Authenticated Chunks for the Stream Control Transmission Protocol (SCTP)

6.1. Establishment of an Association Shared Key

An SCTP endpoint willing to receive or send authenticated chunks MUST send one RANDOM parameter in its INIT or INIT-ACK chunk. The RANDOM parameter MUST contain a 32-byte Random Number. The Random Number should be generated in accordance with RFC 4086 [RFC4086]. If the Random Number is not 32 bytes, the association MUST be aborted. The ABORT chunk SHOULD contain the error cause 'Protocol Violation'. In case of INIT collision, the rules governing the handling of this Random Number follow the same pattern as those for the Verification Tag, as explained in Section 5.2.4 of RFC 9260 [RFC9260]. Therefore, each endpoint knows its own Random Number and the peer's Random Number after the association has been established.¶

An SCTP endpoint has a list of chunks it only accepts if they are received in an authenticated way. This list is included in the INIT and INIT-ACK, and MAY be omitted if it is empty. Since this list does not change during the lifetime of the SCTP endpoint there is no problem in case of INIT collision.¶

Each SCTP endpoint MUST include in the INIT and INIT-ACK a HMAC-ALGO parameter containing a list of HMAC Identifiers it requests the peer to use. The receiver of an HMAC-ALGO parameter SHOULD use the first listed algorithm it supports. The HMAC algorithm based on SHA-1 MUST be supported and included in the HMAC-ALGO parameter. An SCTP endpoint MUST NOT change the parameters listed in the HMAC-ALGO parameter during the lifetime of the endpoint.¶

Both endpoints of an association MAY have endpoint pair shared keys that are byte vectors and pre-configured or established by another mechanism. They are identified by the Shared Key Identifier. For each endpoint pair shared key, an association shared key is computed. If there is no endpoint pair shared key, only one association shared key is computed by using an empty byte vector as the endpoint pair shared key.¶

The RANDOM parameter, the CHUNKS parameter, and the HMAC-ALGO parameter sent by each endpoint are concatenated as byte vectors. These parameters include the parameter type, parameter length, and the parameter value, but padding is omitted; all padding MUST be removed from this concatenation before proceeding with further computation of keys. Parameters that were not sent are simply omitted from the concatenation process. The resulting two vectors are called the two key vectors.¶

From the endpoint pair shared keys and the key vectors, the association shared keys are computed. This is performed by selecting the numerically smaller key vector and concatenating it to the endpoint pair shared key, and then concatenating the numerically larger key vector to that. If the key vectors are equal as numbers but differ in length, then the concatenation order is the endpoint shared key, followed by the shorter key vector, followed by the longer key vector. Otherwise, the key vectors are identical, and may be concatenated to the endpoint pair key in any order. The concatenation is performed on byte vectors, and all numerical comparisons use network byte order to convert the key vectors to a number. The result of the concatenation is the association shared key.¶

6.2. Sending Authenticated Chunks

Endpoints MUST send all requested chunks that have been authenticated where this has been requested by the peer. The other chunks MAY be sent whether or not they have been authenticated. If endpoint pair shared keys are used, one of them MUST be selected for authentication.¶

To send chunks in an authenticated way, the sender MUST include these chunks after an AUTH chunk. This means that a sender MUST bundle chunks in order to authenticate them.¶

If the endpoint has no endpoint pair shared key for the peer, it MUST use Shared Key Identifier zero with an empty endpoint pair shared key. If there are multiple endpoint shared keys the sender selects one and uses the corresponding Shared Key Identifier.¶

The sender MUST calculate the Message Authentication Code (MAC) (as described in RFC 2104 [RFC2104]) using the hash function H as described by the HMAC Identifier and the shared association key K based on the endpoint pair shared key described by the Shared Key Identifier. The 'data' used for the computation of the AUTH-chunk is given by the AUTH chunk with its HMAC field set to zero (as shown in Figure 1) followed by all the chunks that are placed after the AUTH chunk in the SCTP packet.¶

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 0x0F   |   Flags=0     |         Chunk Length          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|    Shared Key Identifier      |        HMAC Identifier        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
\                               0                               /
/                               +-------------------------------\
|                               |           Padding             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Please note that all fields are in network byte order and that the field that will contain the complete HMAC is filled with zeroes. The length of the field shown as zero is the length of the HMAC described by the HMAC Identifier. The padding of all chunks being authenticated MUST be included in the HMAC computation.¶

The sender fills the HMAC into the HMAC field and sends the packet.¶

6.3. Receiving Authenticated Chunks

The receiver has a list of chunk types that it expects to be received only after an AUTH-chunk. This list has been sent to the peer during the association setup. It MUST silently discard these chunks if they are not placed after an AUTH chunk in the packet.¶

The receiver MUST use the HMAC algorithm indicated in the HMAC Identifier field. If this algorithm was not specified by the receiver in the HMAC-ALGO parameter in the INIT or INIT-ACK chunk during association setup, the AUTH chunk and all the chunks after it MUST be discarded and an ERROR chunk SHOULD be sent with the error cause defined in Section 4.1.¶

If an endpoint with no shared key receives a Shared Key Identifier other than 0, it MUST silently discard all authenticated chunks. If the endpoint has at least one endpoint pair shared key for the peer, it MUST use the key specified by the Shared Key Identifier if a key has been configured for that Shared Key Identifier. If no endpoint pair shared key has been configured for that Shared Key Identifier, all authenticated chunks MUST be silently discarded.¶

The receiver now performs the same calculation as described for the sender based on Figure 1. If the result of the calculation is the same as given in the HMAC field, all the chunks following the AUTH chunk are processed. If the field does not match the result of the calculation, all the chunks following the AUTH chunk MUST be silently discarded.¶

It should be noted that if the receiver wants to tear down an association in an authenticated way only, the handling of malformed packets should not result in tearing down the association.¶

An SCTP implementation has to maintain state for each SCTP association. In the following, we call this data structure the SCTP transmission control block (STCB).¶

When an endpoint requires COOKIE-ECHO chunks to be authenticated, some special procedures have to be followed because the reception of a COOKIE-ECHO chunk might result in the creation of an SCTP association. If a packet arrives containing an AUTH chunk as a first chunk, a COOKIE-ECHO chunk as the second chunk, and possibly more chunks after them, and the receiver does not have an STCB for that packet, then authentication is based on the contents of the COOKIE-ECHO chunk. In this situation, the receiver MUST authenticate the chunks in the packet by using the RANDOM parameters, CHUNKS parameters and HMAC_ALGO parameters obtained from the COOKIE-ECHO chunk, and possibly a local shared secret as inputs to the authentication procedure specified in Section 6.3. If authentication fails, then the packet is discarded. If the authentication is successful, the COOKIE-ECHO and all the chunks after the COOKIE-ECHO MUST be processed. If the receiver has an STCB, it MUST process the AUTH chunk as described above using the STCB from the existing association to authenticate the COOKIE-ECHO chunk and all the chunks after it.¶

If the receiver does not find an STCB for a packet containing an AUTH chunk as the first chunk and does not find a COOKIE-ECHO chunk as the second chunk, it MUST use the chunks after the AUTH chunk to look up an existing association. If no association is found, the packet MUST be considered as out of the blue. The out of the blue handling MUST be based on the packet without taking the AUTH chunk into account. If an association is found, it MUST process the AUTH chunk using the STCB from the existing association as described earlier.¶

Requiring ABORT chunks and COOKIE-ECHO chunks to be authenticated makes it impossible for an attacker to bring down or restart an association as long as the attacker does not know the association shared key. But it should also be noted that if an endpoint accepts ABORT chunks only in an authenticated way, it may take longer to detect that the peer is no longer available. If an endpoint accepts COOKIE-ECHO chunks only in an authenticated way, the restart procedure does not work, because the restarting endpoint most likely does not know the association shared key of the old association to be restarted. However, if the restarting endpoint does know the old association shared key, he can successfully send the COOKIE-ECHO chunk in a way that it is accepted by the peer by using this old association shared key for the packet containing the AUTH chunk. After this operation, both endpoints have to use the new association shared key.¶

If a server has an endpoint pair shared key with some clients, it can request the COOKIE_ECHO chunk to be authenticated and can ensure that only associations from clients with a correct endpoint pair shared key are accepted.¶

Furthermore, it is important that the cookie contained in an INIT-ACK chunk and in a COOKIE-ECHO chunk MUST NOT contain any endpoint pair shared keys.¶

8.1. Extending the SCTP_AUTHENTICATION_EVENT event

Section 6.1.8 of [RFC6458] defines the SCTP_AUTHENTICATION_EVENT event, which uses the following structure:¶

struct sctp_authkey_event {
    uint16_t auth_type;
    uint16_t auth_flags;
    uint32_t auth_length;
    uint16_t auth_keynumber;
    uint32_t auth_indication;
    sctp_assoc_t auth_assoc_id;
};

This document updates this structure to¶

struct sctp_authkey_event {
    uint16_t auth_type;
    uint16_t auth_flags;
    uint32_t auth_length;
    uint16_t auth_identifier; /* formerly auth_keynumber */
    uint16_t auth_reserved; /* Avoid hole in structure */
    uint32_t auth_indication;
    sctp_assoc_t auth_assoc_id;
};

by renaming auth_keynumber to auth_identifier. auth_identifier just replaces auth_keynumber in the context of [RFC6458]. In addition to that, the SCTP_AUTHENTICATION_EVENT event is extended to also indicate when a new HMAC Identifier is received and such reporting is explicitly enabled as described in Section 8.2. In this case auth_indication is SCTP_AUTH_NEW_HMAC and the new HMAC identifier is reported in auth_identifier.¶

8.2. Expose HMAC Identifier Usage (SCTP_EXPOSE_HMAC_IDENT_CHANGES)

This option allows the application to enable and disable the reception of SCTP_AUTHENTICATION_EVENT events when a new HMAC Identifiers has been received in an AUTH chunk (see Section 8.1). This read/write socket option uses the level IPPROTO_SCTP and the name SCTP_EXPOSE_HMAC_IDENT_CHANGES. It is needed to provide backwards compatibility and the default is that these events are not reported.¶

The following structure is used to enable or disable the reporting of newly received HMAC Identifiers in AUTH chunks:¶

struct sctp_assoc_value {
    sctp_assoc_t assoc_id;
    uint32_t assoc_value;
};

assoc_id:

This parameter is ignored for one-to-one style sockets. For one-to-many style sockets, the application may fill in an association identifier or SCTP_{FUTURE|CURRENT|ALL}_ASSOC.¶

assoc_value:

Newly received HMAC Identifiers are reported if, and only if, this parameter is non-zero.¶