Internet DRAFT - draft-ietf-tsvwg-rfc4895-bis
draft-ietf-tsvwg-rfc4895-bis
Network Working Group M. Tüxen
Internet-Draft Münster Univ. of Applied Sciences
Obsoletes: 4895 (if approved) R. Stewart
Intended status: Standards Track P. Lei
Expires: 5 September 2024 Netflix, Inc.
H. Tschofenig
4 March 2024
Authenticated Chunks for the Stream Control Transmission Protocol (SCTP)
draft-ietf-tsvwg-rfc4895-bis-02
Abstract
This document describes a new chunk type, several parameters, and
procedures for the Stream Control Transmission Protocol (SCTP). This
new chunk type can be used to authenticate SCTP chunks by using
shared keys between the sender and receiver. The new parameters are
used to establish the shared keys. This document obsoletes RFC 4895.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 5 September 2024.
Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
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than English.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. New Parameter Types . . . . . . . . . . . . . . . . . . . . . 4
3.1. Random Parameter (RANDOM) . . . . . . . . . . . . . . . . 4
3.2. Chunk List Parameter (CHUNKS) . . . . . . . . . . . . . . 5
3.3. Requested HMAC Algorithm Parameter (HMAC ALGO) . . . . . 6
4. New Error Causes . . . . . . . . . . . . . . . . . . . . . . 8
4.1. RANDOM Collision Error Cause . . . . . . . . . . . . . . 8
4.2. Unsupported HMAC Identifier Error Cause . . . . . . . . . 9
5. New Chunk Type . . . . . . . . . . . . . . . . . . . . . . . 9
5.1. Authentication Chunk (AUTH) . . . . . . . . . . . . . . . 10
6. Procedures . . . . . . . . . . . . . . . . . . . . . . . . . 11
6.1. Association Shared Send and Receive Keys . . . . . . . . 11
6.1.1. Handling of RANDOM parameters . . . . . . . . . . . . 11
6.1.2. Computation of the Local and Remote Key Vectors . . . 12
6.1.3. Derivation of Association Send and Receive Keys . . . 12
6.2. Sending Authenticated Chunks . . . . . . . . . . . . . . 13
6.3. Receiving Authenticated Chunks . . . . . . . . . . . . . 14
7. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 16
8. Socket API Considerations . . . . . . . . . . . . . . . . . . 17
8.1. Extending the SCTP_AUTHENTICATION_EVENT event . . . . . . 17
8.2. Expose HMAC Identifier Usage
(SCTP_EXPOSE_HMAC_IDENT_CHANGES) . . . . . . . . . . . . 18
8.3. Get the HMAC Identifier being Sent
(SCTP_SEND_HMAC_IDENT) . . . . . . . . . . . . . . . . . 19
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9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
9.1. IETF-Defined HMAC Identifiers . . . . . . . . . . . . . . 20
10. Security Considerations . . . . . . . . . . . . . . . . . . . 20
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 21
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
12.1. Normative References . . . . . . . . . . . . . . . . . . 21
12.2. Informative References . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
1. Introduction
SCTP uses 32-bit verification tags to protect itself against blind
attackers. These values are not changed during the lifetime of an
SCTP association.
Looking at new SCTP extensions, there is the need to have a method of
proving that an SCTP chunk(s) was really sent by the original peer
that started the association and not by a malicious attacker.
Since it is required to protect SCTP control data, any solution only
protecting SCTP user data is not sufficient.
Therefore, an SCTP extension that provides a mechanism for deriving
shared keys for each association is presented. These association
shared keys are derived from endpoint pair shared keys, which are
configured and might be empty, and data that is exchanged during the
SCTP association setup.
The extension presented in this document allows an SCTP sender to
authenticate chunks using shared keys between the sender and
receiver. The receiver can then verify that the chunks are sent from
the sender and not from a malicious attacker (as long as the attacker
does not know an association shared key).
The extension described in this document places the result of a
Hashed Message Authentication Code (HMAC) computation before the data
covered by that computation. Placing it at the end of the packet
would have required placing a control chunk after DATA chunks in case
of authenticating DATA chunks. This would break the rule that
control chunks occur before DATA chunks in SCTP packets. It should
also be noted that putting the result of the HMAC computation after
the data being covered would not allow sending the packet during the
computation of the HMAC because the result of the HMAC computation is
needed to compute the CRC32C checksum of the SCTP packet, which is
placed in the common header of the SCTP packet.
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The protocol extension defined in this document can be used in
combination with any other currently defined SCTP extension. Its
usage is required by the SCTP extension for Dynamic Address
Reconfiguration as specified in [RFC5061].
2. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. New Parameter Types
This section defines the new parameter types that will be used to
negotiate the authentication during association setup. Table 1
illustrates the new parameter types.
+----------------+------------------------------------------------+
| Parameter Type | Parameter Name |
+----------------+------------------------------------------------+
| 0x8002 | Random Parameter (RANDOM) |
+----------------+------------------------------------------------+
| 0x8003 | Chunk List Parameter (CHUNKS) |
+----------------+------------------------------------------------+
| 0x8004 | Requested HMAC Algorithm Parameter (HMAC ALGO) |
+----------------+------------------------------------------------+
Table 1
Note that the parameter format requires the receiver to ignore the
parameter and continue processing if the parameter is not understood.
This is accomplished (as described in RFC 9260 [RFC9260],
Section 3.2.1.) by the use of the upper bits of the parameter type.
3.1. Random Parameter (RANDOM)
This parameter is used to carry a random number of an arbitrary
length.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Parameter Type = 0x8002 | Parameter Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ Random Number /
/ +-------------------------------\
| | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Parameter Type: 2 bytes (unsigned integer)
This value MUST be set to 0x8002.
Parameter Length: 2 bytes (unsigned integer)
This value is the length of the Random Number in bytes plus 4.
Random Number: n bytes (unsigned integer)
This value represents an arbitrary Random Number in network byte
order.
Padding: 0, 1, 2, or 3 bytes (unsigned integer)
If the length of the Random Number is not a multiple of 4 bytes,
the sender MUST pad the parameter with all zero bytes to make the
parameter 32-bit aligned. The Padding MUST NOT be longer than 3
bytes and it MUST be ignored by the receiver.
The RANDOM parameter MUST be included once in the INIT or INIT ACK
chunk, if the sender wants to send or receive authenticated chunks,
to provide a 32-byte Random Number. For 32-byte Random Numbers, the
Padding is empty.
3.2. Chunk List Parameter (CHUNKS)
This parameter is used to specify which chunk types are required to
be authenticated before being sent by the peer.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Parameter Type = 0x8003 | Parameter Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Chunk Type 1 | Chunk Type 2 | Chunk Type 3 | Chunk Type 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
\ ... \
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Chunk Type n | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Parameter Type: 2 bytes (unsigned integer)
This value MUST be set to 0x8003.
Parameter Length: 2 bytes (unsigned integer)
This value is the number of listed Chunk Types plus 4.
Chunk Type n: 1 byte (unsigned integer)
Each Chunk Type listed is required to be authenticated when sent
by the peer.
Padding: 0, 1, 2, or 3 bytes (unsigned integer)
If the number of Chunk Types is not a multiple of 4, the sender
MUST pad the parameter with all zero bytes to make the parameter
32-bit aligned. The Padding MUST NOT be longer than 3 bytes and
it MUST be ignored by the receiver.
The CHUNKS parameter MUST be included once in the INIT or INIT ACK
chunk if the sender wants to receive authenticated chunks. Its
maximum length is 260 bytes.
The chunk types for INIT, INIT ACK, SHUTDOWN COMPLETE, and AUTH
chunks MUST NOT be listed in the CHUNKS parameter. However, if a
CHUNKS parameter is received then the types for INIT, INIT ACK,
SHUTDOWN COMPLETE, and AUTH chunks MUST be ignored.
A CHUNKS parameter MAY contain chunk types being unknown to the
receiver.
3.3. Requested HMAC Algorithm Parameter (HMAC ALGO)
This parameter is used to list the HMAC Identifiers the peer MUST
use.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Parameter Type = 0x8004 | Parameter Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HMAC Identifier 1 | HMAC Identifier 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
\ ... \
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HMAC Identifier n | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Parameter Type: 2 bytes (unsigned integer)
This value MUST be set to 0x8004.
Parameter Length: 2 bytes (unsigned integer)
This value is the number of HMAC Identifiers multiplied by 2, plus
4.
HMAC Identifier n: 2 bytes (unsigned integer)
The values expressed are a list of HMAC Identifiers that may be
used by the peer. The values are listed by preference, with
respect to the sender, where the first HMAC Identifier listed is
the one most preferable to the sender. Any non-deprecated HMAC
Identifier MUST be listed before any deprecated HMAC Identifier.
Padding: 0 or 2 bytes (unsigned integer)
If the number of HMAC Identifiers is not even, the sender MUST pad
the parameter with all zero bytes to make the parameter 32-bit
aligned. The Padding MUST be 0 or 2 bytes long and it MUST be
ignored by the receiver.
The HMAC ALGO parameter MUST be included once in the INIT or INIT ACK
chunk if the sender wants to send or receive authenticated chunks.
Table 2 shows the currently defined values for HMAC Identifiers.
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+-----------------+--------------------------------------+
| HMAC Identifier | Message Digest Algorithm |
+-----------------+--------------------------------------+
| 0 | Reserved |
+-----------------+--------------------------------------+
| 1 (deprecated) | SHA-1 defined in [NIST_FIPS_180_4] |
+-----------------+--------------------------------------+
| 2 | Reserved |
+-----------------+--------------------------------------+
| 3 (deprecated) | SHA-256 defined in [NIST_FIPS_180_4] |
+-----------------+--------------------------------------+
| 4 (suggested) | SHA-256 defined in [NIST_FIPS_180_4] |
| | with directional keys |
+-----------------+--------------------------------------+
Table 2
Every endpoint supporting SCTP chunk authentication MUST support the
HMAC based on the SHA-256 algorithm with directional keys.
4. New Error Causes
This section defines two new error cause that will be sent if there
is a collision related to the RANDOM parameters or an AUTH chunk is
received with an unsupported HMAC Identifier. Table 3 illustrates
the new error cause.
+------------+-----------------------------+
| Cause Code | Error Cause Name |
+------------+-----------------------------+
| 0x0100 | RANDOM Collision |
+------------+-----------------------------+
| 0x0105 | Unsupported HMAC Identifier |
+------------+-----------------------------+
Table 3
4.1. RANDOM Collision Error Cause
This error cause is used to indicate that an AUTH chunk has been
received with an unsupported HMAC Identifier.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Cause Code = 0x0100 (suggested)| Cause Length = 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Cause Code: 2 bytes (unsigned integer)
This value MUST be set to the IANA assigned cause code for the
'RANDOM Collision' error cause. IANA is requested to assign the
value 0x0100 (suggested) for this cause code
Cause Length: 2 bytes (unsigned integer)
This value MUST be set to 4.
4.2. Unsupported HMAC Identifier Error Cause
This error cause is used to indicate that an AUTH chunk has been
received with an unsupported HMAC Identifier.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cause Code = 0x0105 | Cause Length = 6 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HMAC Identifier | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Cause Code: 2 bytes (unsigned integer)
This value MUST be set to 0x0105.
Cause Length: 2 bytes (unsigned integer)
This value MUST be set to 6.
HMAC Identifier: 2 bytes (unsigned integer)
This value is the HMAC Identifier which is not supported.
Padding: 2 bytes (unsigned integer)
The sender MUST pad the error cause with all zero bytes to make
the cause 32-bit aligned. The Padding MUST be 2 bytes long and it
MUST be ignored by the receiver.
5. New Chunk Type
This section defines the new chunk type that will be used to
authenticate chunks. Table 4 illustrates the new chunk type.
+------------+-----------------------------+
| Chunk Type | Chunk Name |
+------------+-----------------------------+
| 0x0F | Authentication Chunk (AUTH) |
+------------+-----------------------------+
Table 4
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It should be noted that the AUTH-chunk format requires the receiver
to ignore the chunk if it is not understood and silently discard all
chunks that follow. This is accomplished (as described in RFC 9260
[RFC9260], Section 3.2.) by the use of the upper bits of the chunk
type.
5.1. Authentication Chunk (AUTH)
This chunk is used to hold the result of the HMAC calculation.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 0x0F | Flags=0 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Shared Key Identifier | HMAC Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
\ HMAC /
/ \
/ +-------------------------------\
| | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: 1 byte (unsigned integer)
This value MUST be set to 0x0F for all AUTH-chunks.
Flags: 1 byte (unsigned integer)
SHOULD be set to zero on transmit and MUST be ignored on receipt.
Length: 2 bytes (unsigned integer)
This value holds the length of the HMAC in bytes plus 8.
Shared Key Identifier: 2 bytes (unsigned integer)
This value describes which endpoint pair shared key is used.
HMAC Identifier: 2 bytes (unsigned integer)
This value describes which message digest is being used. Table 2
shows the currently defined values.
HMAC: n bytes (unsigned integer)
This holds the result of the HMAC calculation.
Padding: 0, 1, 2, or 3 bytes (unsigned integer)
If the length of the HMAC is not a multiple of 4 bytes, the sender
MUST pad the chunk with all zero bytes to make the chunk 32-bit
aligned. The Padding MUST NOT be longer than 3 bytes and it MUST
be ignored by the receiver.
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The control chunk AUTH MUST NOT appear more than once in an SCTP
packet. All control and data chunks that are placed after the AUTH
chunk in the packet are sent in an authenticated way. Those chunks
placed in a packet before the AUTH chunk are not authenticated.
Please note that DATA chunks can not appear before control chunks in
an SCTP packet.
6. Procedures
6.1. Association Shared Send and Receive Keys
6.1.1. Handling of RANDOM parameters
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 length of the received Random Number is not 32 bytes, the
association MUST be aborted. The ABORT chunk SHOULD contain the
error cause 'Protocol Violation'.
In the unlikely event that an endpoint receives a packet containing
an INIT chunk and all of the following three conditions are
fulfilled:
1. The endpoint is in the COOKIE_WAIT or COOKIE-ECHOED state.
2. The HMAC ALGO parameter contained in the INIT chunk lists at
least one non-deprecated HMAC algorithm.
3. The RANDOM parameter received in the INIT chunk is the same as
the one sent in the INIT chunk it sent earlier.
If all of these conditions are fulfilled, the endpoint MUST send a
packet containing an ABORT chunk with the 'RANDOM Collision' error
cause. The endpoint SHOULD retry setting up an association by
sending packets with new INIT chunks.
In any other case the an endpoint receiving a packet containing an
INIT chunk with a RANDOM parameter MUST select a Random Number to be
included in the INIT ACK it sends different from the Random Number
received.
These rules are similar to the one for the Verification Tag in case
of INIT collisions, as explained in Section 5.2.4 of RFC 9260
[RFC9260].
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If an endpoint lists at least one non-deprecated HMAC Identifier in
the HMAC ALGO parameter it sends in its INIT or INIT ACK chunk, it
operates not in legacy mode. Only if all HMAC Identifiers listed are
deprecated, the endpoint operates in legacy mode. Operating in
legacy mode means supporting SCTP authentication as specified in
[RFC4895] and not as specified by this document.
Therefore, each endpoint knows its own Random Number and the peer's
Random Number after the association has been established and these
two Random Numbers are different if both endpoints are not operating
in legacy mode.
6.1.2. Computation of the Local and Remote Key Vectors
An SCTP endpoint has a list of chunks it only accepts if they are
received in an authenticated way. These chunk types are listed in
the CHUNKS parameter, which is included in the INIT or INIT ACK
chunk. The CHUNKS parameter 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 an 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.
The RANDOM parameter, the CHUNKS parameter, and the HMAC ALGO
parameter sent by each endpoint MUST be concatenated in this sequence
as byte vectors. Parameters that were not sent MUST be omitted from
the concatenation process. 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. The resulting vector
based on the parameters sent by an endpoint are called the local key
vector and the resulting vector based on the parameters received by
an endpint are called the remote key vector.
6.1.3. Derivation of Association Send and Receive Keys
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 send and receive
key are computed. If there is are no endpoint pair shared keys
configured, only one association shared send key and one association
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receive key is computed by using an empty byte vector as the endpoint
pair shared key.
The way association shared send and receive keys are computed from
the endpoint shared keys depends on the peer operating in legacy more
or not.
If the peer operates in legacy mode, the association shared send key
and the association shared receive key are the same. They are
computed 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 send key and the association shared recv
key.
If the peer does not operate in legacy mode, the send context is
defined as the concatenation of local key vector followed by the
remote key vector. The receive context is defined as the
concatenation of the remote key vector followed by the local key
vector. For deriving the association shared send and receive keys, a
method described in Section 3.1 of [RFC5926] is used. The
association shared send key is the result of using HMAC-SHA512 as the
key derivation function with the endpoint shared key as the
Master_Key, the send context as the Context and 512 as the
Output_Length. The association shared receive key is computed the
same way, just using the receive context as the Context. In both
cases "SCTP-AUTH" is used as the Label.
6.2. Sending Authenticated Chunks
All chunks, that have been requested by the peer to be sent
authenticated, MUST be sent authenticated. The other chunks MAY be
sent authenticated when possible. If endpoint pair shared keys are
configured, 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.
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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 send 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 depicted
below) followed by all the chunks that are placed after the AUTH
chunk in the SCTP packet.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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 4.2.
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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. It uses the shared association receive key K based on the
endpoint pair shared key described by the Shared Key Identifier. 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
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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.
7. Examples
This section gives examples of message exchanges for association
setup.
The simplest way of using the extension described in this document is
given by the following message exchange.
---------- INIT[RANDOM; CHUNKS; HMAC ALGO] ---------->
<------- INIT ACK[RANDOM; CHUNKS; HMAC ALGO] ---------
-------------------- COOKIE ECHO -------------------->
<-------------------- COOKIE ACK ---------------------
Please note that the CHUNKS parameter is optional in the INIT and
INIT ACK.
If the server wants to receive DATA chunks in an authenticated way,
the following message exchange is possible:
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---------- INIT[RANDOM; CHUNKS; HMAC ALGO] ---------->
<------- INIT ACK[RANDOM; CHUNKS; HMAC ALGO] ---------
--------------- COOKIE ECHO; AUTH; DATA ------------->
<----------------- COOKIE ACK; SACK ------------------
Please note that if the endpoint pair shared key depends on the
client and the server, and is only known by the upper layer, this
message exchange requires an upper layer intervention between the
processing of the COOKIE ECHO chunk and the processing of the AUTH
and DATA chunk at the server side. This intervention may be realized
by a COMMUNICATION-UP notification followed by the presentation of
the endpoint pair shared key by the upper layer to the SCTP stack,
see for example Section 11 of RFC 9260 [RFC9260]. If this
intervention is not possible due to limitations of the API (for
example, the socket API), the server might discard the AUTH and DATA
chunk, making a retransmission of the DATA chunk necessary. If the
same endpoint pair shared key is used for multiple endpoints and does
not depend on the client, this intervention might not be necessary.
8. Socket API Considerations
This section describes how the socket API defined in [RFC6458] needs
to be extended to provide a way for the application to observe the
HMAC algorithms used for sending and receiving of AUTH chunks.
Please note that this section is informational only.
A socket API implementation based on [RFC6458] is, by means of the
existing SCTP_AUTHENTICATION_EVENT event, extended to provide the
event notification whenever a new HMAC algorithm is used in a
received AUTH chunk.
Furthermore, two new socket options for the level IPPROTO_SCTP and
the name SCTP_EXPOSE_HMAC_IDENT_CHANGES and SCTP_SEND_HMAC_IDENT are
defined as described below. The first socket option enables the
monitoring of HMAC algorithms used in received AUTH chunks via the
SCTP_AUTHENTICATION_EVENT event. The second socket option is used to
query the HMAC algorithm used for sending AUTH chunks.
Support for the SCTP_SEND_HMAC_IDENT and
SCTP_EXPOSE_HMAC_IDENT_CHANGES socket options also needs to be added
to the function sctp_opt_info().
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:
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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.
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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.
8.3. Get the HMAC Identifier being Sent (SCTP_SEND_HMAC_IDENT)
During the SCTP association establishment a HMAC Identifier is
selected which is used by an SCTP endpoint when sending AUTH chunks.
An application can access the result of this selection by using this
read-only socket option, which uses the level IPPROTO_SCTP and the
name SCTP_SEND_HMAC_IDENT.
The following structure is used to access HMAC Identifier used for
sending 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 fills in an
association identifier. It is an error to use
SCTP_{FUTURE|CURRENT|ALL}_ASSOC in assoc_id.
assoc_value: This parameter contains the HMAC Identifier used for
sending AUTH chunks.
9. IANA Considerations
[NOTE to RFC-Editor: "RFCXXXX" is to be replaced by the RFC number
you assign this document.]
[NOTE to RFC-Editor: The requested values for the cause code are
tentative and to be confirmed by IANA.]
IANA is requested to perform the following updates to SCTP-parameters
(http://www.iana.org/assignments/sctp-parameters):
* In the Chunk Types Registry replace in the entry for the AUTH
Chunk Type the reference to [RFC4895] by a reference to RFCXXXX.
* In the Parameter Types Registry replace in the entry for the
Random, the Chunk List, and the Requested HMAC Algorithm Parameter
Chunk Parameter Type the reference to [RFC4895] by a reference to
RFCXXXX.
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* In the Error Cause Codes Registry replace in the entry for the
Unsupported HMAC Identifier Cause Code the reference to [RFC4895]
by a reference to RFCXXXX.
* In the Hashed Message Authentication Code (HMAC) Identifiers
Registry replace in the Reference section the reference to
[RFC4895] by a reference to RFCXXXX.
Replace each reference to [RFC4895] by a reference to RFCXXXX for
all currently registered Message Digest Algorithms.
A new error cause code has to be assigned by IANA. This requires an
additional line in the "Error Cause Codes" registry for SCTP:
+====================+==================+===========+
| Value | Cause Code | Reference |
+====================+==================+===========+
| 0x0100 (suggested) | RANDOM Collision | [RFCXXXX] |
+--------------------+------------------+-----------+
Table 5: New Entry in Error Cause Codes Registry
This document also defines one registry that IANA maintains through
the definition of additional HMAC Identifiers.
9.1. IETF-Defined HMAC Identifiers
Additional HMAC Identifier can be allocated in the range 4 to 65535
through a Specification Required action as defined in [RFC8126].
Provided documentation MUST include the following information:
a) A message digest algorithm usable with the HMAC defined in
[RFC2104] MUST be specified.
b) A short name of the message digest algorithm.
10. Security Considerations
Without using endpoint shared keys, this extension only protects
against modification or injection of authenticated chunks by
attackers who did not capture the initial handshake setting up the
SCTP association.
If an endpoint pair shared key is used, even a true man in the middle
cannot inject chunks, which are required to be authenticated, even if
he intercepts the initial message exchange. The endpoint also knows
that it is accepting authenticated chunks from a peer who knows the
endpoint pair shared key.
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The establishment of endpoint pair shared keys is out of the scope of
this document. Other mechanisms can be used, like using TLS or
manual configuration.
When an endpoint accepts COOKIE ECHO chunks only in an authenticated
way the restart procedure does not work. Neither an attacker nor a
restarted endpoint not knowing the association shared key can perform
an restart. However, if the association shared key is known, it is
possible to restart the association.
Because SCTP already has a built-in mechanism that handles the
reception of duplicated chunks, the presented solution makes use of
this functionality and does not provide a method to avoid replay
attacks by itself. Of course, this only works within each SCTP
association. Therefore, a separate shared key is used for each SCTP
association to handle replay attacks covering multiple SCTP
associations.
Each endpoint presenting a list of more than one element in the HMAC
ALGO parameter must be prepared for the peer using the weakest
algorithm listed.
When an endpoint pair uses non-NULL endpoint pair shared keys and one
of the endpoints still accepts a NULL key, an attacker who captured
the initial handshake can still inject or modify authenticated chunks
by using the NULL key.
11. Acknowledgments
The authors wish to thank Eric Rescorla for being a coauthor of
[RFC4895], which is the basis of this document.
The authors wish to thank David Black, Sascha Grau, Russ Housley,
Ivan Arias Rodriguez, Irene Rüngeler, Nagesh Shamnur, and Magnus
Westerlund for their invaluable comments on [RFC4895].
12. References
12.1. Normative References
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
DOI 10.17487/RFC2104, February 1997,
<https://www.rfc-editor.org/info/rfc2104>.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005,
<https://www.rfc-editor.org/info/rfc4086>.
[RFC5926] Lebovitz, G. and E. Rescorla, "Cryptographic Algorithms
for the TCP Authentication Option (TCP-AO)", RFC 5926,
DOI 10.17487/RFC5926, June 2010,
<https://www.rfc-editor.org/info/rfc5926>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC9260] Stewart, R., Tüxen, M., and K. Nielsen, "Stream Control
Transmission Protocol", RFC 9260, DOI 10.17487/RFC9260,
June 2022, <https://www.rfc-editor.org/info/rfc9260>.
[NIST_FIPS_180_4]
Dang, Q. H. and NIST, "Secure Hash Standard", NIST Federal
Information Processing Standards Publications 180-4,
DOI 10.6028/NIST.FIPS.180-4, July 2015,
<https://nvlpubs.nist.gov/nistpubs/FIPS/
NIST.FIPS.180-4.pdf>.
12.2. Informative References
[RFC4895] Tuexen, M., Stewart, R., Lei, P., and E. Rescorla,
"Authenticated Chunks for the Stream Control Transmission
Protocol (SCTP)", RFC 4895, DOI 10.17487/RFC4895, August
2007, <https://www.rfc-editor.org/info/rfc4895>.
[RFC5061] Stewart, R., Xie, Q., Tuexen, M., Maruyama, S., and M.
Kozuka, "Stream Control Transmission Protocol (SCTP)
Dynamic Address Reconfiguration", RFC 5061,
DOI 10.17487/RFC5061, September 2007,
<https://www.rfc-editor.org/info/rfc5061>.
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[RFC6458] Stewart, R., Tuexen, M., Poon, K., Lei, P., and V.
Yasevich, "Sockets API Extensions for the Stream Control
Transmission Protocol (SCTP)", RFC 6458,
DOI 10.17487/RFC6458, December 2011,
<https://www.rfc-editor.org/info/rfc6458>.
Authors' Addresses
Michael Tüxen
Münster Univ. of Applied Sciences
Stegerwaldstr. 39
48565 Steinfurt
Germany
Email: tuexen@fh-muenster.de
Randall R. Stewart
Netflix, Inc.
15214 Pendio Drive
Bella Collina, FL 34756
United States of America
Email: randall@lakerest.net
Peter Lei
Netflix, Inc.
8735 West Higgins Road
Suite 300
Chicago, IL 60631
United States of America
Email: peterlei@netflix.com
Hannes Tschofenig
Email: hannes.tschofenig@gmx.net
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