implementation, see Section 4.3.
* an MKI value,
* <From, To> values, specifying the lifetime for a master key,
expressed in terms of the two 48-bit index values inside whose
range (including the range end-points) the master key is valid.
For the use of <From, To>, see Section 8.1.1. <From, To> is an
alternative to the MKI and assumes that a master key is in one-
to-one correspondence with the SRTP session key on which the
<From, To> range is defined.
SRTCP SHALL by default share the crypto context with SRTP, except:
* no rollover counter and s_l-value need to be maintained as the
RTCP index is explicitly carried in each SRTCP packet,
* a separate replay list is maintained (when replay protection is
provided),
* SRTCP maintains a separate counter for its master key (even if the
master key is the same as that for SRTP, see below), as a means to
maintain a count of the number of SRTCP packets that have been
processed with that key.
Note in particular that the master key(s) MAY be shared between SRTP
and the corresponding SRTCP, if the pre-defined transforms (including
the key derivation) are used but the session key(s) MUST NOT be so
shared.
In addition, there can be cases (see Sections 8 and 9.1) where
several SRTP streams within a given RTP session, identified by their
synchronization source (SSRCs, which is part of the RTP header),
share most of the crypto context parameters (including possibly
master and session keys). In such cases, just as in the normal
SRTP/SRTCP parameter sharing above, separate replay lists and packet
counters for each stream (SSRC) MUST still be maintained. Also,
separate SRTP indices MUST then be maintained.
A summary of parameters, pre-defined transforms, and default values
for the above parameters (and other SRTP parameters) can be found in
Sections 5 and 8.2.
3.2.2. Transform-dependent parameters
All encryption, authentication/integrity, and key derivation
parameters are defined in the transforms section (Section 4).
Typical examples of such parameters are block size of ciphers,
session keys, data for the Initialization Vector (IV) formation, etc.
Future SRTP transform specifications MUST include a section to list
the additional cryptographic context’s parameters for that transform,
if any.
3.2.3. Mapping SRTP Packets to Cryptographic Contexts
Recall that an RTP session for each participant is defined [RFC3550]
by a pair of destination transport addresses (one network address
plus a port pair for RTP and RTCP), and that a multimedia session is
defined as a collection of RTP sessions. For example, a particular
multimedia session could include an audio RTP session, a video RTP
session, and a text RTP session.
A cryptographic context SHALL be uniquely identified by the triplet
context identifier:
context id = <SSRC, destination network address, destination
transport port number>
where the destination network address and the destination transport
port are the ones in the SRTP packet. It is assumed that, when
presented with this information, the key management returns a context
with the information as described in Section 3.2.
As noted above, SRTP and SRTCP by default share the bulk of the
parameters in the cryptographic context. Thus, retrieving the crypto
context parameters for an SRTCP stream in practice may imply a
binding to the correspondent SRTP crypto context. It is up to the
implementation to assure such binding, since the RTCP port may not be
directly deducible from the RTP port only. Alternatively, the key
management may choose to provide separate SRTP- and SRTCP- contexts,
duplicating the common parameters (such as master key(s)). The
latter approach then also enables SRTP and SRTCP to use, e.g.,
distinct transforms, if so desired. Similar considerations arise
when multiple SRTP streams, forming part of one single RTP session,
share keys and other parameters.
If no valid context can be found for a packet corresponding to a
certain context identifier, that packet MUST be discarded.
3.3. SRTP Packet Processing
The following applies to SRTP. SRTCP is described in Section 3.4.
Assuming initialization of the cryptographic context(s) has taken
place via key management, the sender SHALL do the following to
construct an SRTP packet:
1. Determine which cryptographic context to use as described in
Section 3.2.3.
2. Determine the index of the SRTP packet using the rollover counter,
the highest sequence number in the cryptographic context, and the
sequence number in the RTP packet, as described in Section 3.3.1.
3. Determine the master key and master salt. This is done using the
index determined in the previous step or the current MKI in the
cryptographic context, according to Section 8.1.
4. Determine the session keys and session salt (if they are used by
the transform) as described in Section 4.3, using master key,
master salt, key_derivation_rate, and session key-lengths in the
cryptographic context with the index, determined in Steps 2 and 3.
5. Encrypt the RTP payload to produce the Encrypted Portion of the
packet (see Section 4.1, for the defined ciphers). This step uses
the encryption algorithm indicated in the cryptographic context,
the session encryption key and the session salt (if used) found in
Step 4 together with the index found in Step 2.
6. If the MKI indicator is set to one, append the MKI to the packet.
7. For message authentication, compute the authentication tag for the
Authenticated Portion of the packet, as described in Section 4.2.
This step uses the current rollover counter, the authentication
algorithm indicated in the cryptographic context, and the session
authentication key found in Step 4. Append the authentication tag
to the packet.
8. If necessary, update the ROC as in Section 3.3.1, using the packet
index determined in Step 2.
To authenticate and decrypt an SRTP packet, the receiver SHALL do the
following:
1. Determine which cryptographic context to use as described in
Section 3.2.3.
2. Run the algorithm in Section 3.3.1 to get the index of the SRTP
packet. The algorithm uses the rollover counter and highest
sequence number in the cryptographic context with the sequence
number in the SRTP packet, as described in Section 3.3.1.
3. Determine the master key and master salt. If the MKI indicator in
the context is set to one, use the MKI in the SRTP packet,
otherwise use the index from the previous step, according to
Section 8.1.
4. Determine the session keys, and session salt (if used by the
transform) as described in Section 4.3, using master key, master
salt, key_derivation_rate and session key-lengths in the
cryptographic context with the index, determined in Steps 2 and 3.
5. For message authentication and replay protection, first check if
the packet has been replayed (Section 3.3.2), using the Replay
List and the index as determined in Step 2. If the packet is
judged to be replayed, then the packet MUST be discarded, and the
event SHOULD be logged.
Next, perform verification of the authentication tag, using the
rollover counter from Step 2, the authentication algorithm
indicated in the cryptographic context, and the session
authentication key from Step 4. If the result is "AUTHENTICATION
FAILURE" (see Section 4.2), the packet MUST be discarded from
further processing and the event SHOULD be logged.
6. Decrypt the Encrypted Portion of the packet (see Section 4.1, for
the defined ciphers), using the decryption algorithm indicated in
the cryptographic context, the session encryption key and salt (if
used) found in Step 4 with the index from Step 2.
7. Update the rollover counter and highest sequence number, s_l, in
the cryptographic context as in Section 3.3.1, using the packet
index estimated in Step 2. If replay protection is provided, also
update the Replay List as described in Section 3.3.2.
8. When present, remove the MKI and authentication tag fields from
the packet.
3.3.1. Packet Index Determination, and ROC, s_l Update
SRTP implementations use an "implicit" packet index for sequencing,
i.e., not all of the index is explicitly carried in the SRTP packet.
For the pre-defined transforms, the index i is used in replay
protection (Section 3.3.2), encryption (Section 4.1), message
authentication (Section 4.2), and for the key derivation (Section
4.3).
When the session starts, the sender side MUST set the rollover
counter, ROC, to zero. Each time the RTP sequence number, SEQ, wraps
modulo 2^16, the sender side MUST increment ROC by one, modulo 2^32
(see security aspects below). The sender’s packet index is then
defined as
i = 2^16 * ROC + SEQ.
Receiver-side implementations use the RTP sequence number to
determine the correct index of a packet, which is the location of the
packet in the sequence of all SRTP packets. A robust approach for
the proper use of a rollover counter requires its handling and use to
be well defined. In particular, out-of-order RTP packets with
sequence numbers close to 2^16 or zero must be properly handled.
The index estimate is based on the receiver’s locally maintained ROC
and s_l values. At the setup of the session, the ROC MUST be set to
zero. Receivers joining an on-going session MUST be given the
current ROC value using out-of-band signaling such as key-management
signaling. Furthermore, the receiver SHALL initialize s_l to the RTP
sequence number (SEQ) of the first observed SRTP packet (unless the
initial value is provided by out of band signaling such as key
management).
On consecutive SRTP packets, the receiver SHOULD estimate the index
as
i = 2^16 * v + SEQ,
where v is chosen from the set { ROC-1, ROC, ROC+1 } (modulo 2^32)
such that i is closest (in modulo 2^48 sense) to the value 2^16 * ROC
+ s_l (see Appendix A for pseudocode).
After the packet has been processed and authenticated (when enabled
for SRTP packets for the session), the receiver MUST use v to
conditionally update its s_l and ROC variables as follows. If
v=(ROC-1) mod 2^32, then there is no update to s_l or ROC. If v=ROC,
then s_l is set to SEQ if and only if SEQ is larger than the current
s_l; there is no change to ROC. If v=(ROC+1) mod 2^32, then s_l is
set to SEQ and ROC is set to v.
After a re-keying occurs (changing to a new master key), the rollover
counter always maintains its sequence of values, i.e., it MUST NOT be
reset to zero.
As the rollover counter is 32 bits long and the sequence number is 16
bits long, the maximum number of packets belonging to a given SRTP
stream that can be secured with the same key is 2^48 using the pre-
defined transforms. After that number of SRTP packets have been sent
with a given (master or session) key, the sender MUST NOT send any
more packets with that key. (There exists a similar limit for SRTCP,
which in practice may be more restrictive, see Section 9.2.) This
limitation enforces a security benefit by providing an upper bound on
the amount of traffic that can pass before cryptographic keys are
changed. Re-keying (see Section 8.1) MUST be triggered, before this
amount of traffic, and MAY be triggered earlier, e.g., for increased
security and access control to media. Recurring key derivation by
means of a non-zero key_derivation_rate (see Section 4.3), also gives
stronger security but does not change the above absolute maximum
value.
On the receiver side, there is a caveat to updating s_l and ROC: if
message authentication is not present, neither the initialization of
s_l, nor the ROC update can be made completely robust. The
receiver’s "implicit index" approach works for the pre-defined
transforms as long as the reorder and loss of the packets are not too
great and bit-errors do not occur in unfortunate ways. In
particular, 2^15 packets would need to be lost, or a packet would
need to be 2^15 packets out of sequence before synchronization is
lost. Such drastic loss or reorder is likely to disrupt the RTP
application itself.
The algorithm for the index estimate and ROC update is a matter of
implementation, and should take into consideration the environment
(e.g., packet loss rate) and the cases when synchronization is likely
to be lost, e.g., when the initial sequence number (randomly chosen
by RTP) is not known in advance (not sent in the key management
protocol) but may be near to wrap modulo 2^16.
A more elaborate and more robust scheme than the one given above is
the handling of RTP’s own "rollover counter", see Appendix A.1 of
[RFC3550].
3.3.2. Replay Protection
Secure replay protection is only possible when integrity protection
is present. It is RECOMMENDED to use replay protection, both for RTP
and RTCP, as integrity protection alone cannot assure security
against replay attacks.
A packet is "replayed" when it is stored by an adversary, and then
re-injected into the network. When message authentication is
provided, SRTP protects against such attacks through a Replay List.
Each SRTP receiver maintains a Replay List, which conceptually
contains the indices of all of the packets which have been received
and authenticated. In practice, the list can use a "sliding window"
approach, so that a fixed amount of storage suffices for replay
protection. Packet indices which lag behind the packet index in the
context by more than SRTP-WINDOW-SIZE can be assumed to have been
received, where SRTP-WINDOW-SIZE is a receiver-side, implementation-
dependent parameter and MUST be at least 64, but which MAY be set to
a higher value.
The receiver checks the index of an incoming packet against the
replay list and the window. Only packets with index ahead of the
window, or, inside the window but not already received, SHALL be
accepted.
After the packet has been authenticated (if necessary the window is
first moved ahead), the replay list SHALL be updated with the new
index.
The Replay List can be efficiently implemented by using a bitmap to
represent which packets have been received, as described in the
Security Architecture for IP [RFC2401].
3.4. Secure RTCP
Secure RTCP follows the definition of Secure RTP. SRTCP adds three
mandatory new fields (the SRTCP index, an "encrypt-flag", and the
authentication tag) and one optional field (the MKI) to the RTCP
packet definition. The three mandatory fields MUST be appended to an
RTCP packet in order to form an equivalent SRTCP packet. The added
fields follow any other profile-specific extensions.
According to Section 6.1 of [RFC3550], there is a REQUIRED packet
format for compound packets. SRTCP MUST be given packets according
to that requirement in the sense that the first part MUST be a sender
report or a receiver report. However, the RTCP encryption prefix (a
random 32-bit quantity) specified in that Section MUST NOT be used
since, as is stated there, it is only applicable to the encryption
method specified in [RFC3550] and is not needed by the cryptographic
mechanisms used in SRTP.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+
|V=2|P| RC | PT=SR or RR | length | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| SSRC of sender | |
+>+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ |
| ~ sender info ~ |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| ~ report block 1 ~ |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| ~ report block 2 ~ |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| ~ ... ~ |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |V=2|P| SC | PT=SDES=202 | length | |
| +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ |
| | SSRC/CSRC_1 | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| ~ SDES items ~ |
| +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ |
| ~ ... ~ |
+>+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ |
| |E| SRTCP index | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+
| ~ SRTCP MKI (OPTIONAL) ~ |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| : authentication tag : |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
+-- Encrypted Portion Authenticated Portion -----+
Figure 2. An example of the format of a Secure RTCP packet,
consisting of an underlying RTCP compound packet with a Sender Report
and SDES packet.
The Encrypted Portion of an SRTCP packet consists of the encryption
(Section 4.1) of the RTCP payload of the equivalent compound RTCP
packet, from the first RTCP packet, i.e., from the ninth (9) octet to
the end of the compound packet. The Authenticated Portion of an
SRTCP packet consists of the entire equivalent (eventually compound)
RTCP packet, the E flag, and the SRTCP index (after any encryption
has been applied to the payload).
The added fields are:
E-flag: 1 bit, REQUIRED
The E-flag indicates if the current SRTCP packet is
encrypted or unencrypted. Section 9.1 of [RFC3550] allows
the split of a compound RTCP packet into two lower-layer
packets, one to be encrypted and one to be sent in the
clear. The E bit set to "1" indicates encrypted packet, and
"0" indicates non-encrypted packet.
SRTCP index: 31 bits, REQUIRED
The SRTCP index is a 31-bit counter for the SRTCP packet.
The index is explicitly included in each packet, in contrast
to the "implicit" index approach used for SRTP. The SRTCP
index MUST be set to zero before the first SRTCP packet is
sent, and MUST be incremented by one, modulo 2^31, after
each SRTCP packet is sent. In particular, after a re-key,
the SRTCP index MUST NOT be reset to zero again.
Authentication Tag: configurable length, REQUIRED
The authentication tag is used to carry message
authentication data.
MKI: configurable length, OPTIONAL
The MKI is the Master Key Indicator, and functions according
to the MKI definition in Section 3.
SRTCP uses the cryptographic context parameters and packet processing
of SRTP by default, with the following changes:
* The receiver does not need to "estimate" the index, as it is
explicitly signaled in the packet.
* Pre-defined SRTCP encryption is as specified in Section 4.1, but
using the definition of the SRTCP Encrypted Portion given in this
section, and using the SRTCP index as the index i. The encryption
transform and related parameters SHALL by default be the same
selected for the protection of the associated SRTP stream(s),
while the NULL algorithm SHALL be applied to the RTCP packets not
to be encrypted. SRTCP may have a different encryption transform
than the one used by the corresponding SRTP. The expected use for
this feature is when the former has NULL-encryption and the latter
has a non NULL-encryption.
The E-flag is assigned a value by the sender depending on whether the
packet was encrypted or not.
* SRTCP decryption is performed as in Section 4, but only if the E
flag is equal to 1. If so, the Encrypted Portion is decrypted,
using the SRTCP index as the index i. In case the E-flag is 0,
the payload is simply left unmodified.
* SRTCP replay protection is as defined in Section 3.3.2, but using
the SRTCP index as the index i and a separate Replay List that is
specific to SRTCP.
* The pre-defined SRTCP authentication tag is specified as in
Section 4.2, but with the Authenticated Portion of the SRTCP
packet given in this section (which includes the index). The
authentication transform and related parameters (e.g., key size)
SHALL by default be the same as selected for the protection of the
associated SRTP stream(s).
* In the last step of the processing, only the sender needs to
update the value of the SRTCP index by incrementing it modulo 2^31
and for security reasons the sender MUST also check the number of
SRTCP packets processed, see Section 9.2.
Message authentication for RTCP is REQUIRED, as it is the control
protocol (e.g., it has a BYE packet) for RTP.
Precautions must be taken so that the packet expansion in SRTCP (due
to the added fields) does not cause SRTCP messages to use more than
their share of RTCP bandwidth. To avoid this, the following two
measures MUST be taken:
1. When initializing the RTCP variable "avg_rtcp_size" defined in
chapter 6.3 of [RFC3550], it MUST include the size of the fields
that will be added by SRTCP (index, E-bit, authentication tag, and
when present, the MKI).
2. When updating the "avg_rtcp_size" using the variable "packet_size"
(section 6.3.3 of [RFC3550]), the value of "packet_size" MUST
include the size of the additional fields added by SRTCP.
With these measures in place the SRTCP messages will not use more