RFC 2522
Photuris: Session-Key Management Protocol
Pages: 80
Experimental
Experimental
Part 1 of 3 – Pages 1 to 25
Network Working Group P. Karn Request for Comments: 2522 Qualcomm Category: Experimental W. Simpson DayDreamer March 1999 Photuris: Session-Key Management Protocol Status of this Memo This document defines an Experimental Protocol for the Internet community. It does not specify an Internet standard of any kind. Discussion and suggestions for improvement are requested. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (1999). Copyright (C) Philip Karn and William Allen Simpson (1994-1999). All Rights Reserved.Abstract
Photuris is a session-key management protocol intended for use with the IP Security Protocols (AH and ESP). This document defines the basic protocol mechanisms.
Table of Contents
1. Introduction .......................................... 1 1.1 Terminology ..................................... 1 1.2 Protocol Overview ............................... 3 1.3 Security Parameters ............................. 5 1.4 LifeTimes ....................................... 6 1.4.1 Exchange LifeTimes .............................. 6 1.4.2 SPI LifeTimes ................................... 7 1.5 Random Number Generation ........................ 8 2. Protocol Details ...................................... 9 2.1 UDP ............................................. 9 2.2 Header Format ................................... 10 2.3 Variable Precision Integers ..................... 11 2.4 Exchange-Schemes ................................ 13 2.5 Attributes ...................................... 13 3. Cookie Exchange ....................................... 14 3.0.1 Send Cookie_Request ............................. 14 3.0.2 Receive Cookie_Request .......................... 15 3.0.3 Send Cookie_Response ............................ 15 3.0.4 Receive Cookie_Response ......................... 16 3.1 Cookie_Request .................................. 17 3.2 Cookie_Response ................................. 18 3.3 Cookie Generation ............................... 19 3.3.1 Initiator Cookie ................................ 19 3.3.2 Responder Cookie ................................ 20 4. Value Exchange ........................................ 21 4.0.1 Send Value_Request .............................. 21 4.0.2 Receive Value_Request ........................... 22 4.0.3 Send Value_Response ............................. 22 4.0.4 Receive Value_Response .......................... 23 4.1 Value_Request ................................... 24 4.2 Value_Response .................................. 25 4.3 Offered Attribute List .......................... 26 5. Identification Exchange ............................... 28 5.0.1 Send Identity_Request ........................... 29 5.0.2 Receive Identity_Request ........................ 29 5.0.3 Send Identity_Response .......................... 30 5.0.4 Receive Identity_Response ....................... 30 5.1 Identity_Messages ............................... 31 5.2 Attribute Choices List .......................... 33 5.3 Shared-Secret ................................... 34 5.4 Identity Verification ........................... 34
5.5 Privacy-Key Computation ......................... 36
5.6 Session-Key Computation ......................... 37
6. SPI Messages .......................................... 38
6.0.1 Send SPI_Needed ................................. 38
6.0.2 Receive SPI_Needed .............................. 39
6.0.3 Send SPI_Update ................................. 39
6.0.4 Receive SPI_Update .............................. 39
6.0.5 Automated SPI_Updates ........................... 40
6.1 SPI_Needed ...................................... 41
6.2 SPI_Update ...................................... 43
6.2.1 Creation ........................................ 44
6.2.2 Deletion ........................................ 45
6.2.3 Modification .................................... 45
6.3 Validity Verification ........................... 45
7. Error Messages ........................................ 46
7.1 Bad_Cookie ...................................... 47
7.2 Resource_Limit .................................. 47
7.3 Verification_Failure ............................ 48
7.4 Message_Reject .................................. 49
8. Public Value Exchanges ................................ 50
8.1 Modular Exponentiation Groups ................... 50
8.2 Moduli Selection ................................ 50
8.2.1 Bootstrap Moduli ................................ 51
8.2.2 Learning Moduli ................................. 51
8.3 Generator Selection ............................. 51
8.4 Exponent Selection .............................. 52
8.5 Defective Exchange Values ....................... 53
9. Basic Exchange-Schemes ................................ 54
10. Basic Key-Generation-Function ......................... 55
10.1 MD5 Hash ........................................ 55
11. Basic Privacy-Method .................................. 55
11.1 Simple Masking .................................. 55
12. Basic Validity-Method ................................. 55
12.1 MD5-IPMAC Check ................................. 55
13. Basic Attributes ...................................... 56
13.1 Padding ......................................... 56
13.2 AH-Attributes ................................... 57
13.3 ESP-Attributes .................................. 57
13.4 MD5-IPMAC ....................................... 58
13.4.1 Symmetric Identification ........................ 58
13.4.2 Authentication .................................. 59
13.5 Organizational .................................. 60
APPENDICES ................................................... 61
A. Automaton ............................................. 61
A.1 State Transition Table .......................... 62
A.2 States .......................................... 65
A.2.1 Initial ......................................... 65
A.2.2 Cookie .......................................... 66
A.2.3 Value ........................................... 66
A.2.4 Identity ........................................ 66
A.2.5 Ready ........................................... 66
A.2.6 Update .......................................... 66
B. Use of Identification and Secrets ..................... 67
B.1 Identification .................................. 67
B.2 Group Identity With Group Secret ................ 67
B.3 Multiple Identities With Group Secrets .......... 68
B.4 Multiple Identities With Multiple Secrets ....... 69
OPERATIONAL CONSIDERATIONS ................................... 70
SECURITY CONSIDERATIONS ...................................... 70
HISTORY ...................................................... 71
ACKNOWLEDGEMENTS ............................................. 72
REFERENCES ................................................... 73
CONTACTS ..................................................... 75
COPYRIGHT .................................................... 76
1. Introduction
Photuris [Firefly] establishes short-lived session-keys between two parties, without passing the session-keys across the Internet. These session-keys directly replace the long-lived secret-keys (such as passwords and passphrases) that have been historically configured for security purposes. The basic Photuris protocol utilizes these existing previously configured secret-keys for identification of the parties. This is intended to speed deployment and reduce administrative configuration changes. This document is primarily intended for implementing the Photuris protocol. It does not detail service and application interface definitions, although it does mention some basic policy areas required for the proper implementation and operation of the protocol mechanisms. Since the basic Photuris protocol is extensible, new data types and protocol behaviour should be expected. The implementor is especially cautioned not to depend on values that appear in examples to be current or complete, since their purpose is primarily pedagogical.1.1. Terminology
In this document, the key words "MAY", "MUST, "MUST NOT", "optional", "recommended", "SHOULD", and "SHOULD NOT", are to be interpreted as described in [RFC-2119]. byte An 8-bit quantity; also known as "octet" in standardese. exchange-value The publically distributable value used to calculate a shared-secret. As used in this document, refers to a Diffie-Hellman exchange, not the public part of a public/private key-pair. private-key A value that is kept secret, and is part of an asymmetric public/private key-pair. public-key A publically distributable value that is part of an asymmetric public/private key-pair. secret-key A symmetric key that is not publically distributable. As used in this document, this is distinguished from an asymmetric public/private
key-pair. An example is a user password.
Security Association (SA)
A collection of parameters describing the security
relationship between two nodes. These parameters
include the identities of the parties, the transform
(including algorithm and algorithm mode), the key(s)
(such as a session-key, secret-key, or appropriate
public/private key-pair), and possibly other
information such as sensitivity labelling.
Security Parameters Index (SPI)
A number that indicates a particular set of uni-
directional attributes used under a Security
Association, such as transform(s) and session-
key(s). The number is relative to the IP
Destination, which is the SPI Owner, and is unique
per IP (Next Header) Protocol. That is, the same
value MAY be used by multiple protocols to
concurrently indicate different Security Association
parameters.
session-key A key that is independently derived from a shared-
secret by the parties, and used for keying one
direction of traffic. This key is changed
frequently.
shared-secret As used in this document, the calculated result of
the Photuris exchange.
SPI Owner The party that corresponds to the IP Destination;
the intended recipient of a protected datagram.
SPI User The party that corresponds to the IP Source; the
sender of a protected datagram.
transform A cryptographic manipulation of a particular set of
data. As used in this document, refers to certain
well-specified methods (defined elsewhere). For
example, AH-MD5 [RFC-1828] transforms an IP datagram
into a cryptographic hash, and ESP-DES-CBC [RFC-
1829] transforms plaintext to ciphertext and back
again.
Many of these terms are hierarchically related:
Security Association (bi-directional)
- one or more lists of Security Parameters (uni-directional)
-- one or more Attributes
--- may have a key
--- may indicate a transform
Implementors will find details of cryptographic hashing (such as
MD5), encryption algorithms and modes (such as DES), digital
signatures (such as DSS), and other algorithms in [Schneier95].
1.2. Protocol Overview
The Photuris protocol consists of several simple phases:
1. A "Cookie" Exchange guards against simple flooding attacks sent
with bogus IP Sources or UDP Ports. Each party passes a "cookie"
to the other.
In return, a list of supported Exchange-Schemes are offered by the
Responder for calculating a shared-secret.
2. A Value Exchange establishes a shared-secret between the parties.
Each party passes an Exchange-Value to the other. These values
are used to calculate a shared-secret. The Responder remains
stateless until a shared-secret has been created.
In addition, supported attributes are offered by each party for
use in establishing new Security Parameters.
3. An Identification Exchange identifies the parties to each other,
and verifies the integrity of values sent in phases 1 and 2.
In addition, the shared-secret provides a basis to generate
separate session-keys in each direction, which are in turn used
for conventional authentication or encryption. Additional
security attributes are also exchanged as needed.
This exchange is masked for party privacy protection using a
message privacy-key based on the shared-secret. This protects the
identities of the parties, hides the Security Parameter attribute
values, and improves security for the exchange protocol and
security transforms.
4. Additional messages may be exchanged to periodically change the
session-keys, and to establish new or revised Security Parameters.
These exchanges are also masked for party privacy protection in
the same fashion as above.
The sequence of message types and their purposes are summarized in
the diagram below. The first three phases (cookie, exchange, and
identification) must be carried out in their entirety before any
Security Association can be used.
Initiator Responder
========= =========
Cookie_Request ->
<- Cookie_Response
offer schemes
Value_Request ->
pick scheme
offer value
offer attributes
<- Value_Response
offer value
offer attributes
[generate shared-secret from exchanged values]
Identity_Request ->
make SPI
pick SPI attribute(s)
identify self
authenticate
make privacy key(s)
mask/encrypt message
<- Identity_Response
make SPI
pick SPI attribute(s)
identify self
authenticate
make privacy key(s)
mask/encrypt message
[make SPI session-keys in each direction]
SPI User SPI Owner
======== =========
SPI_Needed ->
list SPI attribute(s)
make validity key
authenticate
make privacy key(s)
mask/encrypt message
<- SPI_Update
make SPI
pick SPI attribute(s)
make SPI session-key(s)
make validity key
authenticate
make privacy key(s)
mask/encrypt message
Either party may initiate an exchange at any time. For example, the
Initiator need not be a "caller" in a telephony link.
The Initiator is responsible for recovering from all message losses
by retransmission.
1.3. Security Parameters
A Photuris exchange between two parties results in a pair of SPI
values (one in each direction). Each SPI is used in creating
separate session-key(s) in each direction.
The SPI is assigned by the entity controlling the IP Destination: the
SPI Owner (receiver). The parties use the combination of IP
Destination, IP (Next Header) Protocol, and SPI to distinguish the
correct Security Association.
When both parties initiate Photuris exchanges concurrently, or one
party initiates more than one Photuris exchange, the Initiator
Cookies (and UDP Ports) keep the exchanges separate. This results in
more than one initial SPI for each Destination.
To create multiple SPIs with different parameters, the parties may
also send SPI_Updates.
There is no requirement that all such outstanding SPIs be used. The
SPI User (sender) selects an appropriate SPI for each datagram
transmission.
Implementation Notes:
The method used for SPI assignment is implementation dependent.
The only requirement is that the SPI be unique for the IP
Destination and IP (Next Header) Protocol.
However, selection of a cryptographically random SPI value can
help prevent attacks that depend on a predicatable sequence of
values. The implementor MUST NOT expect SPI values to have a
particular order or range.
1.4. LifeTimes
The Photuris exchange results in two kinds of state, each with
separate LifeTimes.
1) The Exchange LifeTime of the small amount of state associated with
the Photuris exchange itself. This state may be viewed as between
Internet nodes.
2) The SPI LifeTimes of the individual SPIs that are established.
This state may be viewed as between users and nodes.
The SPI LifeTimes may be shorter or longer than the Exchange
LifeTime. These LifeTimes are not required to be related to each
other.
When an Exchange-Value expires (or is replaced by a newer value), any
unexpired derived SPIs are not affected. This is important to allow
traffic to continue without interruption during new Photuris
exchanges.
1.4.1. Exchange LifeTimes
All retained exchange state of both parties has an associated
Exchange LifeTime (ELT), and is subject to periodic expiration. This
depends on the physical and logistical security of the machine, and
is typically in the range of 10 minutes to one day (default 30
minutes).
In addition, during a Photuris exchange, an Exchange TimeOut (ETO)
limits the wait for the exchange to complete. This timeout includes
the packet round trips, and the time for completing the
Identification Exchange calculations. The time is bounded by both
the maximum amount of calculation delay expected for the processing
power of an unknown peer, and the minimum user expectation for
results (default 30 seconds).
These Exchange LifeTimes and TimeOuts are implementation dependent
and are not disclosed in any Photuris message. The paranoid operator
will have a fairly short Exchange LifeTime, but it MUST NOT be less
than twice the ETO.
To prevent synchronization between Photuris exchanges, the
implementation SHOULD randomly vary each Exchange LifeTime within
twice the range of seconds that are required to calculate a new
Exchange-Value. For example, when the Responder uses a base ELT of
30 minutes, and takes 10 seconds to calculate the new Exchange-Value,
the equation might be (in milliseconds):
1790000 + urandom(20000)
The Exchange-Scheme, Exchange-Values, and resulting shared-secret MAY
be cached in short-term storage for the Exchange LifeTime. When
repetitive Photuris exchanges occur between the same parties, and the
Exchange-Values are discovered to be unchanged, the previously
calculated shared-secret can be used to rapidly generate new
session-keys.
1.4.2. SPI LifeTimes
Each SPI has an associated LifeTime, specified by the SPI owner
(receiver). This SPI LifeTime (SPILT) is usually related to the
speed of the link (typically 2 to 30 minutes), but it MUST NOT be
less than thrice the ETO.
The SPI can also be deleted by the SPI Owner using the SPI_Update.
Once the SPI has expired or been deleted, the parties cease using the
SPI.
To prevent synchronization between multiple Photuris exchanges, the
implementation SHOULD randomly vary each SPI LifeTime. For example,
when the Responder uses a base SPILT of 5 minutes, and 30 seconds for
the ETO, the equation might be (in milliseconds):
285000 + urandom(30000)
There is no requirement that a long LifeTime be accepted by the SPI
User. The SPI User might never use an established SPI, or cease
using the SPI at any time.
When more than one unexpired SPI is available to the SPI User for the
same function, a common implementation technique is to select the SPI
with the greatest remaining LifeTime. However, selecting randomly among a large number of SPIs might provide some defense against traffic analysis. To prevent resurrection of deleted or expired SPIs, SPI Owners SHOULD remember those SPIs, but mark them as unusable until the Photuris exchange shared-secret used to create them also expires and purges the associated state. When the SPI Owner detects an incoming SPI that has recently expired, but the associated exchange state has not yet been purged, the implementation MAY accept the SPI. The length of time allowed is highly dependent on clock drift and variable packet round trip time, and is therefore implementation dependent.1.5. Random Number Generation
The security of Photuris critically depends on the quality of the secret random numbers generated by each party. A poor random number generator at either party will compromise the shared-secret produced by the algorithm. Generating cryptographic quality random numbers on a general purpose computer without hardware assistance is a very tricky problem. In general, this requires using a cryptographic hashing function to "distill" the entropy from a large number of semi-random external events, such as the timing of key strokes. An excellent discussion can be found in [RFC-1750].
2. Protocol Details
The Initiator begins a Photuris exchange under several circumstances: - The Initiator has a datagram that it wishes to send with confidentiality, and has no current Photuris exchange state with the IP Destination. This datagram is discarded, and a Cookie_Request is sent instead. - The Initiator has received the ICMP message [RFC-1812] Destination Unreachable: Communication Administratively Prohibited (Type 3, Code 13), and has no current Photuris exchange state with the ICMP Source. - The Initiator has received the ICMP message [RFC-2521] Security Failures: Bad SPI (Type 40, Code 0), that matches current Photuris exchange state with the ICMP Source. - The Initiator has received the ICMP message [RFC-2521] Security Failures: Need Authentication (Type 40, Code 4), and has no current Photuris exchange state with the ICMP Source. - The Initiator has received the ICMP message [RFC-2521] Security Failures: Need Authorization (Type 40, Code 5), that matches current Photuris exchange state with the ICMP Source. When the event is an ICMP message, special care MUST be taken that the ICMP message actually includes information that matches a previously sent IP datagram. Otherwise, this could provide an opportunity for a clogging attack, by stimulating a new Photuris Exchange.2.1. UDP
All Photuris messages use the User Datagram Protocol header [RFC- 768]. The Initiator sends to UDP Destination Port 468. When replying to the Initiator, the Responder swaps the IP Source and Destination, and the UDP Source and Destination Ports. The UDP checksum MUST be correctly calculated when sent. When a message is received with an incorrect UDP checksum, it is silently discarded.
Implementation Notes:
It is expected that installation of Photuris will ensure that UDP
checksum calculations are enabled for the computer operating
system and later disabling by operators is prevented.
Internet Protocol version 4 [RFC-791] restricts the maximum
reassembled datagram to 576 bytes.
When processing datagrams containing variable size values, the
length must be checked against the overall datagram length. An
invalid size (too long or short) that causes a poorly coded
receiver to abort could be used as a denial of service attack.
2.2. Header Format
All of the messages have a format similar to the following, as
transmitted left to right in network order (most significant to least
significant):
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Initiator-Cookie ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Responder-Cookie ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message |
+-+-+-+-+-+-+-+-+
Initiator-Cookie 16 bytes.
Responder-Cookie 16 bytes.
Message 1 byte. Each message type has a unique value.
Initial values are assigned as follows:
0 Cookie_Request
1 Cookie_Response
2 Value_Request
3 Value_Response
4 Identity_Request
5 Secret_Response (optional)
6 Secret_Request (optional)
7 Identity_Response
8 SPI_Needed
9 SPI_Update
10 Bad_Cookie
11 Resource_Limit
12 Verification_Failure
13 Message_Reject
Further details and differences are elaborated in the individual
messages.
2.3. Variable Precision Integers
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Size | Value ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Size 2, 4, or 8 bytes. The number of significant bits
used in the Value field. Always transmitted most
significant byte first.
When the Size is zero, no Value field is present;
there are no significant bits. This means "missing"
or "null". It should not be confused with the value
zero, which includes an indication of the number of
significant bits.
When the most significant byte is in the range 0
through 254 (0xfe), the field is 2 bytes. Both
bytes are used to indicate the size of the Value
field, which ranges from 1 to 65,279 significant
bits (in 1 to 8,160 bytes).
When the most significant byte is 255 (0xff), the
field is 4 bytes. The remaining 3 bytes are added
to 65,280 to indicate the size of the Value field,
which is limited to 16,776,959 significant bits (in
2,097,120 bytes).
When the most significant 2 bytes are 65,535
(0xffff), the field is 8 bytes. The remaining 6
bytes are added to 16,776,960 to indicate the size
of the Value field.
Value 0 or more bytes. Always transmitted most
significant byte first.
The bits used are right justified within byte
boundaries; that is, any unused bits are in the most
significant byte. When there are no unused bits, or
unused bits are zero filled, the value is assumed to
be an unsigned positive integer.
When the leading unused bits are ones filled, the
number is assumed to be a two's-complement negative
integer. A negative integer will always have at
least one unused leading sign bit in the most
significant byte.
Shortened forms SHOULD NOT be used when the Value includes a number
of leading zero significant bits. The Size SHOULD indicate the
correct number of significant bits.
Implementation Notes:
Negative integers are not required to be supported, but are
included for completeness.
No more than 65,279 significant bits are required to be supported.
Other ranges are vastly too long for these UDP messages, but are
included for completeness.
2.4. Exchange-Schemes
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Scheme | Size | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Value ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Scheme 2 bytes. A unique value indicating the Exchange- Scheme. See the "Basic Exchange-Schemes" for details. Size 2 bytes, ranging from 0 to 65,279. See "Variable Precision Integer". Value 0 or more bytes. See "Variable Precision Integer". The Size MUST NOT be assumed to be constant for a particular Scheme. Multiple kinds of the same Scheme with varying Sizes MAY be present in any list of schemes. However, only one of each Scheme and Size combination will be present in any list of schemes.2.5. Attributes
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Attribute | Length | Value(s) ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Attribute 1 byte. A unique value indicating the kind of attribute. See the "Basic Attributes" for details. When the value is zero (padding), no Length field is present (always zero). Length 1 byte. The size of the Value(s) field in bytes. When the Length is zero, no Value(s) field is present. Value(s) 0 or more bytes. See the "Basic Attributes" for details. The Length MUST NOT be assumed to be constant for a particular
Attribute. Multiple kinds of the same Attribute with varying Lengths MAY be present in any list of attributes.3. Cookie Exchange
Initiator Responder ========= ========= Cookie_Request -> <- Cookie_Response offer schemes3.0.1. Send Cookie_Request
The Initiator initializes local state, and generates a unique "cookie". The Initiator-Cookie MUST be different in each new Cookie_Request between the same parties. See "Cookie Generation" for details. - If any previous exchange between the peer IP nodes has not expired in which this party was the Initiator, this Responder-Cookie is set to the most recent Responder-Cookie, and this Counter is set to the corresponding Counter. For example, a new Virtual Private Network (VPN) tunnel is about to be established to an existing partner. The Counter is the same value received in the prior Cookie_Response, the Responder-Cookie remains the same, and a new Initiator-Cookie is generated. - If the new Cookie_Request is in response to a message of a previous exchange in which this party was the Responder, this Responder-Cookie is set to the previous Initiator-Cookie, and this Counter is set to zero. For example, a Bad_Cookie message was received from the previous Initiator in response to SPI_Needed. The Responder-Cookie is replaced with the Initiator-Cookie, and a new Initiator-Cookie is generated. This provides bookkeeping to detect bogus Bad_Cookie messages. Also, can be used for bi-directional User, Transport, and Process oriented keying. Such mechanisms are outside the scope of this document. - Otherwise, this Responder-Cookie and Counter are both set to zero.
By default, the Initiator operates in the same manner as when all
of its previous exchange state has expired. The Responder will
send a Resource_Limit when its own exchange state has not expired.
The Initiator also starts a retransmission timer. If no valid
Cookie_Response arrives within the time limit, the same
Cookie_Request is retransmitted for the remaining number of
Retransmissions. The Initiator-Cookie value MUST be the same in each
such retransmission to the same IP Destination and UDP Port.
When Retransmissions have been exceeded, if a Resource_Limit message
has been received during the exchange, the Initiator SHOULD begin the
Photuris exchange again by sending a new Cookie_Request with updated
values.
3.0.2. Receive Cookie_Request
On receipt of a Cookie_Request, the Responder determines whether
there are sufficient resources to begin another Photuris exchange.
- When too many SPI values are already in use for this particular
peer, or too many concurrent exchanges are in progress, or some
other resource limit is reached, a Resource_Limit message is sent.
- When any previous exchange initiated by this particular peer has
not exceeded the Exchange TimeOut, and the Responder-Cookie does
not specify one of these previous exchanges, a Resource_Limit
message is sent.
Otherwise, the Responder returns a Cookie_Response.
Note that the Responder creates no additional state at this time.
3.0.3. Send Cookie_Response
The IP Source for the Initiator is examined. If any previous
exchange between the peer IP nodes has not expired, the response
Counter is set to the most recent exchange Counter plus one (allowing
for out of order retransmissions). Otherwise, the response Counter
is set to the request Counter plus one.
If (through rollover of the Counter) the new Counter value is zero
(modulo 256), the value is set to one.
If this new Counter value matches some previous exchange initiated by
this particular peer that has not yet exceeded the Exchange TimeOut,
the Counter is incremented again, until a unique Counter value is
reached.
Nota Bene:
No more than 254 concurrent exchanges between the same two peers
are supported.
The Responder generates a unique cookie. The Responder-Cookie value
in each successive response SHOULD be different. See "Cookie
Generation" for details.
The Exchange-Schemes available between the peers are listed in the
Offered-Schemes.
3.0.4. Receive Cookie_Response
The Initiator validates the Initiator-Cookie, and the Offered-
Schemes.
- When an invalid/expired Initiator-Cookie is detected, the message
is silently discarded.
- When the variable length Offered-Schemes do not match the UDP
Length, or all Offered-Schemes are obviously defective and/or
insufficient for the purposes intended, the message is silently
discarded; the implementation SHOULD log the occurance, and notify
an operator as appropriate.
- Once a valid message has been received, later Cookie_Responses
with matching Initiator-Cookies are also silently discarded, until
a new Cookie_Request is sent.
When the message is valid, an Exchange-Scheme is chosen from the list
of Offered-Schemes.
This Scheme-Choice may affect the next Photuris message sent. By
default, the next Photuris message is a Value_Request.
Implementation Notes:
Only the Initiator-Cookie is used to identify the exchange. The
Counter and Responder-Cookie will both be different from the
Cookie_Request.
Various proposals for extensions utilize the Scheme-Choice to
indicate a different message sequence. Such mechanisms are
outside the scope of this document.
3.1. Cookie_Request
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Initiator-Cookie ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Responder-Cookie ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Message | Counter | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Initiator-Cookie 16 bytes. A randomized value that identifies the exchange. The value MUST NOT be zero. See "Cookie Generation" for details. Responder-Cookie 16 bytes. Identifies a specific previous exchange. Copied from a previous Cookie_Response. When zero, no previous exchange is specified. When non-zero, and the Counter is zero, contains the Initiator-Cookie of a previous exchange. The specified party is requested to be the Responder in this exchange, to retain previous party pairings. When non-zero, and the Counter is also non-zero, contains the Responder-Cookie of a previous exchange. The specified party is requested to be the Responder in this exchange, to retain previous party pairings. Message 0 Counter 1 byte. Indicates the number of previous exchanges. When zero, the Responder-Cookie indicates the Initiator of a previous exchange, or no previous exchange is specified. When non-zero, the Responder-Cookie indicates the Responder to a previous exchange. This value is set to the Counter from the corresponding Cookie_Response or from a Resource_Limit.
3.2. Cookie_Response
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Initiator-Cookie ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Responder-Cookie ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Message | Counter | Offered-Schemes ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Initiator-Cookie 16 bytes. Copied from the Cookie_Request. Responder-Cookie 16 bytes. A randomized value that identifies the exchange. The value MUST NOT be zero. See "Cookie Generation" for details. Message 1 Counter 1 byte. Indicates the number of the current exchange. Must be greater than zero. Offered-Schemes 4 or more bytes. A list of one or more Exchange- Schemes supported by the Responder, ordered from most to least preferable. See the "Basic Exchange- Schemes" for details. Only one Scheme (#2) is required to be supported, and SHOULD be present in every Offered-Schemes list. More than one of each kind of Scheme may be offered, but each is distinguished by its Size. The end of the list is indicated by the UDP Length.
3.3. Cookie Generation
The exact technique by which a Photuris party generates a cookie is implementation dependent. The method chosen must satisfy some basic requirements: 1. The cookie MUST depend on the specific parties. This prevents an attacker from obtaining a cookie using a real IP address and UDP port, and then using it to swamp the victim with requests from randomly chosen IP addresses or ports. 2. It MUST NOT be possible for anyone other than the issuing entity to generate cookies that will be accepted by that entity. This implies that the issuing entity will use local secret information in the generation and subsequent verification of a cookie. It must not be possible to deduce this secret information from any particular cookie. 3. The cookie generation and verification methods MUST be fast to thwart attacks intended to sabotage CPU resources. A recommended technique is to use a cryptographic hashing function (such as MD5). An incoming cookie can be verified at any time by regenerating it locally from values contained in the incoming datagram and the local secret random value.3.3.1. Initiator Cookie
The Initiator secret value that affects its cookie SHOULD change for each new Photuris exchange, and is thereafter internally cached on a per Responder basis. This provides improved synchronization and protection against replay attacks. An alternative is to cache the cookie instead of the secret value. Incoming cookies can be compared directly without the computational cost of regeneration. It is recommended that the cookie be calculated over the secret value, the IP Source and Destination addresses, and the UDP Source and Destination ports.
Implementation Notes:
Although the recommendation includes the UDP Source port, this is
very implementation specific. For example, it might not be
included when the value is constant.
However, it is important that the implementation protect mutually
suspicious users of the same machine from generating the same
cookie.
3.3.2. Responder Cookie
The Responder secret value that affects its cookies MAY remain the
same for many different Initiators. However, this secret SHOULD be
changed periodically to limit the time for use of its cookies
(typically each 60 seconds).
The Responder-Cookie SHOULD include the Initiator-Cookie. The
Responder-Cookie MUST include the Counter (that is returned in the
Cookie_Response). This provides improved synchronization and
protection against replay attacks.
It is recommended that the cookie be calculated over the secret
value, the IP Source and Destination addresses, its own UDP
Destination port, the Counter, the Initiator-Cookie, and the
currently Offered-Schemes.
The cookie is not cached per Initiator to avoid saving state during
the initial Cookie Exchange. On receipt of a Value_Request
(described later), the Responder regenerates its cookie for
validation.
Once the Value_Response is sent (also described later), both
Initiator and Responder cookies are cached to identify the exchange.
Implementation Notes:
Although the recommendation does not include the UDP Source port,
this is very implementation specific. It might be successfully
included in some variants.
However, it is important that the UDP Source port not be included
when matching existing Photuris exchanges for determining the
appropriate Counter.
The recommendation includes the Offered-Schemes to detect a
dynamic change of scheme value between the Cookie_Response and
Value_Response.
Some mechanism MAY be needed to detect a dynamic change of pre-
calculated Responder Exchange-Value between the Value_Response and
Identity_Response. For example, change the secret value to render
the cookie invalid, or explicitly mark the Photuris exchange state
as expired.
(page 25 continued on part 2)