RFC 2522
Photuris: Session-Key Management Protocol
Pages: 80
Experimental
Experimental
Part 2 of 3 – Pages 25 to 53
4. Value Exchange
Initiator Responder ========= ========= Value_Request -> pick scheme offer value offer attributes <- Value_Response offer value offer attributes [generate shared-secret from exchanged values]4.0.1. Send Value_Request
The Initiator generates an appropriate Exchange-Value for the Scheme-Choice. This Exchange-Value may be pre-calculated and used for multiple Responders. The IP Destination for the Responder is examined, and the attributes available between the parties are listed in the Offered-Attributes. The Initiator also starts a retransmission timer. If no valid Value_Response arrives within the time limit, the same Value_Request is retransmitted for the remaining number of Retransmissions. When Retransmissions have been exceeded, if a Bad_Cookie or Resource_Limit message has been received during the exchange, the Initiator SHOULD begin the Photuris exchange again by sending a new Cookie_Request.
4.0.2. Receive Value_Request
The Responder validates the Responder-Cookie, the Counter, the Scheme-Choice, the Exchange-Value, and the Offered-Attributes. - When an invalid/expired Responder-Cookie is detected, a Bad_Cookie message is sent. - 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 an invalid Scheme-Choice is detected, or the Exchange-Value is obviously defective, or the variable length Offered-Attributes do not match the UDP Length, the message is silently discarded; the implementation SHOULD log the occurance, and notify an operator as appropriate. When the message is valid, the Responder sets its Exchange timer to the Exchange TimeOut, and returns a Value_Response. The Responder keeps a copy of the incoming Value_Request cookie pair, and its Value_Response. If a duplicate Value_Request is received, it merely resends its previous Value_Response, and takes no further action.4.0.3. Send Value_Response
The Responder generates an appropriate Exchange-Value for the Scheme-Choice. This Exchange-Value may be pre-calculated and used for multiple Initiators. The IP Source for the Initiator is examined, and the attributes available between the parties are listed in the Offered-Attributes. Implementation Notes: At this time, the Responder begins calculation of the shared- secret. Calculation of the shared-secret is executed in parallel to minimize delay. This may take a substantial amount of time. The implementor should ensure that retransmission is not blocked by this calculation. This is not usually a problem, as retransmission timeouts typically exceed calculation time.
4.0.4. Receive Value_Response
The Initiator validates the pair of Cookies, the Exchange-Value, and the Offered-Attributes. - When an invalid/expired cookie is detected, the message is silently discarded. - When the Exchange-Value is obviously defective, or the variable length Offered-Attributes do not match the UDP Length, 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 Value_Responses with both matching cookies are also silently discarded, until a new Cookie_Request is sent. When the message is valid, the Initiator begins its parallel computation of the shared-secret. When the Initiator completes computation, it sends an Identity_Request to the Responder.
4.1. Value_Request
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Initiator-Cookie ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Responder-Cookie ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Message | Counter | Scheme-Choice | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Initiator-Exchange-Value ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Initiator-Offered-Attributes ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- Initiator-Cookie 16 bytes. Copied from the Cookie_Response. Responder-Cookie 16 bytes. Copied from the Cookie_Response. Message 2 Counter 1 byte. Copied from the Cookie_Response. Scheme-Choice 2 bytes. A value selected by the Initiator from the list of Offered-Schemes in the Cookie_Response. Only the Scheme is specified; the Size will match the Initiator-Exchange-Value, and the Value(s) are implicit. Initiator-Exchange-Value Variable Precision Integer. Provided by the Initiator for calculating a shared-secret between the parties. The Value format is indicated by the Scheme-Choice. The field may be any integral number of bytes in length, as indicated by its Size field. It does not require any particular alignment. The 32-bit alignment shown is for convenience in the illustration.
Initiator-Offered-Attributes
4 or more bytes. A list of Security Parameter
attributes supported by the Initiator.
The contents and usage of this list are further
described in "Offered Attributes List". The end of
the list is indicated by the UDP Length.
4.2. Value_Response
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Initiator-Cookie ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Responder-Cookie ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Responder-Exchange-Value ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Responder-Offered-Attributes ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
Initiator-Cookie 16 bytes. Copied from the Value_Request.
Responder-Cookie 16 bytes. Copied from the Value_Request.
Message 3
Reserved 3 bytes. For future use; MUST be set to zero when
transmitted, and MUST be ignored when received.
Responder-Exchange-Value
Variable Precision Integer. Provided by the
Responder for calculating a shared-secret between
the parties. The Value format is indicated by the
current Scheme-Choice specified in the
Value_Request.
The field may be any integral number of bytes in
length, as indicated by its Size field. It does not
require any particular alignment. The 32-bit
alignment shown is for convenience in the
illustration.
Responder-Offered-Attributes
4 or more bytes. A list of Security Parameter
attributes supported by the Responder.
The contents and usage of this list are further
described in "Offered Attributes List". The end of
the list is indicated by the UDP Length.
4.3. Offered Attribute List
This list includes those attributes supported by the party that are
available to the other party. The attribute formats are specified in
the "Basic Attributes".
The list is composed of two or three sections: Identification-
Attributes, Authentication-Attributes, and (optional) Encapsulation-
Attributes. Within each section, the attributes are ordered from
most to least preferable.
The first section of the list includes methods of identification. An
Identity-Choice is selected from this list.
The second section of the list begins with "AH-Attributes" (#1). It
includes methods of authentication, and other operational types.
The third section of the list begins with "ESP-Attributes" (#2). It
includes methods of authentication, compression, encryption, and
other operational types. When no Encapsulation-Attributes are
offered, the "ESP-Attributes" attribute itself is omitted from the
list.
Attribute-Choices are selected from the latter two sections of the
list.
Support is required for the "MD5-IPMAC" (#5) attribute for both
"Symmetric Identification" and "Authentication" and they SHOULD be
present in every Offered-Attributes list.
Implementation Notes:
For example,
"MD5-IPMAC" (Symmetric Identification),
"AH-Attributes",
"MD5-IPMAC" (Authentication).
Since the offer is made by the prospective SPI User (sender),
order of preference likely reflects the capabilities and
engineering tradeoffs of a particular implementation.
However, the critical processing bottlenecks are frequently in the
receiver. The SPI Owner (receiver) may express its needs by
choosing a less preferable attribute.
The order may also be affected by operational policy and requested
services for an application. Such considerations are outside the
scope of this document.
The list may be divided into additional sections. These sections
will always follow the ESP-Attributes section, and are
indistinguishable from unrecognized attributes.
The authentication, compression, encryption and identification
mechanisms chosen, as well as the encapsulation modes (if any),
need not be the same in both directions.
5. Identification Exchange
Initiator Responder ========= ========= 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] The exchange of messages is ordered, although the formats and meanings of the messages are identical in each direction. The messages are easily distinguished by the parties themselves, by examining the Message and Identification fields. Implementation Notes: The amount of time for the calculation may be dependent on the value of particular bits in secret values used in generating the shared-secret or identity verification. To prevent analysis of these secret bits by recording the time for calculation, sending of the Identity_Messages SHOULD be delayed until the time expected for the longest calculation. This will be different for different processor speeds, different algorithms, and different length variables. Therefore, the method for estimating time is implementation dependent. Any authenticated and/or encrypted user datagrams received before the completion of identity verification can be placed on a queue pending completion of this step. If verification succeeds, the queue is processed as though the datagrams had arrived subsequent to the verification. If verification fails, the queue is discarded.
5.0.1. Send Identity_Request
The Initiator chooses an appropriate Identification, the SPI and SPILT, a set of Attributes for the SPI, calculates the Verification, and masks the message using the Privacy-Method indicated by the current Scheme-Choice. The Initiator also starts a retransmission timer. If no valid Identity_Response arrives within the time limit, its previous Identity_Request is retransmitted for the remaining number of Retransmissions. When Retransmissions have been exceeded, if a Bad_Cookie message has been received during the exchange, the Initiator SHOULD begin the Photuris exchange again by sending a new Cookie_Request.5.0.2. Receive Identity_Request
The Responder validates the pair of Cookies, the Padding, the Identification, the Verification, and the Attribute-Choices. - When an invalid/expired cookie is detected, a Bad_Cookie message is sent. - After unmasking, when invalid Padding is detected, the variable length Attribute-Choices do not match the UDP Length, or an attribute was not in the Offered-Attributes, the message is silently discarded. - When an invalid Identification is detected, or the message verification fails, a Verification_Failure message is sent. - Whenever such a problem is detected, the Security Association is not established; the implementation SHOULD log the occurance, and notify an operator as appropriate. When the message is valid, the Responder sets its Exchange timer to the Exchange LifeTime (if this has not already been done for a previous exchange). When its parallel computation of the shared- secret is complete, the Responder returns an Identity_Response. The Responder keeps a copy of the incoming Identity_Request values, and its Identity_Response. If a duplicate Identity_Request is received, it merely resends its previous Identity_Response, and takes no further action.
5.0.3. Send Identity_Response
The Responder chooses an appropriate Identification, the SPI and SPILT, a set of Attributes for the SPI, calculates the Verification, and masks the message using the Privacy-Method indicated by the current Scheme-Choice. The Responder calculates the SPI session-keys in both directions. At this time, the Responder begins the authentication and/or encryption of user datagrams.5.0.4. Receive Identity_Response
The Initiator validates the pair of Cookies, the Padding, the Identification, the Verification, and the Attribute-Choices. - When an invalid/expired cookie is detected, the message is silently discarded. - After unmasking, when invalid Padding is detected, the variable length Attribute-Choices do not match the UDP Length, or an attribute was not in the Offered-Attributes, the message is silently discarded. - When an invalid Identification is detected, or the message verification fails, a Verification_Failure message is sent. - Whenever such a problem is detected, the Security Association is not established; the implementation SHOULD log the occurance, and notify an operator as appropriate. - Once a valid message has been received, later Identity_Responses with both matching cookies are also silently discarded, until a new Cookie_Request is sent. When the message is valid, the Initiator sets its Exchange timer to the Exchange LifeTime (if this has not already been done for a previous exchange). The Initiator calculates the SPI session-keys in both directions. At this time, the Initiator begins the authentication and/or encryption of user datagrams.
5.1. Identity_Messages
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Initiator-Cookie ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Responder-Cookie ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Message | LifeTime | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Security-Parameters-Index | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | Identity-Choice | | + + + + + + + + + + + + + + + + + + | | ~ Identification ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Verification ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Attribute-Choices ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ... Padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Initiator-Cookie 16 bytes. Copied from the Value_Request. Responder-Cookie 16 bytes. Copied from the Value_Request. Message 4 (Request) or 7 (Response) LifeTime 3 bytes. The number of seconds remaining before the indicated SPI expires. When the SPI is zero, this field MUST be filled with a random non-zero value. Security-Parameters-Index (SPI) 4 bytes. The SPI to be used for incoming communications. When zero, indicates that no SPI is created in this
direction.
Identity-Choice 2 or more bytes. An identity attribute is selected
from the list of Offered-Attributes sent by the
peer, and is used to calculate the Verification.
The field may be any integral number of bytes in
length, as indicated by its Length field. It does
not require any particular alignment. The 16-bit
alignment shown is for convenience in the
illustration.
Identification Variable Precision Integer, or alternative format
indicated by the Identity-Choice. See the "Basic
Attributes" for details.
The field may be any integral number of bytes in
length. It does not require any particular
alignment. The 32-bit alignment shown is for
convenience in the illustration.
Verification Variable Precision Integer, or alternative format
indicated by the Identity-Choice. The calculation
of the value is described in "Identity
Verification".
The field may be any integral number of bytes in
length. It does not require any particular
alignment. The 32-bit alignment shown is for
convenience in the illustration.
Attribute-Choices
0 or more bytes. When the SPI is non-zero, a list
of attributes selected from the list of Offered-
Attributes supported by the peer.
The contents and usage of this list are further
described in "Attribute Choices List". The end of
the list is indicated by the UDP Length after
removing the Padding (UDP Length - last Padding
value).
Padding 8 to 255 bytes. This field is filled up to at least
a 128 byte boundary, measured from the beginning of
the message. The number of pad bytes are chosen
randomly.
In addition, when a Privacy-Method indicated by the
current Scheme-Choice requires the plaintext to be a
multiple of some number of bytes (the block size of
a block cipher), this field is adjusted as necessary
to the size required by the algorithm.
Self-Describing-Padding begins with the value 1.
Each byte contains the index of that byte. Thus,
the final pad byte indicates the number of pad bytes
to remove. For example, when the unpadded message
length is 120 bytes, the padding values might be 1,
2, 3, 4, 5, 6, 7, and 8.
The portion of the message after the SPI field is masked using the
Privacy-Method indicated by the current Scheme-Choice.
The fields following the SPI are opaque. That is, the values are set
prior to masking (and optional encryption), and examined only after
unmasking (and optional decryption).
5.2. Attribute Choices List
This list specifies the attributes of the SPI. The attribute formats
are specified in the "Basic Attributes".
The list is composed of one or two sections: Authentication-
Attributes, and/or Encapsulation-Attributes.
When sending from the SPI User to the SPI Owner, the attributes are
processed in the order listed. For example,
"ESP-Attributes",
"Deflate" (Compression),
"XOR" (Encryption),
"DES-CBC" (Encryption),
"XOR" (Encryption),
"AH-Attributes",
"AH-Sequence",
"MD5-IPMAC" (Authentication),
would result in ESP with compression and triple encryption (inside),
and then AH authentication with sequence numbers (outside) of the ESP
payload.
The SPI Owner will naturally process the datagram in the reverse
order.
This ordering also affects the order of key generation. Both SPI
Owner and SPI User generate the keys in the order listed.
Implementation Notes:
When choices are made from the list of Offered-Attributes, it is
not required that any Security Association include every kind of
offered attribute in any single SPI, or that a separate SPI be
created for every offered attribute.
Some kinds of attributes may be included more than once in a
single SPI. The set of allowable combinations of attributes are
dependent on implementation and operational policy. Such
considerations are outside the scope of this document.
The list may be divided into additional sections. This can occur
only when both parties recognize the affected attributes.
The authentication, compression, encryption and identification
mechanisms chosen, as well as the encapsulation modes (if any),
need not be the same in both directions.
5.3. Shared-Secret
A shared-secret is used in a number of calculations. Regardless of
the internal representation of the shared-secret, when used in
calculations it is in the same form as the Value part of a Variable
Precision Integer:
- most significant byte first.
- bits used are right justified within byte boundaries.
- any unused bits are in the most significant byte.
- unused bits are zero filled.
The shared-secret does not include a Size field.
5.4. Identity Verification
These messages are authenticated using the Identity-Choice. The
Verification value is calculated prior to masking (and optional
encryption), and verified after unmasking (and optional decryption).
The Identity-Choice authentication function is supplied with two
input values:
- the sender (SPI Owner) verification-key,
- the data to be verified (as a concatenated sequence of bytes).
The resulting output value is stored in the Verification field.
The Identity-Choice verification data consists of the following
concatenated values:
+ the Initiator Cookie,
+ the Responder Cookie,
+ the Message, LifeTime and SPI fields,
+ the Identity-Choice and Identification,
+ the SPI User Identity Verification (response only),
+ the Attribute-Choices following the Verification field,
+ the Padding,
+ the SPI Owner TBV,
+ the SPI Owner Exchange-Value,
+ the SPI Owner Offered-Attributes,
+ the SPI User TBV,
+ the SPI User Exchange-Value,
+ the SPI User Offered-Attributes,
+ the Responder Offered-Schemes.
The TBV (Three Byte Value) consists of the Counter and Scheme-Choice
fields from the Value_Request, or the Reserved field from the
Value_Response, immediately preceding the associated Exchange-Value.
Note that the order of the Exchange-Value and Offered-Attributes
fields is different in each direction, and the Identification and SPI
fields are also likely to be different in each direction. Note also
that the SPI User Identity Verification (from the Identity_Request)
is present only in the Identity_Response.
If the verification fails, the users are notified, and a
Verification_Failure message is sent, without adding any SPI. On
success, normal operation begins with the authentication and/or
encryption of user datagrams.
Implementation Notes:
This is distinct from any authentication method specified for the
SPI.
The exact details of the Identification and verification-key
included in the Verification calculation are dependent on the
Identity-Choice, as described in the "Basic Attributes".
Each party may wish to keep their own trusted databases, such as
the Pretty Good Privacy (PGP) web of trust, and accept only those
identities found there. Failure to find the Identification in
either an internal or external database results in the same
Verification_Failure message as failure of the verification
computation.
The Exchange-Value data includes both the Size and Value fields.
The Offered-Attributes and Attribute-Choices data includes the
Attribute, Length and Value fields.
5.5. Privacy-Key Computation
Identification Exchange messages are masked using the Privacy-Method
indicated by the current Scheme-Choice. Masking begins with the next
field after the SPI, and continues to the end of the data indicated
by the UDP Length, including the Padding.
The Scheme-Choice specified Key-Generation-Function is used to create
a special privacy-key for each message. This function is calculated
over the following concatenated values:
+ the SPI Owner Exchange-Value,
+ the SPI User Exchange-Value,
+ the Initiator Cookie,
+ the Responder Cookie,
+ the Message, LifeTime and SPI (or Reserved) fields,
+ the computed shared-secret.
Since the order of the Exchange-Value fields is different in each
direction, and the Message, LifeTime and SPI fields are also
different in each direction, the resulting privacy-key will usually
be different in each direction.
When a larger number of keying-bits are needed than are available
from one iteration of the specified Key-Generation-Function, more
keying-bits are generated by duplicating the trailing shared-secret,
and recalculating the function. That is, the first iteration will
have one trailing copy of the shared-secret, the second iteration
will have two trailing copies of the shared-secret, and so forth.
Implementation Notes:
This is distinct from any encryption method specified for the SPI.
The length of the Padding, and other details, are dependent on the
Privacy-Method. See the "Basic Privacy-Method" list for details.
To avoid keeping the Exchange-Values in memory after the initial
verification, it is often possible to pre-compute the function
over the initial bytes of the concatenated data values for each
direction, and append the trailing copies of the shared-secret.
The Exchange-Value data includes both the Size and Value fields.
5.6. Session-Key Computation
Each SPI has one or more session-keys. These keys are generated
based on the attributes of the SPI. See the "Basic Attributes" for
details.
The Scheme-Choice specified Key-Generation-Function is used to create
the SPI session-key for that particular attribute. This function is
calculated over the following concatenated values:
+ the Initiator Cookie,
+ the Responder Cookie,
+ the SPI Owner generation-key,
+ the SPI User generation-key,
+ the message Verification field,
+ the computed shared-secret.
Since the order of the generation-keys is different in each
direction, and the Verification field is also likely to be different
in each direction, the resulting session-key will usually be
different in each direction.
When a larger number of keying-bits are needed than are available
from one iteration of the specified Key-Generation-Function, more
keying-bits are generated by duplicating the trailing shared-secret,
and recalculating the function. That is, the first iteration will
have one trailing copy of the shared-secret, the second iteration
will have two trailing copies of the shared-secret, and so forth.
Implementation Notes:
This is distinct from any privacy-key generated for the Photuris
exchange. Different initialization data is used, and iterations
are maintained separately.
The exact details of the Verification field and generation-keys
that are included in the session-key calculation are dependent on
the Identity-Choices, as described in the "Basic Attributes".
To avoid keeping the generation-keys in memory after the initial
verification, it is often possible to pre-compute the function
over the initial bytes of the concatenated data values for each
direction, and append the trailing copies of the shared-secret.
When both authentication and encryption attributes are used for
the same SPI, there may be multiple session-keys associated with
the same SPI. These session-keys are generated in the order of
the Attribute-Choices list.
6. SPI Messages
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
The exchange of messages is not related to the Initiator and
Responder. Instead, either party may send one of these messages at
any time. The messages are easily distinguished by the parties.
6.0.1. Send SPI_Needed
At any time after completion of the Identification Exchange, either
party can send SPI_Needed. This message is sent when a prospective
SPI User needs particular attributes for a datagram (such as
confidentiality), and no current SPI has those attributes.
The prospective SPI User selects from the intersection of attributes
that both parties have previously offered, calculates the
Verification, and masks the message using the Privacy-Method
indicated by the current Scheme-Choice.
6.0.2. Receive SPI_Needed
The potential SPI Owner validates the pair of Cookies, the Padding, the Verification, and the Attributes-Needed. - When an invalid/expired cookie is detected, a Bad_Cookie message is sent. - When too many SPI values are already in use for this particular peer, or some other resource limit is reached, a Resource_Limit message is sent. - After unmasking, when invalid Padding is detected, the variable length Attributes-Needed do not match the UDP Length, or an attribute was not in the Offered-Attributes, the message is silently discarded. - When the message verification fails, a Verification_Failure message is sent. - Whenever such a problem is detected, the SPI is not established; the implementation SHOULD log the occurance, and notify an operator as appropriate. When the message is valid, the party SHOULD send SPI_Update with the necessary attributes. If an existing SPI has those attributes, that SPI is returned in the SPI_Update with the remaining SPILT.6.0.3. Send SPI_Update
At any time after completion of the Identification Exchange, either party can send SPI_Update. This message has effect in only one direction, from the SPI Owner to the SPI User. The SPI Owner chooses the SPI and SPILT, a set of Attributes for the SPI, calculates the Verification, and masks the message using the Privacy-Method indicated by the current Scheme-Choice.6.0.4. Receive SPI_Update
The prospective SPI User validates the pair of Cookies, the Padding, the Verification, and the Attributes-Needed. - When an invalid/expired cookie is detected, a Bad_Cookie message
is sent.
- After unmasking, when invalid Padding is detected, the variable
length Attribute-Choices do not match the UDP Length, an attribute
was not in the Offered-Attributes, or the message modifies an
existing SPI, the message is silently discarded.
- When the message verification fails, a Verification_Failure
message is sent.
- Whenever such a problem is detected, the SPI is not established;
the implementation SHOULD log the occurance, and notify an
operator as appropriate.
When the message is valid, further actions are dependent on the value
of the LifeTime field, as described later.
6.0.5. Automated SPI_Updates
Each SPI requires replacement under several circumstances:
- the volume of data processed (inhibiting probability
cryptanalysis),
- exhaustion of available anti-replay Sequence Numbers,
- and expiration of the LifeTime.
In general, a determination is made upon receipt of a datagram. If
the transform specific processing finds that refreshment is needed,
an automated SPI_Update is triggered.
In addition, automated SPI_Updates allow rapid SPI refreshment for
high bandwidth applications in a high delay environment. The update
messages flow in the opposite direction from the primary traffic,
conserving bandwidth and avoiding service interruption.
When creating each SPI, the implementation MAY optionally set an
Update TimeOut (UTO); by default, to half the value of the LifeTime
(SPILT/2). This time is highly dynamic, and adjustable to provide an
automated SPI_Update long before transform specific processing. If
no new Photuris exchange occurs within the time limit, and the
current exchange state has not expired, an automated SPI_Update is
sent.
6.1. SPI_Needed
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Initiator-Cookie ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Responder-Cookie ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Message | Reserved-LT | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Reserved-SPI | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | | ~ Verification ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Attributes-Needed ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ... Padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Initiator-Cookie 16 bytes. Copied from the Value_Request. Responder-Cookie 16 bytes. Copied from the Value_Request. Message 8 Reserved-LT 3 bytes. For future use; MUST be filled with a random non-zero value when transmitted, and MUST be ignored when received. Reserved-SPI 4 bytes. For future use; MUST be set to zero when transmitted, and MUST be ignored when received. Verification Variable Precision Integer, or other format indicated by the current Scheme-Choice. The calculation of the value is described in "Validity Verification". The field may be any integral number of bytes in length. It does not require any particular alignment. The 32-bit alignment shown is for convenience in the illustration.
Attributes-Needed
4 or more bytes. A list of two or more attributes,
selected from the list of Offered-Attributes
supported by the peer.
The contents and usage of this list are as
previously described in "Attribute Choices List".
The end of the list is indicated by the UDP Length
after removing the Padding (UDP Length - last
Padding value).
Padding 8 or more bytes. The message is padded in the same
fashion specified for Identification Exchange
messages.
The portion of the message after the SPI field is masked using the
Privacy-Method indicated by the current Scheme-Choice.
The fields following the SPI are opaque. That is, the values are set
prior to masking (and optional encryption), and examined only after
unmasking (and optional decryption).
6.2. SPI_Update
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Initiator-Cookie ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Responder-Cookie ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Message | LifeTime | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Security-Parameters-Index | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | | ~ Verification ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Attribute-Choices ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ... Padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Initiator-Cookie 16 bytes. Copied from the Value_Request. Responder-Cookie 16 bytes. Copied from the Value_Request. Message 9 LifeTime 3 bytes. The number of seconds remaining before the indicated SPI expires. The value zero indicates deletion of the indicated SPI. Security-Parameters-Index (SPI) 4 bytes. The SPI to be used for incoming communications. This may be a new SPI value (for creation), or an existing SPI value (for deletion). The value zero indicates special processing. Verification Variable Precision Integer, or other format indicated by the current Scheme-Choice. The calculation of the value is described in "Validity Verification".
The field may be any integral number of bytes in
length. It does not require any particular
alignment. The 32-bit alignment shown is for
convenience in the illustration.
Attribute-Choices
0 or more bytes. When the SPI and SPILT are non-
zero, a list of attributes selected from the list of
Offered-Attributes supported by the peer.
The contents and usage of this list are as
previously described in "Attribute Choices List".
The end of the list is indicated by the UDP Length
after removing the Padding (UDP Length - last
Padding value).
Padding 8 or more bytes. The message is padded in the same
fashion specified for Identification Exchange
messages.
The portion of the message after the SPI field is masked using the
Privacy-Method indicated by the current Scheme-Choice.
The fields following the SPI are opaque. That is, the values are set
prior to masking (and optional encryption), and examined only after
unmasking (and optional decryption).
6.2.1. Creation
When the LifeTime is non-zero, and the SPI is also non-zero, the
SPI_Update can be used to create a new SPI. When the SPI is zero,
the SPI_Update is silently discarded.
The new session-keys are calculated in the same fashion as the
Identity_Messages. Since the SPI value is always different than any
previous SPI during the Exchange LifeTime of the shared-secret, the
resulting session-keys will necessarily be different from all others
used in the same direction.
No retransmission timer is necessary. Success is indicated by the
peer use of the new SPI.
Should all creation attempts fail, eventually the peer will find that
all existing SPIs have expired, and will begin the Photuris exchange
again by sending a new Cookie_Request. When appropriate, this
Cookie_Request MAY include a Responder-Cookie to retain previous
party pairings.
6.2.2. Deletion
When the LifeTime is zero, the SPI_Update can be used to delete a single existing SPI. When the SPI is also zero, the SPI_Update will delete all existing SPIs related to this Security Association, and mark the Photuris exchange state as expired. This is especially useful when the application that needed them terminates. No retransmission timer is necessary. This message is advisory, to reduce the number of ICMP Security Failures messages. Should any deletion attempts fail, the peer will learn that the deleted SPIs are invalid through the normal ICMP Security Failures messages, and will initiate a Photuris exchange by sending a new Cookie_Request.6.2.3. Modification
The SPI_Update cannot be used to modify existing SPIs, such as lengthen an existing SPI LifeTime, resurrect an expired SPI, or add/remove an Attribute-Choice. On receipt, such an otherwise valid message is silently discarded.6.3. Validity Verification
These messages are authenticated using the Validity-Method indicated by the current Scheme-Choice. The Verification value is calculated prior to masking (and optional encryption), and verified after unmasking (and optional decryption). The Validity-Method authentication function is supplied with two input values: - the sender (SPI Owner) verification-key, - the data to be verified (as a concatenated sequence of bytes). The resulting output value is stored in the Verification field. The Validity-Method verification data consists of the following concatenated values:
+ the Initiator Cookie,
+ the Responder Cookie,
+ the Message, LifeTime and SPI (or Reserved) fields,
+ the SPI Owner Identity Verification,
+ the SPI User Identity Verification,
+ the Attribute-Choices following the Verification field,
+ the Padding.
Note that the order of the Identity Verification fields (from the
Identity_Messages) is different in each direction, and the Message,
LifeTime and SPI fields are also likely to be different in each
direction.
If the verification fails, the users are notified, and a
Verification_Failure message is sent, without adding or deleting any
SPIs. On success, normal operation begins with the authentication
and/or encryption of user datagrams.
Implementation Notes:
This is distinct from any authentication method specified for the
SPI.
The Identity Verification data includes both the Size and Value
fields. The Attribute-Choices data includes the Attribute, Length
and Value fields.
7. Error Messages
These messages are issued in response to Photuris state loss or other
problems. A message has effect in only one direction. No
retransmission timer is necessary.
These messages are not masked.
The receiver checks the Cookies for validity. Special care MUST be
taken that the Cookie pair in the Error Message actually match a pair
currently in use, and that the protocol is currently in a state where
such an Error Message might be expected. Otherwise, these messages
could provide an opportunity for a denial of service attack. Invalid
messages are silently discarded.
7.1. Bad_Cookie
For the format of the 33 byte message, see "Header Format". There are no additional fields. Initiator-Cookie 16 bytes. Copied from the offending message. Responder-Cookie 16 bytes. Copied from the offending message. Message 10 This error message is sent when a Value_Request, Identity_Request, SPI_Needed, or SPI_Update is received, and the receiver specific Cookie is invalid or the associated exchange state has expired. During the Photuris exchange, when this error message is received, it has no immediate effect on the operation of the protocol phases. Later, when Retransmissions have been exceeded, and this error message has been received, the Initiator SHOULD begin the Photuris exchange again by sending a new Cookie_Request with the Responder- Cookie and Counter updated appropriately. When this error message is received in response to SPI_Needed, the exchange state SHOULD NOT be marked as expired, but the party SHOULD initiate a Photuris exchange by sending a new Cookie_Request. When this error message is received in response to SPI_Update, the exchange state SHOULD NOT be marked as expired, and no further action is taken. A new exchange will be initiated later when needed by the peer to send authenticated and/or encrypted data. Existing SPIs are not deleted. They expire normally, and are purged sometime later.7.2. Resource_Limit
For the format of the 34 byte message, see "Cookie_Request". There are no additional fields. Initiator-Cookie 16 bytes. Copied from the offending message. Responder-Cookie 16 bytes. Copied from the offending message. Special processing is applied to a Cookie_Request. When the offending message Responder-Cookie and Counter were both zero, and an existing exchange has not yet been purged, this field is replaced with the
Responder-Cookie from the existing exchange.
Message 11
Counter 1 byte. Copied from the offending message.
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.
This error message is sent when a Cookie_Request, Value_Request or
SPI_Needed is received, and too many SPI values are already in use
for that peer, or some other Photuris resource is unavailable.
During the Photuris exchange, when this error message is received in
response to a Cookie_Request or Value_Request, the implementation
SHOULD double the retransmission timeout (as usual) for sending
another Cookie_Request or Value_Request. Otherwise, it has no
immediate effect on the operation of the protocol phases. Later,
when Retransmissions have been exceeded, and this error message has
been received, the Initiator SHOULD begin the Photuris exchange again
by sending a new Cookie_Request with the Responder-Cookie and Counter
updated appropriately.
When this error message is received in response to SPI_Needed, the
implementation SHOULD NOT send another SPI_Needed until one of the
existing SPIs associated with this exchange is deleted or has
expired.
7.3. Verification_Failure
For the format of the 33 byte message, see "Header Format". There
are no additional fields.
Initiator-Cookie 16 bytes. Copied from the offending message.
Responder-Cookie 16 bytes. Copied from the offending message.
Message 12
This error message is sent when an Identity_Message, SPI_Needed or
SPI_Update is received, and verification fails.
When this error message is received, the implementation SHOULD log the occurance, and notify an operator as appropriate. However, receipt has no effect on the operation of the protocol.7.4. Message_Reject
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Initiator-Cookie ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Responder-Cookie ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Message | Bad-Message | Offset | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Initiator-Cookie 16 bytes. Copied from the offending message. Responder-Cookie 16 bytes. Copied from the offending message. Message 13 Bad-Message 1 byte. Indicates the Message number of the offending message. Offset 2 bytes. The number of bytes from the beginning of the offending message where the unrecognized field starts. The minimum value is 32. This error message is sent when an optional Message type is received that is not supported, or an optional format of a supported Message is not recognized. When this error message is received, the implementation SHOULD log the occurance, and notify an operator as appropriate. However, receipt has no effect on the operation of the protocol.
(next page on part 3)