RFC 5934
Trust Anchor Management Protocol (TAMP)
Internet Engineering Task Force (IETF) R. Housley Request for Comments: 5934 Vigil Security, LLC Category: Standards Track S. Ashmore ISSN: 2070-1721 National Security Agency C. Wallace Cygnacom Solutions August 2010 Trust Anchor Management Protocol (TAMP)Abstract
This document describes a transport independent protocol for the management of trust anchors (TAs) and community identifiers stored in a trust anchor store. The protocol makes use of the Cryptographic Message Syntax (CMS), and a digital signature is used to provide integrity protection and data origin authentication. The protocol can be used to manage trust anchor stores containing trust anchors represented as Certificate, TBSCertificate, or TrustAnchorInfo objects. Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc5934.
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Table of Contents
1. Introduction ....................................................4 1.1. Terminology ................................................5 1.2. Trust Anchors ..............................................5 1.2.1. Apex Trust Anchors ..................................6 1.2.2. Management Trust Anchors ............................7 1.2.3. Identity Trust Anchors ..............................7 1.3. Architectural Elements .....................................8 1.3.1. Cryptographic Module ................................8 1.3.2. Trust Anchor Store ..................................9 1.3.3. TAMP Processing Dependencies ........................9 1.3.4. Application-Specific Protocol Processing ...........10 1.4. ASN.1 Encoding ............................................11 2. Cryptographic Message Syntax Profile ...........................12 2.1. ContentInfo ...............................................13 2.2. SignedData Info ...........................................14 2.2.1. SignerInfo .........................................15 2.2.2. EncapsulatedContentInfo ............................16 2.2.3. Signed Attributes ..................................16 2.2.4. Unsigned Attributes ................................18 3. Trust Anchor Formats ...........................................18 4. Trust Anchor Management Protocol Messages ......................19 4.1. TAMP Status Query .........................................21 4.2. TAMP Status Query Response ................................24 4.3. Trust Anchor Update .......................................27 4.3.1. Trust Anchor List ..................................31 4.4. Trust Anchor Update Confirm ...............................32 4.5. Apex Trust Anchor Update ..................................34 4.6. Apex Trust Anchor Update Confirm ..........................36 4.7. Community Update ..........................................38 4.8. Community Update Confirm ..................................40 4.9. Sequence Number Adjust ....................................42 4.10. Sequence Number Adjust Confirm ...........................43 4.11. TAMP Error ...............................................44 5. Status Codes ...................................................45 6. Sequence Number Processing .....................................50 7. Subordination Processing .......................................51 8. Implementation Considerations ..................................54 9. Wrapped Apex Contingency Key Certificate Extension .............54 10. Security Considerations .......................................55 11. IANA Considerations ...........................................58 12. References ....................................................58 12.1. Normative References .....................................58 12.2. Informative References ...................................59
Appendix A. ASN.1 Modules ........................................61 A.1. ASN.1 Module Using 1993 Syntax ............................61 A.2. ASN.1 Module Using 1988 Syntax ............................70 Appendix B. Media Type Registrations .............................77 B.1. application/tamp-status-query .............................77 B.2. application/tamp-status-response ..........................78 B.3. application/tamp-update ...................................79 B.4. application/tamp-update-confirm ...........................80 B.5. application/tamp-apex-update ..............................81 B.6. application/tamp-apex-update-confirm ......................82 B.7. application/tamp-community-update .........................83 B.8. application/tamp-community-update-confirm .................84 B.9. application/tamp-sequence-adjust ..........................85 B.10. application/tamp-sequence-adjust-confirm ..................86 B.11. application/tamp-error ....................................87 Appendix C. TAMP over HTTP .......................................88 C.1. TAMP Status Query Message .................................89 C.2. TAMP Status Response Message ..............................89 C.3. Trust Anchor Update Message ...............................89 C.4. Trust Anchor Update Confirm Message .......................89 C.5. Apex Trust Anchor Update Message ..........................89 C.6. Apex Trust Anchor Update Confirm Message ..................90 C.7. Community Update Message ..................................90 C.8. Community Update Confirm Message ..........................90 C.9. Sequence Number Adjust Message ............................90 C.10. Sequence Number Adjust Confirm Message ....................90 C.11. TAMP Error Message ........................................911. Introduction
This document describes the Trust Anchor Management Protocol (TAMP). TAMP may be used to manage the trust anchors and community identifiers in any device that uses digital signatures; however, this specification was written with the requirements of cryptographic modules in mind. For example, TAMP can support signed firmware packages [RFC4108], where the trust anchor public key can be used to validate digital signatures on firmware packages or validate the X.509 certification path [RFC5280][X.509] of the firmware package signer. Most TAMP messages are digitally signed to provide integrity protection and data origin authentication. Both signed and unsigned TAMP messages employ the Cryptographic Message Syntax (CMS) [RFC5652]. The CMS is a data protection encapsulation syntax that makes use of ASN.1 [X.680].
This specification does not provide for confidentiality of TAMP messages. If confidentiality is required, then the communications environment that is used to transfer TAMP messages must provide it. This specification is intended to satisfy the protocol-related requirements expressed in "Trust Anchor Management Requirements" [TA-MGMT-REQS] and uses vocabulary from that document. TAMP messages may be exchanged in real time over a network, such as via HTTP as described in Appendix A, or may be stored and transferred using other means. TAMP exchanges consist of a request message that includes instructions for a trust anchor store and, optionally, a corresponding response message that reports the result of carrying out the instructions in the request. Response messages need not be propagated in all cases. For example, a GPS receiver may be unable to transmit a response and may instead use an attached display to indicate the results of processing a TAMP request.1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].1.2. Trust Anchors
TAMP manages trust anchors. A trust anchor contains a public key that is used to validate digital signatures. TAMP recognizes three formats for representing trust anchor information: Certificate [RFC5280], TBSCertificate [RFC5280], and TrustAnchorInfo [RFC5914]. All trust anchors are distinguished by the public key, and all trust anchors consist of the following components: o A public key signature algorithm identifier and associated public key, which MAY include parameters o A public key identifier Other information may appear in a trust anchor, including certification path processing controls and a human readable name. TAMP recognizes three types of trust anchors based on functionality: apex trust anchors, management trust anchors, and identity trust anchors. In addition to the information described above, apex trust anchors and management trust anchors that sign TAMP messages have an associated sequence number that is used for replay detection.
The public key is used to name a trust anchor, and the public key identifier is used to identify the trust anchor as a signer of a particular object, such as a SignedData object or a public key certificate. This public key identifier can be stored with the trust anchor, or in most public key identifier assignment methods, it can be computed from the public key whenever needed. A trust anchor public key can be used in two different ways to support digital signature validation. In the first approach, the trust anchor public key is used directly to validate the digital signature. In the second approach, the trust anchor public key is used to validate an X.509 certification path, and then the subject public key in the final certificate in the certification path is used to validate the digital signature. When the second approach is employed, the certified public key may be used for things other than digital signature validation; the other possible actions are constrained by the key usage certificate extension. TAMP implementations MUST support validation of TAMP messages that are directly validated using a trust anchor. Support for TAMP messages validated using an X.509 certificate validated using a trust anchor, or using longer certification paths, is OPTIONAL. The CMS provides a location to carry X.509 certificates, and this facility can be used to transfer certificates to aid in the construction of the certification path.1.2.1. Apex Trust Anchors
Within the context of a single trust anchor store, one trust anchor is superior to all others. This trust anchor is referred to as the apex trust anchor. This trust anchor represents the ultimate authority over the trust anchor store. Much of this authority can be delegated to other trust anchors. The apex trust anchor private key is expected to be controlled by an entity with information assurance responsibility for the trust anchor store. The apex trust anchor is by definition unconstrained and therefore does not have explicit authorization information associated with it. Due to the special nature of the apex trust anchor, TAMP includes separate facilities to change it. In particular, TAMP includes a facility to securely replace the apex trust anchor. This action might be taken for one or more of the following reasons: o The crypto period for the apex trust anchor public/private key pair has come to an end
o The apex trust anchor private key is no longer available
o The apex trust anchor public/private key pair needs to be revoked
o The authority has decided to use a different digital signature
algorithm or the same digital signature algorithm with different
parameters, such as a different elliptic curve
o The authority has decided to use a different key size
o The authority has decided to transfer control to another authority
To accommodate these requirements, the apex trust anchor MAY include
two public keys. Whenever the apex trust anchor is updated, both
public keys will be replaced. The first public key, called the
operational public key, is used in the same manner as other trust
anchors. Any type of TAMP message, including an Apex Trust Anchor
Update message, can be validated with the operational public key.
The second public key, called the contingency public key, can only be
used to update the apex trust anchor. The contingency private key
SHOULD be used at only one point in time; it is used only to sign an
Apex Trust Anchor Update message that results in its own replacement
(as well as the replacement of the operational public key). The
contingency public key is distributed in encrypted form. When the
contingency public key is used to validate an Apex Trust Anchor
Update message, the symmetric key needed to decrypt the contingency
public key is provided as part of the signed Apex Trust Anchor Update
message that is to be verified with the contingency public key.
1.2.2. Management Trust Anchors
Management trust anchors are used in the management of cryptographic
modules. For example, the TAMP messages specified in this document
are validated to a management trust anchor. Likewise, a signed
firmware package as specified in [RFC4108] is validated to a
management trust anchor.
1.2.3. Identity Trust Anchors
Identity trust anchors are used to validate certification paths, and
they represent the trust anchor for a public key infrastructure.
They are most often used in the validation of certificates associated
with non-management applications.
1.3. Architectural Elements
TAMP does not assume any particular architecture. However, TAMP REQUIRES the following architectural elements: a cryptographic module, a trust anchor store, TAMP protocol processing, and other application-specific protocol processing. A globally unique algorithm identifier MUST be assigned for each one- way hash function, digital signature generation/validation algorithm, and symmetric key unwrapping algorithm that is implemented. To support CMS, an object identifier (OID) is assigned to name a one-way hash function, and another OID is assigned to name each combination of a one-way hash function when used with a digital signature algorithm. Similarly, certificates associate OIDs assigned to public key algorithms with subject public keys, and certificates make use of an OID that names both the one-way hash function and the digital signature algorithm for the certificate issuer digital signature. [RFC3279], [RFC3370], [RFC5753], and [RFC5754] provide OIDs for a number of commonly used algorithms; however, OIDs may be defined in later or different specifications.1.3.1. Cryptographic Module
The cryptographic module MUST include the following capabilities: o The cryptographic module SHOULD support the secure storage of a digital signature private key to sign TAMP responses and either a certificate containing the associated public key or a certificate designator. In the latter case, the certificate is stored elsewhere but is available to parties that need to validate cryptographic module digital signatures. The designator is a public key identifier. o The cryptographic module MUST support at least one one-way hash function, one digital signature validation algorithm, one digital signature generation algorithm, and, if contingency keys are supported, one symmetric key unwrapping algorithm. If only one one-way hash function is present, it MUST be consistent with the digital signature validation and digital signature generation algorithms. If only one digital signature validation algorithm is present, it MUST be consistent with the apex trust anchor operational public key. If only one digital signature generation algorithm is present, it MUST be consistent with the cryptographic module digital signature private key. These algorithms MUST be available for processing TAMP messages, including the content types defined in [RFC5652], and for validation of X.509
certification paths. As with similar specifications, such as
RFC 5280, this specification does not mandate support for any
cryptographic algorithms. However, algorithm requirements may be
imposed by specifications that use trust anchors managed via TAMP.
1.3.2. Trust Anchor Store
The trust anchor store MUST include the following capabilities:
o Each trust anchor store MUST have a unique name. For example, a
cryptographic module containing a single trust anchor store may be
identified by a unique serial number with respect to other modules
within the same family where the family is represented as an ASN.1
object identifier (OID) and the unique serial number is
represented as a string of octets. Other means of establishing a
unique name are also possible.
o Each trust anchor store SHOULD have the capability to securely
store one or more community identifiers. The community identifier
is an OID, and it identifies a collection of cryptographic modules
that can be the target of a single TAMP message or the intended
recipients for a particular management message.
o The trust anchor store SHOULD support the use of an apex trust
anchor. If apex support is provided, the trust anchor store MUST
support the secure storage of exactly one apex trust anchor. The
trust anchor store SHOULD support the secure storage of at least
one additional trust anchor. Each trust anchor MUST contain a
unique public key. A public key MUST NOT appear more than once in
a trust anchor store.
o The trust anchor store MUST have the capability to securely store
a sequence number for each trust anchor authorized to generate
TAMP messages and be able to report the sequence number along with
the key identifier of the trust anchor.
1.3.3. TAMP Processing Dependencies
TAMP processing MUST include the following capabilities:
o TAMP processing MUST have a means of locating an appropriate trust
anchor. Two mechanisms are available. The first mechanism is
based on the public key identifier for digital signature
verification, and the second mechanism is based on the trust
anchor X.500 distinguished name and other X.509 certification path
controls for certificate path discovery and validation. The first
mechanism MUST be supported, but the second mechanism MAY be
supported.
o TAMP processing MUST be able to invoke the digital signature
validation algorithm using the public key held in secure storage
for trust anchors.
o TAMP processing MUST have read and write access to secure storage
for sequence numbers associated with each TAMP message signer as
described in Section 6.
o TAMP processing MUST have read and write access to secure storage
for trust anchors in order to update them. Update operations
include adding trust anchors, removing trust anchors, and
modifying trust anchors. Application-specific constraints MUST be
securely stored with each management trust anchor as described in
Section 1.3.4.
o TAMP processing MUST have read access to secure storage for the
community membership list, if any, to determine whether a targeted
message ought to be accepted.
o To implement the OPTIONAL community identifier update feature,
TAMP processing MUST have read and write access to secure storage
for the community membership list.
o To generate signed confirmation messages, TAMP processing MUST be
able to invoke the digital signature generation algorithm using
the cryptographic module digital signature private key, and it
MUST have read access to the cryptographic module certificate or
its designator. TAMP uses X.509 certificates [RFC5280].
o The TAMP processing MUST have read access to the trust anchor
store unique name.
1.3.4. Application-Specific Protocol Processing
The apex trust anchor and management trust anchors managed with TAMP
can be used by the TAMP application. Other management applications
MAY make use of all three types of trust anchors, but non-management
applications SHOULD only make use of identity trust anchors.
Applications MUST ensure that usage of a trust anchor is consistent
with any constraints associated with the trust anchor. For example,
if name constraints are associated with a trust anchor, certification
paths that start with the trust anchor and contain certificates with
names that violate the name constraints MUST be rejected.
The application-specific protocol processing MUST be provided with
the following services:
o The application-specific protocol processing MUST have a means of
locating an appropriate trust anchor. Two mechanisms are
available to applications. The first mechanism is based on the
public key identifier for digital signature verification, and the
second mechanism is based on the trust anchor X.500 distinguished
name and other X.509 certification path controls for certificate
path discovery and validation.
o The application-specific protocol processing MUST be able to
invoke the digital signature validation algorithm using the public
key held in secure storage for trust anchors.
o The application-specific protocol processing MUST have read access
to data associated with trust anchors to ensure that constraints
can be enforced appropriately. For example, an application MUST
have read access to any name constraints associated with a TA to
ensure that certification paths terminated by that TA do not
include certificates issued to entities outside the TA manager-
designated namespace.
o The application-specific protocol processing MUST have read access
to secure storage for the community membership list, if any, to
determine whether a targeted message ought to be accepted.
o If the application-specific protocol requires digital signatures
on confirmation messages or receipts, then the application-
specific protocol processing MUST be able to invoke the digital
signature generation algorithm with the cryptographic module
digital signature private key and its associated certificate or
certificate designator. Digital signature generation MUST be
controlled in a manner that ensures that the content type of
signed confirmation messages or receipts is appropriate for the
application-specific protocol processing.
o The application-specific protocol processing MUST have read access
to the trust anchor store unique name.
1.4. ASN.1 Encoding
The CMS uses Abstract Syntax Notation One (ASN.1) [X.680]. ASN.1 is
a formal notation used for describing data protocols, regardless of
the programming language used by the implementation. Encoding rules
describe how the values defined in ASN.1 will be represented for
transmission. The Basic Encoding Rules (BER) [X.690] are the most
widely employed rule set, but they offer more than one way to
represent data structures. For example, definite-length encoding and
indefinite-length encoding are supported. This flexibility is not
desirable when digital signatures are used. As a result, the
Distinguished Encoding Rules (DER) [X.690] were invented. DER is a subset of BER that ensures a single way to represent a given value. For example, DER always employs definite-length encoding. Digitally signed structures MUST be encoded with DER. In other specifications, structures that are not digitally signed do not require DER, but in this specification, DER is REQUIRED for all structures. By always using DER, the TAMP processor will have fewer options to implement. ASN.1 is used throughout the text of this document for illustrative purposes. The authoritative source of ASN.1 for the structures defined in this document is Appendix A.2. Cryptographic Message Syntax Profile
TAMP makes use of signed and unsigned messages. The Cryptographic Message Syntax (CMS) is used in both cases. A digital signature is used to protect the message from undetected modification and provide data origin authentication. TAMP makes no general provision for encryption of content. CMS is used to construct a signed TAMP message. The CMS ContentInfo content type MUST always be present. For signed messages, ContentInfo MUST encapsulate the CMS SignedData content type; for unsigned messages, ContentInfo MUST encapsulate the TAMP message directly. The CMS SignedData content type MUST encapsulate the TAMP message. A unique content type identifier identifies the particular type of TAMP message. The CMS encapsulation of a signed TAMP message is summarized by: ContentInfo { contentType id-signedData, -- (1.2.840.113549.1.7.2) content SignedData } SignedData { version CMSVersion, -- Always set to 3 digestAlgorithms DigestAlgorithmIdentifiers, -- Only one encapContentInfo EncapsulatedContentInfo, certificates CertificateSet, -- OPTIONAL signer certificates crls CertificateRevocationLists, -- OPTIONAL signerInfos SET OF SignerInfo -- Only one }
SignerInfo {
version CMSVersion, -- Always set to 3
sid SignerIdentifier,
digestAlgorithm DigestAlgorithmIdentifier,
signedAttrs SignedAttributes,
-- REQUIRED in TAMP messages
signatureAlgorithm SignatureAlgorithmIdentifier,
signature SignatureValue,
unsignedAttrs UnsignedAttributes -- OPTIONAL; may only be
} -- present in Apex Trust
-- Anchor Update messages
EncapsulatedContentInfo {
eContentType OBJECT IDENTIFIER, -- Names TAMP message type
eContent OCTET STRING -- Contains TAMP message
}
When a TAMP message is used to update the apex trust anchor, this
same structure is used; however, the digital signature will be
validated with either the apex trust anchor operational public key or
the contingency public key. When the contingency public key is used,
the symmetric key needed to decrypt the previously stored contingency
public key is provided as a contingency-public-key-decrypt-key
unsigned attribute. Section 4.5 of this document describes the Apex
Trust Anchor Update message.
CMS is also used to construct an unsigned TAMP message. The CMS
ContentInfo structure MUST always be present, and it MUST be the
outermost layer of encapsulation. A unique content type identifier
identifies the particular TAMP message. The CMS encapsulation of an
unsigned TAMP message is summarized by:
ContentInfo {
contentType OBJECT IDENTIFIER, -- Names TAMP message type
content OCTET STRING -- Contains TAMP message
}
2.1. ContentInfo
CMS requires the outermost encapsulation to be ContentInfo [RFC5652].
The fields of ContentInfo are used as follows:
o contentType indicates the type of the associated content, and for
TAMP, the encapsulated type is either SignedData or the content
type identifier associated with an unsigned TAMP message. When
the id-signedData (1.2.840.113549.1.7.2) object identifier is
present in this field, then a signed TAMP message is in the
content. Otherwise, an unsigned TAMP message is in the content.
o content holds the content, and for TAMP, the content is either a
SignedData content or an unsigned TAMP message.
2.2. SignedData Info
The SignedData content type [RFC5652] contains the signed TAMP
message and a digital signature value; the SignedData content type
MAY also contain the certificates needed to validate the digital
signature. The fields of SignedData are used as follows:
o version is the syntax version number, and for TAMP, the version
number MUST be set to 3.
o digestAlgorithms is a collection of one-way hash function
identifiers, and for TAMP, it contains a single one-way hash
function identifier. The one-way hash function employed by the
TAMP message originator in generating the digital signature MUST
be present.
o encapContentInfo is the signed content, consisting of a content
type identifier and the content itself. The use of the
EncapsulatedContentInfo type is discussed further in
Section 2.2.2.
o certificates is an OPTIONAL collection of certificates. It MAY be
omitted, or it MAY include the X.509 certificates needed to
construct the certification path of the TAMP message originator.
For TAMP messages sent to a trust anchor store where an apex trust
anchor or management trust anchor is used directly to validate the
TAMP message digital signature, this field SHOULD be omitted.
When an apex trust anchor or management trust anchor is used to
validate an X.509 certification path [RFC5280], and the subject
public key from the final certificate in the certification path is
used to validate the TAMP message digital signature, the
certificate of the TAMP message originator SHOULD be included, and
additional certificates to support certification path construction
MAY be included. For TAMP messages sent by a trust anchor store,
this field SHOULD include only the signer's certificate or should
be omitted. A TAMP message recipient MUST NOT reject a valid TAMP
message that contains certificates that are not needed to validate
the digital signature. PKCS#6 extended certificates [PKCS#6] and
attribute certificates (either version 1 or version 2) [RFC5755]
MUST NOT be included in the set of certificates; these certificate
formats are not used in TAMP. Certification authority (CA)
certificates and end entity certificates MUST conform to the
profiles defined in [RFC5280].
o crls is an OPTIONAL collection of certificate revocation lists
(CRLs).
o signerInfos is a collection of per-signer information, and for
TAMP, the collection MUST contain exactly one SignerInfo. The use
of the SignerInfo type is discussed further in Section 2.2.1.
2.2.1. SignerInfo
The TAMP message originator is represented in the SignerInfo type.
The fields of SignerInfo are used as follows:
o version is the syntax version number. With TAMP, the version MUST
be set to 3.
o sid identifies the TAMP message originator's public key. The
subjectKeyIdentifier alternative is always used with TAMP, which
identifies the public key directly. When the public key is
included in a TrustAnchorInfo object, this identifier is included
in the keyId field. When the public key is included in a
Certificate or TBSCertificate, this identifier is included in the
subjectKeyIdentifier certificate extension.
o digestAlgorithm identifies the one-way hash function, and any
associated parameters, used by the TAMP message originator. It
MUST contain the one-way hash functions employed by the
originator. This message digest algorithm identifier MUST match
the one carried in the digestAlgorithms field in SignedData. The
message digest algorithm identifier is carried in two places to
facilitate stream processing by the receiver.
o signedAttrs is an OPTIONAL set of attributes that are signed along
with the content. The signedAttrs are OPTIONAL in the CMS, but
signedAttrs is REQUIRED for all signed TAMP messages. The SET OF
Attribute MUST be encoded with the Distinguished Encoding Rules
(DER) [X.690]. Section 2.2.3 of this document lists the signed
attributes that MUST be included in the collection. Other signed
attributes MAY be included, but any unrecognized signed attributes
MUST be ignored.
o signatureAlgorithm identifies the digital signature algorithm, and
any associated parameters, used by the TAMP message originator to
generate the digital signature.
o signature is the digital signature value generated by the TAMP
message originator.
o unsignedAttrs is an OPTIONAL set of attributes that are not
signed. For TAMP, this field is usually omitted. It is present
only in Apex Trust Anchor Update messages that are to be validated
using the apex trust anchor contingency public key. In this case,
the SET OF Attribute MUST include the symmetric key needed to
decrypt the contingency public key in the contingency-public-key-
decrypt-key unsigned attribute. Section 2.2.4 of this document
describes this unsigned attribute.
2.2.2. EncapsulatedContentInfo
The EncapsulatedContentInfo structure contains the TAMP message. The
fields of EncapsulatedContentInfo are used as follows:
o eContentType is an object identifier that uniquely specifies the
content type, and for TAMP, the value identifies the TAMP message.
The list of TAMP message content types is provided in Section 4.
o eContent is the TAMP message, encoded as an octet string. In
general, the CMS does not require the eContent to be DER-encoded
before constructing the octet string. However, TAMP messages MUST
be DER-encoded.
2.2.3. Signed Attributes
The TAMP message originator MUST digitally sign a collection of
attributes along with the TAMP message. Each attribute in the
collection MUST be DER-encoded. The syntax for attributes is defined
in [RFC5912].
Each of the attributes used with this CMS profile has a single
attribute value. Even though the syntax is defined as a SET OF
AttributeValue, there MUST be exactly one instance of AttributeValue
present.
The SignedAttributes syntax within SignerInfo is defined as a SET OF
Attribute. The SignedAttributes MUST include only one instance of
any particular attribute. TAMP messages that violate this rule MUST
be rejected as malformed.
The TAMP message originator MUST include the content-type and
message-digest attributes. The TAMP message originator MAY also
include the binary-signing-time attribute.
The TAMP message originator MAY include any other attribute that it
deems appropriate. The intent is to allow additional signed
attributes to be included if a future need is identified. This does
not cause an interoperability concern because unrecognized signed
attributes MUST be ignored.
The following summarizes the signed attribute requirements for TAMP
messages:
o content-type MUST be supported.
o message-digest MUST be supported.
o binary-signing-time MAY be supported. When present, it is
generally ignored by the recipient.
o other attributes MAY be supported. Unrecognized attributes MUST
be ignored by the recipient.
2.2.3.1. Content-Type Attribute
The TAMP message originator MUST include a content-type attribute; it
is an object identifier that uniquely specifies the content type.
Section 11.1 of [RFC5652] defines the content-type attribute. For
TAMP, the value identifies the TAMP message. The list of TAMP
message content types and their identifiers is provided in Section 4.
A content-type attribute MUST contain the same object identifier as
the content type contained in the EncapsulatedContentInfo.
2.2.3.2. Message-Digest Attribute
The TAMP message originator MUST include a message-digest attribute,
having as its value the output of a one-way hash function computed on
the TAMP message that is being signed. Section 11.2 of [RFC5652]
defines the message-digest attribute.
2.2.3.3. Binary-Signing-Time Attribute
The TAMP message originator MAY include a binary-signing-time
attribute, specifying the time at which the digital signature was
applied to the TAMP message. The binary-signing-time attribute is
defined in [RFC4049].
No processing of the binary-signing-time attribute is REQUIRED of a
TAMP message recipient; however, the binary-signing-time attribute
MAY be included by the TAMP message originator as a form of message
identifier.
2.2.4. Unsigned Attributes
For TAMP, unsigned attributes are usually omitted. An unsigned attribute is present only in Apex Trust Anchor Update messages that are to be validated by the apex trust anchor contingency public key. In this case, the symmetric key to decrypt the previous contingency public key is provided in the contingency-public-key-decrypt-key unsigned attribute. This attribute MUST be supported, and it is described in Section 2.2.4.1. The TAMP message originator SHOULD NOT include other unsigned attributes, and any unrecognized unsigned attributes MUST be ignored. The UnsignedAttributes syntax within SignerInfo is defined as a SET OF Attribute. The UnsignedAttributes MUST include only one instance of any particular attribute. TAMP messages that violate this rule MUST be rejected as malformed.2.2.4.1. Contingency-Public-Key-Decrypt-Key Attribute
The contingency-public-key-decrypt-key attribute provides the plaintext symmetric key needed to decrypt the previously distributed apex trust anchor contingency public key. The symmetric key MUST be useable with the symmetric algorithm used to previously encrypt the contingency public key. The contingency-public-key-decrypt-key attribute has the following syntax: contingency-public-key-decrypt-key ATTRIBUTE ::= { WITH SYNTAX PlaintextSymmetricKey SINGLE VALUE TRUE ID id-aa-TAMP-contingencyPublicKeyDecryptKey } id-aa-TAMP-contingencyPublicKeyDecryptKey OBJECT IDENTIFIER ::= { id-attributes 63 } PlaintextSymmetricKey ::= OCTET STRING3. Trust Anchor Formats
TAMP recognizes three formats for representing trust anchor information within the protocol itself: Certificate [RFC5280], TBSCertificate [RFC5280], and TrustAnchorInfo [RFC5914]. The TrustAnchorChoice structure, defined in [RFC5914], is used to select one of these options.
TrustAnchorChoice ::= CHOICE {
certificate Certificate,
tbsCert [1] EXPLICIT TBSCertificate,
taInfo [2] EXPLICIT TrustAnchorInfo }
The Certificate structure is commonly used to represent trust
anchors. Certificates include a signature, which removes the ability
for relying parties to customize the information within the structure
itself. TBSCertificate contains all of the information of the
Certificate structure except for the signature, enabling tailoring of
the information. TrustAnchorInfo is intended to serve as a
minimalist representation of trust anchor information for scenarios
where storage or bandwidth is highly constrained.
Implementations are not required to support all three options. The
unsupportedTrustAnchorFormat error code should be indicated when
generating a TAMPError due to receipt of an unsupported trust anchor
format.
(page 19 continued on part 2)
