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Internet Engineering Task Force (IETF) D. McGrew
Request for Comments: 7714 Cisco Systems, Inc.
Category: Standards Track K. Igoe
ISSN: 2070-1721 National Security Agency
December 2015
AES-GCM Authenticated Encryption in the Secure Real-time Transport Protocol (SRTP)
Abstract
This document defines how the AES-GCM Authenticated Encryption with Associated Data family of algorithms can be used to provide confidentiality and data authentication in the Secure Real-time Transport Protocol (SRTP).
Status of This Memo
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 5741.
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7714.
Copyright Notice
Copyright (c) 2015 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 (http://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.
The Secure Real-time Transport Protocol (SRTP) [RFC3711] is a profile of the Real-time Transport Protocol (RTP) [RFC3550], which can provide confidentiality, message authentication, and replay protection to the RTP traffic and to the control traffic for RTP, the Real-time Transport Control Protocol (RTCP). It is important to note that the outgoing SRTP packets from a single endpoint may be originating from several independent data sources.
Authenticated Encryption [BN00] is a form of encryption that, in addition to providing confidentiality for the Plaintext that is encrypted, provides a way to check its integrity and authenticity. Authenticated Encryption with Associated Data, or AEAD [R02], adds the ability to check the integrity and authenticity of some Associated Data (AD), also called "Additional Authenticated Data" (AAD), that is not encrypted. This specification makes use of the interface to a generic AEAD algorithm as defined in [RFC5116].
The Advanced Encryption Standard (AES) is a block cipher that provides a high level of security and can accept different key sizes. AES Galois/Counter Mode (AES-GCM) [GCM] is a family of AEAD algorithms based upon AES. This specification makes use of the AES versions that use 128-bit and 256-bit keys, which we call "AES-128" and "AES-256", respectively.
Any AEAD algorithm provides an intrinsic authentication tag. In many applications, the authentication tag is truncated to less than full length. In this specification, the authentication tag MUST NOT be truncated. The authentications tags MUST be a full 16 octets in length. When used in SRTP/SRTCP, AES-GCM will have two configurations:
AEAD_AES_128_GCM AES-128 with a 16-octet authentication tag AEAD_AES_256_GCM AES-256 with a 16-octet authentication tag The key size is set when the session is initiated and SHOULD NOT be altered.
The Galois/Counter Mode of operation (GCM) is an AEAD mode of operation for block ciphers. GCM uses Counter Mode to encrypt the data, an operation that can be efficiently pipelined. Further, GCM authentication uses operations that are particularly well suited to efficient implementation in hardware, making it especially appealing for high-speed implementations, or for implementations in an efficient and compact circuit.
In summary, this document defines how to use an AEAD algorithm, particularly AES-GCM, to provide confidentiality and message authentication within SRTP and SRTCP packets.
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 [RFC2119].
3. Overview of the SRTP/SRTCP AEAD Security Architecture
SRTP/SRTCP AEAD security is based upon the following principles:
a) Both privacy and authentication are based upon the use of symmetric algorithms. An AEAD algorithm such as AES-GCM combines privacy and authentication into a single process.
b) A secret master key is shared by all participating endpoints -- both those originating SRTP/SRTCP packets and those receiving these packets. Any given master key MAY be used simultaneously by several endpoints to originate SRTP/SRTCP packets (as well as one or more endpoints using this master key to process inbound data).
c) A Key Derivation Function (KDF) is applied to the shared master key value to form separate encryption keys, authentication keys, and salting keys for SRTP and for SRTCP (a total of six keys). This process is described in Section 4.3 of [RFC3711]. The master key MUST be at least as large as the encryption key derived from it. Since AEAD algorithms such as AES-GCM combine encryption and authentication into a single process, AEAD algorithms do not make use of separate authentication keys.
d) Aside from making modifications to IANA registries to allow AES-GCM to work with Security Descriptions (SDES), Datagram Transport Layer Security for Secure RTP (DTLS-SRTP), and Multimedia Internet KEYing (MIKEY), the details of how the master key is established and shared between the participants are outside the scope of this document. Similarly, any mechanism for rekeying an existing session is outside the scope of the document.
e) Each time an instantiation of AES-GCM is invoked to encrypt and authenticate an SRTP or SRTCP data packet, a new Initialization Vector (IV) is used. SRTP combines the 4-octet Synchronization Source (SSRC) identifier, the 4-octet Rollover Counter (ROC), and the 2-octet Sequence Number (SEQ) with the 12-octet encryption salt to form a 12-octet IV (see Section 8.1). SRTCP combines the SSRC and 31-bit SRTCP index with the encryption salt to form a 12-octet IV (see Section 9.1).
The following terms have very specific meanings in the context of this RFC:
Instantiation: In AEAD, an instantiation is an (Encryption_key,
salt) pair together with all of the data structures (for example, counters) needed for it to function properly. In SRTP/SRTCP, each endpoint will need two instantiations of the AEAD algorithm for each master key in its possession: one instantiation for SRTP traffic and one instantiation for SRTCP traffic.
Invocation: SRTP/SRTCP data streams are broken into packets.
Each packet is processed by a single invocation of the appropriate instantiation of the AEAD algorithm.
In many applications, each endpoint will have one master key for processing outbound data but may have one or more separate master keys for processing inbound data.
McGrew & Igoe Standards Track [Page 5]
RFC 7714 AES-GCM for SRTP December 20155. Generic AEAD Processing5.1. Types of Input Data
Associated Data: Data that is to be authenticated but not
encrypted.
Plaintext: Data that is to be both encrypted and
authenticated.
Raw Data: Data that is to be neither encrypted nor
authenticated.
Which portions of SRTP/SRTCP packets that are to be treated as Associated Data, which are to be treated as Plaintext, and which are to be treated as Raw Data are covered in Sections 8.2, 9.2, and 9.3.
5.2. AEAD Invocation Inputs and Outputs5.2.1. Encrypt Mode
Inputs:
Encryption_key Octet string, either 16 or
32 octets long
Initialization_Vector Octet string, 12 octets long Associated_Data Octet string of variable length Plaintext Octet string of variable length
(*): In AEAD, the authentication tag in embedded in the
ciphertext. When GCM is being used, the ciphertext consists of the encrypted Plaintext followed by the authentication tag.
McGrew & Igoe Standards Track [Page 6]
RFC 7714 AES-GCM for SRTP December 20155.2.2. Decrypt Mode
Inputs:
Encryption_key Octet string, either 16 or
32 octets long
Initialization_Vector Octet string, 12 octets long Associated_Data Octet string of variable length Ciphertext Octet string of variable length
AEAD requires that all incoming packets MUST pass AEAD authentication before any other action takes place. Plaintext and Associated Data MUST NOT be released until the AEAD authentication tag has been validated. Further, the ciphertext MUST NOT be decrypted until the AEAD tag has been validated.
Should the AEAD tag prove to be invalid, the packet in question is to be discarded and a Validation Error flag raised. Local policy determines how this flag is to be handled and is outside the scope of this document.
Each outbound packet uses a 12-octet IV and an encryption key to form two outputs:
o a 16-octet first_key_block, which is used in forming the authentication tag, and
o a keystream of octets, formed in blocks of 16 octets each The first 16-octet block of the key is saved for use in forming the authentication tag, and the remainder of the keystream is XORed to the Plaintext to form the cipher. This keystream is formed one block at a time by inputting the concatenation of a 12-octet IV (see Sections 8.1 and 9.1) with a 4-octet block to AES. The pseudocode below illustrates this process:
In theory, this keystream generation process allows for the encryption of up to (2^36) - 32 octets per invocation (i.e., per packet), far longer than is actually required.
With any counter mode, if the same (IV, Encryption_key) pair is used twice, precisely the same keystream is formed. As explained in Section 9.1 of [RFC3711], this is a cryptographic disaster. For GCM, the consequences are even worse, since such a reuse compromises GCM's integrity mechanism not only for the current packet stream but for all future uses of the current encryption_key.
The AEAD message authentication mechanism MUST be the primary message authentication mechanism for AEAD SRTP/SRTCP. Additional SRTP/SRTCP authentication mechanisms SHOULD NOT be used with any AEAD algorithm, and the optional SRTP/SRTCP authentication tags are NOT RECOMMENDED and SHOULD NOT be present. Note that this contradicts Section 3.4 of [RFC3711], which makes the use of the SRTCP authentication tag field mandatory, but the presence of the AEAD authentication renders the older authentication methods redundant. Rationale: Some applications use the SRTP/SRTCP authentication tag as a means of conveying additional information, notably [RFC4771]. This document retains the authentication tag field primarily to preserve compatibility with these applications.
McGrew & Igoe Standards Track [Page 8]
RFC 7714 AES-GCM for SRTP December 20157.2. RTP Padding
AES-GCM does not require that the data be padded out to a specific block size, reducing the need to use the padding mechanism provided by RTP. It is RECOMMENDED that the RTP padding mechanism not be used unless it is necessary to disguise the length of the underlying Plaintext.
8. AES-GCM Processing for SRTP8.1. SRTP IV Formation for AES-GCM
The 12-octet IV used by AES-GCM SRTP is formed by first concatenating 2 octets of zeroes, the 4-octet SSRC, the 4-octet rollover counter (ROC), and the 2-octet sequence number (SEQ). The resulting 12-octet value is then XORed to the 12-octet salt to form the 12-octet IV.
McGrew & Igoe Standards Track [Page 9]
RFC 7714 AES-GCM for SRTP December 20158.2. Data Types in SRTP Packets
All SRTP packets MUST be both authenticated and encrypted. The data fields within the RTP packets are broken into Associated Data, Plaintext, and Raw Data, as follows (see Figure 2):
Associated Data: The version V (2 bits), padding flag P (1 bit),
extension flag X (1 bit), Contributing Source (CSRC) count CC (4 bits), marker M (1 bit), Payload Type PT (7 bits), sequence number (16 bits), timestamp (32 bits), SSRC (32 bits), optional CSRC identifiers (32 bits each), and optional RTP extension (variable length).
Plaintext: The RTP payload (variable length), RTP padding
(if used, variable length), and RTP pad count (if used, 1 octet).
Raw Data: The optional variable-length SRTP Master Key
Identifier (MKI) and SRTP authentication tag (whose use is NOT RECOMMENDED). These fields are appended after encryption has been performed.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A |V=2|P|X| CC |M| PT | sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A | timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A | synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
A | contributing source (CSRC) identifiers (optional) |
A | .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A | RTP extension (OPTIONAL) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
P | payload ... |
P | +-------------------------------+
P | | RTP padding | RTP pad count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
P = Plaintext (to be encrypted and authenticated) A = Associated Data (to be authenticated only)
Figure 2: Structure of an RTP Packet before Authenticated Encryption Since the AEAD ciphertext is larger than the Plaintext by exactly the length of the AEAD authentication tag, the corresponding SRTP-encrypted packet replaces the Plaintext field with a slightly larger field containing the cipher. Even if the Plaintext field is empty, AEAD encryption must still be performed, with the resulting cipher consisting solely of the authentication tag. This tag is to be placed immediately before the optional variable-length SRTP MKI and SRTP authentication tag fields.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A |V=2|P|X| CC |M| PT | sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A | timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A | synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
A | contributing source (CSRC) identifiers (optional) |
A | .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A | RTP extension (OPTIONAL) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
C | cipher |
C | ... |
C | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
R : SRTP MKI (OPTIONAL) :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
R : SRTP authentication tag (NOT RECOMMENDED) :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
C = Ciphertext (encrypted and authenticated) A = Associated Data (authenticated only) R = neither encrypted nor authenticated, added
after Authenticated Encryption completed
Figure 3: Structure of an SRTP Packet after Authenticated Encryption
RTP header extensions were first defined in [RFC3550]. [RFC6904] describes how these header extensions are to be encrypted in SRTP.
When RFC 6904 is in use, a separate keystream is generated to encrypt selected RTP header extension elements. For the AEAD_AES_128_GCM algorithm, this keystream MUST be generated in the manner defined in [RFC6904], using the AES Counter Mode (AES-CM) transform. For the AEAD_AES_256_GCM algorithm, the keystream MUST be generated in the manner defined for the AES_256_CM transform. The originator must perform any required header extension encryption before the AEAD algorithm is invoked.
As with the other fields contained within the RTP header, both encrypted and unencrypted header extensions are to be treated by the AEAD algorithm as Associated Data (AD). Thus, the AEAD algorithm does not provide any additional privacy for the header extensions, but it does provide integrity and authentication.
In order to prevent IV reuse, we must ensure that the (ROC,SEQ,SSRC) triple is never used twice with the same master key. The following two scenarios illustrate this issue:
Counter Management: A rekey MUST be performed to establish a new
master key before the (ROC,SEQ) pair cycles back to its original value. Note that this scenario implicitly assumes that either (1) the outgoing RTP process is trusted to not attempt to repeat a (ROC,SEQ) value or (2) the encryption process ensures that both the SEQ and ROC numbers of the packets presented to it are always incremented in the proper fashion. This is particularly important for GCM, since using the same (ROC,SEQ) value twice compromises the authentication mechanism. For GCM, the (ROC,SEQ) and SSRC values used MUST be generated or checked by either the SRTP implementation or a module (e.g., the RTP application) that can be considered equally trustworthy. While [RFC3711] allows the detection of SSRC collisions after they happen, SRTP using GCM with shared master keys MUST prevent an SSRC collision from happening even once.
SSRC Management: For a given master key, the set of all SSRC
values used with that master key must be partitioned into disjoint pools, one pool for each endpoint using that master key to originate outbound data. Each such originating endpoint MUST only issue SSRC values from the pool it has been assigned. Further, each originating endpoint MUST maintain a history of outbound SSRC identifiers that it has issued within the lifetime of the current master key, and when a new SSRC requests an SSRC identifier it MUST NOT be given an identifier that has been previously issued. A rekey MUST be performed before any of the originating endpoints using that master key exhaust their pools of SSRC values. Further, the identity of the entity giving out SSRC values MUST be verified, and the SSRC signaling MUST be integrity protected.
McGrew & Igoe Standards Track [Page 12]
RFC 7714 AES-GCM for SRTP December 20159. AES-GCM Processing of SRTCP Compound Packets
All SRTCP compound packets MUST be authenticated, but unlike SRTP, SRTCP packet encryption is optional. A sender can select which packets to encrypt and indicates this choice with a 1-bit Encryption flag (located just before the 31-bit SRTCP index).
The 12-octet IV used by AES-GCM SRTCP is formed by first concatenating 2 octets of zeroes, the 4-octet SSRC identifier, 2 octets of zeroes, a single "0" bit, and the 31-bit SRTCP index. The resulting 12-octet value is then XORed to the 12-octet salt to form the 12-octet IV.
RFC 7714 AES-GCM for SRTP December 20159.2. Data Types in Encrypted SRTCP Compound Packets
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A |V=2|P| RC | Packet Type | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A | synchronization source (SSRC) of sender |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
P | sender info :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
P | report block 1 :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
P | report block 2 :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
P | ... :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
P |V=2|P| SC | Packet Type | length |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
P | SSRC/CSRC_1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
P | SDES items :
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
P | ... :
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
A |1| SRTCP index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
R | SRTCP MKI (optional) index :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
R : SRTCP authentication tag (NOT RECOMMENDED) :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
P = Plaintext (to be encrypted and authenticated) A = Associated Data (to be authenticated only) R = neither encrypted nor authenticated, added after
encryption
Figure 5: AEAD SRTCP Inputs When Encryption Flag = 1
(The fields are defined in RFC 3550.)
When the Encryption flag is set to 1, the SRTCP packet is broken into Plaintext, Associated Data, and Raw (untouched) Data (as shown above in Figure 5):
Associated Data: The packet version V (2 bits), padding flag P
(1 bit), reception report count RC (5 bits), Packet Type (8 bits), length (2 octets), SSRC (4 octets), Encryption flag (1 bit), and SRTCP index (31 bits).
Raw Data: The optional variable-length SRTCP MKI and SRTCP
authentication tag (whose use is NOT RECOMMENDED).
Plaintext: All other data.
Note that the Plaintext comes in one contiguous field. Since the AEAD cipher is larger than the Plaintext by exactly the length of the AEAD authentication tag, the corresponding SRTCP-encrypted packet replaces the Plaintext field with a slightly larger field containing the cipher. Even if the Plaintext field is empty, AEAD encryption must still be performed, with the resulting cipher consisting solely of the authentication tag. This tag is to be placed immediately before the Encryption flag and SRTCP index.
McGrew & Igoe Standards Track [Page 15]
RFC 7714 AES-GCM for SRTP December 20159.3. Data Types in Unencrypted SRTCP Compound Packets
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A |V=2|P| RC | Packet Type | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A | synchronization source (SSRC) of sender |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A | sender info :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A | report block 1 :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A | report block 2 :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A | ... :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A |V=2|P| SC | Packet Type | length |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
A | SSRC/CSRC_1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A | SDES items :
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
A | ... :
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
A |0| SRTCP index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
R | SRTCP MKI (optional) index :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
R : authentication tag (NOT RECOMMENDED) :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A = Associated Data (to be authenticated only) R = neither encrypted nor authenticated, added after
encryption
Figure 6: AEAD SRTCP Inputs When Encryption Flag = 0
When the Encryption flag is set to 0, the SRTCP compound packet is broken into Plaintext, Associated Data, and Raw (untouched) Data, as follows (see Figure 6):
Plaintext: None.
Raw Data: The variable-length optional SRTCP MKI and SRTCP authentication tag (whose use is NOT RECOMMENDED).
Associated Data: All other data.
Even though there is no ciphertext in this RTCP packet, AEAD encryption returns a cipher field that is precisely the length of the AEAD authentication tag. This cipher is to be placed before the Encryption flag and the SRTCP index in the authenticated SRTCP packet.
A new master key MUST be established before the 31-bit SRTCP index cycles back to its original value. Ideally, a rekey should be performed and a new master key put in place well before the SRTCP index cycles back to the starting value.
The comments on SSRC management in Section 8.4 also apply.
In general, any AEAD algorithm can accept inputs with varying lengths, but each algorithm can accept only a limited range of lengths for a specific parameter. In this section, we describe the constraints on the parameter lengths that any AEAD algorithm must support to be used in AEAD-SRTP. Additionally, we specify a complete parameter set for one specific family of AEAD algorithms, namely AES-GCM. All AEAD algorithms used with SRTP/SRTCP MUST satisfy the five constraints listed below:
McGrew & Igoe Standards Track [Page 17]
RFC 7714 AES-GCM for SRTP December 2015
Parameter Meaning Value
---------------------------------------------------------------------
A_MAX maximum Associated MUST be at least 12 octets.
Data length
N_MIN minimum nonce (IV) MUST be 12 octets.
length
N_MAX maximum nonce (IV) MUST be 12 octets.
length
P_MAX maximum Plaintext GCM: MUST be <= 2^36 - 32 octets.
length per invocation
C_MAX maximum ciphertext GCM: MUST be <= 2^36 - 16 octets.
length per invocation
For the sake of clarity, we specify three additional parameters:
AEAD authentication tag length MUST be 16 octets
Maximum number of invocations SRTP: MUST be at most 2^48
for a given instantiation SRTCP: MUST be at most 2^31
Block Counter size GCM: MUST be 32 bits
The reader is reminded that the ciphertext is longer than the Plaintext by exactly the length of the AEAD authentication tag.
A Key Derivation Function (KDF) is used to derive all of the required encryption and authentication keys from a secret value shared by the endpoints. The AEAD_AES_128_GCM algorithm MUST use the (128-bit) AES_CM PRF KDF described in [RFC3711]. AEAD_AES_256_GCM MUST use the AES_256_CM_PRF KDF described in [RFC6188].
McGrew & Igoe Standards Track [Page 18]
RFC 7714 AES-GCM for SRTP December 201512. Summary of AES-GCM in SRTP/SRTCP
For convenience, much of the information about the use of the AES-GCM family of algorithms in SRTP is collected in the tables contained in this section.
The AES-GCM family of AEAD algorithms is built around the AES block cipher algorithm. AES-GCM uses AES-CM for encryption and Galois Message Authentication Code (GMAC) for authentication. A detailed description of the AES-GCM family can be found in [RFC5116]. The following members of the AES-GCM family may be used with SRTP/SRTCP:
Any implementation of AES-GCM SRTP MUST support both AEAD_AES_128_GCM and AEAD_AES_256_GCM. Below, we summarize parameters associated with these two GCM algorithms:
+--------------------------------+------------------------------+
| Parameter | Value |
+--------------------------------+------------------------------+
| Master key length | 128 bits |
| Master salt length | 96 bits |
| Key Derivation Function | AES_CM PRF [RFC3711] |
| Maximum key lifetime (SRTP) | 2^48 packets |
| Maximum key lifetime (SRTCP) | 2^31 packets |
| Cipher (for SRTP and SRTCP) | AEAD_AES_128_GCM |
| AEAD authentication tag length | 128 bits |
+--------------------------------+------------------------------+
Table 2: The AEAD_AES_128_GCM Crypto Suite
McGrew & Igoe Standards Track [Page 19]
RFC 7714 AES-GCM for SRTP December 2015
+--------------------------------+------------------------------+
| Parameter | Value |
+--------------------------------+------------------------------+
| Master key length | 256 bits |
| Master salt length | 96 bits |
| Key Derivation Function | AES_256_CM_PRF [RFC6188] |
| Maximum key lifetime (SRTP) | 2^48 packets |
| Maximum key lifetime (SRTCP) | 2^31 packets |
| Cipher (for SRTP and SRTCP) | AEAD_AES_256_GCM |
| AEAD authentication tag length | 128 bits |
+--------------------------------+------------------------------+
Table 3: The AEAD_AES_256_GCM Crypto Suite
13. Security Considerations13.1. Handling of Security-Critical Parameters
As with any security process, the implementer must take care to ensure that cryptographically sensitive parameters are properly handled. Many of these recommendations hold for all SRTP cryptographic algorithms, but we include them here to emphasize their importance.
- If the master salt is to be kept secret, it MUST be properly erased when no longer needed.
- The secret master key and all keys derived from it MUST be kept secret. All keys MUST be properly erased when no longer needed.
- At the start of each packet, the Block Counter MUST be reset to 1. The Block Counter is incremented after each block key has been produced, but it MUST NOT be allowed to exceed 2^32 - 1 for GCM. Note that even though the Block Counter is reset at the start of each packet, IV uniqueness is ensured by the inclusion of SSRC/ROC/SEQ or the SRTCP index in the IV. (The reader is reminded that the first block of key produced is reserved for use in authenticating the packet and is not used to encrypt Plaintext.)
- Each time a rekey occurs, the initial values of both the 31-bit
SRTCP index and the 48-bit SRTP packet index (ROC||SEQ) MUST be
saved in order to prevent IV reuse.
- Processing MUST cease if either the 31-bit SRTCP index or the
48-bit SRTP packet index (ROC||SEQ) cycles back to its initial
value. Processing MUST NOT resume until a new SRTP/SRTCP session
has been established using a new SRTP master key. Ideally, a rekey should be done well before any of these counters cycle.
McGrew & Igoe Standards Track [Page 20]
RFC 7714 AES-GCM for SRTP December 201513.2. Size of the Authentication Tag
We require that the AEAD authentication tag be 16 octets, in order to effectively eliminate the risk of an adversary successfully introducing fraudulent data. Though other protocols may allow the use of truncated authentication tags, the consensus of the authors and the working group is that risks associated with using truncated AES-GCM tags are deemed too high to allow the use of truncated authentication tags in SRTP/SRTCP.
"Session Description Protocol (SDP) Security Descriptions for Media Streams" [RFC4568] defines SRTP "crypto suites". A crypto suite corresponds to a particular AEAD algorithm in SRTP. In order to allow security descriptions to signal the use of the algorithms defined in this document, IANA has registered the following crypto suites in the "SRTP Crypto Suite Registrations" subregistry of the "Session Description Protocol (SDP) Security Descriptions" registry. The ABNF [RFC5234] syntax is as follows:
RFC 7714 AES-GCM for SRTP December 201514.2. DTLS-SRTP
DTLS-SRTP [RFC5764] defines DTLS-SRTP "SRTP protection profiles". These profiles also correspond to the use of an AEAD algorithm in SRTP. In order to allow the use of the algorithms defined in this document in DTLS-SRTP, IANA has registered the following SRTP protection profiles:
Below, we list the SRTP transform parameters for each of these protection profiles. Unless separate parameters for SRTP and SRTCP are explicitly listed, these parameters apply to both SRTP and SRTCP.
auth_tag_length: N/A maximum lifetime: at most 2^31 SRTCP packets and
at most 2^48 SRTP packets
Note that these SRTP protection profiles do not specify an auth_function, auth_key_length, or auth_tag_length, because all of these profiles use AEAD algorithms and thus do not use a separate auth_function, auth_key, or auth_tag. The term "aead_auth_tag_length" is used to emphasize that this refers to the authentication tag provided by the AEAD algorithm and that this tag is not located in the authentication tag field provided by SRTP/SRTCP.
McGrew & Igoe Standards Track [Page 22]
RFC 7714 AES-GCM for SRTP December 201514.3. MIKEY
In accordance with "MIKEY: Multimedia Internet KEYing" [RFC3830], IANA maintains several subregistries under "Multimedia Internet KEYing (MIKEY) Payload Name Spaces". Per this document, additions have been made to two of the MIKEY subregistries.
In the "MIKEY Security Protocol Parameters" subregistry, the following has been added:
Type | Meaning | Possible Values
--------------------------------------------------------
20 | AEAD authentication tag length | 16 octets
This list is, of course, intended for use with GCM. It is conceivable that new AEAD algorithms introduced at some point in the future may require a different set of authentication tag lengths.
In the "Encryption algorithm (Value 0)" subregistry (derived from Table 6.10.1.b of [RFC3830]), the following has been added:
The encryption algorithm, session encryption key length, and AEAD authentication tag sizes received from MIKEY fully determine the AEAD algorithm to be used. The exact mapping is described in Section 15.
MIKEY specifies the algorithm family separately from the key length (which is specified by the Session Encryption key length) and the authentication tag length (specified by the AEAD authentication tag length).
Section 11 of this document restricts the choice of KDF for AEAD algorithms. To enforce this restriction in MIKEY, we require that the SRTP Pseudorandom Function (PRF) has value AES-CM whenever an AEAD algorithm is used. Note that, according to Section 6.10.1 of [RFC3830], the input key length of the KDF (i.e., the SRTP master key length) is always equal to the session encryption key length. This means, for example, that AEAD_AES_256_GCM will use AES_256_CM_PRF as the KDF.
consisting of a 12-octet header (8040f17b 8041f8d3 5501a0b2) and a 38-octet payload (47616c6c 69612065 7374206f 6d6e6973 20646976 69736120 696e2070 61727465 73207472 6573), which is just the ASCII string "Gallia est omnis divisa in partes tres". The salt used (51756964 2070726f 2071756f) comes from the ASCII string "Quid pro quo". The 16-octet (128-bit) key is 00 01 02 ... 0f, and the 32-octet (256-bit) key is 00 01 02 ... 1f. At the time this document was written, the RTP payload type (1000000 binary = 64 decimal) was an unassigned value.
As shown in Section 8.1, the IV is formed by XORing two 12-octet values. The first 12-octet value is formed by concatenating two zero octets, the 4-octet SSRC (found in the ninth through 12th octets of the packet), the 4-octet rollover counter (ROC) maintained at each end of the link, and the 2-octet sequence number (SEQ) (found in the third and fourth octets of the packet). The second 12-octet value is the salt, a value that is held constant at least until the key is changed.
Process the length word
length word: 00000000000001900000000000000000
partial hash: b04200c26b81c98af55cc2eafccd1cbc
Turn GHASH into GMAC
GHASH: b0 42 00 c2 6b 81 c9 8a f5 5c c2 ea fc cd 1c bc
K0: 92 0b 3f 40 b9 3d 2a 1d 1c 8b 5c d1 e5 67 5e aa
full GMAC: 22 49 3f 82 d2 bc e3 97 e9 d7 9e 3b 19 aa 42 16
Cipher with tag
22493f82 d2bce397 e9d79e3b 19aa4216
Turn GHASH into GMAC
GHASH: b0 42 00 c2 6b 81 c9 8a f5 5c c2 ea fc cd 1c bc
K0: 92 0b 3f 40 b9 3d 2a 1d 1c 8b 5c d1 e5 67 5e aa
full GMAC: 22 49 3f 82 d2 bc e3 97 e9 d7 9e 3b 19 aa 42 16
Received tag = 22493f82 d2bce397 e9d79e3b 19aa4216 Computed tag = 22493f82 d2bce397 e9d79e3b 19aa4216
Received tag verified.
As shown in Section 9.1, the IV is formed by XORing two 12-octet values. The first 12-octet value is formed by concatenating two zero octets, the 4-octet SSRC (found in the fifth through eighth octets of the RTP packet), another two padding octets, and the 31-bit SRTCP index, right-justified in a 32-bit = 4-octet field with a single "0" bit prepended as padding. An example of SRTCP IV formation is shown below:
In an SRTCP packet, a 1-bit Encryption flag is prepended to the 31-bit SRTCP index to form a 32-bit value we shall call the "ESRTCP word". The E-flag is one if the SRTCP packet has been encrypted and zero if it has been tagged but not encrypted. Note that the ESRTCP field is only present in an SRTCP packet, not in an RTCP packet. The full ESRTCP word is part of the AAD. When encrypting and tagging an RTCP packet (E-flag = 1), the SRTCP packet consists of the following fields in the following order:
- The first 8 octets of the RTCP packet (part of the AAD).
- The cipher.
- The ESRTCP word (the final part of the AAD).
- Any Raw Data that might have been appended to the end of the original RTCP packet.
Recall that AEAD treats the authentication tag as an integral part of the cipher, and in fact the authentication tag is the last 8 or 16 octets of the cipher.
The reader is reminded that when the RTCP packet is to be tagged but not encrypted (E-flag = 0), GCM will produce a cipher that consists solely of the 8-octet or 16-octet authentication tag. The tagged SRTCP consists of the following fields in the order listed below:
- All of the AAD, except for the ESRTCP word.
- The cipher (= the authentication tag).
- The ESRTCP word (the final part of the AAD).
- Any Raw Data that might have been appended to the end of the original RTCP packet.
McGrew & Igoe Standards Track [Page 38]
RFC 7714 AES-GCM for SRTP December 201517.1. SRTCP AEAD_AES_128_GCM Encryption and Tagging
Process the length word
length word: 00000000000000600000000000000160
partial hash: 7296107c9716534371dfc1a30c5ffeb5
Turn GHASH into GMAC
GHASH: 72 96 10 7c 97 16 53 43 71 df c1 a3 0c 5f fe b5
K0: ba dc b4 24 01 d9 1e 6c b4 74 39 d1 49 86 14 6b
full GMAC: c8 4a a4 58 96 cf 4d 2f c5 ab f8 72 45 d9 ea de
Cipher with tag
63e94885 dcdab67c a727d766 2f6b7e99 7ff5c0f7 6c06f32d c676a5f1 730d6fda 4ce09b46 86303ded 0bb9275b c84aa458 96cf4d2f c5abf872 45d9eade
Append the ESRTCP word with the E-flag set
63e94885 dcdab67c a727d766 2f6b7e99 7ff5c0f7 6c06f32d c676a5f1 730d6fda 4ce09b46 86303ded 0bb9275b c84aa458 96cf4d2f c5abf872 45d9eade 800005d4
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550, July 2003, <http://www.rfc-editor.org/info/rfc3550>.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC 3711, DOI 10.17487/RFC3711, March 2004, <http://www.rfc-editor.org/info/rfc3711>.
[RFC4568] Andreasen, F., Baugher, M., and D. Wing, "Session
Description Protocol (SDP) Security Descriptions for Media Streams", RFC 4568, DOI 10.17487/RFC4568, July 2006, <http://www.rfc-editor.org/info/rfc4568>.
[RFC5764] McGrew, D. and E. Rescorla, "Datagram Transport Layer
Security (DTLS) Extension to Establish Keys for the Secure Real-time Transport Protocol (SRTP)", RFC 5764, DOI 10.17487/RFC5764, May 2010, <http://www.rfc-editor.org/info/rfc5764>.
RFC 7714 AES-GCM for SRTP December 201518.2. Informative References
[BN00] Bellare, M. and C. Namprempre, "Authenticated Encryption:
Relations among notions and analysis of the generic composition paradigm", Proceedings of ASIACRYPT 2000, Springer-Verlag, LNCS 1976, pp. 531-545, DOI 10.1007/3-540-44448-3_41, <http://www-cse.ucsd.edu/users/mihir/papers/oem.html>.
[GCM] Dworkin, M., "NIST Special Publication 800-38D:
Recommendation for Block Cipher Modes of Operation: Galois/Counter Mode (GCM) and GMAC", U.S. National Institute of Standards and Technology, November 2007, <http://csrc.nist.gov/publications/nistpubs/800-38D/SP-800-38D.pdf>.
[R02] Rogaway, P., "Authenticated-Encryption with Associated-
Data", ACM Conference on Computer and Communications Security (CCS'02), pp. 98-107, ACM Press, DOI 10.1145/586110.586125, September 2002, <http://www.cs.ucdavis.edu/~rogaway/papers/ad.html>.
[RFC4771] Lehtovirta, V., Naslund, M., and K. Norrman, "Integrity
Transform Carrying Roll-Over Counter for the Secure Real-time Transport Protocol (SRTP)", RFC 4771, DOI 10.17487/RFC4771, January 2007, <http://www.rfc-editor.org/info/rfc4771>.
The authors would like to thank Michael Peck, Michael Torla, Qin Wu, Magnus Westerlund, Oscar Ohllson, Woo-Hwan Kim, John Mattsson, Richard Barnes, Morris Dworkin, Stephen Farrell, and many other reviewers who provided valuable comments on earlier draft versions of this document.
Authors' Addresses
David A. McGrew Cisco Systems, Inc. 510 McCarthy Blvd. Milpitas, CA 95035 United States Phone: (408) 525 8651
Kevin M. Igoe NSA/CSS Commercial Solutions Center National Security Agency
Email: mythicalkevin@yahoo.com
McGrew & Igoe Standards Track [Page 48]
Ad
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