AES-GCM and AES-CCM Authenticated Encryption in Secure RTP (SRTP)
draft-ietf-avtcore-srtp-aes-gcm-00
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 7714.
|
|
|---|---|---|---|
| Authors | David McGrew , Kevin Igoe | ||
| Last updated | 2012-05-15 (Latest revision 2012-05-03) | ||
| Replaces | draft-ietf-avt-srtp-aes-gcm | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews |
GENART Last Call review
(of
-14)
by Ben Campbell
Almost ready
|
||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | Adopted by a WG | |
| Document shepherd | Magnus Westerlund | ||
| IESG | IESG state | Became RFC 7714 (Proposed Standard) | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-ietf-avtcore-srtp-aes-gcm-00
Network Working Group D. McGrew
Internet Draft Cisco Systems, Inc.
Intended Status: Informational K.M. Igoe
Expires: August 20, 2012 National Security Agency
February 17, 2012
AES-GCM and AES-CCM Authenticated Encryption in Secure RTP (SRTP)
draft-ietf-avtcore-srtp-aes-gcm-00
Status of this Memo
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provisions of BCP 78 and BCP 79.
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 20, 2012.
Copyright Notice
Copyright (c) 2012 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
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described in the Simplified BSD License.
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Abstract
This document defines how AES-GCM, AES-CCM, and other Authenticated
Encryption with Associated Data (AEAD) algorithms, can be used to
provide confidentiality and data authentication mechanisms in the
SRTP protocol.
Table of Contents
1. Introduction.....................................................2
1.1. Conventions Used In This Document...........................3
1.2. AEAD processing for SRTP....................................4
1.2.1. AEAD versus SRTP/SRTCP Authentication..................5
1.2.2. Values used to form the Initialization Vector (IV).....6
1.3. SRTP IV formation for AES-GCM and AES-CCM...................6
1.4. SRTCP IV formation for AES-GCM and AES-CCM..................7
1.5. AEAD Processing of SRTP Packets.............................8
1.6. AEAD Processing of SRTCP Packets............................8
1.6.1. Encrypted SRTCP packets................................9
1.6.2. Unencrypted SRTCP packets.............................10
1.7. Initialization of the Counters.............................10
1.8. Prevention of IV Reuse.....................................11
2. AEAD parameters for SRTP and SRTCP..............................11
2.1. Generic AEAD Parameter Constraints.........................11
2.2. AES-GCM for SRTP/SRTCP.....................................12
2.3. AES-CCM for SRTP/SRTCP.....................................13
2.4. Key Derivation Functions...................................13
3. Security Considerations.........................................14
3.1. Handling of Security Critical Parameters...................14
3.2. Size of the Authentication Tag.............................14
4. IANA Considerations.............................................15
5. Acknowledgements................................................16
6. References......................................................17
6.1. Normative References.......................................17
6.2. Informative References.....................................18
1. Introduction
The Secure Real-time Transport Protocol (SRTP) is a profile of the
Real-time Transport Protocol (RTP), 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).
SRTP/SRTCP assumes that both the sender and recipient have a shared
secret master key and a shared master salt. As described in sections
4.3.1 and 4.3.3 of [RFC3711], a Key Derivation Function is applied to
these values to obtain separate encryption keys, authentication keys
and salting keys for SRTP and for SRTCP. (Note: As will be explained
below, AEAD SRTP/SRTCP does not make use of these authentication
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keys.)
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",
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. Two families of AEAD algorithm families, AES Galois/Counter
Mode (AES-GCM) and AES Counter with Cipher Block Chaining-Message
Authentication Code (AES-CCM), are 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.
The Galois/Counter Mode of operation (GCM) and the Counter with
Cipher Block Chaining-Message Authentication Code mode of operation
(CCM) are both AEAD modes of operation for block ciphers. Both use
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. CCM is well suited for use in compact software
implementations. This specification uses GCM and CCM with both
AES-128 and AES-256.
In summary, this document defines how to use AEAD algorithms,
particularly AES-GCM and AES-CCM, to provide confidentiality and
message authentication within SRTP and SRTCP packets.
1.1. Conventions Used In This Document
The following terms have very specific meanings in the context of
this RFC:
Crypto Context For the purposes of this document a crypto context
is the outcome of any process which results in
authentication of each participant in the SRTP
session and possession by each partcipant of a
shared secret master key and a shared master
salt. Details of how the maser key and master
salt are established are outside the scope of this
document. Similarly any mechanism for rekeying an
existing Ciper Contest is outside the scope of the
document. The master key MUST be at least as
large as the encryption key. The SRTP/SRTCP Key
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Derivation Function (KDF) defined in [RFC3711] is
applied to the master key and master SALT to
derive the SRTP_encr_key, SRTCP_encr_key,
SRTP_SALT, and SRTCP_SALT. Authentication keys
are not used in AEAD.
Instantiation Once keys have been established, an instance of
the AEAD algorithm is created using the
appropriate key and salt. In a point-to-point
scenario, each participant in the SRTP/SRTCP
session will need four instantiations of the AEAD
algorithm; one for inbound SRTP traffic, one for
outbound SRTP traffic source, one for inbound
SRTCP traffic, and one for outbound SRTCP traffic
source. See section 1.2 for details on what is
required of each instantiation.
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.
Each AEAD instantiation has its own key, a 48-bit zero-based packet
counter that is incremented after that particular instantiation has
been invoked to process an SRTP packet, and a 31-bit zero-based SRTCP
index that is incremented each time an SRTCP packet is processed. A
32-bit Block Counter is incremented each time a block of key is
produced and is reset (to zero for CCM and to one for GCM) at the
start of each packet. As we shall see in sections 1.3 and 1.4, the
packet counter and SRTCP counter play a crucial role in the formation
of each packet's IV.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
1.2. AEAD processing for SRTP
We first define how to use a generic AEAD algorithm in SRTP, then we
describe the specific use of the AES-128-GCM and AES-256-GCM
algorithms.
The use of an AEAD algorithm is defined by expressing the AEAD
encryption algorithm inputs in terms of SRTP fields and data
structures. The AEAD encryption inputs are as follows:
Key This input is the SRTP encryption key
(SRTP_encr_key) produced from the shared
secret master key using the key derivation
process. (Note that the SRTP_auth_key is
not used).
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Associated Data This is data that is to be authenticated
but not encrypted. In SRTP, the associated
data consists of the entire RTP header,
including the list of CSRC identifiers (if
present) and the RTP header extension (if
present), as shown in Figure 3.
Plaintext Data that is to be both encrypted and
authenticated. In SRTP this consists of
the RTP payload, the RTP padding and the
RTP pad count fields (if the latter two
fields are present) as shown in Figure 3.
The padding service provided by RTP is not
needed by the AEAD encryption algorithm, so
the RTP padding and RTP pad count fields
SHOULD be omitted.
Initialization Vector Each SRTP/SRTCP packet has its own 12-octet
initialization vector (IV). Construction
of this IV is covered in more detail
below.
The AEAD encryption algorithm accepts these four inputs and returns a
Ciphertext field.
1.2.1. AEAD versus SRTP/SRTCP Authentication
The reader is reminded that in addition to providing confidentiality
for the plaintext that is encrypted, an AEAD algorithm also provides
a mechanism that allows the intended recipient to check the data
integrity and authenticity of the plaintext and associated data. The
AEAD authentication tag is incorporated into the Ciphertext field by
RFC 5116, thus AEAD does not make use of the SRTP/SRTCP
Authentication Tag fields defined in RFC 3711. (Note that this means
that the cipher text will be longer than the plain text by precisely
the length of the AEAD authentication tag.)
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 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.
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1.2.2. Values used to form the Initialization Vector (IV)
The initialization vector for an SRTP packet is formed from the:
SSRC The 4-octet Synchronization Source identifier
(SSRC), found in the RTP header.
Packet Counter Each AEAD instantiation MUST maintain a 6 octet
zero-based packet counter which is incremented
each time an SRTP packet is sent. As we shall
see below, the packet counter is used to insure
each SRTP packet gets a unique initialization
vector.
Sequence Number The 2-octet RTP Sequence Number (SEQ), found in
the RTP header. The SEQ is just the 16 least
significant bits of the packet counter.
Rollover Counter A 4-octet Rollover Counter (ROC), maintained
independently by both sides of the link,
incremented each time the Sequence Number cycles
back to 0. The ROC is just the 32 most
significant bits of the Packet Counter.
SRTCP index The SRTCP index is a 31-bit counter that plays
the same role for SRTCP packets that the Packet
Counter does for SRTP packets. Unlike the
Packet Counter, the SRTCP index is explicitly
included in each STRCP packet. The sender MUST
increment the SRTCP index by one after each SRTP
packet is sent.
SALT A 12-octet SRTP session encryption salt produced
by the SRTP Key Derivation Function (KDF) (see
section 2.4).
The reader is reminded that both SRTP and SRTCP allow packets to
arrive out of order, presenting the receiver with a synchronization
problem. The 31-bit SRTCP index is contained in the unencrypted (but
authenticated) portion of the SRTCP header, allowing the recipient to
read the SRTCP index directly from the header. But only the low 16
bits of the SRTP Packet counter are contained in the SRTP header (in
the sequence number field). Section 3.3.1 of [RFC3711] explains in
great detail how the 16-bit sequence number and 32-bit Rollover
Counter are to be used to recover the 48-bit Packet Counter.
1.3. SRTP IV formation for AES-GCM and AES-CCM
AES-GCM and AES-CCM SRTP use a 12 byte initialization vector which is
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formed as follows. A 12-octet string is formed by concatenating
2-octets of zeroes, the 4-octet SSRC, and the 6-octet Packet
Counter. The resulting string is bitwise exclusive-ored with the
12-octet salt to form the 12-octet IV.
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 | Packet_Counter |---+
+--+--+--+--+--+--+--+--+--+--+--+--+ |
|
+--+--+--+--+--+--+--+--+--+--+--+--+ |
| Encryption Salt |->(+)
+--+--+--+--+--+--+--+--+--+--+--+--+ |
|
+--+--+--+--+--+--+--+--+--+--+--+--+ |
| Initialization Vector |<--+
+--+--+--+--+--+--+--+--+--+--+--+--+
Figure 1: AES-GCM and AES-CCM SRTP
Initialization Vector formation.
Using the terminology of section 8.2.1. of [GCM], the first six
octets of the IV are the fixed field and the last six bytes are the
invocation field.
1.4. SRTCP IV formation for AES-GCM and AES-CCM
The initialization vector for an SRTCP packet is formed from the
4-octet Synchronization Source identifier (SSRC), 31-bit SRTCP Index
(packed zero-filled, right justified into a 4-octet field), and a
12-octet SRTCP session encryption salt produced by the SRTP Key
Derivation Function (KDF).
0 1 2 3 4 5 6 7 8 9 10 11
+--+--+--+--+--+--+--+--+--+--+--+--+
|00|00| SSRC |00|00|SRTCP Index|---+
+--+--+--+--+--+--+--+--+--+--+--+--+ |
|
+--+--+--+--+--+--+--+--+--+--+--+--+ |
| Encryption Salt |->(+)
+--+--+--+--+--+--+--+--+--+--+--+--+ |
|
+--+--+--+--+--+--+--+--+--+--+--+--+ |
| Initialization Vector |<--+
+--+--+--+--+--+--+--+--+--+--+--+--+
Figure 2: SRTCP Initialization Vector formation.
As shown if figure 2, a 12-octet string is formed by concatenating in
order 2-octets of zeroes, the 4-octet SSRC, 2 more zero octets, and
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the 4-octet SRTCP index. The resulting 12-octet string is bitwise
exclusive-ored into salt; the output of that process is the IV. The
IV is always exactly 12 octets in length. Using the terminology of
section 8.2.1. of [GCM], the first eight octets of the IV are the
fixed field and the last four bytes are the invocation field.
1.5. AEAD Processing of SRTP Packets
All SRTP packets MUST be authenticated and encrypted. Figure 3 below
shows which fields of AEAD SRTP packet are to be treated as plaintext
and which are to be treated as additional authenticated data.
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| Packet Type | 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
X : authentication tag (NOT RECOMMENDED) :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
P = Plaintext (to be encrypted and authenticated)
A = Associated Data (to be authenticated only)
X = neither encrypted nor authenticated
Note: The RTP padding and RP padding count fields are optional
and are not recommended
Figure 3: AEAD inputs from an SRTP packet.
1.6. AEAD Processing of SRTCP Packets
All SRTCP 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 in the leftmost bit of the 32-bit word that contains the
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SRTCP index)
1.6.1. Encrypted SRTCP packets
When the encryption flag is set to 1, the first 8-octets, the
encryption flag and 31-bit SRTCP index MUST be treated as AAD. The
remaining data MUST be treated as plaintext, and hence is to be both
encrypted and AEAD authenticated, save for the optional SRTCP MKI
index and optional SRTCP authentication tag, which MUST be neither
encrypted nor AEAD authenticated. Figure 4 below shows how fields of
an RTCP packet are to be treated when the encryption flag is set to
1.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
X | SRTCP MKI (optional)index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
X : authentication tag (NOT RECOMMENDED) :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
P = Plaintext (to be encrypted and authenticated)
A = Associated Data (to be authenticated only)
X = neither encrypted nor authenticated
Figure 4: AEAD SRTCP inputs when encryption flag = 1.
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1.6.2. Unencrypted SRTCP packets
When the encryption flag is set to 0, all of the data up to and
including the SRTCP index is treated as AAD. Figure 5 shows how the
fields of an RTCP packet are to be treated when 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
X | SRTCP MKI (optional)index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
X : authentication tag (NOT RECOMMENDED) :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A = Associated Data (to be authenticated only)
X = neither encrypted nor authenticated
Figure 5: AEAD SRTCP inputs when encryption flag = 0.
1.7. Initialization of the Counters
When an AEAD Crypto Context is first established, both the SRTCP
index and the rollover counter are set to zero. The Sequence Number
is set to a value passed to it by RTP. When the context is rekeyed
these counters keep their current values and are not reset to zero.
These conventions assist in making a seamless transition from the old
key (if any) to the new key despite of the fact that packets are
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allowed to arrive out of order.
As mentioned in section 1.1, AES-GCM and AES-CCM both use a Block
Counter which is reset at the start of each packet. For AES-CCM it
is reset to 0 and for AES-GCM it is reset to 1.
1.8. Prevention of IV Reuse
For a given key it is critical that the IV not repeat. This reduces
to the problem of insuring neither the Packet Counter nor the SRTCP
index do not repeat before the AEAD instantiation is rekeyed.
Processing MUST cease if either the 48-bit Packet Counter or the
31-bit SRTCP index cycles back to their initial value. Processing
MUST NOT resume until a new SRTP/SRTCP session has been established
using a new shared secret master key and shares master salt.
Ideally, a rekey should be done well before either of these counters
cycle.
2. AEAD parameters for SRTP and SRTCP
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 two specific AEAD algorithms, namely AES-GCM and
AES-CCM.
2.1. Generic AEAD Parameter Constraints
All AEAD algorithms used with SRTP/SRTCP MUST satisfy the three
constraints listed below:
PARAMETER Meaning Value
A_MAX maximum additional MUST be at least 12 octets
authenticated data
length
N_MIN minimum nonce (IV) MUST be no more than 12 octets
length
N_MAX maximum nonce (IV) MUST be at least 12 octets
length
C_MAX maximum ciphertext MUST be at most 2^16-40 octets
length per invocation SHOULD be at least 2232
The upper bound on C_MAX is obtained by subtracting away a 20-octet
IP header, an 8-octet UDP header, and a 12-octet RTP header out of
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the largest possible IP packet, the total length of which is 2^16
octets.
Similarly the lower bound on C_MAX is based on the maximum
transmission unit (MTU) of 2272 octets in IEEE 802.11. Because many
RTP applications use very short payloads (for example, the G.729
codec used in VoIP can be as short as 20 octets), implementations
that only support a maximum ciphertext length smaller than 2232
octets are permitted under this RFC. However, in the interest of
maximizing interoperability between various AEAD implementations, the
use of C_MAX values less than 2232 is discouraged.
For sake of clarity we specify two additional parameters:
Authentication Tag Length MUST be either 8, 12, or 16
octets
Maximum number of invocations MUST be at most 2^48 for SRTP
for a given instantiation MUST be at most 2^31 for SRTCP
The reader is reminded that the plaintext is shorter than the
ciphertext by exactly the length of the AEAD authentication tag.
2.2. AES-GCM for SRTP/SRTCP
AES-GCM is a family of AEAD algorithms built around the AES block
cipher algorithm. AES-GCM uses AES counter mode 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:
Table 1: AES-GCM algorithms for SRTP/SRTCP
Name Key Size Auth. Tag Size Reference
================================================================
AEAD_AES_128_GCM 16 octets 16 octets [RFC5116]
AEAD_AES_256_GCM 32 octets 16 octets [RFC5116]
AEAD_AES_128_GCM_8 16 octets 8 octets [RFC5282]
AEAD_AES_256_GCM_8 32 octets 8 octets [RFC5282]
AEAD_AES_128_GCM_12 16 octets 12 octets [RFC5282]
AEAD_AES_256_GCM_12 32 octets 12 octets [RFC5282]
Any implementation of AES-GCM SRTP SHOULD support both
AEAD_AES_128_GCM_8 and AEAD_AES_256_GCM_8, and it MAY support the
four other variants shown in the table.
In addition to the Packet Counter used in the formation of IVs, each
instantiation of AES-GCM has a block counter which is incremented
each time AES is called to produce a 16-octet output block. The
block counter is reset to "1" each time AES-GCM is invoked to process
a new packet. The 128-bit concatentation of the IV and the block
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counter is input to AES and the output is used as a block of key that
is XORed to the next block of data to be encrypted/decypted.
1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
----+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| initialization | block |
| vector | counter |
----+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
Figure 6: AES Inputs for Counter Mode Encryption
2.3. AES-CCM for SRTP/SRTCP
AES-CCM is another family of AEAD algorithms built around the AES
block cipher algorithm. AES-GCM uses AES counter mode for encryption
and AES Cipher Block Chaining Message Authentication Code (CBC MAC)
for authentication. A detailed description of the AES-CCM family can
be found in [RFC5116]. The following members of the AES-CCM family
may be used with SRTP/SRTCP:
Table 2: AES-CCM algorithms for SRTP/SRTCP
Name Key Size Auth. Tag Size Reference
================================================================
AEAD_AES_128_CCM 16 octets 16 octets [RFC5116]
AEAD_AES_256_CCM 32 octets 16 octets [RFC5116]
Any implementation of AES-CCM SRTP/SRTCP SHOULD support both
AEAD_AES_128_CCM and AEAD_AES_256_CCM.
In addition to the Packet Counter used in the formation of IVs,
each instantiation of AES-CCM has a block counter which is
incremented each time AES is called to produce a 16-octet output
block. The block counter is reset to "0" each time AES-CCM is
invoked to process a new packet. As with AES-GCM, the 128-bit
concatentation of the IV abd the block counter is input to AES to
produce a block of key that is XORed to the next block of data to
be encrypted/decypted.
AES-CCM uses a flag octet that conveys information about the length
of the authentication tag, length of the block counter, and presence
of additional authenticated data. For AES-CCM in SRTP/SRTCP, the
flag octet has the hex value 5A if an 8-octet authentication tag is
used, 6A if a 12-octet authentication tag is used, and 7A if a
16-octet authentication tag is used. The flag octet is one of the
inputs to AES during the counter mode encryption of the plaintext.
2.4. 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
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two endpoints. Both the AEAD_AES_128_GCM algorithms and the
AEAD_AES_128_CCM algorithms MUST use the (128-bit) AES_CM_PRF Key
Derivation Function described in [RFC3711]. Both the
AEAD_AES_256_GCM algorithms and the AEAD_AES_256_CCM algorithms MUST
use the AES_256_CM_PRF Key Derivation Function described in [RFC
6188].
3. Security Considerations
3.1. Handling of Security Critical Parameters
As with any security process, the implementer must take care to
ensure 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.
- Packets that fail the authentication check SHOULD be silently
discarded.
- The sender MUST increment the Packet Counter after each SRTP
packet is processed.
- The sender MUST increment the SRTCP index after each SRTCP
packet is processed.
- At the start of each packet the block counter MUST be reset (to
0 for CCM, to 1 for GCM). The block counter is incremented
after each block key has been produced, but it MUST NOT be
allowed to exceed 2^32-1.
- Each time a rekey occurs the initial values of the invocation
counter and SRTCP index MUST be saved.
- Processing MUST cease if the 48-bit Packet Counter or the 31-bit
SRTCP index 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 either of these counters cycle.
3.2. Size of the Authentication Tag
We require that the AEAD authentication tag must be at least 8
octets, significantly reducing the probability of an adversary
successfully introducing fraudulent data. The goal of an
authentication tag is to minimize the probability of a successful
forgery occurring anywhere in the network we are attempting to
defend. There are three relevant factors: how low we wish the
probability of successful forgery to be (prob_success), how many
attempts the adversary can make (N_tries) and the size of the
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authentication tag in bits (N_tag_bits). Then
prob_success < expected number of successes
= N_tries * 2^-N_tag_bits.
Suppose an adversary wishes to introduce a forged or altered packet
into a target network by randomly selecting an authentication value
until by chance they hit a valid authentication tag. The table below
summarizes the relationship between the number of forged packets the
adversary has tried, the size of the authentication tag, and the
probability of a compromise occurring (i.e. at least one of the
attempted forgeries having a valid authentication tag). The reader
is reminded that the forgery attempts can be made over the entire
network, not just a single link, and that frequently changing the key
does not decrease the probability of a compromise occurring.
|==================+========================================|
| Authentication | Probability of a Compromise Occurring |
| Tag Size |------------+-------------+-------------|
| (octets) | 2^-30 | 2^-20 | 2^-10 |
|==================+=============+=============+============|
| 4 | 2^2 tries | 2^12 tries | 2^22 tries |
|==================+============+=============+=============|
| 8 | 2^34 tries | 2^44 tries | 2^54 tries |
|==================+============+=============+=============|
| 12 | 2^66 tries | 2^76 tries | 2^86 tries |
|==================+============+=============+=============|
| 16 | 2^98 tries | 2^108 tries | 2^118 tries |
|==================+============+=============+=============|
Table 1: Probability of a compromise occurring for a given
number of forgery attempts and tag size.
4. IANA Considerations
RFC 4568 defines SRTP "crypto suites"; a crypto suite corresponds to
a particular AEAD algorithm in SRTP. In order to allow SDP to signal
the use of the algorithms defined in this document, IANA will
register the following crypto suites into the subregistry for SRTP
crypto suites under the SRTP transport of the SDP Security
Descriptions:
srtp-crypto-suite-ext = "AEAD_AES_128_GCM" /
"AEAD_AES_256_GCM" /
"AEAD_AES_128_GCM_8" /
"AEAD_AES_256_GCM_8" /
"AEAD_AES_128_GCM_12" /
"AEAD_AES_256_GCM_12" /
"AEAD_AES_128_CCM" /
"AEAD_AES_256_CCM" /
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srtp-crypto-suite-ext
DTLS-SRTP [RFC5764] defines a DTLS-SRTP "SRTP Protection Profile"; it
also corresponds 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 will also register the following SRTP Protection
Profiles:
SRTP_AEAD_AES_128_GCM
SRTP_AEAD_AES_256_GCM
SRTP_AEAD_AES_128_GCM_8
SRTP_AEAD_AES_256_GCM_8
SRTP_AEAD_AES_128_GCM_12
SRTP_AEAD_AES_256_GCM_12
SRTP_AEAD_AES_128_CCM
SRTP_AEAD_AES_256_CCM
5. Acknowledgements
The authors would like to thank Michael Peck, Michael Torla, Qin Wu,
and many other reviewers who provided valuable comments on earlier
drafts of this document.
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6. References
6.1. Normative References
[CCM] Dworkin, M., "NIST Special Publication 800-38C: The CCM
Mode for Authentication and Confidentiality", U.S.
National Institute of Standards and Technology http://
csrc.nist.gov/publications/nistpubs/800-38C/SP800-38C.pdf.
[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 http://
csrc.nist.gov/publications/nistpubs/800-38D/SP800-38D.pdf.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and
K. Norrman, "The Secure Real-time Transport Protocol
(SRTP)", RFC 3711, March 2004.
[RFC4658] Andreasen, F., Baugher, M., and D.Wing, "Session
Description Protocol (SDP): Security Descriptions for
Media Streams", RFC 4568, July 2006.
[RFC5116] McGrew, D., "An Interface and Algorithms for
Authenticated Encryption with Associated Data", RFC 5116,
January 2008.
[RFC5116] McGrew, D., "An Interface and Algorithms for
Authenticated Encryption with Associated Data", RFC 5116,
January 2008.
[RFC5282] McGrew, D. and D. Black, "Using Authenticated Encryption
Algorithms with the Encrypted Payload of the Internet Key
Exchange version 2 (IKEv2) Protocol", RFC 5282, August 2008.
[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, May 2010.
[RFC6188] McGrew,D.,"The Use of AES-192 and AES-256 in Secure RTP"
RFC 6811, March 2011
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6.2. 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 http://
www-cse.ucsd.edu/users/mihir/papers/oem.html.
[BOYD] Boyd, C. and A. Mathuria, "Protocols for Authentication
and Key Establishment", Springer, 2003 .
[CMAC] "NIST Special Publication 800-38B", http://csrc.nist.gov/
CryptoToolkit/modes/800-38_Series_Publications/
SP800-38B.pdf.
[EEM04] Bellare, M., Namprempre, C., and T. Kohno, "Breaking and
provably repairing the SSH authenticated encryption
scheme: A case study of the Encode-then-Encrypt-and-MAC
paradigm", ACM Transactions on Information and System Secu
rity, http://www-cse.ucsd.edu/users/tkohno/papers/
TISSEC04/.
[GR05] Garfinkel, T. and M. Rosenblum, "When Virtual is Harder
than Real: Security Challenges in Virtual Machine Based
Computing Environments", Proceedings of the 10th Workshop
on Hot Topics in Operating Systems http://
www.stanford.edu/~talg/papers/HOTOS05/
virtual-harder-hotos05.pdf.
[J02] Jonsson, J., "On the Security of CTR + CBC-MAC",
Proceedings of the 9th Annual Workshop on Selected Areas
on Cryptography, http://csrc.nist.gov/CryptoToolkit/modes/
proposedmodes/ccm/ccm-ad1.pdf, 2002.
[MODES] Dworkin, M., "NIST Special Publication 800-38:
Recommendation for Block Cipher Modes of Operation", U.S.
National Institute of Standards and Technology http://
csrc.nist.gov/publications/nistpubs/800-38a/sp800-38a.pdf.
[MV04] McGrew, D. and J. Viega, "The Security and Performance of
the Galois/Counter Mode (GCM)", Proceedings of INDOCRYPT
'04, http://eprint.iacr.org/2004/193, December 2004.
[R02] Rogaway, P., "Authenticated encryption with Associated-
Data", ACM Conference on Computer and Communication
Security (CCS'02), pp. 98-107, ACM Press,
2002. http://www.cs.ucdavis.edu/~rogaway/papers/ad.html.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
February 1997.
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[RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Requirements for Security", BCP 106, RFC 4086, June 2005.
[RFC4106] Viega, J. and D. McGrew, "The Use of Galois/Counter Mode
(GCM) in IPsec Encapsulating Security Payload (ESP)",
RFC 4106, June 2005.
[RFC4107] Bellovin, S. and R. Housley, "Guidelines for Cryptographic
Key Management", BCP 107, RFC 4107, June 2005.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, December 2005.
[RFC4309] Housley, R., "Using Advanced Encryption Standard (AES) CCM
Mode with IPsec Encapsulating Security Payload (ESP)",
RFC 4309, December 2005.
[RFC4771] Lehtovirta, V., Naslund, M., and K. Norrman, "Integrity
Transform Carrying Roll-Over Counter for the Secure Real-
time Transport Protocol (SRTP)", RFC 4771, January 2007.
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Author's Address
David A. McGrew
Cisco Systems, Inc.
510 McCarthy Blvd.
Milpitas, CA 95035
US
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: kmigoe@nsa.gov
Acknowledgement
Funding for the RFC Editor function is provided by the IETF
Administrative Support Activity (IASA).
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