docname: draft-ietf-avtcore-cryptex-08 title: Completely Encrypting RTP Header Extensions and Contributing Sources category: std ipr: trust200902 area: ART workgroup: AVTCORE keyword: SRTP stand_alone: yes updates: 3711
pi: [toc, sortrefs, symrefs]
ins: J. Uberti
name: Justin Uberti
organization: Clubhouse
email: [email protected]
-
ins: C. Jennings name: Cullen Jennings org: Cisco email: [email protected]
-
ins: S. Garcia Murillo name: Sergio Garcia Murillo org: Millicast email: [email protected]
normative: RFC3550: RFC3711: RFC8285: RFC8859:
informative: RFC6464: RFC6465: RFC6904:
--- abstract
While the Secure Real-time Transport Protocol (SRTP) provides confidentiality for the contents of a media packet, a significant amount of metadata is left unprotected, including RTP header extensions and contributing sources (CSRCs). However, this data can be moderately sensitive in many applications. While there have been previous attempts to protect this data, they have had limited deployment, due to complexity as well as technical limitations.
This document updates RFC 3711, the SRTP specification, and defines Cryptex as a new mechanism that completely encrypts header extensions and CSRCs and uses simpler Session Description Protocol (SDP) signaling with the goal of facilitating deployment.
--- middle
The Secure Real-time Transport Protocol (SRTP) {{RFC3711}} mechanism provides message authentication for the entire RTP packet, but only encrypts the RTP payload. This has not historically been a problem, as much of the information carried in the header has minimal sensitivity (e.g., RTP timestamp); in addition, certain fields need to remain as cleartext because they are used for key scheduling (e.g., RTP SSRC and sequence number).
However, as noted in {{RFC6904}}, the security requirements can be different for information carried in RTP header extensions, including the per-packet sound levels defined in {{RFC6464}} and {{RFC6465}}, which are specifically noted as being sensitive in the Security Considerations section of those RFCs.
In addition to the contents of the header extensions, there are now enough header extensions in active use that the header extension identifiers themselves can provide meaningful information in terms of determining the identity of the endpoint and/or application. Accordingly, these identifiers can be considered a fingerprinting issue.
Finally, the CSRCs included in RTP packets can also be sensitive, potentially allowing a network eavesdropper to determine who was speaking and when during an otherwise secure conference call.
Encryption of Header Extensions in SRTP {{RFC6904}} was proposed in 2013 as a solution to the problem of unprotected header extension values. However, it has not seen significant adoption, and has a few technical shortcomings.
First, the mechanism is complicated. Since it allows encryption to be negotiated on a per-extension basis, a fair amount of signaling logic is required. And in the SRTP layer, a somewhat complex transform is required to allow only the selected header extension values to be encrypted. One of the most popular SRTP implementations had a significant bug in this area that was not detected for five years.
Second, it only protects the header extension values, and not their ids or lengths. It also does not protect the CSRCs. As noted above, this leaves a fair amount of potentially sensitive information exposed.
Third, it bloats the header extension space. Because each extension must be offered in both unencrypted and encrypted forms, twice as many header extensions must be offered, which will in many cases push implementations past the 14-extension limit for the use of one-byte extension headers defined in {{RFC8285}}. Accordingly, implementations will need to use two-byte headers in many cases, which are not supported well by some existing implementations.
Finally, the header extension bloat combined with the need for backwards compatibility results in additional wire overhead. Because two-byte extension headers may not be handled well by existing implementations, one-byte extension identifiers will need to be used for the unencrypted (backwards compatible) forms, and two-byte for the encrypted forms. Thus, deployment of {{RFC6904}} encryption for header extensions will typically result in multiple extra bytes in each RTP packet, compared to the present situation.
From the previous analysis, the desired properties of a solution are:
- Build on existing {{RFC3711}} SRTP framework (simple to understand)
- Build on existing {{RFC8285}} header extension framework (simple to implement)
- Protection of header extension ids, lengths, and values
- Protection of CSRCs when present
- Simple signaling
- Simple crypto transform and SRTP interactions
- Backward compatible with unencrypted endpoints, if desired
- Backward compatible with existing RTP tooling
The last point deserves further discussion. While considering possible solutions that would have encrypted more of the RTP header (e.g., the number of CSRCs), lack of support on current tools was inevitable and the additional complexity outweighed the slight improvement in confidentiality by fixing previous solutions. Hence, a new approach was needed to solve the described problem in {{problem-statement}}.
{::boilerplate bcp14-tagged}
This specification proposes a mechanism to negotiate encryption of all RTP header extensions (ids, lengths, and values) as well as CSRC values. It reuses the existing SRTP framework, is accordingly simple to implement, and is backward compatible with existing RTP packet parsing code, even when support for the mechanism has been negotiated.
Except when explicitly stated otherwise, Cryptex reuses all the framework procedures, transforms and considerations described in {{!RFC3711}}.
Cryptex support is indicated via a new "a=cryptex" SDP attribute defined in this specification.
The new "a=cryptex" attribute is a property attribute as defined in {{!RFC8866}} section 5.13 and therefore takes no value, and can be used at the session level or media level.
The presence of the "a=cryptex" attribute in the SDP (either in an offer or answer) indicates that the endpoint is capable of receiving RTP packets encrypted with Cryptex, as defined below.
Once each peer has verified that the other party supports receiving RTP packets encrypted with Cryptex, senders can unilaterally decide whether to use or not the Cryptex mechanism on a per packet basis.
If BUNDLE is in use as per {{?RFC9143}} and the "a=cryptex" attribute is present for a media line, it MUST be present for all RTP-based "m=" sections belonging to the same bundle group. This ensures that the encrypted MID header extensions can be processed, allowing to associate RTP streams with the correct "m=" section in each BUNDLE group as specified in {{!RFC9143}} section 9.2. When used with BUNDLE, this attribute is assigned to the TRANSPORT category {{RFC8859}}.
Both endpoints can change the Cryptex support status by modifying the session as specified in {{!RFC3264}} section 8. Generating subsequent SDP offers and answers MUST use the same procedures for including the "a=cryptex" attribute as the ones on the initial offer and answer.
A General Mechanism for RTP Header Extensions {{RFC8285}} defines two values for the "defined by profile" field for carrying one-byte and two-byte header extensions. In order to allow a receiver to determine if an incoming RTP packet is using the encryption scheme in this specification, two new values are defined:
- 0xC0DE for the encrypted version of the one-byte header extensions (instead of 0xBEDE).
- 0xC2DE for the encrypted versions of the two-byte header extensions (instead of 0x100).
In the case of using two-byte header extensions, the extension id with value 256 MUST NOT be negotiated, as the value of this id is meant to be contained in the "appbits" of the "defined by profile" field, which are not available when using the values above.
Note that as per {{!RFC8285}} it is not possible to mix one-byte and two-byte headers on the same RTP packet. Mixing one-byte and two-byte headers on the same RTP stream requires negotiation of the "extmap-allow-mixed" SDP attribute as defined in {{!RFC8285}} section 4.1.2.
Peers MAY negotiate both Cryptex and the Encryption of Header Extensions mechanism defined in {{RFC6904}} via SDP offer/answer as described in {{sdp-considerations}}, and if both mechanisms are supported, either one can be used for any given packet. However, if a packet is encrypted with Cryptex, it MUST NOT also use {{RFC6904}} header extension encryption, and vice versa.
If one of the peers has advertised both the ability to receive cryptex and the ability to receive header extensions encrypted as per {{RFC6904}} in the SDP exchange, it is RECOMMENDED for the other peer to use Cryptex rather than {{RFC6904}} when sending RTP packets so all the header extensions and CSRCS are encrypted unless there is a compelling reason to use {{RFC6904}} (e.g. a need for some header extensions to be sent in the clear so that so they are processable by RTP middleboxes) in which case, it SHOULD use {{RFC6904}} instead.
When the mechanism defined by this specification has been negotiated, sending an RTP packet that has any CSRCs or contains any {{RFC8285}} header extensions follows the steps below. This mechanism MUST NOT be used with header extensions other than the {{RFC8285}} variety.
If the RTP packet contains one-byte headers, the 16-bit RTP header extension tag MUST be set to 0xC0DE to indicate that the encryption has been applied, and the one-byte framing is being used. Otherwise, the header extension tag MUST be set to 0xC2DE to indicate encryption has been applied, and the two-byte framing is being used.
If the packet contains CSRCs but no header extensions, an empty extension block consisting of the 0xC0DE tag and a 16-bit length field set to zero (explicitly permitted by {{RFC3550}}) MUST be appended, and the X bit MUST be set to 1 to indicate an extension block is present. This is necessary to provide the receiver an indication that the CSRCs in the packet are encrypted.
The RTP packet MUST then be encrypted as described in Encryption Procedure.
When receiving an RTP packet that contains header extensions, the "defined by profile" field MUST be checked to ensure the payload is formatted according to this specification. If the field does not match one of the values defined above, the implementation MUST instead handle it according to the specification that defines that value.
Alternatively, if the implementation considers the use of this specification mandatory and the "defined by profile" field does not match one of the values defined above, it MUST stop the processing of the RTP packet and report an error for the RTP stream.
If the RTP packet passes this check, it is then decrypted according to Decryption Procedure, and passed to the next layer to process the packet and its extensions. In the event that a zero-length extension block was added as indicated above, it can be left as-is and will be processed normally.
When this mechanism is active, the SRTP packet is protected as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+
|V=2|P|X| CC |M| PT | sequence number | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| timestamp | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| synchronization source (SSRC) identifier | |
+>+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ |
| | contributing source (CSRC) identifiers | |
| | .... | |
+>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
X | 0xC0 or 0xC2 | 0xDE | length | |
+>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | RFC 8285 header extensions | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | payload ... | |
| | +-------------------------------+ |
| | | RTP padding | RTP pad count | |
+>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+
| ~ SRTP MKI (OPTIONAL) ~ |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| : authentication tag (RECOMMENDED) : |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
+- Encrypted Portion* Authenticated Portion ---+
{: #srtp-packet artwork-align="center" title="SRTP packet}
- Note that, as required by {{RFC8285}}, the 4 bytes at the start of the extension block are not encrypted.
Specifically, the encrypted portion MUST include any CSRC identifiers, any RTP header extension (except for the first 4 bytes), and the RTP payload.
The encryption procedure is identical to that of {{RFC3711}} except for the Encrypted Portion of the SRTP packet. The plaintext input to the cipher is as follows:
Plaintext = CSRC identifiers (if used) || header extension data ||
RTP payload || RTP padding (if used) || RTP pad count (if used).
Here "header extension data" refers to the content of the RTP extension field,
excluding the first four bytes (the RFC 8285 extension header). The first
4 * CSRC count (CC) bytes of the ciphertext are placed in the CSRC field of the RTP header.
The remainder of the ciphertext is the RTP payload of the encrypted packet.
To minimize changes to surrounding code, the encryption mechanism can choose to replace a "defined by profile" field from {{RFC8285}} with its counterpart defined in RTP Header Processing above and encrypt at the same time.
For AEAD ciphers (e.g., GCM), the 12-byte fixed header and the four-byte header extension header (the "defined by profile" field and the length) are considered AAD, even though they are non-contiguous in the packet if CSRCs are present.
Associated Data: fixed header || extension header (if X=1)
Here "fixed header" refers to the 12-byte fixed portion of the RTP header, and "extension header" refers to the four-byte RFC 8285 extension header ("defined by profile" and extension length).
Implementations can rearrange a packet so that the AAD and plaintext are contiguous by swapping the order of the extension header and the CSRC identifiers, resulting in an intermediate representation of the form shown in {{intermediate-packet}}. After encryption, the CSRCs (now encrypted) and extension header would need to be swapped back to their original positions. A similar operation can be done when decrypting to create contiguous ciphertext and AAD inputs.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+
|V=2|P|X| CC |M| PT | sequence number | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| timestamp | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| synchronization source (SSRC) identifier | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| 0xC0 or 0xC2 | 0xDE | length | |
+>+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+<+
| | contributing source (CSRC) identifiers | |
| | .... | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | RFC 8285 header extensions | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | payload ... | |
| | +-------------------------------+ |
| | | RTP padding | RTP pad count | |
+>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
+- Plaintext Input AAD Input ---+
{: #intermediate-packet artwork-align="center" title="An RTP packet transformed to make Cryptex cipher inputs contiguous"}
Note: This intermediate representation is only displayed as reference for implementations and is not meant to be sent on the wire.
The decryption procedure is identical to that of {{RFC3711}} except for the Encrypted Portion of the SRTP packet, which is as shown in the section above.
To minimize changes to surrounding code, the decryption mechanism can choose to replace the "defined by profile" field with its no-encryption counterpart from {{RFC8285}} and decrypt at the same time.
This specification attempts to encrypt as much as possible without interfering with backwards compatibility for systems that expect a certain structure from an RTPv2 packet, including systems that perform demultiplexing based on packet headers. Accordingly, the first two bytes of the RTP packet are not encrypted.
This specification also attempts to reuse the key scheduling from SRTP, which depends on the RTP packet sequence number and SSRC identifier. Accordingly, these values are also not encrypted.
All security considerations in {{!RFC3711}} section 9 are applicable to this specification, except section 9.4. Confidentiality of the RTP Header which is the purpose of this specification.
The risks of using weak or NULL authentication with SRTP, described in Section 9.5 of {{!RFC3711}}, apply to encrypted header extensions as well.
This specification extends SRTP by expanding the Encrypted Portion of the RTP packet, as shown in Packet Structure. It does not change how SRTP authentication works in any way. Given that more of the packet is being encrypted than before, this is necessarily an improvement.
The RTP fields that are left unencrypted (see rationale above) are as follows:
- RTP version
- padding bit
- extension bit
- number of CSRCs
- marker bit
- payload type
- sequence number
- timestamp
- SSRC identifier
- number of {{RFC8285}} header extensions
These values contain a fixed set (i.e., one that won't be changed by extensions) of information that, at present, is observed to have low sensitivity. In the event any of these values need to be encrypted, SRTP is likely the wrong protocol to use and a fully-encapsulating protocol such as DTLS is preferred (with its attendant per-packet overhead).
This document updates the "Session Description Protocol Parameters" as specified in Section 8.2.4 of {{!RFC8866}}. Specifically, it adds the SDP "a=cryptex" attribute to the Attribute Names (<attribute-name>) registry for both media and session level usage.
Contact name: IETF AVT Working Group or IESG if AVT is closed
Contact email address: [email protected]
Attribute name: cryptex
Attribute syntax: This attribute takes no values.
Attribute semantics: N/A
Attribute value: N/A
Usage level: session, media
Charset dependent: No
Purpose: The presence of this attribute in the SDP indicates that the endpoint is capable of receiving RTP packets encrypted with Cryptex as described in this document.
O/A procedures: SDP O/A procedures are described in Section 4 of this document.
Mux Category: TRANSPORT
The authors wish to thank Lennart Grahl for pointing out many of the issues with the existing header encryption mechanism, as well as suggestions for this proposal. Thanks also to Jonathan Lennox, Iñaki Castillo, and Bernard Aboba for their review and suggestions.
--- back
All values are in hexadecimal and represented in network order (big endian).
The following section list the test vectors for using cryptex with AES-CTR as per {{RFC3711}}
Common values are organized as follows:
Rollover Counter: 00000000
Master Key: e1f97a0d3e018be0d64fa32c06de4139
Master Salt: 0ec675ad498afeebb6960b3aabe6
Crypto Suite: AES_CM_128_HMAC_SHA1_80
Session Key: c61e7a93744f39ee10734afe3ff7a087
Session Salt: 30cbbc08863d8c85d49db34a9ae1
Authentication Key: cebe321f6ff7716b6fd4ab49af256a156d38baa4
RTP Packet:
900f1235
decafbad
cafebabe
bede0001
51000200
abababab
abababab
abababab
abababab
Encrypted RTP Packet:
900f1235
decafbad
cafebabe
c0de0001
eb923652
51c3e036
f8de27e9
c27ee3e0
b4651d9f
bc4218a7
0244522f
34a5
RTP Packet:
900f1236
decafbad
cafebabe
10000001
05020002
abababab
abababab
abababab
abababab
Encrypted RTP Packet:
900f1236
decafbad
cafebabe
c2de0001
4ed9cc4e
6a712b30
96c5ca77
339d4204
ce0d7739
6cab6958
5fbce381
94a5
RTP Packet:
920f1238
decafbad
cafebabe
0001e240
0000b26e
bede0001
51000200
abababab
abababab
abababab
abababab
Encrypted RTP Packet:
920f1238
decafbad
cafebabe
8bb6e12b
5cff16dd
c0de0001
92838c8c
09e58393
e1de3a9a
74734d67
45671338
c3acf11d
a2df8423
bee0
RTP Packet:
920f1239
decafbad
cafebabe
0001e240
0000b26e
10000001
05020002
abababab
abababab
abababab
abababab
Encrypted RTP Packet:
920f1239
decafbad
cafebabe
f70e513e
b90b9b25
c2de0001
bbed4848
faa64466
5f3d7f34
125914e9
f4d0ae92
3c6f479b
95a0f7b5
3133
RTP Packet:
920f123a
decafbad
cafebabe
0001e240
0000b26e
bede0000
abababab
abababab
abababab
abababab
Encrypted RTP Packet:
920f123a
decafbad
cafebabe
7130b6ab
fe2ab0e3
c0de0000
e3d9f64b
25c9e74c
b4cf8e43
fb92e378
1c2c0cea
b6b3a499
a14c
RTP Packet:
920f123b
decafbad
cafebabe
0001e240
0000b26e
10000000
abababab
abababab
abababab
abababab
Encrypted RTP Packet:
920f123b
decafbad
cafebabe
cbf24c12
4330e1c8
c2de0000
599dd45b
c9d687b6
03e8b59d
771fd38e
88b170e0
cd31e125
eabe
The following section list the test vectors for using cryptex with AES-GCM as per {{?RFC7714}}
Common values are organized as follows:
Rollover Counter: 00000000
Master Key: 000102030405060708090a0b0c0d0e0f
Master Salt: a0a1a2a3a4a5a6a7a8a9aaab
Crypto Suite: AEAD_AES_128_GCM
Session Key: 077c6143cb221bc355ff23d5f984a16e
Session Salt: 9af3e95364ebac9c99c5a7c4
RTP Packet:
900f1235
decafbad
cafebabe
bede0001
51000200
abababab
abababab
abababab
abababab
Encrypted RTP Packet:
900f1235
decafbad
cafebabe
c0de0001
39972dc9
572c4d99
e8fc355d
e743fb2e
94f9d8ff
54e72f41
93bbc5c7
4ffab0fa
9fa0fbeb
RTP Packet:
900f1236
decafbad
cafebabe
10000001
05020002
abababab
abababab
abababab
abababab
Encrypted RTP Packet:
900f1236
decafbad
cafebabe
c2de0001
bb75a4c5
45cd1f41
3bdb7daa
2b1e3263
de313667
c9632490
81b35a65
f5cb6c88
b394235f
RTP Packet:
920f1238
decafbad
cafebabe
0001e240
0000b26e
bede0001
51000200
abababab
abababab
abababab
abababab
Encrypted RTP Packet:
920f1238
decafbad
cafebabe
63bbccc4
a7f695c4
c0de0001
8ad7c71f
ac70a80c
92866b4c
6ba98546
ef913586
e95ffaaf
fe956885
bb0647a8
bc094ac8
RTP Packet:
920f1239
decafbad
cafebabe
0001e240
0000b26e
10000001
05020002
abababab
abababab
abababab
abababab
Encrypted RTP Packet:
920f1239
decafbad
cafebabe
3680524f
8d312b00
c2de0001
c78d1200
38422bc1
11a7187a
18246f98
0c059cc6
bc9df8b6
26394eca
344e4b05
d80fea83
RTP Packet:
920f123a
decafbad
cafebabe
0001e240
0000b26e
bede0000
abababab
abababab
abababab
abababab
Encrypted RTP Packet:
920f123a
decafbad
cafebabe
15b6bb43
37906fff
c0de0000
b7b96453
7a2b03ab
7ba5389c
e9331712
6b5d974d
f30c6884
dcb651c5
e120c1da
RTP Packet:
920f123b
decafbad
cafebabe
0001e240
0000b26e
10000000
abababab
abababab
abababab
abababab
Encrypted RTP Packet:
920f123b
decafbad
cafebabe
dcb38c9e
48bf95f4
c2de0000
61ee432c
f9203170
76613258
d3ce4236
c06ac429
681ad084
13512dc9
8b5207d8
--- back