RFC 7714
AES-GCM Authenticated Encryption in the Secure Real-time Transport Protocol (SRTP)
Part 1 of 9: RFC 7714 💖🎀
okay bestie, let's get into it ✨ This RFC is called AES-GCM Authenticated Encryption in the Secure Real-time Transport Protocol (SRTP) 🦋 and it's giving "how do we lock down our live audio and video so nobody can peek or mess with it" 👑
The authors are D. McGrew from Cisco Systems, Inc. 🌸 and K. Igoe from the National Security Agency 🫶 It's a Standards Track document, which means it's an official Internet standard, not just a vibe 💅 It was published in December 2015 🎀 and it's part of the IETF's stuff, with ISSN 2070-1721 🧚 It doesn't obsolete or update any other RFCs in its title block, so it's standing on its own, iykyk 💜
Here's the table of contents so you can see the whole shape of it 🌈
1. Introduction ....................................................3
2. Conventions Used in This Document ...............................4
3. Overview of the SRTP/SRTCP AEAD Security Architecture ...........4
4. Terminology .....................................................5
5. Generic AEAD Processing .........................................6
5.1. Types of Input Data ........................................6
5.2. AEAD Invocation Inputs and Outputs .........................6
5.2.1. Encrypt Mode ........................................6
5.2.2. Decrypt Mode ........................................7
5.3. Handling of AEAD Authentication ............................7
6. Counter Mode Encryption .........................................7
7. Unneeded SRTP/SRTCP Fields ......................................8
7.1. SRTP/SRTCP Authentication Tag Field ........................8
7.2. RTP Padding ................................................9
8. AES-GCM Processing for SRTP .....................................9
8.1. SRTP IV Formation for AES-GCM ..............................9
8.2. Data Types in SRTP Packets ................................10
8.3. Handling Header Extensions ................................11
8.4. Prevention of SRTP IV Reuse ...............................12
9. AES-GCM Processing of SRTCP Compound Packets ...................13
9.1. SRTCP IV Formation for AES-GCM ............................13
9.2. Data Types in Encrypted SRTCP Compound Packets ............14
9.3. Data Types in Unencrypted SRTCP Compound Packets ..........16
9.4. Prevention of SRTCP IV Reuse ..............................17
10. Constraints on AEAD for SRTP and SRTCP ........................17
11. Key Derivation Functions ......................................18
12. Summary of AES-GCM in SRTP/SRTCP ..............................19
13. Security Considerations .......................................20
13.1. Handling of Security-Critical Parameters .................20
13.2. Size of the Authentication Tag ...........................21
14. IANA Considerations ...........................................21
14.1. SDES .....................................................21
14.2. DTLS-SRTP ................................................22
14.3. MIKEY ....................................................23
15. Parameters for Use with MIKEY .................................23
16. Some RTP Test Vectors .........................................24
16.1. SRTP AEAD_AES_128_GCM ....................................25
16.1.1. SRTP AEAD_AES_128_GCM Encryption ..................25
16.1.2. SRTP AEAD_AES_128_GCM Decryption ..................27
16.1.3. SRTP AEAD_AES_128_GCM Authentication Tagging ......29
16.1.4. SRTP AEAD_AES_128_GCM Tag Verification ............30
16.2. SRTP AEAD_AES_256_GCM ....................................31
16.2.1. SRTP AEAD_AES_256_GCM Encryption ..................31
16.2.2. SRTP AEAD_AES_256_GCM Decryption ..................33
16.2.3. SRTP AEAD_AES_256_GCM Authentication Tagging ......35
16.2.4. SRTP AEAD_AES_256_GCM Tag Verification ............36
17. RTCP Test Vectors ...........................................37
17.1. SRTCP AEAD_AES_128_GCM Encryption and Tagging ............39
17.2. SRTCP AEAD_AES_256_GCM Verification and Decryption .......41
17.3. SRTCP AEAD_AES_128_GCM Tagging Only ......................43
17.4. SRTCP AEAD_AES_256_GCM Tag Verification ..................44
18. References ....................................................45
18.1. Normative References .....................................45
18.2. Informative References ...................................47
Acknowledgements ..................................................48
Authors' Addresses ................................................481. Introduction 🌸
Okay so SRTP [RFC3711] is basically a profile of RTP [RFC3550] 🎧 It can give you confidentiality, message authentication, and replay protection for RTP traffic, and also for RTCP, which is the control traffic for RTP 🔐 Also, fun fact: the outgoing SRTP packets from one endpoint might be coming from several independent data sources at once 🦋
Now, Authenticated Encryption [BN00] is a form of encryption that not only hides the Plaintext it encrypts, but also gives you a way to check that it's intact and really from who it says it's from 💅 Then there's AEAD [R02], which stands for Authenticated Encryption with Associated Data 💜 It adds the ability to check the integrity and authenticity of some Associated Data (AD), also called "Additional Authenticated Data" (AAD), that doesn't get encrypted 🫶 This spec uses the generic AEAD interface defined in [RFC5116] ✨
The Advanced Encryption Standard (AES) is a block cipher 🔥 It's super secure and accepts different key sizes 👑 AES Galois/Counter Mode (AES-GCM) [GCM] is a family of AEAD algorithms built on AES 🌈 This spec uses the AES versions with 128-bit and 256-bit keys, which we call "AES-128" and "AES-256", respectively 🎀
Any AEAD algorithm gives you an intrinsic authentication tag 🧚 In lots of apps, that tag gets chopped down to less than full length, but not here!! 🙈 In this spec, the authentication tag MUST NOT be truncated. The authentication tags MUST be a full 16 octets in length. 😭 When it's used in SRTP/SRTCP, AES-GCM comes in 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 tagThe key size gets set when the session starts, and it SHOULD NOT be altered. 💅
Okay, the Galois/Counter Mode (GCM) is an AEAD mode of operation for block ciphers ✨ GCM uses Counter Mode to encrypt the data, and that part can be efficiently pipelined 🌸 The GCM authentication uses operations that are really well suited to efficient hardware implementation, so it's especially appealing for high-speed stuff or for tiny, efficient circuits 🔥
Tl;dr: this document explains how to use an AEAD algorithm, especially AES-GCM, to get confidentiality and message authentication inside SRTP and SRTCP packets 💖
2. Conventions Used in This Document 🎀
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 built on these principles: 👑
- a) Both privacy and authentication rely on symmetric algorithms 🔐 An AEAD algorithm like AES-GCM squishes privacy and authentication into one single process 💅
- b) A secret master key is shared by all the participating endpoints, both the ones originating SRTP/SRTCP packets and the ones receiving them 🫶 Any given master key MAY be used at the same time by several endpoints to originate SRTP/SRTCP packets, as well as by one or more endpoints using that master key to process inbound data ✨
- c) A Key Derivation Function (KDF) gets applied to the shared master key value to make separate encryption keys, authentication keys, and salting keys for SRTP and for SRTCP, six keys total 🌈 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 like AES-GCM combine encryption and authentication into one process, they do not use separate authentication keys 🌸
- d) Other than making IANA registry changes so AES-GCM works 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 gets established and shared between participants are outside the scope of this doc 🧚 Similarly, any mechanism for rekeying an existing session is also outside the scope 🙈
- e) Every time an instance 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) 💖
That's the end of Part 1 of 9, bestie 🫶 Keep an eye out for Part 2 🦋✨
4. Terminology ✨
okay so these terms have very specific meanings in this RFC, so pay attention bestie 🎀
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. 💅
5. Generic AEAD Processing 🌸
5.1. 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 get treated as Associated Data, which get treated as Plaintext, and which get treated as Raw Data are covered in Sections 8.2, 9.2, and 9.3. It's giving "read the next sections" energy 📖✨
5.2. AEAD Invocation Inputs and Outputs
5.2.1. 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
Outputs:
Ciphertext* Octet string, length =
length(Plaintext) + tag_length(*): In AEAD, the authentication tag is embedded in the ciphertext. When GCM is being used, the ciphertext consists of the encrypted Plaintext followed by the authentication tag. 🎀
5.2.2. 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
Outputs:
Plaintext Octet string, length =
length(Ciphertext) - tag_length
Validity_Flag Boolean, TRUE if valid,
FALSE otherwise5.3. Handling of AEAD Authentication
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. No peeking, no shortcuts, periodt 🙅♀️💖
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. 🚩
6. Counter Mode Encryption 🦋
Each outbound packet uses a 12-octet IV and an encryption key to form two outputs:
- a 16-octet first_key_block, which is used in forming the authentication tag, and
- 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: 🧁
def GCM_keystream( Plaintext_len, IV, Encryption_key ):
assert Plaintext_len <= (2**36) - 32 ## measured in octets
key_stream = ""
block_counter = 1
first_key_block = AES_ENC( data=IV||block_counter,
key=Encryption_key )
while len(key_stream) < Plaintext_len:
block_counter = block_counter + 1
key_block = AES_ENC( data=IV||block_counter,
key=Encryption_key )
key_stream = key_stream||key_block
key_stream = truncate( key_stream, Plaintext_len )
return( first_key_block, key_stream )In theory, this keystream generation process allows for the encryption of up to (2^36) - 32 octets per invocation (i.e., per packet), which is way more 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. Bestie, do not reuse that pair 😭🙅♀️
7. Unneeded SRTP/SRTCP Fields 🧚
AEAD Counter Mode encryption removes the need for certain existing SRTP/SRTCP mechanisms. Some old friends get retired here 🫶
7.1. SRTP/SRTCP Authentication Tag Field
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. 🎀
7.2. 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 SRTP 💜
8.1. SRTP IV Formation for AES-GCM
0 0 0 0 0 0 0 0 0 0 1 1
0 1 2 3 4 5 6 7 8 9 0 1
+--+--+--+--+--+--+--+--+--+--+--+--+
|00|00| SSRC | ROC | SEQ |---+
+--+--+--+--+--+--+--+--+--+--+--+--+ |
|
+--+--+--+--+--+--+--+--+--+--+--+--+ |
| Encryption Salt |->(+)
+--+--+--+--+--+--+--+--+--+--+--+--+ |
|
+--+--+--+--+--+--+--+--+--+--+--+--+ |
| Initialization Vector |<--+
+--+--+--+--+--+--+--+--+--+--+--+--+
Figure 1: AES-GCM SRTP Initialization Vector FormationThe 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. ✨🦋
8.2. Data Types in SRTP Packets 🎀
Okay bestie, this part is where we sort every field in an RTP packet into one of three bins 🗂️. Every SRTP packet MUST be both authenticated and encrypted 🔐, no exceptions, periodt. The fields get split into Associated Data, Plaintext, and Raw Data like this (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). These are authenticated only, never encrypted.
- Plaintext 🤫: the RTP payload (variable length), RTP padding (if used, variable length), and RTP pad count (if used, 1 octet). These get encrypted and authenticated.
- Raw Data 📦: the optional variable-length SRTP Master Key Identifier (MKI) and SRTP authentication tag (whose use is NOT RECOMMENDED). These get tacked on after encryption is done.
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 SRTP-encrypted packet swaps the Plaintext field for a slightly bigger chunk holding the cipher ✨. Even when the Plaintext field is empty, AEAD encryption must still be performed, and the result is just the authentication tag 🫶. That tag goes immediately before the optional variable-length SRTP MKI and SRTP authentication tag fields.
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) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 completedFigure 3: Structure of an SRTP Packet after Authenticated Encryption
8.3. Handling Header Extensions 💅
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 gets generated to encrypt selected RTP header extension elements 🔑. For the AEAD_AES_128_GCM algorithm, this keystream MUST be generated the way [RFC6904] describes, using the AES Counter Mode (AES-CM) transform. For the AEAD_AES_256_GCM algorithm, the keystream MUST be generated the way AES_256_CM defines it. The originator must do any required header extension encryption before the AEAD algorithm gets invoked.
Here's the vibe: header extensions, encrypted or not, get treated as Associated Data (AD) by the AEAD algorithm, same as the other RTP header fields 🧚. So the AEAD algorithm doesn't add any extra privacy for header extensions, but it does give them integrity and authentication 🔒.
8.4. Prevention of SRTP IV Reuse 🙈
Okay this one is SERIOUS. To prevent IV reuse, we must ensure the (ROC,SEQ,SSRC) triple is never used twice with the same master key 😭. The following two scenarios show what goes wrong:
- Counter Management: A rekey MUST be performed to establish a new master key before the (ROC,SEQ) pair cycles back to its original value. This scenario quietly assumes either (1) the outgoing RTP process is trusted to never repeat a (ROC,SEQ) value, or (2) the encryption process makes sure that both the SEQ and ROC numbers of the packets handed to it are always incremented properly. This matters a lot for GCM, because using the same (ROC,SEQ) value twice wrecks 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 (for example, the RTP application) that can be considered equally trustworthy. [RFC3711] lets you detect SSRC collisions after they happen, but SRTP using GCM with shared master keys MUST prevent an SSRC collision from happening even once 🚫.
- SSRC Management: For a given master key, all the SSRC values used with that master key must be split into disjoint pools, one pool for each endpoint that uses that master key to send outbound data 🍓. Each originating endpoint MUST only issue SSRC values from the pool it was assigned. Further, each originating endpoint MUST keep a history of the outbound SSRC identifiers it has issued during the lifetime of the current master key, and when a new SSRC asks for an identifier, it MUST NOT be given one that was already issued before. A rekey MUST be performed before any of the originating endpoints using that master key run out of their SSRC pools ⏰. Further, the identity of the entity handing out SSRC values MUST be verified, and the SSRC signaling MUST be integrity protected 👑.
9. 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 pick which packets to encrypt and signals that choice with a 1-bit Encryption flag, sitting just before the 31-bit SRTCP index.
9.1. SRTCP IV Formation for AES-GCM ✨
The 12-octet IV used by AES-GCM SRTCP gets built by first concatenating these pieces, in order:
- 2 octets of zeroes
- the 4-octet SSRC identifier
- 2 octets of zeroes
- a single "0" bit
- the 31-bit SRTCP index
Then the resulting 12-octet value gets XORed with the 12-octet salt, and that gives you the 12-octet IV 🧁.
0 1 2 3 4 5 6 7 8 9 10 11
+--+--+--+--+--+--+--+--+--+--+--+--+
|00|00| SSRC |00|00|0+SRTCP Idx|---+
+--+--+--+--+--+--+--+--+--+--+--+--+ |
|
+--+--+--+--+--+--+--+--+--+--+--+--+ |
| Encryption Salt |->(+)
+--+--+--+--+--+--+--+--+--+--+--+--+ |
|
+--+--+--+--+--+--+--+--+--+--+--+--+ |
| Initialization Vector |<--+
+--+--+--+--+--+--+--+--+--+--+--+--+Figure 4: SRTCP Initialization Vector Formation
9.2. Data Types in Encrypted SRTCP Compound Packets 🎀
okay so here's the encrypted version 💖 When the Encryption flag is set to 1, the SRTCP packet gets split into three bits: Plaintext, Associated Data, and Raw (untouched) Data (check the picture below 👀):
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.)So here's the breakdown ✨
- 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 🧚 The AEAD cipher is bigger than the Plaintext by exactly the length of the AEAD authentication tag, so the encrypted packet swaps the Plaintext field for a slightly bigger field holding the cipher. Even if the Plaintext field is empty, AEAD encryption must still be performed, and the resulting cipher is made up of nothing but the authentication tag 🥹 That tag goes right before the Encryption flag and SRTCP index.
9.3. Data Types in Unencrypted SRTCP Compound Packets 🌸
now the unencrypted version, where the Encryption flag is set to 0 🦋
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 = 0When the Encryption flag is set to 0, the SRTCP compound packet is split into Plaintext, Associated Data, and Raw (untouched) Data, like this (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's no ciphertext in this one, AEAD encryption still hands back a cipher field that's exactly the length of the AEAD authentication tag 💅 That cipher goes before the Encryption flag and the SRTCP index in the authenticated SRTCP packet.
9.4. Prevention of SRTCP IV Reuse 🔐
okay this one is SO important 😭 A new master key MUST be established before the 31-bit SRTCP index cycles back to its original value. Ideally, you'd do a rekey and get a fresh master key in place well before the SRTCP index wraps around to the starting value 💜
The comments on SSRC management in Section 8.4 also apply.
10. Constraints on AEAD for SRTP and SRTCP 👑
in general, any AEAD algorithm can take inputs of different lengths, but each algorithm can only handle a limited range of lengths for a given parameter 🫶 In this section, we go over the constraints on those parameter lengths that any AEAD algorithm has to support to be used in AEAD-SRTP. We also spell out a complete parameter set for one specific family of AEAD algorithms, AES-GCM.
All AEAD algorithms used with SRTP/SRTCP MUST satisfy the five constraints listed below:
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 invocationFor the sake of clarity, we also 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 bitsReminder, the ciphertext is longer than the Plaintext by exactly the length of the AEAD authentication tag 🎀
11. Key Derivation Functions 🗝️
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]. 💖
12. Summary of AES-GCM in SRTP/SRTCP 🍓
For convenience, most of the info about using the AES-GCM family in SRTP lives in the tables in this section, so bookmark it bestie 📌
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. You can find a detailed description of the AES-GCM family in [RFC5116]. The following members of the AES-GCM family may be used with SRTP/SRTCP:
Name Key Size AEAD Tag Size Reference
================================================================
AEAD_AES_128_GCM 16 octets 16 octets [RFC5116]
AEAD_AES_256_GCM 32 octets 16 octets [RFC5116]
Table 1: AES-GCM Algorithms for SRTP/SRTCPAny implementation of AES-GCM SRTP MUST support both AEAD_AES_128_GCM and AEAD_AES_256_GCM. Below, we sum up the parameters for 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 +--------------------------------+------------------------------+
| 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 Suite13. Security Considerations 🛡️
13.1. Handling of Security-Critical Parameters ✨
okay bestie, this is the serious part 🔒 security stuff needs the implementer to be extra careful with the cryptographically sensitive parameters 🫶 a lot of this applies to every SRTP crypto algorithm, but we're repeating it here bc it's THAT important 💀
- 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 goes up after each block key is produced, but it MUST NOT be allowed to exceed 2^32 - 1 for GCM. Even though the Block Counter resets at the start of every packet, IV uniqueness is still handled by including SSRC/ROC/SEQ or the SRTCP index in the IV. 🔢 (Quick reminder: the first block of key produced is reserved for authenticating the packet and is NOT used to encrypt Plaintext 🙈)
- Each time a rekey happens, the initial values of both the 31-bit SRTCP index and the 48-bit SRTP packet index (ROC||SEQ) MUST be saved 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. ⏳💀
13.2. Size of the Authentication Tag 🏷️
we require the AEAD authentication tag to be 16 octets, bc that effectively wipes out the risk of an attacker sneaking in fraudulent data 🚫 other protocols might allow truncated authentication tags, but the authors and the working group agreed the risks of truncated AES-GCM tags are way too high for SRTP/SRTCP, so truncated tags are a no-go here 🙅♀️
14. IANA Considerations 📋
14.1. SDES
"Session Description Protocol (SDP) Security Descriptions for Media Streams" [RFC4568] defines SRTP "crypto suites" 🎀 a crypto suite lines up with a particular AEAD algorithm in SRTP. So security descriptions can signal the algorithms defined in this document, IANA 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 looks like this:
srtp-crypto-suite-ext = "AEAD_AES_128_GCM" /
"AEAD_AES_256_GCM" /
srtp-crypto-suite-ext14.2. DTLS-SRTP
DTLS-SRTP [RFC5764] defines DTLS-SRTP "SRTP protection profiles" 🧁 these profiles also line up with using an AEAD algorithm in SRTP. To use the algorithms from this document in DTLS-SRTP, IANA registered these SRTP protection profiles:
SRTP_AEAD_AES_128_GCM = {0x00, 0x07}
SRTP_AEAD_AES_256_GCM = {0x00, 0x08}Here are the SRTP transform parameters for each profile 👀 unless SRTP and SRTCP get separate parameters that are explicitly listed, these apply to both SRTP and SRTCP.
SRTP_AEAD_AES_128_GCM
cipher: AES_128_GCM
cipher_key_length: 128 bits
cipher_salt_length: 96 bits
aead_auth_tag_length: 16 octets
auth_function: NULL
auth_key_length: N/A
auth_tag_length: N/A
maximum lifetime: at most 2^31 SRTCP packets and
at most 2^48 SRTP packets
SRTP_AEAD_AES_256_GCM
cipher: AES_256_GCM
cipher_key_length: 256 bits
cipher_salt_length: 96 bits
aead_auth_tag_length: 16 octets
auth_function: NULL
auth_key_length: N/A
auth_tag_length: N/A
maximum lifetime: at most 2^31 SRTCP packets and
at most 2^48 SRTP packetsthese profiles don't specify an auth_function, auth_key_length, or auth_tag_length 🙃 that's because they all use AEAD algorithms, so there's no separate auth_function, auth_key, or auth_tag. The term aead_auth_tag_length is there to make it crystal clear that it refers to the authentication tag the AEAD algorithm gives you, and that this tag is NOT in the authentication tag field provided by SRTP/SRTCP 💅
14.3. MIKEY
okay so following "MIKEY: Multimedia Internet KEYing" [RFC3830], IANA maintains several subregistries under "Multimedia Internet KEYing (MIKEY) Payload Name Spaces" 🌸 this document adds stuff to two of the MIKEY subregistries.
In the "MIKEY Security Protocol Parameters" subregistry, this got added:
Type | Meaning | Possible Values
--------------------------------------------------------
20 | AEAD authentication tag length | 16 octetsthis list is, of course, meant for use with GCM 🎯 it's conceivable that new AEAD algorithms introduced later might need a different set of authentication tag lengths 🔮
In the "Encryption algorithm (Value 0)" subregistry (derived from Table 6.10.1.b of [RFC3830]), this got added:
SRTP Encr. | Value | Default Session | Default Auth.
Algorithm | | Encr. Key Length | Tag Length
-----------------------------------------------------------
AES-GCM | 6 | 16 octets | 16 octetsthe encryption algorithm, the session encryption key length, and the AEAD authentication tag sizes received from MIKEY fully determine which AEAD algorithm gets used 🧩 the exact mapping is described in Section 15.
15. Parameters for Use with MIKEY 🎀
MIKEY sets the algorithm family separately from the key length (which is set by the Session Encryption key length) and the authentication tag length (set by the AEAD authentication tag length) 💁♀️
+------------+-------------+-------------+
| Encryption | Encryption | AEAD Auth. |
| Algorithm | Key Length | Tag Length |
+============+=============+=============+
AEAD_AES_128_GCM | AES-GCM | 16 octets | 16 octets |
+------------+-------------+-------------+
AEAD_AES_256_GCM | AES-GCM | 32 octets | 16 octets |
+============+=============+=============+
Table 4: Mapping MIKEY Parameters to AEAD AlgorithmsSection 11 of this document restricts which KDF you can pick for AEAD algorithms 🔐 to enforce that in MIKEY, we require the SRTP Pseudorandom Function (PRF) to have the value AES-CM whenever an AEAD algorithm is used. Also, according to Section 6.10.1 of [RFC3830], the input key length of the KDF (aka the SRTP master key length) is always equal to the session encryption key length. So for example, AEAD_AES_256_GCM will use AES_256_CM_PRF as the KDF 🧠
16. Some RTP Test Vectors 🧪
the examples in this section are all built on the same RTP packet 📦
8040f17b 8041f8d3 5501a0b2 47616c6c
69612065 7374206f 6d6e6973 20646976
69736120 696e2070 61727465 73207472
6573it's made 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 (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 🔑 when 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) kept 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, which stays constant at least until the key changes ✨
| Pad | SSRC | ROC | SEQ |
00 00 55 01 a0 b2 00 00 00 00 f1 7b
salt 51 75 69 64 20 70 72 6f 20 71 75 6f
------------------------------------
IV 51 75 3c 65 80 c2 72 6f 20 71 84 14every single RTP example here uses this IV 💖
16.1. SRTP AEAD_AES_128_GCM ✨
16.1.1. SRTP AEAD_AES_128_GCM Encryption ✨
okay bestie, let's go! 🎀 first up we encrypt this packet 💖
8040f17b 8041f8d3 5501a0b2 47616c6c
69612065 7374206f 6d6e6973 20646976
69736120 696e2070 61727465 73207472
6573step one: build the IV 🧚 it's made from the salt plus the SSRC, ROC and SEQ fields, and it's giving very organized energy 💅
| Pad | SSRC | ROC | SEQ |
00 00 55 01 a0 b2 00 00 00 00 f1 7b
salt: 51 75 69 64 20 70 72 6f 20 71 75 6f
IV: 51 75 3c 65 80 c2 72 6f 20 71 84 14next, the key, the AAD, the plaintext and the hash subkey H 🔑
Key: 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f
AAD: 8040f17b 8041f8d3 5501a0b2
PT: 47616c6c 69612065 7374206f 6d6e6973
20646976 69736120 696e2070 61727465
73207472 6573
IV: 51 75 3c 65 80 c2 72 6f 20 71 84 14
H: c6a13b37878f5b826f4f8162a1c8d879now we encrypt the plaintext block by block 🦋 each block gets its own counter and key block, and then we XOR it with the plaintext to get the ciphertext
block # 0
IV||blk_cntr: 51753c6580c2726f2071841400000002
key_block: b5 2c 8f cf 92 55 fe 09 df ce a6 73 f0 10 22 b9
plain_block: 47 61 6c 6c 69 61 20 65 73 74 20 6f 6d 6e 69 73
cipher_block: f2 4d e3 a3 fb 34 de 6c ac ba 86 1c 9d 7e 4b ca
block # 1
IV||blk_cntr: 51753c6580c2726f2071841400000003
key_block: 9e 07 52 a3 64 5a 2f 4f 2b cb d4 0a 30 b5 a5 fe
plain_block: 20 64 69 76 69 73 61 20 69 6e 20 70 61 72 74 65
cipher_block: be 63 3b d5 0d 29 4e 6f 42 a5 f4 7a 51 c7 d1 9b
block # 2
IV||blk_cntr: 51753c6580c2726f2071841400000004
key_block: 45 fe 4e ad ed 40 0a 5d 1a f3 63 f9 0c e1 49 3b
plain_block: 73 20 74 72 65 73
cipher_block: 36 de 3a df 88 33here's the cipher before the tag gets appended 🍓
f24de3a3 fb34de6c acba861c 9d7e4bca
be633bd5 0d294e6f 42a5f47a 51c7d19b
36de3adf 8833now the GMAC tag 😭 we hash the AAD first, then the ciphertext, then the length word
Process the AAD
AAD word: 8040f17b8041f8d35501a0b200000000
partial hash: bcfb3d1d0e6e3e78ba45403377dba11b
Process the cipher
cipher word: f24de3a3fb34de6cacba861c9d7e4bca
partial hash: 0ebc0abe1b15b32fedd2b07888c1ef61
cipher word: be633bd50d294e6f42a5f47a51c7d19b
partial hash: 438e5797011ea860585709a2899f4685
cipher word: 36de3adf883300000000000000000000
partial hash: 336fb643310d7bac2aeaa76247f6036d
Process the length word
length word: 00000000000000600000000000000130
partial hash: 1b964067078c408c4e442a8f015e5264then we turn GHASH into GMAC by XORing it with K0 👑
GHASH: 1b 96 40 67 07 8c 40 8c 4e 44 2a 8f 01 5e 52 64
K0: 92 0b 3f 40 b9 3d 2a 1d 1c 8b 5c d1 e5 67 5e aa
full GMAC: 89 9d 7f 27 be b1 6a 91 52 cf 76 5e e4 39 0c ceand the cipher with the tag stuck on the end 🌈
f24de3a3 fb34de6c acba861c 9d7e4bca
be633bd5 0d294e6f 42a5f47a 51c7d19b
36de3adf 8833899d 7f27beb1 6a9152cf
765ee439 0ccethe final encrypted and tagged packet, slay 💅✨
8040f17b 8041f8d3 5501a0b2 f24de3a3
fb34de6c acba861c 9d7e4bca be633bd5
0d294e6f 42a5f47a 51c7d19b 36de3adf
8833899d 7f27beb1 6a9152cf 765ee439
0cce16.1.2. SRTP AEAD_AES_128_GCM Decryption 🦋
now the reverse: we decrypt that same packet 🔄
8040f17b 8041f8d3 5501a0b2 f24de3a3
fb34de6c acba861c 9d7e4bca be633bd5
0d294e6f 42a5f47a 51c7d19b 36de3adf
8833899d 7f27beb1 6a9152cf 765ee439
0ccesame IV, same key, same AAD, but this time the input is the ciphertext 🧚
| Pad | SSRC | ROC | SEQ |
00 00 55 01 a0 b2 00 00 00 00 f1 7b
salt: 51 75 69 64 20 70 72 6f 20 71 75 6f
IV: 51 75 3c 65 80 c2 72 6f 20 71 84 14Key: 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f
AAD: 8040f17b 8041f8d3 5501a0b2
CT: f24de3a3 fb34de6c acba861c 9d7e4bca
be633bd5 0d294e6f 42a5f47a 51c7d19b
36de3adf 8833899d 7f27beb1 6a9152cf
765ee439 0cce
IV: 51 75 3c 65 80 c2 72 6f 20 71 84 14
H: c6a13b37878f5b826f4f8162a1c8d879before we decrypt anything, we verify the tag 🔍 if it doesn't match, no decryption, no vibes 💀
89 9d 7f 27 be b1 6a 91 52 cf 76 5e e4 39 0c ce
Process the AAD
AAD word: 8040f17b8041f8d35501a0b200000000
partial hash: bcfb3d1d0e6e3e78ba45403377dba11b
Process the cipher
cipher word: f24de3a3fb34de6cacba861c9d7e4bca
partial hash: 0ebc0abe1b15b32fedd2b07888c1ef61
cipher word: be633bd50d294e6f42a5f47a51c7d19b
partial hash: 438e5797011ea860585709a2899f4685
cipher word: 36de3adf883300000000000000000000
partial hash: 336fb643310d7bac2aeaa76247f6036d
Process the length word
length word: 00000000000000600000000000000130
partial hash: 1b964067078c408c4e442a8f015e5264
Turn GHASH into GMAC
GHASH: 1b 96 40 67 07 8c 40 8c 4e 44 2a 8f 01 5e 52 64
K0: 92 0b 3f 40 b9 3d 2a 1d 1c 8b 5c d1 e5 67 5e aa
full GMAC: 89 9d 7f 27 be b1 6a 91 52 cf 76 5e e4 39 0c ce
Received tag = 899d7f27 beb16a91 52cf765e e4390cce
Computed tag = 899d7f27 beb16a91 52cf765e e4390cce
Received tag verified.tags match, so we're good to decrypt 🎉
Decrypt the cipher
block # 0
IV||blk_cntr: 51753c6580c2726f2071841400000002
key_block: b5 2c 8f cf 92 55 fe 09 df ce a6 73 f0 10 22 b9
cipher_block: f2 4d e3 a3 fb 34 de 6c ac ba 86 1c 9d 7e 4b ca
plain_block: 47 61 6c 6c 69 61 20 65 73 74 20 6f 6d 6e 69 73
block # 1
IV||blk_cntr: 51753c6580c2726f2071841400000003
key_block: 9e 07 52 a3 64 5a 2f 4f 2b cb d4 0a 30 b5 a5 fe
cipher_block: be 63 3b d5 0d 29 4e 6f 42 a5 f4 7a 51 c7 d1 9b
plain_block: 20 64 69 76 69 73 61 20 69 6e 20 70 61 72 74 65
block # 2
IV||blk_cntr: 51753c6580c2726f2071841400000004
key_block: 45 fe 4e ad ed 40 0a 5d 1a f3 63 f9 0c e1 49 3b
cipher_block: 36 de 3a df 88 33
plain_block: 73 20 74 72 65 73and we're back to the original plaintext, verified and tagged 🌸✨
47616c6c 69612065 7374206f 6d6e6973
20646976 69736120 696e2070 61727465
73207472 657316.1.3. SRTP AEAD_AES_128_GCM Authentication Tagging 🏷️
this one's just tagging, no encryption 🎀 we take the same plaintext packet, but now the whole thing is AAD, so it just gets authenticated
8040f17b 8041f8d3 5501a0b2 47616c6c
69612065 7374206f 6d6e6973 20646976
69736120 696e2070 61727465 73207472
6573 | Pad | SSRC | ROC | SEQ |
00 00 55 01 a0 b2 00 00 00 00 f1 7b
salt: 51 75 69 64 20 70 72 6f 20 71 75 6f
IV: 51 75 3c 65 80 c2 72 6f 20 71 84 14Key: 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f
AAD: 8040f17b 8041f8d3 5501a0b2 47616c6c
69612065 7374206f 6d6e6973 20646976
69736120 696e2070 61727465 73207472
6573
IV: 51 75 3c 65 80 c2 72 6f 20 71 84 14
H: c6a13b37878f5b826f4f8162a1c8d879compute the GMAC tag over the AAD (which is the whole packet here) 🧮
Process the AAD
AAD word: 8040f17b8041f8d35501a0b247616c6c
partial hash: 79f41fea34a474a77609d8925e9f2b22
AAD word: 696120657374206f6d6e697320646976
partial hash: 84093a2f85abf17ab37d3ce2f706138f
AAD word: 69736120696e20706172746573207472
partial hash: ab2760fee24e6dec754739d8059cd144
AAD word: 65730000000000000000000000000000
partial hash: e84f3c55d287fc561c41d09a8aada4be
Process the length word
length word: 00000000000001900000000000000000
partial hash: b04200c26b81c98af55cc2eafccd1cbc 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 16we keep the first 16 bytes of the GMAC (sorry, those are literally the tag bytes shown below) and append them to the packet 💅
22493f82 d2bce397 e9d79e3b 19aa4216tagged packet, ready to go 🦋✨
8040f17b 8041f8d3 5501a0b2 47616c6c
69612065 7374206f 6d6e6973 20646976
69736120 696e2070 61727465 73207472
65732249 3f82d2bc e397e9d7 9e3b19aa
421616.1.4. SRTP AEAD_AES_128_GCM Tag Verification 🔍
last one, and it's a verify-only vibe 🫶 we check the tag on the tagged packet from 16.1.3
8040f17b 8041f8d3 5501a0b2 47616c6c
69612065 7374206f 6d6e6973 20646976
69736120 696e2070 61727465 73207472
65732249 3f82d2bc e397e9d7 9e3b19aa
4216 | Pad | SSRC | ROC | SEQ |
00 00 55 01 a0 b2 00 00 00 00 f1 7b
salt: 51 75 69 64 20 70 72 6f 20 71 75 6f
IV: 51 75 3c 65 80 c2 72 6f 20 71 84 14Key: 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f
AAD: 8040f17b 8041f8d3 5501a0b2 47616c6c
69612065 7374206f 6d6e6973 20646976
69736120 696e2070 61727465 73207472
6573
CT: 22493f82 d2bce397 e9d79e3b 19aa4216
IV: 51 75 3c 65 80 c2 72 6f 20 71 84 14
H: c6a13b37878f5b826f4f8162a1c8d879 22 49 3f 82 d2 bc e3 97 e9 d7 9e 3b 19 aa 42 16
Process the AAD
AAD word: 8040f17b8041f8d35501a0b247616c6c
partial hash: 79f41fea34a474a77609d8925e9f2b22
AAD word: 696120657374206f6d6e697320646976
partial hash: 84093a2f85abf17ab37d3ce2f706138f
AAD word: 69736120696e20706172746573207472
partial hash: ab2760fee24e6dec754739d8059cd144
AAD word: 65730000000000000000000000000000
partial hash: e84f3c55d287fc561c41d09a8aada4be
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
Received tag = 22493f82 d2bce397 e9d79e3b 19aa4216
Computed tag = 22493f82 d2bce397 e9d79e3b 19aa4216
Received tag verified.the received and computed tags match, so the packet is verified 👑💖 that's the whole AES-128-GCM set, bestie!
16.2. SRTP AEAD_AES_256_GCM 🌈
16.2.1. SRTP AEAD_AES_256_GCM Encryption
Okay bestie, we're encrypting this packet 🎀 Here's the packet we're starting with:
8040f17b 8041f8d3 5501a0b2 47616c6c
69612065 7374206f 6d6e6973 20646976
69736120 696e2070 61727465 73207472
6573First we need to form the IV, which is basically the nonce made from the salt plus our header fields 💅
| Pad | SSRC | ROC | SEQ |
00 00 55 01 a0 b2 00 00 00 00 f1 7b
salt: 51 75 69 64 20 70 72 6f 20 71 75 6f
IV: 51 75 3c 65 80 c2 72 6f 20 71 84 14Now the key, the AAD, the plaintext, the IV, and H, all lined up together 🦋
Key: 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f
10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f
AAD: 8040f17b 8041f8d3 5501a0b2
PT: 47616c6c 69612065 7374206f 6d6e6973
20646976 69736120 696e2070 61727465
73207472 6573
IV: 51 75 3c 65 80 c2 72 6f 20 71 84 14
H: f29000b62a499fd0a9f39a6add2e7780Next, we encrypt the plaintext one block at a time. Each block gets its own counter, so it's a whole little assembly line 🏭✨
Encrypt the Plaintext
block # 0
IV||blk_cntr: 51753c6580c2726f2071841400000002
key_block: 75 d0 b2 14 c1 43 de 77 9c eb 58 95 5e 40 5a d9
plain_block: 47 61 6c 6c 69 61 20 65 73 74 20 6f 6d 6e 69 73
cipher_block: 32 b1 de 78 a8 22 fe 12 ef 9f 78 fa 33 2e 33 aa
block # 1
IV||blk_cntr: 51753c6580c2726f2071841400000003
key_block: 91 e4 7b 4e f3 2b 83 d3 dc 65 0a 72 17 8d da 6a
plain_block: 20 64 69 76 69 73 61 20 69 6e 20 70 61 72 74 65
cipher_block: b1 80 12 38 9a 58 e2 f3 b5 0b 2a 02 76 ff ae 0f
block # 2
IV||blk_cntr: 51753c6580c2726f2071841400000004
key_block: 68 86 43 eb dd 08 07 98 16 3a 16 d5 e5 04 f6 3a
plain_block: 73 20 74 72 65 73
cipher_block: 1b a6 37 99 b8 7bAll the cipher blocks glued together make up the ciphertext before the tag gets added 🌸
Cipher before tag appended
32b1de78 a822fe12 ef9f78fa 332e33aa
b1801238 9a58e2f3 b50b2a02 76ffae0f
1ba63799 b87bNow for the tag. We run GHASH over the AAD and then over the cipher, add the length word, and then turn the result into a GMAC 🔥
Compute the GMAC tag
Process the AAD
AAD word: 8040f17b8041f8d35501a0b200000000
partial hash: 0154dcb75485b71880e1957c877351bd
Process the cipher
cipher word: 32b1de78a822fe12ef9f78fa332e33aa
partial hash: c3f07db9a8b9cb4345eb07f793d322d2
cipher word: b18012389a58e2f3b50b2a0276ffae0f
partial hash: 6d1e66fe32eb32ecd8906ceab09db996
cipher word: 1ba63799b87b00000000000000000000
partial hash: b3d1d2f1fa3b366619bc42cd2eedafee
Process the length word
length word: 00000000000000600000000000000130
partial hash: 7debf5fa1fac3bd318d5e1a7ee401091
Turn GHASH into GMAC
GHASH: 7d eb f5 fa 1f ac 3b d3 18 d5 e1 a7 ee 40 10 91
K0: 07 48 2e cc c0 53 ed 63 e1 6e 99 df 39 e7 7c 82
full GMAC: 7a a3 db 36 df ff d6 b0 f9 bb 78 78 d7 a7 6c 13Once the tag is stuck onto the end of the ciphertext, the whole thing is the finished, encrypted packet 👑
Cipher with tag
32b1de78 a822fe12 ef9f78fa 332e33aa
b1801238 9a58e2f3 b50b2a02 76ffae0f
1ba63799 b87b7aa3 db36dfff d6b0f9bb
7878d7a7 6c13And here it is, all encrypted and tagged, ready to go 💖
Encrypted and tagged packet:
8040f17b 8041f8d3 5501a0b2 32b1de78
a822fe12 ef9f78fa 332e33aa b1801238
9a58e2f3 b50b2a02 76ffae0f 1ba63799
b87b7aa3 db36dfff d6b0f9bb 7878d7a7
6c1316.2.2. SRTP AEAD_AES_256_GCM Decryption
Now we flip it around and decrypt that same packet 🦋 Here's what we're starting with:
8040f17b 8041f8d3 5501a0b2 32b1de78
a822fe12 ef9f78fa 332e33aa b1801238
9a58e2f3 b50b2a02 76ffae0f 1ba63799
b87b7aa3 db36dfff d6b0f9bb 7878d7a7
6c13Same as before, we start by forming the IV 💅
| Pad | SSRC | ROC | SEQ |
00 00 55 01 a0 b2 00 00 00 00 f1 7b
salt: 51 75 69 64 20 70 72 6f 20 71 75 6f
IV: 51 75 3c 65 80 c2 72 6f 20 71 84 14Then we set up the key, the AAD, the ciphertext, the IV, and H 🌈
Key: 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f
10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f
AAD: 8040f17b 8041f8d3 5501a0b2
CT: 32b1de78 a822fe12 ef9f78fa 332e33aa
b1801238 9a58e2f3 b50b2a02 76ffae0f
1ba63799 b87b7aa3 db36dfff d6b0f9bb
7878d7a7 6c13
IV: 51 75 3c 65 80 c2 72 6f 20 71 84 14
H: f29000b62a499fd0a9f39a6add2e7780Before we decrypt anything, we check the tag. We're verifying the received tag by recomputing it, bestie, so nothing gets trusted just because it showed up 🙈
Verify the received tag
7a a3 db 36 df ff d6 b0 f9 bb 78 78 d7 a7 6c 13
Process the AAD
AAD word: 8040f17b8041f8d35501a0b200000000
partial hash: 0154dcb75485b71880e1957c877351bd
Process the cipher
cipher word: 32b1de78a822fe12ef9f78fa332e33aa
partial hash: c3f07db9a8b9cb4345eb07f793d322d2
cipher word: b18012389a58e2f3b50b2a0276ffae0f
partial hash: 6d1e66fe32eb32ecd8906ceab09db996
cipher word: 1ba63799b87b00000000000000000000
partial hash: b3d1d2f1fa3b366619bc42cd2eedafee
Process the length word
length word: 00000000000000600000000000000130
partial hash: 7debf5fa1fac3bd318d5e1a7ee401091
Turn GHASH into GMAC
GHASH: 7d eb f5 fa 1f ac 3b d3 18 d5 e1 a7 ee 40 10 91
K0: 07 48 2e cc c0 53 ed 63 e1 6e 99 df 39 e7 7c 82
full GMAC: 7a a3 db 36 df ff d6 b0 f9 bb 78 78 d7 a7 6c 13
Received tag = 7aa3db36 dfffd6b0 f9bb7878 d7a76c13
Computed tag = 7aa3db36 dfffd6b0 f9bb7878 d7a76c13
Received tag verified.The received tag and the computed tag match, so the verification passes ✨
Now we decrypt the blocks. Same counters, same key blocks, just going the other direction 🍓
Decrypt the cipher
block # 0
IV||blk_cntr: 51753c6580c2726f2071841400000002
key_block: 75 d0 b2 14 c1 43 de 77 9c eb 58 95 5e 40 5a d9
cipher_block: 32 b1 de 78 a8 22 fe 12 ef 9f 78 fa 33 2e 33 aa
plain_block: 47 61 6c 6c 69 61 20 65 73 74 20 6f 6d 6e 69 73
block # 1
IV||blk_cntr: 51753c6580c2726f2071841400000003
key_block: 91 e4 7b 4e f3 2b 83 d3 dc 65 0a 72 17 8d da 6a
cipher_block: b1 80 12 38 9a 58 e2 f3 b5 0b 2a 02 76 ff ae 0f
plain_block: 20 64 69 76 69 73 61 20 69 6e 20 70 61 72 74 65
block # 2
IV||blk_cntr: 51753c6580c2726f2071841400000004
key_block: 68 86 43 eb dd 08 07 98 16 3a 16 d5 e5 04 f6 3a
cipher_block: 1b a6 37 99 b8 7b
plain_block: 73 20 74 72 65 73And the verified, decrypted packet comes out the other end 👑
Verified and tagged packet:
47616c6c 69612065 7374206f 6d6e6973
20646976 69736120 696e2070 61727465
73207472 657316.2.3. SRTP AEAD_AES_256_GCM Authentication Tagging
This time we skip the encryption and only tag the packet 🫶 The payload stays readable, but the tag still covers it. Here's the packet:
8040f17b 8041f8d3 5501a0b2 47616c6c
69612065 7374206f 6d6e6973 20646976
69736120 696e2070 61727465 73207472
6573Forming the IV, as always 💜
| Pad | SSRC | ROC | SEQ |
00 00 55 01 a0 b2 00 00 00 00 f1 7b
salt: 51 75 69 64 20 70 72 6f 20 71 75 6f
IV: 51 75 3c 65 80 c2 72 6f 20 71 84 14Key, AAD, IV, and H. Notice the AAD is the whole packet this time, since nothing is encrypted 🍓
Key: 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f
10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f
AAD: 8040f17b 8041f8d3 5501a0b2 47616c6c
69612065 7374206f 6d6e6973 20646976
69736120 696e2070 61727465 73207472
6573
IV: 51 75 3c 65 80 c2 72 6f 20 71 84 14
H: f29000b62a499fd0a9f39a6add2e7780Now we compute the GMAC tag over all that AAD 🔥
Compute the GMAC tag
Process the AAD
AAD word: 8040f17b8041f8d35501a0b247616c6c
partial hash: c059753e6763791762ca630d8ef97714
AAD word: 696120657374206f6d6e697320646976
partial hash: a4e3401e712900dc4f1d2303bc4b2675
AAD word: 69736120696e20706172746573207472
partial hash: 1c8c1af883de0d67878f379a19c65987
AAD word: 65730000000000000000000000000000
partial hash: 958462781aa8e8feacce6d93b54472ac
Process the length word
length word: 00000000000001900000000000000000
partial hash: af2efb5dcfdb9900e7127721fdb56956
Turn GHASH into GMAC
GHASH: af 2e fb 5d cf db 99 00 e7 12 77 21 fd b5 69 56
K0: 07 48 2e cc c0 53 ed 63 e1 6e 99 df 39 e7 7c 82
full GMAC: a8 66 d5 91 0f 88 74 63 06 7c ee fe c4 52 15 d4Then we stick the tag on the end of the packet 🦋
Cipher with tag
a866d591 0f887463 067ceefe c45215d4And here is the tagged packet, all done ⭐
Tagged packet:
8040f17b 8041f8d3 5501a0b2 47616c6c
69612065 7374206f 6d6e6973 20646976
69736120 696e2070 61727465 73207472
6573a866 d5910f88 7463067c eefec452
15d416.2.4. SRTP AEAD_AES_256_GCM Tag Verification
Last one, and it's the receiver's side 🧚 Here's the tagged packet coming in:
8040f17b 8041f8d3 5501a0b2 47616c6c
69612065 7374206f 6d6e6973 20646976
69736120 696e2070 61727465 73207472
6573a866 d5910f88 7463067c eefec452
15d4Forming the IV one more time, because we love a routine 💅
| Pad | SSRC | ROC | SEQ |
00 00 55 01 a0 b2 00 00 00 00 f1 7b
salt: 51 75 69 64 20 70 72 6f 20 71 75 6f
IV: 51 75 3c 65 80 c2 72 6f 20 71 84 14Here's the key, the AAD, the CT, the IV, and H 🌸
Key: 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f
10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f
AAD: 8040f17b 8041f8d3 5501a0b2 47616c6c
69612065 7374206f 6d6e6973 20646976
69736120 696e2070 61727465 73207472
6573
CT: a866d591 0f887463 067ceefe c45215d4
IV: 51 75 3c 65 80 c2 72 6f 20 71 84 14
H: f29000b62a499fd0a9f39a6add2e7780Now we check the received tag by computing it ourselves and comparing 👀
Verify the received tag
a8 66 d5 91 0f 88 74 63 06 7c ee fe c4 52 15 d4
Process the AAD
AAD word: 8040f17b8041f8d35501a0b247616c6c
partial hash: c059753e6763791762ca630d8ef97714
AAD word: 696120657374206f6d6e697320646976
partial hash: a4e3401e712900dc4f1d2303bc4b2675
AAD word: 69736120696e20706172746573207472
partial hash: 1c8c1af883de0d67878f379a19c65987
AAD word: 65730000000000000000000000000000
partial hash: 958462781aa8e8feacce6d93b54472ac
Process the length word
length word: 00000000000001900000000000000000
partial hash: af2efb5dcfdb9900e7127721fdb56956
Turn GHASH into GMAC
GHASH: af 2e fb 5d cf db 99 00 e7 12 77 21 fd b5 69 56
K0: 07 48 2e cc c0 53 ed 63 e1 6e 99 df 39 e7 7c 82
full GMAC: a8 66 d5 91 0f 88 74 63 06 7c ee fe c4 52 15 d4
Received tag = a866d591 0f887463 067ceefe c45215d4
Computed tag = a866d591 0f887463 067ceefe c45215d4
Received tag verified.The received and computed tags are identical, so the tag verifies 🌈💖✨
17. RTCP Test Vectors ✨
okay bestie, this section is all about RTCP test vectors 🦋 every example here is built off the same RTCP packet, so you can follow the whole thing without losing track 💅
81c8000e 4d617273 4e545031 4e545031
52545020 0000042a 0000eb98 4c756e61
deadbeef deadbeef deadbeef deadbeef
deadbeefand the SRTCP index is 000005d4 (32-bit) 🌸
As shown in Section 9.1, the IV is made by XORing two 12-octet values 🔥 the first one is built from two zero octets, the 4-octet SSRC (which lives in the fifth through eighth octets of the RTP packet), two more padding octets, and the 31-bit SRTCP index. That index gets right-justified into a 32-bit (4-octet) field with a single 0 bit stuck on the front as padding. Here's the SRTCP IV formation example, it's giving very clear 🧚
| Pad | SSRC | Pad | 0+SRTCP |
00 00 4d 61 72 73 00 00 00 00 05 d4
salt 51 75 69 64 20 70 72 6f 20 71 75 6f
------------------------------------
IV 51 75 24 05 52 03 72 6f 20 71 70 bbNow, in an SRTCP packet, you prepend a 1-bit Encryption flag to the 31-bit SRTCP index, and that makes a 32-bit value we'll call the "ESRTCP word" 💖 the E-flag is one if the SRTCP packet has been encrypted, and zero if it's been tagged but not encrypted. Also, the ESRTCP field only shows up in an SRTCP packet, never in a plain RTCP packet. The full ESRTCP word is part of the AAD ✨
When you're encrypting and tagging an RTCP packet (E-flag = 1), the SRTCP packet has these fields, in this exact 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 honestly, the authentication tag is the last 8 or 16 octets of the cipher. Iconic 👑
The reader is reminded that when the RTCP packet is tagged but not encrypted (E-flag = 0), GCM gives you a cipher that's made up of nothing but the 8-octet or 16-octet authentication tag 🙈 so the tagged SRTCP has these fields, in this order:
- 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.
17.1. SRTCP AEAD_AES_128_GCM Encryption and Tagging 🦋
Okay, first up, we're encrypting this packet:
81c8000d 4d617273 4e545031 4e545032
52545020 0000042a 0000e930 4c756e61
deadbeef deadbeef deadbeef deadbeef
deadbeefHere are the parameters, bestie:
Key size = 128 bits
Tag size = 16 octetsForm the IV 💅
| Pad | SSRC | Pad | SRTCP |
00 00 4d 61 72 73 00 00 00 00 05 d4
salt: 51 75 69 64 20 70 72 6f 20 71 75 6f
IV: 51 75 24 05 52 03 72 6f 20 71 70 bbNext up are the key, AAD, plaintext, IV, and H values 🌈
Key: 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f
AAD: 81c8000d 4d617273 800005d4
PT: 4e545031 4e545032 52545020 0000042a
0000e930 4c756e61 deadbeef deadbeef
deadbeef deadbeef deadbeef
IV: 51 75 24 05 52 03 72 6f 20 71 70 bb
H: c6a13b37878f5b826f4f8162a1c8d879Now we encrypt the plaintext, block by block ✨
Encrypt the Plaintext
block # 0
IV||blk_cntr: 517524055203726f207170bb00000002
key_block: 2d bd 18 b4 92 8e e6 4e f5 73 87 46 2f 6b 7a b3
plain_block: 4e 54 50 31 4e 54 50 32 52 54 50 20 00 00 04 2a
cipher_block: 63 e9 48 85 dc da b6 7c a7 27 d7 66 2f 6b 7e 99
block # 1
IV||blk_cntr: 517524055203726f207170bb00000003
key_block: 7f f5 29 c7 20 73 9d 4c 18 db 1b 1e ad a0 d1 35
plain_block: 00 00 e9 30 4c 75 6e 61 de ad be ef de ad be ef
cipher_block: 7f f5 c0 f7 6c 06 f3 2d c6 76 a5 f1 73 0d 6f da
block # 2
IV||blk_cntr: 517524055203726f207170bb00000004
key_block: 92 4d 25 a9 58 9d 83 02 d5 14 99 b4 e0 14 78 15
plain_block: de ad be ef de ad be ef de ad be ef
cipher_block: 4c e0 9b 46 86 30 3d ed 0b b9 27 5bHere's the cipher before the tag gets appended 🍓
Cipher before tag appended
63e94885 dcdab67c a727d766 2f6b7e99
7ff5c0f7 6c06f32d c676a5f1 730d6fda
4ce09b46 86303ded 0bb9275bTime to compute the GMAC tag 🔥 first we process the AAD, then the cipher, then the length word:
Compute the GMAC tag
Process the AAD
AAD word: 81c8000d4d617273800005d400000000
partial hash: 085d6eb166c555aa62982f630430ec6e
Process the cipher
cipher word: 63e94885dcdab67ca727d7662f6b7e99
partial hash: 8c9221be93466d68bbb16fa0d42b0187
cipher word: 7ff5c0f76c06f32dc676a5f1730d6fda
partial hash: 221ebb044ec9fd0bf116d7780f198792
cipher word: 4ce09b4686303ded0bb9275b00000000
partial hash: 50f70b9ca110ab312dce212657328dae
Process the length word
length word: 00000000000000600000000000000160
partial hash: 7296107c9716534371dfc1a30c5ffeb5Turning GHASH into GMAC is the last step of the tag math 💖
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 deNow the cipher has the tag stuck on the end 🦋
Cipher with tag
63e94885 dcdab67c a727d766 2f6b7e99
7ff5c0f7 6c06f32d c676a5f1 730d6fda
4ce09b46 86303ded 0bb9275b c84aa458
96cf4d2f c5abf872 45d9eadeThen we append the ESRTCP word with the E-flag set 🧚
Append the ESRTCP word with the E-flag set
63e94885 dcdab67c a727d766 2f6b7e99
7ff5c0f7 6c06f32d c676a5f1 730d6fda
4ce09b46 86303ded 0bb9275b c84aa458
96cf4d2f c5abf872 45d9eade 800005d4And that's the finished encrypted and tagged packet 👑
Encrypted and tagged packet:
81c8000d 4d617273 63e94885 dcdab67c
a727d766 2f6b7e99 7ff5c0f7 6c06f32d
c676a5f1 730d6fda 4ce09b46 86303ded
0bb9275b c84aa458 96cf4d2f c5abf872
45d9eade 800005d417.2. SRTCP AEAD_AES_256_GCM Verification and Decryption 🎀
Next we're going the other way, so we verify and decrypt. Here are the parameters for this one 🌸
Key size = 256 bits
Tag size = 16 octets
Process the length wordNow we're decrypting this packet 🙈
81c8000d 4d617273 d50ae4d1 f5ce5d30
4ba297e4 7d470c28 2c3ece5d bffe0a50
a2eaa5c1 110555be 8415f658 c61de047
6f1b6fad 1d1eb30c 4446839f 57ff6f6c
b26ac3be 800005d4These are the key settings from the original, and yes, they're listed twice, so read them as written 💀
Key size = 256 bits
Key size = 16 octetsForm the IV ✨
| Pad | SSRC | Pad | SRTCP |
00 00 4d 61 72 73 00 00 00 00 05 d4
salt: 51 75 69 64 20 70 72 6f 20 71 75 6f
IV: 51 75 24 05 52 03 72 6f 20 71 70 bbNext up are the key, AAD, ciphertext, IV, and H values 🌈
Key: 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f
10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f
AAD: 81c8000d 4d617273 800005d4
CT: d50ae4d1 f5ce5d30 4ba297e4 7d470c28
2c3ece5d bffe0a50 a2eaa5c1 110555be
8415f658 c61de047 6f1b6fad 1d1eb30c
4446839f 57ff6f6c b26ac3be
IV: 51 75 24 05 52 03 72 6f 20 71 70 bb
H: f29000b62a499fd0a9f39a6add2e7780First we verify the received tag, bestie. This is the most important part, no cap 🔥
Verify the received tag
1d 1e b3 0c 44 46 83 9f 57 ff 6f 6c b2 6a c3 be
Process the AAD
AAD word: 81c8000d4d617273800005d400000000
partial hash: 3ae5afd36dead5280b18950400176b5b
Process the cipher
cipher word: d50ae4d1f5ce5d304ba297e47d470c28
partial hash: e90fab7546f6940781227227ac926ebe
cipher word: 2c3ece5dbffe0a50a2eaa5c1110555be
partial hash: 9b236807d8b2dab07583adce367aa88f
cipher word: 8415f658c61de0476f1b6fad00000000
partial hash: e69313f423a75e3e0b7eb93321700e86
Process the length word
length word: 00000000000000600000000000000160
partial hash: 3a284af2616fdf505faf37eec39fbc8b
Turn GHASH into GMAC
GHASH: 3a 28 4a f2 61 6f df 50 5f af 37 ee c3 9f bc 8b
K0: 27 36 f9 fe 25 29 5c cf 08 50 58 82 71 f5 7f 35
full GMAC: 1d 1e b3 0c 44 46 83 9f 57 ff 6f 6c b2 6a c3 be
Received tag = 1d1eb30c 4446839f 57ff6f6c b26ac3be
Computed tag = 1d1eb30c 4446839f 57ff6f6c b26ac3be
Received tag verified.The tags match, so we can decrypt the cipher now 💖
Decrypt the cipher
block # 0
IV||blk_cntr: 517524055203726f207170bb00000002
key_block: 9b 5e b4 e0 bb 9a 0d 02 19 f6 c7 c4 7d 47 08 02
cipher_block: d5 0a e4 d1 f5 ce 5d 30 4b a2 97 e4 7d 47 0c 28
plain_block: 4e 54 50 31 4e 54 50 32 52 54 50 20 00 00 04 2a
block # 1
IV||blk_cntr: 517524055203726f207170bb00000003
key_block: 2c 3e 27 6d f3 8b 64 31 7c 47 1b 2e cf a8 eb 51
cipher_block: 2c 3e ce 5d bf fe 0a 50 a2 ea a5 c1 11 05 55 be
plain_block: 00 00 e9 30 4c 75 6e 61 de ad be ef de ad be ef
block # 2
IV||blk_cntr: 517524055203726f207170bb00000004
key_block: 5a b8 48 b7 18 b0 5e a8 b1 b6 d1 42 3b 74 39 55
cipher_block: 84 15 f6 58 c6 1d e0 47 6f 1b 6f ad
plain_block: de ad be ef de ad be ef de ad be efAnd here's the verified and decrypted packet 🦋
Verified and decrypted packet:
81c8000d 4d617273 4e545031 4e545032
52545020 0000042a 0000e930 4c756e61
deadbeef deadbeef deadbeef deadbeef
deadbeef17.3. SRTCP AEAD_AES_128_GCM Tagging Only 🧚
This time we're only tagging the packet, not encrypting it, so no cipher text, just the tag 🌸
Tagging the following packet:
81c8000d 4d617273 4e545031 4e545032
52545020 0000042a 0000e930 4c756e61
deadbeef deadbeef deadbeef deadbeef
deadbeefParameters first 💅
Key size = 128 bits
Tag size = 16 octetsForm the IV ✨
| Pad | SSRC | Pad | SRTCP |
00 00 4d 61 72 73 00 00 00 00 05 d4
salt: 51 75 69 64 20 70 72 6f 20 71 75 6f
IV: 51 75 24 05 52 03 72 6f 20 71 70 bbKey, AAD, IV, and H values 🌈 notice that the AAD here is the whole packet except for the ESRTCP word, since the E-flag is zero 🙈
Key: 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f
AAD: 81c8000d 4d617273 4e545031 4e545032
52545020 0000042a 0000e930 4c756e61
deadbeef deadbeef deadbeef deadbeef
deadbeef 000005d4
IV: 51 75 24 05 52 03 72 6f 20 71 70 bb
H: c6a13b37878f5b826f4f8162a1c8d879Now we compute the GMAC tag 🔥
Compute the GMAC tag
Process the AAD
AAD word: 81c8000d4d6172734e5450314e545032
partial hash: f8dbbe278e06afe17fb4fb2e67f0a22e
AAD word: 525450200000042a0000e9304c756e61
partial hash: 6ccd900dfd0eb292f68f8a410d0648ec
AAD word: deadbeefdeadbeefdeadbeefdeadbeef
partial hash: 6a14be0ea384c6b746235ba955a57ff5
AAD word: deadbeef000005d40000000000000000
partial hash: cc81f14905670a1e37f8bc81a91997cd
Process the length word
length word: 00000000000001c00000000000000000
partial hash: 3ec16d4c3c0e90a59e91be415bd976d8
Turn GHASH into GMAC
GHASH: 3e c1 6d 4c 3c 0e 90 a5 9e 91 be 41 5b d9 76 d8
K0: ba dc b4 24 01 d9 1e 6c b4 74 39 d1 49 86 14 6b
full GMAC: 84 1d d9 68 3d d7 8e c9 2a e5 87 90 12 5f 62 b3With the tag computed, the cipher is just the tag itself 💖
Cipher with tag
841dd968 3dd78ec9 2ae58790 125f62b3And here's the final tagged packet, with the ESRTCP word (E-flag zero) sitting at the very end 👑
Tagged packet:
81c8000d 4d617273 4e545031 4e545032
52545020 0000042a 0000e930 4c756e61
deadbeef deadbeef deadbeef deadbeef
deadbeef 841dd968 3dd78ec9 2ae58790
125f62b3 000005d417.4. SRTCP AEAD_AES_256_GCM Tag Verification 💅
okay bestie, last test vector arc 🧚 this one checks the tag on an SRTCP packet that uses AES-256 GCM, and the key is 32 bytes of 🌈 pattern. here are the basics:
Key size = 256 bitsTag size = 16 octets
time to process the length word and verify the packet 🔥
Process the length word
Verifying the following packet:
81c8000d 4d617273 4e545031 4e545032
52545020 0000042a 0000e930 4c756e61
deadbeef deadbeef deadbeef deadbeef
deadbeef 91db4afb feee5a97 8fab4393
ed2615fe 000005d4the next step is forming the IV, and it's giving puzzle energy 🧩 the salt gets XORed with the Pad, SSRC, and SRTCP fields 🎀
Key size = 256 bits
Key size = 16 octets
Form the IV
| Pad | SSRC | Pad | SRTCP |
00 00 4d 61 72 73 00 00 00 00 05 d4
salt: 51 75 69 64 20 70 72 6f 20 71 75 6f
IV: 51 75 24 05 52 03 72 6f 20 71 70 bbnow the key, AAD, ciphertext, IV, and hash subkey are all lined up together 🦋
Key: 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f
10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f
AAD: 81c8000d 4d617273 4e545031 4e545032
52545020 0000042a 0000e930 4c756e61
deadbeef deadbeef deadbeef deadbeef
deadbeef 000005d4
CT: 91db4afb feee5a97 8fab4393 ed2615fe
IV: 51 75 24 05 52 03 72 6f 20 71 70 bb
H: f29000b62a499fd0a9f39a6add2e7780next up is verifying the received tag 👑
Verify the received tag
91 db 4a fb fe ee 5a 97 8f ab 43 93 ed 26 15 feprocessing the AAD is a whole mood 😭 it's done in 16-byte words, with a partial hash after each one, then the length word goes at the end
Process the AAD
AAD word: 81c8000d4d6172734e5450314e545032
partial hash: 7bc665c71676a5a5f663b3229af4b85c
AAD word: 525450200000042a0000e9304c756e61
partial hash: 34ed77752703ab7d69f44237910e3bc0
AAD word: deadbeefdeadbeefdeadbeefdeadbeef
partial hash: 74a59f1a99282344d64ab1c8a2be6cf8
AAD word: deadbeef000005d40000000000000000
partial hash: 126335c0baa7ab1b79416ceeb9f7a518
Process the length word
length word: 00000000000001c00000000000000000
partial hash: b6edb305dbc7065887fb1b119cd36acbthen we turn GHASH into GMAC by XORing with K0, and the full GMAC is the tag we compare against 🌸
Turn GHASH into GMAC
GHASH: b6 ed b3 05 db c7 06 58 87 fb 1b 11 9c d3 6a cb
K0: 27 36 f9 fe 25 29 5c cf 08 50 58 82 71 f5 7f 35
full GMAC: 91 db 4a fb fe ee 5a 97 8f ab 43 93 ed 26 15 fe
Received tag = 91db4afb feee5a97 8fab4393 ed2615fe
Computed tag = 91db4afb feee5a97 8fab4393 ed2615fe
Received tag verified.the received and computed tags match, so the packet passes ✨ here's the verified packet:
Verified packet:
81c8000d 4d617273 4e545031 4e545032
52545020 0000042a 0000e930 4c756e61
deadbeef deadbeef deadbeef deadbeef
deadbeef18. References 📚
18.1. Normative References
these are the must-reads, the ones this whole spec leans on 🫶
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[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>.
[RFC3830] Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K.
Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830,
DOI 10.17487/RFC3830, August 2004,
<http://www.rfc-editor.org/info/rfc3830>.
[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>.
[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
<http://www.rfc-editor.org/info/rfc5116>.
[RFC5234] Crocker, D., Ed., and P. Overell, "Augmented BNF for
Syntax Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<http://www.rfc-editor.org/info/rfc5234>.
[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>.
[RFC6188] McGrew, D., "The Use of AES-192 and AES-256 in Secure
RTP", RFC 6188, DOI 10.17487/RFC6188, March 2011,
<http://www.rfc-editor.org/info/rfc6188>.
[RFC6904] Lennox, J., "Encryption of Header Extensions in the Secure
Real-time Transport Protocol (SRTP)", RFC 6904,
DOI 10.17487/RFC6904, April 2013,
<http://www.rfc-editor.org/info/rfc6904>.18.2. Informative References
these ones are good to know for the deeper lore 🔮
[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>.Acknowledgements 🫶
the authors want to give a huge thank you to Michael Peck, Michael Torla, Qin Wu, Magnus Westerlund, Oscar Ohllson, Woo-Hwan Kim, John Mattsson, Richard Barnes, Morris Dworkin, Stephen Farrell, and so many other reviewers who dropped 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
Email: mcgrew@cisco.com
URI: http://www.mindspring.com/~dmcgrew/dam.htm
Kevin M. Igoe
NSA/CSS Commercial Solutions Center
National Security Agency
Email: mythicalkevin@yahoo.com📦 where this page comes from
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