   message sent from the CT-KIP server to the CT-KIP client).  It is
   sent upon reception of a <ClientHello> message.

   <xs:element name="ServerHello" type="ServerHelloPDU"/>

   <xs:complexType name="ServerHelloPDU">
     <xs:annotation>
       <xs:documentation xml:lang="en">
         Message sent from CT-KIP server to CT-KIP

         client in response to a received ClientHello
         PDU.
       </xs:documentation>
     </xs:annotation>
     <xs:complexContent>
       <xs:extension base="AbstractResponseType">
         <xs:sequence minOccurs="0">
           <xs:element name="KeyType"
             type="AlgorithmType"/>
           <xs:element name="EncryptionAlgorithm"
             type="AlgorithmType"/>
           <xs:element name="MacAlgorithm"
             type="AlgorithmType"/>
           <xs:element name="EncryptionKey"
             type="ds:KeyInfoType"/>
           <xs:element name="Payload"
             type="PayloadType"/>
           <xs:element name="Extensions"
             type="ExtensionsType" minOccurs="0"/>
           <xs:element name="Mac" type="MacType"
             minOccurs="0"/>
         </xs:sequence>
       </xs:extension>
     </xs:complexContent>
   </xs:complexType>


   <xs:complexType name="PayloadType">
     <xs:annotation>
       <xs:documentation xml:lang="en">
         Currently, only the nonce is defined.  In future versions,
         other payloads may be defined, e.g., for one-roundtrip
         initialization protocols.
       </xs:documentation>
     </xs:annotation>
     <xs:choice>
       <xs:element name="Nonce" type="NonceType"/>
       <any namespace="##other" processContents="strict"/>
     </xs:choice>
   </xs:complexType>

   <xs:complexType name="MacType">
     <xs:simpleContent>
       <xs:extension base="xs:base64Binary">
         <xs:attribute name="MacAlgorithm" type="xs:anyURI"/>
       </xs:extension>
     </xs:simpleContent>
   </xs:complexType>

   The components of this message have the following meaning:

   o  Version: (attribute inherited from the AbstractResponseType type)
      The version selected by the CT-KIP server.  May be lower than the
      version indicated by the CT-KIP client, in which case, local
      policy at the client will determine whether or not to continue the
      session.

   o  SessionID: (attribute inherited from the AbstractResponseType
      type) An identifier for this session.

   o  Status: (attribute inherited from the abstract
      AbstractResponseType type) Return code for the <ClientHello>.  If
      Status is not "Continue", only the Status and Version attributes
      will be present; otherwise, all the other elements must be present
      as well.

   o  <KeyType>: The type of the key to be generated.

   o  <EncryptionAlgorithm>: The encryption algorithm to use when
      protecting R_C.

   o  <MacAlgorithm>: The MAC algorithm to be used by the CT-KIP server.

   o  <EncryptionKey>: Information about the key to use when encrypting
      R_C.  It will either be the server's public key (the <ds:KeyValue>
      alternative of ds:KeyInfoType) or an identifier for a shared
      secret key (the <ds:KeyName> alternative of ds:KeyInfoType).

   o  <Payload>: The actual payload.  For this version of the protocol,
      only one payload is defined: the pseudorandom string R_S.

   o  <Extensions>: A list of server extensions.  Two extensions are
      defined for this message in this version of CT-KIP: the
      ClientInfoType and the ServerInfoType (see Section 3.9).

   o  <Mac>: The MAC must be present if the CT-KIP run will result in
      the replacement of an existing token key with a new one (i.e., if
      the <KeyID> element was present in the <ClientHello> message).  In
      this case, the CT-KIP server must prove to the cryptographic token
      that it is authorized to replace it.  The MAC value shall be
      computed on the (ASCII) string "MAC 1 computation", the client's
      nonce R (if sent), and the server's nonce R_S using an
      authentication key K_AUTH that should be a special authentication
      key used only for this purpose but may be the current K_TOKEN.

      The MAC value may be computed by using the CT-KIP-PRF function of
      Section 3.4, in which case the input parameter s shall be set to
      the concatenation of the (ASCII) string "MAC 1 computation", R (if
      sent by the client), and R_S, and k shall be set to K_AUTH.  The
      input parameter dsLen shall be set to the length of R_S:

      dsLen = len(R_S)

      MAC = CT-KIP-PRF (K_AUTH, "MAC 1 computation" || [R ||] R_S,
      dsLen)

      The CT-KIP client must verify the MAC if the successful execution
      of the protocol will result in the replacement of an existing
      token key with a newly generated one.  The CT-KIP client must
      terminate the CT-KIP session if the MAC does not verify, and must
      delete any nonces, keys, and/or secrets associated with the failed
      run of the CT-KIP protocol.

      The MacType's MacAlgorithm attribute shall, when present, identify
      the negotiated MAC algorithm.

3.8.5.  The CT-KIP Client's Second PDU

   This message contains the nonce chosen by the cryptographic token,
   R_C, encrypted by the specified encryption key and encryption
   algorithm.

   <xs:element name="ClientNonce" type="ClientNoncePDU"/>

   <xs:complexType name="ClientNoncePDU">
     <xs:annotation>
       <xs:documentation xml:lang="en">
         Second message sent from CT-KIP client to
         CT-KIP server in a CT-KIP session.
       </xs:documentation>
     </xs:annotation>
     <xs:complexContent>
       <xs:extension base="AbstractRequestType">
         <xs:sequence>
           <xs:element name="EncryptedNonce"
             type="xs:base64Binary"/>
           <xs:element name="Extensions"
             type="ExtensionsType" minOccurs="0"/>
         </xs:sequence>
         <xs:attribute name="SessionID" type="IdentifierType"
           use="required"/>
       </xs:extension>
     </xs:complexContent>

   </xs:complexType>

   The components of this message have the following meaning:

   o  Version: (inherited from the AbstractRequestType type) Shall be
      the same version as in the <ServerHello> message.

   o  SessionID: Shall have the same value as the SessionID attribute in
      the received <ServerHello> message.

   o  <EncryptedNonce>: The nonce generated and encrypted by the token.
      The encryption shall be made using the selected encryption
      algorithm and identified key, and as specified in Section 3.4.

   o  <Extensions>: A list of extensions.  Two extensions are defined
      for this message in this version of CT-KIP: the ClientInfoType and
      the ServerInfoType (see Section 3.9).

3.8.6.  The CT-KIP Server's Final PDU

   This message is the last message of a two roundtrip CT-KIP exchange.
   The CT-KIP server sends this message to the CT-KIP client in response
   to the <ClientNonce> message.

   <xs:element name="ServerFinished" type="ServerFinishedPDU"/>

   <xs:complexType name="ServerFinishedPDU">
     <xs:annotation>
       <xs:documentation xml:lang="en">
         Final message sent from CT-KIP server to
         CT-KIP client in a CT-KIP session.
       </xs:documentation>
     </xs:annotation>
     <xs:complexContent>
       <xs:extension base="AbstractResponseType">
         <xs:sequence minOccurs="0">
           <xs:element name="TokenID"
             type="xs:base64Binary"/>
           <xs:element name="KeyID"
             type="xs:base64Binary"/>
           <xs:element name="KeyExpiryDate"
             type="xs:dateTime" minOccurs="0"/>
           <xs:element name="ServiceID"
             type="IdentifierType" minOccurs="0"/>
           <xs:element name="ServiceLogo"
             type="LogoType" minOccurs="0"/>
           <xs:element name="UserID"
             type="IdentifierType" minOccurs="0"/>

           <xs:element name="Extensions"
             type="ExtensionsType" minOccurs="0"/>
           <xs:element name="Mac"
             type="MacType"/>
         </xs:sequence>
       </xs:extension>
     </xs:complexContent>
   </xs:complexType>

   The components of this message have the following meaning:

   o  Version: (inherited from the AbstractResponseType type) The CT-KIP
      version used in this session.

   o  SessionID: (inherited from the AbstractResponseType type) The
      previously established identifier for this session.

   o  Status: (inherited from the AbstractResponseType type) Return code
      for the <ServerFinished> message.  If Status is not "Success",
      only the Status, SessionID, and Version attributes will be present
      (the presence of the SessionID attribute is dependent on the type
      of reported error); otherwise, all the other elements must be
      present as well.  In this latter case, the <ServerFinished>
      message can be seen as a "Commit" message, instructing the
      cryptographic token to store the generated key and associate the
      given key identifier with this key.

   o  <TokenID>: An identifier for the token carrying the generated key.
      Must have the same value as the <TokenID> element of the
      <ClientHello> message, if one was provided.  When assigned by the
      CT-KIP server, the <TokenID> element shall be unique within the
      domain of the CT-KIP server.

   o  <KeyID>: An identifier for the newly generated key.  The
      identifier shall be globally unique.  Must have the same value as
      any key identifier provided by the CT-KIP client in the
      <ClientHello> message.

      The reason for requiring globally unique key identifiers is that
      it avoids potential conflicts when associating key holders with
      key identifiers.  One way of achieving global uniqueness with
      reasonable certainty is to hash the combination of the issuer's
      fully qualified domain name with an (issuer-specific) serial
      number, assuming that each issuer makes sure their serial numbers
      never repeat.

      CT-KIP clients must support key identifiers at least 64 octets
      long.  CT-KIP servers should not generate key identifiers longer
      than 64 octets.

   o  <KeyExpiryDate>: This optional element provides the date and time
      after which the generated key should be treated as expired and
      invalid.

   o  <ServiceID>: An optional identifier for the service that has
      stored the generated key.  The cryptographic token may store this
      identifier associated with the key in order to simplify later
      lookups.  The identifier shall be a printable string.

   o  <ServiceLogo>: This optional element provides a graphical logo
      image for the service that can be displayed in user interfaces,
      e.g., to help a user select a certain key.  The logo should
      contain an image within the size range of 60 pixels wide by 45
      pixels high, and 200 pixels wide by 150 pixels high.  The required
      MimeType attribute of this type provides information about the
      MIME type of the image.  This specification supports both the JPEG
      and GIF image formats (with MIME types of "image/jpeg" and "image/
      gif").

   o  <UserID>: An optional identifier for the user associated with the
      generated key in the authentication service.  The cryptographic
      token may store this identifier associated with the generated key
      in order to enhance later user experiences.  The identifier shall
      be a printable string.

   o  <Extensions>: A list of extensions chosen by the CT-KIP server.
      For this message, this version of CT-KIP defines two extensions,
      the OTPKeyConfigurationDataType and the ClientInfoType (see
      Section 3.9).

   o  <Mac>: To avoid a false "Commit" message causing the token to end
      up in an initialized state for which the server does not know the
      stored key, <ServerFinished> messages must always be authenticated
      with a MAC.  The MAC shall be made using the already established
      MAC algorithm.  The MAC value shall be computed on the (ASCII)
      string "MAC 2 computation" and R_C using an authentication key
      K_AUTH.  Again, this should be a special authentication key used
      only for this purpose, but may also be an existing K_TOKEN.  (In
      this case, implementations must protect against attacks where
      K_TOKEN is used to pre-compute MAC values.)  If no authentication
      key is present in the token, and no K_TOKEN existed before the CT-
      KIP run, K_AUTH shall be the newly generated K_TOKEN.

      If CT-KIP-PRF is used as the MAC algorithm, then the input
      parameter s shall consist of the concatenation of the (ASCII)
      string "MAC 2 computation" and R_C, and the parameter dsLen shall
      be set to the length of R_C:

      dsLen = len(R_C)

      MAC = CT-KIP-PRF (K_AUTH, "MAC 2 computation" || R_C, dsLen)

      When receiving a <ServerFinished> message with Status = "Success"
      for which the MAC verifies, the CT-KIP client shall associate the
      generated key K_TOKEN with the provided key identifier and store
      this data permanently.  After this operation, it shall not be
      possible to overwrite the key unless knowledge of an authorizing
      key is proven through a MAC on a later <ServerHello> (and
      <ServerFinished>) message.

      The CT-KIP client must verify the MAC.  The CT-KIP client must
      terminate the CT-KIP session if the MAC does not verify, and must,
      in this case, also delete any nonces, keys, and/or secrets
      associated with the failed run of the CT-KIP protocol.

      The MacType's MacAlgorithm attribute shall, when present, identify
      the negotiated MAC algorithm.

3.9.  Protocol Extensions

3.9.1.  The ClientInfoType Type

   When present in a <ClientHello> or a <ClientNonce> message, the
   optional ClientInfoType extension contains CT-KIP client-specific
   information.  CT-KIP servers must support this extension.  CT-KIP
   servers must not attempt to interpret the data it carries and, if
   received, must include it unmodified in the current protocol run's
   next server response.  Servers need not retain the ClientInfoType's
   data after that response has been generated.

   <xs:complexType name="ClientInfoType">
     <xs:complexContent>
       <xs:extension base="AbstractExtensionType">
         <xs:sequence>
           <xs:element name="Data"
             type="xs:base64Binary"/>
         </xs:sequence>
       </xs:extension>
     </xs:complexContent>
   </xs:complexType>

3.9.2.  The ServerInfoType Type

   When present, the optional ServerInfoType extension contains CT-KIP
   server-specific information.  This extension is only valid in
   <ServerHello> messages for which Status = "Continue".  CT-KIP clients
   must support this extension.  CT-KIP clients must not attempt to
   interpret the data it carries and, if received, must include it
   unmodified in the current protocol run's next client request (i.e.,
   the <ClientNonce> message).  CT-KIP clients need not retain the
   ServerInfoType's data after that request has been generated.  This
   extension may be used, e.g., for state management in the CT-KIP
   server.

   <xs:complexType name="ServerInfoType">
     <xs:complexContent>
       <xs:extension base="AbstractExtensionType">
         <xs:sequence>
           <xs:element name="Data"
             type="xs:base64Binary"/>
         </xs:sequence>
       </xs:extension>
     </xs:complexContent>
   </xs:complexType>

3.9.3.  The OTPKeyConfigurationDataType Type

   The optional OTPKeyConfigurationDataType extension contains
   additional key configuration data for OTP keys:

   <xs:complexType name="OTPKeyConfigurationDataType">
     <xs:annotation>
       <xs:documentation xml:lang="en">
         This extension is only valid in ServerFinished
         PDUs.  It carries additional configuration data
         that an OTP token should use (subject to local
         policy) when generating OTP values with a newly
         generated OTP key.
       </xs:documentation>
     </xs:annotation>
     <xs:complexContent>
       <xs:extension base="ExtensionType">
         <xs:sequence>
           <xs:element name="OTPFormat"
             type="OTPFormatType"/>
           <xs:element name="OTPLength"
             type="xs:positiveInteger"/>
           <xs:element name="OTPMode"
             type="OTPModeType" minOccurs="0"/>

         </xs:sequence>
       </xs:extension>
     </xs:complexContent>
   </xs:complexType>

   This extension is only valid in <ServerFinished> messages.  It
   carries additional configuration data that the cryptographic token
   should use (subject to local policy) when generating OTP values from
   the newly generated OTP key.  The components of this extension have
   the following meaning:

   o  OTPFormat: The default format of OTPs produced with this key.

   o  OTPLength: The default length of OTPs produced with this key.

   o  OTPMode: The default mode of operation when producing OTPs with
      this key.

4.  Protocol Bindings

4.1.  General Requirement

   CT-KIP assumes a reliable transport.

4.2.  HTTP/1.1 binding for CT-KIP

4.2.1.  Introduction

   This section presents a binding of the previous messages to HTTP/1.1
   [7].  Note that the HTTP client normally will be different from the
   CT-KIP client, i.e., the HTTP client will only exist to "proxy" CT-
   KIP messages from the CT-KIP client to the CT-KIP server.  Likewise,
   on the HTTP server side, the CT-KIP server may receive CT-KIP PDUs
   from a "front-end" HTTP server.

4.2.2.  Identification of CT-KIP Messages

   The MIME-type for all CT-KIP messages shall be

   application/vnd.otps.ct-kip+xml

4.2.3.  HTTP Headers

   HTTP proxies must not cache responses carrying CT-KIP messages.  For
   this reason, the following holds:

   o  When using HTTP/1.1, requesters should:

      *  Include a Cache-Control header field set to "no-cache,
         no-store".

      *  Include a Pragma header field set to "no-cache".

   o  When using HTTP/1.1, responders should:

      *  Include a Cache-Control header field set to "no-cache,
         no-must-revalidate, private".

      *  Include a Pragma header field set to "no-cache".

      *  NOT include a Validator, such as a Last-Modified or ETag
         header.

   There are no other restrictions on HTTP headers, besides the
   requirement to set the Content-Type header value to application/
   vnd.otps.ct-kip+xml.

4.2.4.  HTTP Operations

   Persistent connections as defined in HTTP/1.1 are assumed but not
   required.  CT-KIP requests are mapped to HTTP POST operations.  CT-
   KIP responses are mapped to HTTP responses.

4.2.5.  HTTP Status Codes

   A CT-KIP HTTP responder that refuses to perform a message exchange
   with a CT-KIP HTTP requester should return a 403 (Forbidden)
   response.  In this case, the content of the HTTP body is not
   significant.  In the case of an HTTP error while processing a CT-KIP
   request, the HTTP server must return a 500 (Internal Server Error)
   response.  This type of error should be returned for HTTP-related
   errors detected before control is passed to the CT-KIP processor, or
   when the CT-KIP processor reports an internal error (for example, the
   CT-KIP XML namespace is incorrect, or the CT-KIP schema cannot be
   located).  If the type of a CT-KIP request cannot be determined, the
   CT-KIP responder must return a 400 (Bad request) response.

   In these cases (i.e., when the HTTP response code is 4xx or 5xx), the
   content of the HTTP body is not significant.

   Redirection status codes (3xx) apply as usual.

   Whenever the HTTP POST is successfully invoked, the CT-KIP HTTP
   responder must use the 200 status code and provide a suitable CT-KIP
   message (possibly with CT-KIP error information included) in the HTTP
   body.

4.2.6.  HTTP Authentication

   No support for HTTP/1.1 authentication is assumed.

4.2.7.  Initialization of CT-KIP

   The CT-KIP server may initialize the CT-KIP protocol by sending an
   HTTP response with Content-Type set to application/
   vnd.otps.ct-kip+xml and response code set to 200 (OK).  This message
   may, e.g., be sent in response to a user requesting token
   initialization in a browsing session.  The initialization message may
   carry data in its body.  If this is the case, the data shall be a
   valid instance of a <CT-KIPTrigger> element.

4.2.8.  Example Messages

   a.  Initialization from CT-KIP server:

   HTTP/1.1 200 OK
   Cache-Control: no-store
   Content-Type: application/vnd.otps.ct-kip+xml
   Content-Length: <some value>

   CT-KIP initialization data in XML form...

   b.  Initial request from CT-KIP client:

   POST http://example.com/cgi-bin/CT-KIP-server HTTP/1.1
   Cache-Control: no-store
   Pragma: no-cache
   Host: example.com
   Content-Type: application/vnd.otps.ct-kip+xml
   Content-Length: <some value>

   CT-KIP data in XML form (supported version, supported algorithms...)

   c.  Initial response from CT-KIP server:

   HTTP/1.1 200 OK
   Cache-Control: no-store
   Content-Type: application/vnd.otps.ct-kip+xml
   Content-Length: <some other value>

   CT-KIP data in XML form (server random nonce, server public key, ...)

5.  Security considerations

5.1.  General

   CT-KIP is designed to protect generated key material from exposure.
   No other entities than the CT-KIP server and the cryptographic token
   will have access to a generated K_TOKEN if the cryptographic
   algorithms used are of sufficient strength and, on the CT-KIP client
   side, generation and encryption of R_C and generation of K_TOKEN take
   place as specified and in the token.  This applies even if malicious
   software is present in the CT-KIP client.  However, as discussed in
   the following, CT-KIP does not protect against certain other threats
   resulting from man-in-the-middle attacks and other forms of attacks.
   CT-KIP should, therefore, be run over a transport providing privacy
   and integrity, such as HTTP over Transport Layer Security (TLS) with
   a suitable ciphersuite, when such threats are a concern.  Note that
   TLS ciphersuites with anonymous key exchanges are not suitable in
   those situations.

5.2.  Active Attacks

5.2.1.  Introduction

   An active attacker may attempt to modify, delete, insert, replay or
   reorder messages for a variety of purposes including service denial
   and compromise of generated key material.  Sections 5.2.2 through
   5.2.7 analyze these attack scenarios.

5.2.2.  Message Modifications

   Modifications to a <CT-KIPTrigger> message will either cause denial-
   of-service (modifications of any of the identifiers or the nonce) or
   the CT-KIP client to contact the wrong CT-KIP server.  The latter is
   in effect a man-in-the-middle attack and is discussed further in
   Section 5.2.7.

   An attacker may modify a <ClientHello> message.  This means that the
   attacker could indicate a different key or token than the one
   intended by the CT-KIP client, and could also suggest other

   cryptographic algorithms than the ones preferred by the CT-KIP
   client, e.g., cryptographically weaker ones.  The attacker could also
   suggest earlier versions of the CT-KIP protocol, in case these
   versions have been shown to have vulnerabilities.  These
   modifications could lead to an attacker succeeding in initializing or
   modifying another token than the one intended (i.e., the server
   assigning the generated key to the wrong token), or gaining access to
   a generated key through the use of weak cryptographic algorithms or
   protocol versions.  CT-KIP implementations may protect against the
   latter by having strict policies about what versions and algorithms
   they support and accept.  The former threat (assignment of a
   generated key to the wrong token) is not possible when the shared-key
   variant of CT-KIP is employed (assuming existing shared keys are
   unique per token) but is possible in the public-key variant.
   Therefore, CT-KIP servers must not accept unilaterally provided token
   identifiers in the public-key variant.  This is also indicated in the
   protocol description.  In the shared-key variant, however, an
   attacker may be able to provide the wrong identifier (possibly also
   leading to the incorrect user being associated with the generated
   key) if the attacker has real-time access to the token with the
   identified key.  In other words, the generated key is associated with
   the correct token but the token is associated with the incorrect
   user.  See further Section 5.5 for a discussion of this threat and
   possible countermeasures.

   An attacker may also modify a <ServerHello> message.  This means that
   the attacker could indicate different key types, algorithms, or
   protocol versions than the legitimate server would, e.g.,
   cryptographically weaker ones.  The attacker could also provide a
   different nonce than the one sent by the legitimate server.  Clients
   will protect against the former through strict adherence to policies
   regarding permissible algorithms and protocol versions.  The latter
   (wrong nonce) will not constitute a security problem, as a generated
   key will not match the key generated on the legitimate server.  Also,
   whenever the CT-KIP run would result in the replacement of an
   existing key, the <Mac> element protects against modifications of
   R_S.

   Modifications of <ClientNonce> messages are also possible.  If an
   attacker modifies the SessionID attribute, then, in effect, a switch
   to another session will occur at the server, assuming the new
   SessionID is valid at that time on the server.  It still will not
   allow the attacker to learn a generated K_TOKEN since R_C has been
   wrapped for the legitimate server.  Modifications of the
   <EncryptedNonce> element, e.g., replacing it with a value for which
   the attacker knows an underlying R'C, will not result in the client
   changing its pre-CT-KIP state, since the server will be unable to
   provide a valid MAC in its final message to the client.  The server

   may, however, end up storing K'TOKEN rather than K_TOKEN.  If the
   token has been associated with a particular user, then this could
   constitute a security problem.  For a further discussion about this
   threat, and a possible countermeasure, see Section 5.5 below.  Note
   that use of Secure Socket Layer (SSL) or TLS does not protect against
   this attack if the attacker has access to the CT-KIP client (e.g.,
   through malicious software, "trojans").

   Finally, attackers may also modify the <ServerFinished> message.
   Replacing the <Mac> element will only result in denial-of-service.
   Replacement of any other element may cause the CT-KIP client to
   associate, e.g., the wrong service with the generated key.  CT-KIP
   should be run over a transport providing privacy and integrity when
   this is a concern.

5.2.3.  Message Deletion

   Message deletion will not cause any other harm than denial-of-
   service, since a token shall not change its state (i.e., "commit" to
   a generated key) until it receives the final message from the CT-KIP
   server and successfully has processed that message, including
   validation of its MAC.  A deleted <ServerFinished> message will not
   cause the server to end up in an inconsistent state vis-a-vis the
   token if the server implements the suggestions in Section 5.5.

5.2.4.  Message Insertion

   An active attacker may initiate a CT-KIP run at any time, and suggest
   any token identifier.  CT-KIP server implementations may receive some
   protection against inadvertently initializing a token or
   inadvertently replacing an existing key or assigning a key to a token
   by initializing the CT-KIP run by use of the <CT-KIPTrigger>.  The
   <TriggerNonce> element allows the server to associate a CT-KIP
   protocol run with, e.g., an earlier user-authenticated session.  The
   security of this method, therefore, depends on the ability to protect
   the <TriggerNonce> element in the CT-KIP initialization message.  If
   an eavesdropper is able to capture this message, he may race the
   legitimate user for a key initialization.  CT-KIP over a transport
   providing privacy and integrity, coupled with the recommendations in
   Section 5.5, is recommended when this is a concern.

   Insertion of other messages into an existing protocol run is seen as
   equivalent to modification of legitimately sent messages.

5.2.5.  Message Replay

   Attempts to replay a previously recorded CT-KIP message will be
   detected, as the use of nonces ensures that both parties are live.

5.2.6.  Message Reordering

   An attacker may attempt to re-order messages but this will be
   detected, as each message is of a unique type.

5.2.7.  Man in the Middle

   In addition to other active attacks, an attacker posing as a man in
   the middle may be able to provide his own public key to the CT-KIP
   client.  This threat and countermeasures to it are discussed in
   Section 3.3.  An attacker posing as a man-in-the-middle may also be
   acting as a proxy and, hence, may not interfere with CT-KIP runs but
   still learn valuable information; see Section 5.3.

5.3.  Passive Attacks

   Passive attackers may eavesdrop on CT-KIP runs to learn information
   that later on may be used to impersonate users, mount active attacks,
   etc.

   If CT-KIP is not run over a transport providing privacy, a passive
   attacker may learn:

   o  What tokens a particular user is in possession of;

   o  The identifiers of keys on those tokens and other attributes
      pertaining to those keys, e.g., the lifetime of the keys; and

   o  CT-KIP versions and cryptographic algorithms supported by a
      particular CT-KIP client or server.

   Whenever the above is a concern, CT-KIP should be run over a
   transport providing privacy.  If man-in-the-middle attacks for the
   purposes described above are a concern, the transport should also
   offer server-side authentication.

5.4.  Cryptographic Attacks

   An attacker with unlimited access to an initialized token may use the
   token as an "oracle" to pre-compute values that later on may be used
   to impersonate the CT-KIP server.  Sections 3.6 and 3.8 contain
   discussions of this threat and steps recommended to protect against
   it.

5.5.  Attacks on the Interaction between CT-KIP and User Authentication

   If keys generated in CT-KIP will be associated with a particular user
   at the CT-KIP server (or a server trusted by, and communicating with
   the CT-KIP server), then in order to protect against threats where an
   attacker replaces a client-provided encrypted R_C with his own R'C
   (regardless of whether the public-key variant or the shared-secret
   variant of CT-KIP is employed to encrypt the client nonce), the
   server should not commit to associate a generated K_TOKEN with the
   given token (user) until the user simultaneously has proven both
   possession of a token containing K_TOKEN and some out-of-band
   provided authenticating information (e.g., a temporary password).
   For example, if the token is a one-time password token, the user
   could be required to authenticate with both a one-time password
   generated by the token and an out-of-band provided temporary PIN in
   order to have the server "commit" to the generated token value for
   the given user.  Preferably, the user should perform this operation
   from another host than the one used to initialize the token, in order
   to minimize the risk of malicious software on the client interfering
   with the process.

   Another threat arises when an attacker is able to trick a user to
   authenticate to the attacker rather than to the legitimate service
   before the CT-KIP protocol run.  If successful, the attacker will
   then be able to impersonate the user towards the legitimate service,
   and subsequently receive a valid CT-KIP trigger.  If the public-key
   variant of CT-KIP is used, this may result in the attacker being able
   to (after a successful CT-KIP protocol run) impersonate the user.
   Ordinary precautions must, therefore, be in place to ensure that
   users authenticate only to legitimate services.

6.  Intellectual Property Considerations

   RSA and SecurID are registered trademarks or trademarks of RSA
   Security Inc. in the United States and/or other countries.  The names
   of other products and services mentioned may be the trademarks of
   their respective owners.

7.  References

7.1.  Normative References

   [1]   Davis, M. and M. Duerst, "Unicode Normalization Forms",
         March 2001,
         <http://www.unicode.org/unicode/reports/tr15/tr15-21.html>.

7.2.  Informative References

   [2]   RSA Laboratories, "PKCS #11 Mechanisms for the Cryptographic
         Token Key Initialization Protocol", PKCS #11 Version 2.20
         Amendment 2, December 2005, <ftp://ftp.rsasecurity.com/pub/
         pkcs/pkcs-11/v2-20/pkcs-11v2-20a2.pdf>.

   [3]   RSA Laboratories, "Cryptographic Token Interface Standard",
         PKCS #11 Version 2.20, June 2004, <ftp://ftp.rsasecurity.com/
         pub/pkcs/pkcs-11/v2-20/pkcs-11v2-20.pdf>.

   [4]   RSA Laboratories, "Frequently Asked Questions About Today's
         Cryptography. Version 4.1", 2000, <http://www.rsasecurity.com/
         rsalabs/faq/files/rsalabs_faq41.pdf>.

   [5]   RSA Laboratories, "Password-Based Cryptography Standard",
         PKCS #5 Version 2.0, March 1999,
         <ftp://ftp.rsasecurity.com/pub/pkcs/pkcs-5v2/pkcs5v2-0.pdf>.

   [6]   RSA Laboratories, "RSA Cryptography Standard", PKCS #1 Version
         2.1, June 2002,
         <ftp://ftp.rsasecurity.com/pub/pkcs/pkcs-1/pkcs-1v2-1.pdf>.

   [7]   Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L.,
         Leach, P., and T. Berners-Lee, "Hypertext Transfer Protocol --
         HTTP/1.1", RFC 2616, June 1999.

   [8]   National Institute of Standards and Technology, "Specification
         for the Advanced Encryption Standard (AES)", FIPS 197,
         November 2001,
         <http://csrc.nist.gov/publications/fips/fips197/fips-197.pdf>.

   [9]   Krawzcyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-Hashing
         for Message Authentication", RFC 2104, February 1997.

   [10]  Iwata, T. and K. Kurosawa, "OMAC: One-Key CBC MAC.  In Fast
         Software Encryption, FSE 2003, pages 129 - 153.
         Springer-Verlag", 2003,
         <http://crypt.cis.ibaraki.ac.jp/omac/docs/omac.pdf>.

   [11]  National Institute of Standards and Technology, "Secure Hash
         Standard", FIPS 197, February 2004, <http://csrc.nist.gov/
         publications/fips/fips180-2/fips180-2withchangenotice.pdf>.

   [12]  RSA Laboratories, "Cryptographic Token Key Initialization
         Protocol", OTPS Version 1.0, December 2005,
         <ftp://ftp.rsasecurity.com/pub/otps/ct-kip/ct-kip-v1-0.pdf>.

Appendix A.  CT-KIP Schema

   <xs:schema
     targetNamespace=
     "http://www.rsasecurity.com/rsalabs/otps/schemas/2005/12/ct-kip#"
     xmlns:xs="http://www.w3.org/2001/XMLSchema"
     xmlns:ds="http://www.w3.org/2000/09/xmldsig#"
     xmlns=
     "http://www.rsasecurity.com/rsalabs/otps/schemas/2005/12/ct-kip#">

   <xs:import namespace="http://www.w3.org/2000/09/xmldsig#"
     schemaLocation=
     "http://www.w3.org/TR/2002/REC-xmldsig-core-20020212/
   xmldsig-core-schema.xsd"/>

   <!-- Basic types -->

   <xs:complexType name="AbstractRequestType" abstract="true">
     <xs:attribute name="Version" type="VersionType" use="required"/>
   </xs:complexType>

   <xs:complexType name="AbstractResponseType" abstract="true">
     <xs:attribute name="Version" type="VersionType" use="required"/>
     <xs:attribute name="SessionID" type="IdentifierType"/>
     <xs:attribute name="Status" type="StatusCode" use="required"/>
   </xs:complexType>

   <xs:simpleType name="StatusCode">
     <xs:restriction base="xs:string">
       <xs:enumeration value="Continue"/>
       <xs:enumeration value="Success"/>
       <xs:enumeration value="Abort"/>
       <xs:enumeration value="AccessDenied"/>
       <xs:enumeration value="MalformedRequest"/>
       <xs:enumeration value="UnknownRequest"/>
       <xs:enumeration value="UnknownCriticalExtension"/>
       <xs:enumeration value="UnsupportedVersion"/>
       <xs:enumeration value="NoSupportedKeyTypes"/>
       <xs:enumeration value="NoSupportedEncryptionAlgorithms"/>
       <xs:enumeration value="NoSupportedMACAlgorithms"/>
       <xs:enumeration value="InitializationFailed"/>
     </xs:restriction>
   </xs:simpleType>

   <xs:simpleType name="VersionType">
     <xs:restriction base="xs:string">
       <xs:pattern value="\d{1,2}\.\d{1,3}"/>
     </xs:restriction>

   </xs:simpleType>

   <xs:simpleType name="IdentifierType">
     <xs:restriction base="xs:string">
       <xs:maxLength value="128"/>
     </xs:restriction>
   </xs:simpleType>

   <xs:simpleType name="NonceType">
     <xs:restriction base="xs:base64Binary">
       <xs:length value="16"/>
     </xs:restriction>
   </xs:simpleType>

   <xs:complexType name="LogoType">
     <xs:simpleContent>
       <xs:extension base="xs:base64Binary">
         <xs:attribute name="MimeType" type="MimeTypeType"
         use="required"/>
       </xs:extension>
     </xs:simpleContent>
   </xs:complexType>

   <xs:simpleType name="MimeTypeType">
     <xs:restriction base="xs:string">
       <xs:enumeration value="image/jpeg"/>
       <xs:enumeration value="image/gif"/>
     </xs:restriction>
   </xs:simpleType>

   <!-- Algorithms are identified through URIs -->
   <xs:complexType name="AlgorithmsType">
     <xs:sequence maxOccurs="unbounded">
       <xs:element name="Algorithm" type="AlgorithmType"/>
     </xs:sequence>
   </xs:complexType>

   <xs:simpleType name="AlgorithmType">
     <xs:restriction base="xs:anyURI"/>
   </xs:simpleType>

   <xs:complexType name="MacType">
     <xs:simpleContent>
       <xs:extension base="xs:base64Binary">
         <xs:attribute name="MacAlgorithm"
         type="xs:anyURI"/>
       </xs:extension>
     </xs:simpleContent>

   </xs:complexType>

   <!-- CT-KIP extensions (for future use) -->
   <xs:complexType name="ExtensionsType">
     <xs:sequence maxOccurs="unbounded">
       <xs:element name="Extension" type="AbstractExtensionType"/>
     </xs:sequence>
   </xs:complexType>

   <xs:complexType name="AbstractExtensionType" abstract="true">
     <xs:attribute name="Critical" type="xs:boolean"/>
   </xs:complexType>

   <xs:complexType name="ClientInfoType">
     <xs:complexContent>
       <xs:extension base="AbstractExtensionType">
         <xs:sequence>
           <xs:element name="Data" type="xs:base64Binary"/>
         </xs:sequence>
       </xs:extension>
     </xs:complexContent>
   </xs:complexType>

   <xs:complexType name="ServerInfoType">
     <xs:complexContent>
       <xs:extension base="AbstractExtensionType">
         <xs:sequence>
           <xs:element name="Data" type="xs:base64Binary"/>
         </xs:sequence>
       </xs:extension>
     </xs:complexContent>
   </xs:complexType>

   <xs:complexType name="OTPKeyConfigurationDataType">
     <xs:annotation>
       <xs:documentation xml:lang="en">
         This extension is only valid in ServerFinished PDUs.  It
         carries additional configuration data that an OTP token should
         use (subject to local policy) when generating OTP values from a
         newly generated OTP key.
       </xs:documentation>
     </xs:annotation>
     <xs:complexContent>
       <xs:extension base="AbstractExtensionType">
         <xs:sequence>
           <xs:element name="OTPFormat" type="OTPFormatType"/>
           <xs:element name="OTPLength" type="xs:positiveInteger"/>
           <xs:element name="OTPMode" type="OTPModeType" minOccurs="0"/>

         </xs:sequence>
       </xs:extension>
     </xs:complexContent>
   </xs:complexType>

   <xs:simpleType name="OTPFormatType">
     <xs:restriction base="xs:string">
       <xs:enumeration value="Decimal"/>
       <xs:enumeration value="Hexadecimal"/>
       <xs:enumeration value="Alphanumeric"/>
       <xs:enumeration value="Binary"/>
     </xs:restriction>
   </xs:simpleType>

   <xs:complexType name="OTPModeType">
     <xs:choice maxOccurs="unbounded">
       <xs:element name="Time" type="TimeType"/>
       <xs:element name="Counter"/>
       <xs:element name="Challenge"/>
       <xs:any namespace="##other" processContents="strict"/>
     </xs:choice>
   </xs:complexType>

   <xs:complexType name="TimeType">
     <xs:complexContent>
       <xs:restriction base="xs:anyType">
         <xs:attribute name="TimeInterval" type="xs:positiveInteger"/>
       </xs:restriction>
     </xs:complexContent>
   </xs:complexType>

   <xs:complexType name="PayloadType">
     <xs:annotation>
       <xs:documentation xml:lang="en">
       </xs:documentation>
     </xs:annotation>
     <xs:choice>
       <xs:element name="Nonce" type="NonceType"/>
       <xs:any namespace="##other" processContents="strict"/>
     </xs:choice>
   </xs:complexType>

   <xs:simpleType name="PlatformType">
     <xs:restriction base="xs:string">
       <xs:enumeration value="Hardware"/>
       <xs:enumeration value="Software"/>
       <xs:enumeration value="Unspecified"/>
     </xs:restriction>

   </xs:simpleType>

   <xs:complexType name="TokenPlatformInfoType">
     <xs:annotation>
       <xs:documentation xml:lang="en">
         Carries token platform information helping the client to select
         a suitable token.
       </xs:documentation>
     </xs:annotation>
     <xs:attribute name="KeyLocation" type="PlatformType"/>
     <xs:attribute name="AlgorithmLocation" type="PlatformType"/>
   </xs:complexType>

   <xs:complexType name="InitializationTriggerType">
     <xs:sequence>
       <xs:element name="TokenID" type="xs:base64Binary" minOccurs="0"/>
       <xs:element name="KeyID" type="xs:base64Binary" minOccurs="0"/>
       <xs:element name="TokenPlatformInfo" type="TokenPlatformInfoType"
         minOccurs="0"/>
       <xs:element name="TriggerNonce" type="NonceType"/>
       <xs:element name="CT-KIPURL" type="xs:anyURI" minOccurs="0"/>
       <xs:any namespace="##other" processContents="strict"
         minOccurs="0"/>
     </xs:sequence>
   </xs:complexType>

   <!-- CT-KIP PDUs -->

   <!-- CT-KIP trigger -->
   <xs:element name="CT-KIPTrigger" type="CT-KIPTriggerType"/>

   <xs:complexType name="CT-KIPTriggerType">
     <xs:annotation>
       <xs:documentation xml:lang="en">
         Message used to trigger the device to initiate a CT-KIP run.
       </xs:documentation>
     </xs:annotation>
     <xs:sequence>
       <xs:choice>
         <xs:element name="InitializationTrigger"
         type="InitializationTriggerType"/>
         <xs:any namespace="##other" processContents="strict"/>
       </xs:choice>
     </xs:sequence>
     <xs:attribute name="Version" type="VersionType"/>
   </xs:complexType>

   <!-- ClientHello PDU -->

   <xs:element name="ClientHello" type="ClientHelloPDU"/>

   <xs:complexType name="ClientHelloPDU">
     <xs:annotation>
       <xs:documentation xml:lang="en">
         Message sent from CT-KIP client to CT-KIP server to initiate an
         CT-KIP session.
       </xs:documentation>
     </xs:annotation>
     <xs:complexContent>
       <xs:extension base="AbstractRequestType">
         <xs:sequence>
           <xs:element name="TokenID" type="xs:base64Binary"
             minOccurs="0"/>
           <xs:element name="KeyID" type="xs:base64Binary"
             minOccurs="0"/>
           <xs:element name="ClientNonce" type="NonceType"
             minOccurs="0"/>
           <xs:element name="TriggerNonce" type="NonceType"
             minOccurs="0"/>
           <xs:element name="SupportedKeyTypes" type="AlgorithmsType"/>
           <xs:element name="SupportedEncryptionAlgorithms"
             type="AlgorithmsType"/>
           <xs:element name="SupportedMACAlgorithms"
             type="AlgorithmsType"/>
           <xs:element name="Extensions" type="ExtensionsType"
             minOccurs="0"/>
         </xs:sequence>
       </xs:extension>
     </xs:complexContent>
   </xs:complexType>

   <!-- ServerHello PDU -->
   <xs:element name="ServerHello" type="ServerHelloPDU"/>

   <xs:complexType name="ServerHelloPDU">
     <xs:annotation>
       <xs:documentation xml:lang="en">
         Message sent from CT-KIP server to CT-KIP client in response to
         a received ClientHello PDU.
       </xs:documentation>
     </xs:annotation>
     <xs:complexContent>
       <xs:extension base="AbstractResponseType">
         <xs:sequence minOccurs="0">
           <xs:element name="KeyType" type="AlgorithmType"/>
           <xs:element name="EncryptionAlgorithm" type="AlgorithmType"/>
           <xs:element name="MacAlgorithm" type="AlgorithmType"/>

           <xs:element name="EncryptionKey" type="ds:KeyInfoType"/>
           <xs:element name="Payload" type="PayloadType"/>
           <xs:element name="Extensions" type="ExtensionsType"
             minOccurs="0"/>
           <xs:element name="Mac" type="MacType" minOccurs="0"/>
         </xs:sequence>
       </xs:extension>
     </xs:complexContent>
   </xs:complexType>

   <!-- ClientNonce PDU -->
   <xs:element name="ClientNonce" type="ClientNoncePDU"/>

   <xs:complexType name="ClientNoncePDU">
     <xs:annotation>
       <xs:documentation xml:lang="en">
         Second message sent from CT-KIP client to CT-KIP server to
         convey the client's chosen secret.
       </xs:documentation>
     </xs:annotation>
     <xs:complexContent>
       <xs:extension base="AbstractRequestType">
         <xs:sequence>
           <xs:element name="EncryptedNonce" type="xs:base64Binary"/>
           <xs:element name="Extensions" type="ExtensionsType"
             minOccurs="0"/>
         </xs:sequence>
         <xs:attribute name="SessionID" type="IdentifierType"
           use="required"/>
       </xs:extension>
     </xs:complexContent>
   </xs:complexType>

   <!-- ServerFinished PDU -->
   <xs:element name="ServerFinished" type="ServerFinishedPDU"/>
   <xs:complexType name="ServerFinishedPDU">
     <xs:annotation>
       <xs:documentation xml:lang="en">
         Final message sent from CT-KIP server to CT-KIP client in an
         CT-KIP session.
       </xs:documentation>
     </xs:annotation>
     <xs:complexContent>
       <xs:extension base="AbstractResponseType">
         <xs:sequence minOccurs="0">
           <xs:element name="TokenID" type="xs:base64Binary"/>
           <xs:element name="KeyID" type="xs:base64Binary"/>
           <xs:element name="KeyExpiryDate" type="xs:dateTime"

             minOccurs="0"/>
           <xs:element name="ServiceID" type="IdentifierType"
             minOccurs="0"/>
           <xs:element name="ServiceLogo" type="LogoType"
             minOccurs="0"/>
           <xs:element name="UserID" type="IdentifierType"
             minOccurs="0"/>
           <xs:element name="Extensions" type="ExtensionsType"
             minOccurs="0"/>
           <xs:element name="Mac" type="MacType"/>
         </xs:sequence>
       </xs:extension>
     </xs:complexContent>
   </xs:complexType>

   </xs:schema>

Appendix B.  Examples of CT-KIP Messages

B.1.  Introduction

   All examples are syntactically correct.  MAC and cipher values are
   fictitious, however.  The examples illustrate a complete CT-KIP
   exchange, starting with an initialization (trigger) message from the
   server.

B.2.  Example of a CT-KIP Initialization (Trigger) Message

   <CT-KIPTrigger
     xmlns=
     "http://www.rsasecurity.com/rsalabs/otps/schemas/2005/12/ct-kip#"
     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     Version="1.0">
     <InitializationTrigger>
       <TokenID>12345678</TokenID>
       <TriggerNonce>112dsdfwf312asder394jw==</TriggerNonce>
     </InitializationTrigger>
   </CT-KIPTrigger>

B.3.  Example of a <ClientHello> Message

   <?xml version="1.0" encoding="UTF-8"?>
   <ClientHello
     xmlns=
     "http://www.rsasecurity.com/rsalabs/otps/schemas/2005/12/ct-kip#"
     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     Version="1.0">
     <TokenID>12345678</TokenID>

    <TriggerNonce>112dsdfwf312asder394jw==</TriggerNonce>
     <SupportedKeyTypes>
       <Algorithm>http://www.rsasecurity.com/rsalabs/otps/schemas
   /2005/09/otps-wst#SecurID-AES</Algorithm>
     </SupportedKeyTypes>
     <SupportedEncryptionAlgorithms>
       <Algorithm>http://www.w3.org/2001/04/xmlenc#rsa-1_5</Algorithm>
       <Algorithm>http://www.rsasecurity.com/rsalabs/otps/schemas/
   2005/12/ct-kip#ct-kip-prf-aes</Algorithm>
     </SupportedEncryptionAlgorithms>
     <SupportedMACAlgorithms>
       <Algorithm>http://www.rsasecurity.com/rsalabs/otps/schemas/
   2005/12/ct-kip#ct-kip-prf-aes</Algorithm>
     </SupportedMACAlgorithms>
   </ClientHello>

B.4.  Example of a <ServerHello> Message

   <?xml version="1.0" encoding="UTF-8"?>
   <ServerHello
     xmlns=
   "http://www.rsasecurity.com/rsalabs/otps/schemas/2005/12/ct-kip#"
     xmlns:ds="http://www.w3.org/2000/09/xmldsig#"
     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     Version="1.0" SessionID="4114" Status="Success">
     <KeyType>http://www.rsasecurity.com/rsalabs/otps/schemas/2005/09/
   otps-wst#SecurID-AES</KeyType>
     <EncryptionAlgorithm>http://www.rsasecurity.com/rsalabs/otps/
   schemas/2005/12/ct-kip#ct-kip-prf-aes</EncryptionAlgorithm>
     <MacAlgorithm>http://www.rsasecurity.com/rsalabs/otps/schemas/
   2005/12/ct-kip#ct-kip-prf-aes</MacAlgorithm>
     <EncryptionKey>
       <ds:KeyName>KEY-1</ds:KeyName>
     </EncryptionKey>
     <Payload>
       <Nonce>qw2ewasde312asder394jw==</Nonce>
     </Payload>
   </ServerHello>

B.5.  Example of a <ClientNonce> Message

   <?xml version="1.0" encoding="UTF-8"?>
   <ClientNonce
     xmlns="http://www.rsasecurity.com/rsalabs/otps/schemas/
   2005/12/ct-kip#"
     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     Version="1.0" SessionID="4114">
     <EncryptedNonce>vXENc+Um/9/NvmYKiHDLaErK0gk=</EncryptedNonce>

   </ClientNonce>

B.6.  Example of a <ServerFinished> Message

   <?xml version="1.0" encoding="UTF-8"?>
   <ServerFinished
     xmlns="http://www.rsasecurity.com/rsalabs/otps/schemas/
   2005/12/ct-kip#"
     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     Version="1.0" SessionID="4114" Status="Success">
     <TokenID>12345678</TokenID>
     <KeyExpiryDate>2009-09-16T03:02:00Z</KeyExpiryDate>
     <KeyID>43212093</KeyID>
     <ServiceID>Example Enterprise Name</ServiceID>
     <UserID>exampleLoginName</UserID>
     <Extensions>
       <Extension xsi:type="OTPKeyConfigurationDataType">
         <OTPFormat>Decimal</OTPFormat>
         <OTPLength>6</OTPLength>
         <OTPMode><Time/></OTPMode>
       </Extension>
     </Extensions>
     <Mac>miidfasde312asder394jw==</Mac>
   </ServerFinished>

Appendix C.  Integration with PKCS #11

   A CT-KIP client that needs to communicate with a connected
   cryptographic token to perform a CT-KIP exchange may use PKCS #11 [3]
   as a programming interface.  When performing CT-KIP with a
   cryptographic token using the PKCS #11 programming interface, the
   procedure described in [2], Appendix B, is recommended.

Appendix D.  Example CT-KIP-PRF Realizations

D.1.  Introduction

   This example appendix defines CT-KIP-PRF in terms of AES [8] and HMAC
   [9].

D.2.  CT-KIP-PRF-AES

D.2.1.  Identification

   For tokens supporting this realization of CT-KIP-PRF, the following
   URI may be used to identify this algorithm in CT-KIP:

   http://www.rsasecurity.com/rsalabs/otps/schemas/2005/12/

   ct-kip#ct-kip-prf-aes

   When this URI is used to identify the encryption algorithm to use,
   the method for encryption of R_C values described in Section 3.6
   shall be used.

D.2.2.  Definition

   CT-KIP-PRF-AES (k, s, dsLen)

   Input:

   k     encryption key to use

   s     octet string consisting of randomizing material.  The length of
         the string s is sLen.

   dsLen desired length of the output

   Output:

   DS    a pseudorandom string, dsLen-octets long

   Steps:

   1.  Let bLen be the output block size of AES in octets:

       bLen = (AES output block length in octets)

       (normally, bLen = 16)

   2.  If dsLen > (2**32 - 1) * bLen, output "derived data too long" and
       stop

   3.  Let n be the number of bLen-octet blocks in the output data,
       rounding up, and let j be the number of octets in the last block:

       n = ROUND( dsLen / bLen )

       j = dsLen - (n - 1) * bLen

   4.  For each block of the pseudorandom string DS, apply the function
       F defined below to the key k, the string s and the block index to
       compute the block:

       B1 = F (k, s, 1) ,

       B2 = F (k, s, 2) ,

       ...

       Bn = F (k, s, n)

   The function F is defined in terms of the OMAC1 construction from
   [10], using AES as the block cipher:

   F (k, s, i) = OMAC1-AES (k, INT (i) || s)

   where INT (i) is a four-octet encoding of the integer i, most
   significant octet first, and the output length of OMAC1 is set to
   bLen.

   Concatenate the blocks and extract the first dsLen octets to produce
   the desired data string DS:

   DS = B1 || B2 || ... || Bn<0..j-1>

   Output the derived data DS.

D.2.3.  Example

   If we assume that dsLen = 16, then:

   n = 16 / 16 = 1

   j = 16 - (1 - 1) * 16 = 16

   DS = B1 = F (k, s, 1) = OMAC1-AES (k, INT (1) || S)

D.3.  CT-KIP-PRF-SHA256

D.3.1.  Identification

   For tokens supporting this realization of CT-KIP-PRF, the following
   URI may be used to identify this algorithm in CT-KIP:

   http://www.rsasecurity.com/rsalabs/otps/schemas/2005/12/
   ct-kip#ct-kip-prf-sha256

   When this URI is used to identify the encryption algorithm to use,
   the method for encryption of R_C values described in Section 3.6
   shall be used.

D.3.2.  Definition

   CT-KIP-PRF-SHA256 (k, s, dsLen)

   Input:

   k     encryption key to use

   s     octet string consisting of randomizing material.  The length of
         the string s is sLen

   dsLen desired length of the output

   Output:

   DS    a pseudorandom string, dsLen-octets long

   Steps:

   1.  Let bLen be the output size in octets of SHA-256 [11] (no
       truncation is done on the HMAC output):

       bLen = 32

   2.  If dsLen > (2**32 - 1) bLen, output "derived data too long" and
       stop

   3.  Let n be the number of bLen-octet blocks in the output data,
       rounding up, and let j be the number of octets in the last block:

       n = ROUND ( dsLen / bLen )

       j = dsLen - (n - 1) * bLen

   4.  For each block of the pseudorandom string DS, apply the function
       F defined below to the key k, the string s and the block index to
       compute the block:

       B1 = F (k, s, 1) ,

       B2 = F (k, s, 2) ,

       ...

       Bn = F (k, s, n)

   The function F is defined in terms of the HMAC construction from [9],
   using SHA-256 as the digest algorithm:

   F (k, s, i) = HMAC-SHA256 (k, INT (i) || s)

   where INT (i) is a four-octet encoding of the integer i, most
   significant octet first, and the output length of HMAC is set to
   bLen.

   Concatenate the blocks and extract the first dsLen octets to produce
   the desired data string DS:

   DS = B1 || B2 || ... || Bn<0..j-1>

   Output the derived data DS.

D.3.3.  Example

   If we assume that sLen = 256 (two 128-octet long values) and dsLen =
   16, then:

   n = ROUND ( 16 / 32 ) = 1

   j = 16 - (1 - 1) * 32 = 16

   B1 = F (k, s, 1) = HMAC-SHA256 (k, INT (1) || s )

   DS = B1<0 ... 15>

   That is, the result will be the first 16 octets of the HMAC output.

Appendix E.  About OTPS

   The One-Time Password Specifications are documents produced by RSA
   Laboratories in cooperation with secure systems developers for the
   purpose of simplifying integration and management of strong
   authentication technology into secure applications, and to enhance
   the user experience of this technology.

   Further development of the OTPS series will occur through mailing
   list discussions and occasional workshops, and suggestions for
   improvement are welcome.  As for our PKCS documents, results may also
   be submitted to standards forums.  For more information, contact:

   OTPS Editor
   RSA Laboratories
   174 Middlesex Turnpike
   Bedford, MA  01730 USA
   otps-editor@rsasecurity.com
   http://www.rsasecurity.com/rsalabs/

Author's Address

   Magnus Nystroem
   RSA Security

   EMail: magnus@rsasecurity.com

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