| RFC 8723 | Double SRTP | April 2020 |
| Jennings, et al. | Standards Track | [Page] |
In some conferencing scenarios, it is desirable for an intermediary to be able to manipulate some parameters in Real-time Transport Protocol (RTP) packets, while still providing strong end-to-end security guarantees. This document defines a cryptographic transform for the Secure Real-time Transport Protocol (SRTP) that uses two separate but related cryptographic operations to provide hop-by-hop and end-to-end security guarantees. Both the end-to-end and hop-by-hop cryptographic algorithms can utilize an authenticated encryption with associated data (AEAD) algorithm or take advantage of future SRTP transforms with different properties.¶
This is an Internet Standards Track document.¶
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.¶
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc8723.¶
Copyright (c) 2020 IETF Trust and the persons identified as the document authors. All rights reserved.¶
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.¶
Cloud conferencing systems that are based on switched conferencing have a central Media Distributor (MD) device that receives media from endpoints and distributes it to other endpoints, but does not need to interpret or change the media content. For these systems, it is desirable to have one cryptographic key that enables encryption and authentication of the media end-to-end while still allowing certain information in the header of an RTP packet to be changed by the MD. At the same time, a separate cryptographic key provides integrity and optional confidentiality for the media flowing between the MD and the endpoints. The framework document [PRIVATE-MEDIA-FRAMEWORK] describes this concept in more detail.¶
This specification defines a transform for SRTP that uses 1) the AES Galois/Counter Mode (AES-GCM) algorithm [RFC7714] to provide encryption and integrity for an RTP packet for the end-to-end cryptographic key and 2) a hop-by-hop cryptographic encryption and integrity between the endpoint and the MD. The MD decrypts and checks integrity of the hop-by-hop security. The MD MAY change some of the RTP header information that would impact the end-to-end integrity. In that case, the original value of any RTP header field that is changed is included in an "Original Header Block" that is added to the packet. The new RTP packet is encrypted with the hop-by-hop cryptographic algorithm before it is sent. The receiving endpoint decrypts and checks integrity using the hop-by-hop cryptographic algorithm and then replaces any parameters the MD changed using the information in the Original Header Block before decrypting and checking the end-to-end integrity.¶
One can think of the double transform as a normal SRTP transform for encrypting the RTP in a way such that things that only know half of the key, can decrypt and modify part of the RTP packet but not other parts, including the media payload.¶
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.¶
Terms used throughout this document include:¶
This specification uses a cryptographic context with two parts:¶
The RECOMMENDED cipher for the hop-by-hop and end-to-end algorithms is AES-GCM. Other combinations of SRTP ciphers that support the procedures in this document can be added to the IANA registry.¶
The keys and salt for these algorithms are generated with the following steps:¶
If the MD is to be able to modify header fields but not decrypt the payload, then it must have a cryptographic key for the outer algorithm but not the inner (end-to-end) algorithm. This document does not define how the MD should be provisioned with this information. One possible way to provide keying material for the outer (hop-by-hop) algorithm is to use [DTLS-TUNNEL].¶
Although SRTP uses a single master key to derive keys for an SRTP
session, this transform requires separate inner and outer keys.
In order to allow the inner and outer keys to be managed
independently via the master key, the transforms defined in this
document MUST be used with the following pseudorandom function
(PRF), which preserves the separation between the two halves of the
key. Given a positive integer n representing the desired output
length, a master key k_master, and an input x:¶
PRF_double_n(k_master,x) = PRF_(n/2)(inner(k_master),x) ||
PRF_(n/2)(outer(k_master),x)
¶
Here PRF_double_n(k_master, x) represents the AES_CM PRF Key Derivation Function (KDF) (see
Section 4.3.3 of [RFC3711]) for
DOUBLE_AEAD_AES_128_GCM_AEAD_AES_128_GCM algorithm and
AES_256_CM_PRF KDF [RFC6188] for DOUBLE_AEAD_AES_256_GCM_AEAD_AES_256_GCM
algorithm. The term inner(k_master) represents the first half of
the key; outer(k_master)
represents the second half of the key.¶
The OHB contains the original values of any modified RTP header fields. In the encryption process, the OHB is included in an SRTP packet as described in Section 5. In the decryption process, the receiving endpoint uses it to reconstruct the original RTP header so that it can pass the proper additional authenticated data (AAD) value to the inner transform.¶
The OHB can reflect modifications to the following fields in an RTP header: the payload type (PT), the SEQ, and the marker bit. All other fields in the RTP header MUST remain unmodified; since the OHB cannot reflect their original values, the receiver will be unable to verify the end-to-end integrity of the packet.¶
The OHB has the following syntax (in ABNF [RFC5234]):¶
OCTET = %x00-FF PT = OCTET SEQ = 2OCTET Config = OCTET OHB = [ PT ] [ SEQ ] Config¶
If present, the PT and SEQ parts of the OHB contain the original payload type and sequence number fields, respectively. The final "Config" octet of the OHB specifies whether these fields are present, and the original value of the marker bit (if necessary):¶
+-+-+-+-+-+-+-+-+ |R R R R B M P Q| +-+-+-+-+-+-+-+-+¶
In particular, an all-zero OHB Config octet (0x00) indicates that
there have been no modifications from the original header.¶
If the marker bit is not present (M=0), then B MUST be set to zero.
That is, if C represents the value of the Config octet, then the
masked value C & 0x0C MUST NOT have the value 0x80.¶
As implied by the use of the word "double" above, this transform applies AES-GCM to the SRTP packet twice. This allows media distributors to be able to modify some header fields while allowing endpoints to verify the end-to-end integrity of a packet.¶
The first, "inner" application of AES-GCM encrypts the SRTP payload and protects the integrity of a version of the SRTP header with extensions truncated. Omitting extensions from the inner integrity check means that they can be modified by an MD holding only the outer key.¶
The second, "outer" application of AES-GCM encrypts the ciphertext produced by the inner encryption (i.e., the encrypted payload and authentication tag), plus an OHB that expresses any changes made between the inner and outer transforms.¶
An MD that has the outer key but not the inner key may modify the header fields that can be included in the OHB by decrypting, modifying, and re-encrypting the packet.¶
An endpoint encrypts a packet by using the inner (end-to-end) cryptographic key and then the outer (hop-by-hop) cryptographic key. The encryption also supports a mode for repair packets that only does the outer (hop-by-hop) encryption. The processes is as follows:¶
Form a synthetic RTP packet with the following contents:¶
0x00) to the encrypted
payload (with the authentication tag) obtained from step 4.¶
When using Encrypted Key Transport (EKT) [EKT-SRTP], the EKTField comes after the SRTP packet, exactly like using EKT with any other SRTP transform.¶
The MD has the part of the key for the outer (hop-by-hop) cryptographic algorithm, but it does not have the part of the key for the inner (end-to-end) cryptographic algorithm. The cryptographic algorithm and key used to decrypt a packet and any encrypted RTP header extensions would be the same as those used in the endpoint's outer algorithm and key.¶
In order to modify a packet, the MD decrypts the received packet, modifies the packet, updates the OHB with any modifications not already present in the OHB, and re-encrypts the packet using the outer (hop-by-hop) cryptographic key before transmitting using the following steps:¶
Reflect any changes to header fields in the OHB:¶
In order to avoid nonce reuse, the cryptographic contexts used in steps 1 and 4 MUST use different, independent master keys. Note that this means that the key used for decryption by the MD MUST be different from the key used for re-encryption to the end recipient.¶
Note that if multiple MDs modify the same packet, then the first MD to alter a given header field is the one that adds it to the OHB. If a subsequent MD changes the value of a header field that has already been changed, then the original value will already be in the OHB, so no update to the OHB is required.¶
An MD that decrypts, modifies, and re-encrypts packets in this way MUST use an independent key for each recipient, and MUST NOT re-encrypt the packet using the sender's keys. If the MD decrypts and re-encrypts with the same key and salt, it will result in the reuse of a (key, nonce) pair, undermining the security of AES-GCM.¶
To decrypt a packet, the endpoint first decrypts and verifies using the outer (hop-by-hop) cryptographic key, then uses the OHB to reconstruct the original packet, which it decrypts and verifies with the inner (end-to-end) cryptographic key using the following steps:¶
Form a new synthetic SRTP packet with:¶
Once the packet has been successfully decrypted, the application needs to be careful about which information it uses to get the correct behavior. The application MUST use only the information found in the synthetic SRTP packet and MUST NOT use the other data that was in the outer SRTP packet with the following exceptions:¶
The PT and sequence number from the inner SRTP packet can be used for collection of various statistics.¶
If the RTP header of the outer packet contains extensions, they MAY be used. However, because extensions are not protected end-to-end, implementations SHOULD reject an RTP packet containing headers that would require end-to-end protection.¶
Unlike RTP, which is encrypted both hop-by-hop and end-to-end using two separate cryptographic keys, RTCP is encrypted using only the outer (hop-by-hop) cryptographic key. The procedures for RTCP encryption are specified in [RFC3711], and this document introduces no additional steps.¶
MDs sometimes interact with RTP media packets sent by endpoints, e.g., to provide recovery or receive commands via dual-tone multi-frequency (DTMF) signaling. When media packets are encrypted end-to-end, these procedures require modification. (End-to-end interactions, including end-to-end recovery, are not affected by end-to-end encryption.)¶
Repair mechanisms, in general, will need to perform recovery on encrypted packets (double-encrypted when using this transform), since the MD does not have access to the plaintext of the packet, only an intermediate, E2E-encrypted form.¶
When the recovery mechanism calls for the recovery packet itself to be encrypted, it is encrypted with only the outer, hop-by-hop key. This allows an MD to generate recovery packets without having access to the inner, end-to-end keys. However, it also results in recovery packets being triple-encrypted, twice for the base transform, and once for the recovery protection.¶
When using RTX [RFC4588] with the double transform, the cached payloads MUST be the double-encrypted packets, i.e., the bits that are sent over the wire to the other side. When encrypting a retransmission packet, it MUST be encrypted like a packet in repair mode (i.e., with only the hop-by-hop key).¶
If the MD were to cache the inner, E2E-encrypted payload and retransmit it with an RTX original sequence number field prepended, then the modifications to the payload would cause the inner integrity check to fail at the receiver.¶
A typical RTX receiver would decrypt the packet, undo the RTX transformation, then process the resulting packet normally by using the steps in Section 5.3.¶
When using RED [RFC2198] with the double transform, the processing at the sender and receiver is the same as when using RED with any other SRTP transform.¶
The main difference between the double transform and any other transform is that in an intermediated environment, usage of RED must be end-to-end. An MD cannot synthesize RED packets, because it lacks access to the plaintext media payloads that are combined to form a RED payload.¶
Note that Flexible Forward Error Correction (Flex FEC) may often provide similar or better repair capabilities compared to RED. For most applications, Flex FEC is a better choice than RED; in particular, Flex FEC has modes in which the MD can synthesize recovery packets.¶
When using Flex FEC [RFC8627] with the double transform, repair packets MUST be constructed by first double-encrypting the packet, then performing FEC. Processing of repair packets proceeds in the opposite order, performing FEC recovery and then decrypting. This ensures that the original media is not revealed to the MD but, at the same time, allows the MD to repair media. When encrypting a packet that contains the Flex FEC data, which is already encrypted, it MUST be encrypted with only the outer, hop-by-hop transform.¶
The algorithm recommended in [WEBRTC-FEC] for repair of video is Flex FEC [RFC8627]. Note that for interoperability with WebRTC, [WEBRTC-FEC] recommends not using additional FEC-only "m=" lines in SDP for the repair packets.¶
This specification recommends and defines AES-GCM as both the inner and outer cryptographic algorithms, identified as DOUBLE_AEAD_AES_128_GCM_AEAD_AES_128_GCM and DOUBLE_AEAD_AES_256_GCM_AEAD_AES_256_GCM. These algorithms provide for authenticated encryption and will consume additional processing time double-encrypting for hop-by-hop and end-to-end. However, the approach is secure and simple; thus, it is viewed as an acceptable trade-off in processing efficiency.¶
Note that names for the cryptographic transforms are of the form DOUBLE_(inner algorithm)_(outer algorithm).¶
While this document only defines a profile based on AES-GCM, it is possible for future documents to define further profiles with different inner and outer algorithms in this same framework. For example, if a new SRTP transform were defined that encrypts some or all of the RTP header, it would be reasonable for systems to have the option of using that for the outer algorithm. Similarly, if a new transform were defined that provided only integrity, that would also be reasonable to use for the outer transform as the payload data is already encrypted by the inner transform.¶
The AES-GCM cryptographic algorithm introduces an additional 16 octets to the length of the packet. When using AES-GCM for both the inner and outer cryptographic algorithms, the total additional length is 32 octets. The OHB will consume an additional 1-4 octets. Packets in repair mode will carry additional repair data, further increasing their size.¶
This SRTP transform provides protection against two classes of attacker: a network attacker that knows neither the inner nor outer keys and a malicious MD that knows the outer key. Obviously, it provides no protections against an attacker that holds both the inner and outer keys.¶
The protections with regard to the network are the same as with the normal SRTP AES-GCM transforms. The major difference is that the double transforms are designed to work better in a group context. In such contexts, it is important to note that because these transforms are symmetric, they do not protect against attacks within the group. Any member of the group can generate valid SRTP packets for any SSRC in use by the group.¶
With regard to a malicious MD, the recipient can verify the integrity of the base header fields and confidentiality and integrity of the payload. The recipient has no assurance, however, of the integrity of the header extensions in the packet.¶
The main innovation of this transform relative to other SRTP transforms is that it allows a partly trusted MD to decrypt, modify, and re-encrypt a packet. When this is done, the cryptographic contexts used for decryption and re-encryption MUST use different, independent master keys. If the same context is used, the nonce formation rules for SRTP will cause the same key and nonce to be used with two different plaintexts, which substantially degrades the security of AES-GCM.¶
In other words, from the perspective of the MD, re-encrypting packets using this protocol will involve the same cryptographic operations as if it had established independent AES-GCM crypto contexts with the sender and the receiver. This property allows the use of an MD that supports AES-GCM but does not modify any header fields, without requiring any modification to the MD.¶
IANA has added the following protection profiles to the "DTLS-SRTP Protection Profiles" registry defined in [RFC5764].¶
| Value | Profile | Reference |
|---|---|---|
| {0x00, 0x09} | DOUBLE_AEAD_AES_128_GCM_AEAD_AES_128_GCM | RFC 8723 |
| {0x00, 0x0A} | DOUBLE_AEAD_AES_256_GCM_AEAD_AES_256_GCM | RFC 8723 |
The SRTP transform parameters for each of these protection profiles are:¶
| DOUBLE_AEAD_AES_128_GCM_AEAD_AES_128_GCM | |
|---|---|
| cipher: | AES_128_GCM then AES_128_GCM |
| cipher_key_length: | 256 bits |
| cipher_salt_length: | 192 bits |
| aead_auth_tag_length: | 256 bits |
| auth_function: | NULL |
| auth_key_length: | N/A |
| auth_tag_length: | N/A |
| maximum lifetime: | at most 231 SRTCP packets and at most 248 SRTP packets |
| DOUBLE_AEAD_AES_256_GCM_AEAD_AES_256_GCM | |
| cipher: | AES_256_GCM then AES_256_GCM |
| cipher_key_length: | 512 bits |
| cipher_salt_length: | 192 bits |
| aead_auth_tag_length: | 256 bits |
| auth_function: | NULL |
| auth_key_length: | N/A |
| auth_tag_length: | N/A |
| maximum lifetime: | at most 231 SRTCP packets and at most 248 SRTP packets |
The first half of the key and salt is used for the inner (end-to-end) algorithm and the second half is used for the outer (hop-by-hop) algorithm.¶
The following figures show a double-encrypted SRTP packet. The sides indicate the parts of the packet that are encrypted and authenticated by the hop-by-hop and end-to-end operations.¶
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 |
| .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTP extension (OPTIONAL) ... |
+>+>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
O I | payload ... |
O I | +-------------------------------+
O I | | RTP padding | RTP pad count |
O +>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
O | | E2E authentication tag |
O | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
O | | OHB ... |
+>| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | | HBH authentication tag |
| | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| +- E2E Encrypted Portion
|
+--- HBH Encrypted Portion
¶
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 | I O
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ I O
| timestamp | I O
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ I O
| synchronization source (SSRC) identifier | I O
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ I O
| contributing source (CSRC) identifiers | I O
| .... | I O
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+ O
| RTP extension (OPTIONAL) ... | | O
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+ O
| payload ... | I O
| +-------------------------------+ I O
| | RTP padding | RTP pad count | I O
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+ O
| E2E authentication tag | | O
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | O
| OHB ... | | O
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |<+
| HBH authentication tag | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| |
E2E Authenticated Portion ---+ |
|
HBH Authenticated Portion -----+
¶
Thank you to Alex Gouaillard, David Benham, Magnus Westerlund, Nils Ohlmeier, Roni Even, and Suhas Nandakumar for reviews and improvements to this specification. In addition, thank you to Sergio Garcia Murillo, who proposed the change of transporting the OHB information in the RTP payload instead of the RTP header.¶