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IETF RFC 4771
Integrity Transform Carrying Roll-Over Counter for the Secure Real-time Transport Protocol (SRTP)
Last modified on Tuesday, January 16th, 2007
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Network Working Group V. Lehtovirta
Request for Comments: 4771 M. Naslund
Category: Standards Track K. Norrman
Ericsson
January 2007
Integrity Transform Carrying Roll-Over Counter
for the Secure Real-time Transport Protocol (SRTP)
 Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright © The IETF Trust (2007).
Abstract
This document defines an integrity transform for Secure Real-time
Transport Protocol (SRTP; see RFC 3711), which allows the roll-over
counter (ROC) to be transmitted in SRTP packets as part of the
authentication tag. The need for sending the ROC in SRTP packets
arises in situations where the receiver joins an ongoing SRTP session
and needs to quickly and robustly synchronize. The mechanism also
enhances SRTP operation in cases where there is a risk of losing
sender-receiver synchronization.
Table of Contents
1. Introduction ....................................................2
1.1. Terminology ................................................3
2. The Transform ...................................................3
3. Transform Modes .................................................5
4. Parameter Negotiation ...........................................5
5. Security Considerations .........................................7
6. IANA Considerations ............................................10
7. Acknowledgements ...............................................10
8. References .....................................................10
8.1. Normative References ......................................10
8.2. Informative References ....................................10
Lehtovirta, et al. Standards Track PAGE 1 
RFC 4771 Roll-Over Counter Carrying Transform January 2007
1. Introduction
When a receiver joins an ongoing SRTP [RFC 3711] session, out-of-band
signaling must provide the receiver with the value of the ROC the
sender is currently using. For instance, it can be transferred in
the Common Header Payload of a MIKEY [RFC 3830] message. In some
cases, the receiver will not be able to synchronize his ROC with the
one used by the sender, even if it is signaled to him out of band.
Examples of where synchronization failure will appear are:
1. The receiver receives the ROC in a MIKEY message together with a
key required for a particular continuous service. He does not,
however, join the service until after a few hours, at which point
the sender's sequence number (SEQ) has wrapped around, and so the
sender, meanwhile, has increased the value of ROC. When the user
joins the service, he grabs the SEQ from the first seen SRTP
packet and prepends the ROC to build the index. If integrity
protection is used, the packet will be discarded. If there is no
integrity protection, the packet may (if key derivation rate is
non-zero) be decrypted using the wrong session key, as ROC is used
as input in session key derivation. In either case, the receiver
will not have its ROC synchronized with the sender, and it is not
possible to recover without out-of-band signaling.
2. If the receiver leaves the session (due to being out of radio
coverage or because of a user action), and does not start
receiving traffic from the service again until after 2^15 packets
have been sent, the receiver will be out of synchronization (for
the same reasons as in example 1).
3. The receiver joins a service when the SEQ has recently wrapped
around (say, SEQ = 0x0001). The sender generates a MIKEY message
and includes the current value of ROC (say, ROC = 1) in the MIKEY
message. The MIKEY message reaches the receiver, who reads the
ROC value and initializes its local ROC to 1. Now, if an SRTP
packet prior to wraparound, i.e., with a SEQ lower than 0 (say,
SEQ = 0xffff), was delayed and reaches the receiver as the first
SRTP packet he sees, the receiver will initialize its highest
received sequence number, s_l, to 0xffff. Next, the receiver will
receive SRTP packets with sequence numbers larger than zero, and
will deduce that the SEQ has wrapped. Hence, the receiver will
incorrectly update the ROC and be out of synchronization.
4. Similarly to (3), since the initial SEQ is selected at random by
the sender, it may happen to be selected as a value very close to
0xffff. In this case, should the first few packets be lost, the
receiver may similarly end up out of synchronization.
Lehtovirta, et al. Standards Track PAGE 2
RFC 4771 Roll-Over Counter Carrying Transform January 2007
These problems have been recognized in, e.g., 3GPP2 and 3GPP, where
SRTP is used for streaming media protection in their respective
multicast/broadcast solutions [BCMCS][MBMS]. Problem 4 actually
exists inherently due to the way SEQ initialization is done in RTP.
One possible approach to address the issue could be to carry the ROC
in the MKI (Master Key Identifier) field of each SRTP packet. This
has the advantage that the receiver immediately knows the entire
index for a packet. Unfortunately, the MKI has no semantics in RFC
3711 (other than specifying master key), and a regular RFC 3711
compliant implementation would not be able to make use of the
information carried in the MKI. Furthermore, the MKI field is not
integrity protected; hence, care must be taken to avoid obvious
attacks against the synchronization.
In this document, a solution is presented where the ROC is carried in
the authentication tag of a special integrity transform in selected
SRTP packets.
The benefit of this approach is that the functionality of fast and
robust synchronization can be achieved as a separate integrity
transform, using the hooks existing in SRTP. Furthermore, when the
ROC is transmitted to the receiver it needs to be integrity protected
to avoid persistent denial-of-service (DoS) attacks or transmission
errors that could bring the receiver out of synchronization. (A DoS
attack is regarded as persistent if it can last after the attacker
has left the area; in this particular case, an attacker could modify
the ROC in one packet and the victim would be out of synchronization
until the next ROC is transmitted). The above discussion leads to
the conclusion that it makes sense to carry the ROC inside the
authentication tag of an integrity transform.
1.1. Terminology
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 RFC 2119 [RFC 2119].
2. The Transform
The transform, hereafter called Roll-over Counter Carrying Transform
(or RCC for short), works as follows.
The sender processes the RTP packet according to RFC 3711. When
applying the message integrity transform, the sender checks if the
SEQ is equal to 0 modulo some non-zero integer constant R. If that
is the case, the sender computes the MAC in the same way as is done
when using the default integrity transform (i.e., HMAC-SHA1(auth_key,
Lehtovirta, et al. Standards Track PAGE 3
RFC 4771 Roll-Over Counter Carrying Transform January 2007
Authenticated_portion || ROC)). Next, the sender truncates the MAC
by 32 bits to generate MAC_tr, i.e., MAC_tr is the tag_length - 32
most significant bits of the MAC. Next, the sender constructs the
tag as TAG = ROC_sender || MAC_tr, where ROC_sender is the value of
his local ROC, and appends the tag to the packet. See the security
considerations section for discussions on the effects of shortening
the MAC. In particular, note that a tag-length of 32 bits gives no
security at all.
If the SEQ is not equal to 0 mod R, the sender just proceeds to
process the packet according to RFC 3711 without performing the
actions in the previous paragraph.
The value R is the rate at which the ROC is included in the SRTP
packets. Since the ROC consumes four octets, this gives the
possibility to use it sparsely.
When the receiver receives an SRTP packet, it processes the packet
according to RFC 3711 except that during authentication processing
ROC_local is replaced by ROC_sender (retrieved from the packet).
This works as follows. In the step where integrity protection is to
be verified, if the SEQ is equal to 0 modulo R, the receiver extracts
ROC_sender from the TAG and verifies the MAC computed (in the same
way as if the default integrity transform was used) over the
authenticated portion of the packet (as defined in [RFC 3711]), but
concatenated with ROC_sender instead of concatenated with the
local_ROC. The receiver generates MAC_tr for the MAC verification in
the same way the sender did. Note that the session key used in the
MAC calculation is dependent on the ROC, and during the derivation of
the session integrity key, the ROC found in the packet under
consideration MUST be used. If the verification is successful, the
receiver sets his local ROC equal to the ROC carried in the packet.
If the MAC does not verify, the packet MUST be dropped. The
rationale for using the ROC from the packet in the MAC calculation is
that if the receiver has an incorrect ROC value, MAC verification
will fail, so the receiver will not correct his ROC.
If the SEQ is not equal to 0 mod R, the receiver just proceeds to
process the packet according to RFC 3711 without performing the
actions in the previous paragraph.
Since Secure Real-time Transport Control Protocol (SRTCP) already
carries the entire index in-band, there is no reason to apply this
transform to SRTCP. Hence, the transform SHALL only be applied to
SRTP, and SHALL NOT be used with SRTCP.
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RFC 4771 Roll-Over Counter Carrying Transform January 2007
3. Transform Modes
The above transform only provides integrity protection for the
packets that carry the ROC (this will be referred to as mode 1). In
the cases where there is a need to integrity protect all the packets,
the packets that do not have SEQ equal to 0 mod R MUST be protected
using the default integrity transform (this will be referred to as
mode 2).
Under some circumstances, it may be acceptable not to use integrity
protection on any of the packets; this will be referred to as mode 3.
Without integrity protection of the packets carrying the ROC, a DoS
attack, which will prevail until the next correctly received ROC, is
possible. Make sure to carefully read the security considerations in
Section 5 before using mode 3.
In case no integrity protection is offered, i.e., mode 3, the
following applies. The receiver's SRTP layer SHOULD ignore the ROC
value from the packet if the application layer can indicate to it
that the local ROC is synchronized with the sender (hence, the packet
would be processed using the local ROC). Note that the received ROC
still MUST be removed from the packet before continued processing.
In this scenario, the application layer feedback to the SRTP layer
need not be on a per-packet basis, and it can consist merely of a
boolean value set by the application layer and read by the SRTP
layer.
Thus, note the following difference. Using mode 2 will integrity
protect all RTP packets, but only add ROC to those having SEQ
divisible by R. Using mode 1 and setting R equal to one will also
integrity protect all packets, but will in addition to that add ROC
to each packet. Modes 1 and 2 MUST compute the MAC in the same way
as the pre-defined authentication transform for SRTP, i.e., HMAC-
SHA1.
To comply with this specification, mode 1, mode 2, and mode 3 are
MANDATORY to implement. However, it is up to local policy to decide
which mode(s) are allowed to be used.
4. Parameter Negotiation
RCC requires that a few parameters are signaled out of band. The
parameters that must be in place before the transform can be used are
integrity transform mode and the rate, R, at which the ROC will be
transmitted. This can be done using, e.g., MIKEY [RFC 3830].
Lehtovirta, et al. Standards Track PAGE 5
RFC 4771 Roll-Over Counter Carrying Transform January 2007
To perform the parameter negotiation using MIKEY, three integrity
transforms have been registered -- RCCm1, RCCm2, and RCCm3 in Table
6.10.1.c of [RFC 3830] -- for the three modes defined.
Table 1. Integrity transforms
SRTP auth alg | Value
--------------+------
RCCm1 | 2
RCCm2 | 3
RCCm3 | 4
Furthermore, the parameter R has been registered in Table 6.10.1.a of
[RFC 3830].
Table 2. Integrity transform parameter
Type | Meaning | Possible values
-----+-----------------------------+----------------
13 | ROC transmission rate | 16-bit integer
The ROC transmission rate, R, is given in network byte order. R MUST
be a non-zero unsigned integer. If the ROC transmission rate is not
included in the negotiation, the default value of 1 SHALL be used.
To have the ability to use different integrity transforms for SRTP
and SRTCP, which is needed in connection to the use of RCC, the
following additional parameters have been registered in Table
6.10.1.a of [RFC 3830]:
Table 3. Integrity parameters
Type | Meaning | Possible values
-----+-----------------------------+----------------
14 | SRTP Auth. algorithm | see below
15 | SRTCP Auth. algorithm | see below
16 | SRTP Session Auth. key len | see below
17 | SRTCP Session Auth. key len | see below
18 | SRTP Authentication tag len | see below
19 | SRTCP Authentication tag len| see below
The possible values for authentication algorithms (types 14 and 15)
are the same as for the "Authentication algorithm" parameter (type 2)
in Table 6.10.1.a of RFC 3830 with the addition of the values found
in Table 1 above.
Lehtovirta, et al. Standards Track PAGE 6
RFC 4771 Roll-Over Counter Carrying Transform January 2007
The possible values for session authentication key lengths (types 16
and 17) are the same as for the "Session Auth. key length" parameter
(type 3) in Table 6.10.1.a of RFC 3830.
The possible values for authentication tag lengths (types 18 and 19)
are the same as for the "Authentication tag length" parameter (type
11) in Table 6.10.1.a of RFC 3830 with the addition that the length
of ROC MUST be included in the "Authentication tag length" parameter.
This means that the minimum tag length when using RCC is 32 bits.
To avoid ambiguities when introducing these new parameters that have
overlapping functionality to existing parameters in Table 6.10.1.a of
RFC 3830, the following approach MUST be taken: If any of the
parameter types 14-19 (specifying behavior specific to SRTP or SRTCP)
and a corresponding general parameter (type 2, 3, or 11) are both
present in the policy, the more specific parameter SHALL have
precedence. For example, if the "Authentication algorithm" parameter
(type 2) is set to HMAC-SHA-1, and the "SRTP Auth. Algorithm" (type
14) is set to RCCm1, SRTP will use the RCCm1 algorithm, but since
there is no specific algorithm chosen for SRTCP, the more generally
specified one (HMAC-SHA-1) is used.
5. Security Considerations
An analogous method already exists in SRTCP (the SRTCP index is
carried in each packet under integrity protection). To the best of
our knowledge, the only security consideration introduced here is
that the entire SRTP index (ROC || SEQ) will become public since it
is transferred without encryption. (In normal SRTP operation, only
the SEQ-part of the index is disclosed.) However, RFC 3711 does not
identify a need for encrypting the SRTP index.
It is important to realize that only every Rth packet is integrity
protected in mode 1, so unless R = 1, the mechanism should be seen
for what it is: a way to improve sender-receiver synchronization, and
not a replacement for integrity protection.
The use of mode 3 (NULL-MAC) introduces a vulnerability not present
in RFC 3711; namely, if an attacker modifies the ROC, the
modification will go undetected by the receiver, and the receiver
will lose cryptographic synchronization until the next correct ROC is
received. This implies that an attacker can perform a DoS attack by
only modifying every Rth packet. Because of this, mode 3 MUST only
be used after proper risk assessment of the underlying network.
Besides the considerations in Section 9.5 and 9.5.1 of RFC 3711,
additional requirements of the underlying transport network must be
met.
Lehtovirta, et al. Standards Track PAGE 7
RFC 4771 Roll-Over Counter Carrying Transform January 2007
o The transport network must only consist of trusted domains. That
means that everyone on the path from the source to the destination
is trusted not to modify or inject packets.
o The transport network must be protected from packet injection,
i.e., it must be ensured that the only packets present on the path
from the source to the destination(s) originate from trusted
sources.
o If the packets, on their way from the source to the
destination(s), travel outside of a trusted domain, their
integrity must be ensured (e.g., by using a Virtual Private
Network (VPN) connection or a trusted leased line).
In the (assumed common) case that the last link to the destination(s)
is a wireless link, the possibility that an attacker injects forged
packets here must be carefully considered before using mode 3.
Especially, if used in a broadcast setting, many destinations would
be affected by the attack. However, unless R is big, this DoS attack
would be similar in effect to radio jamming, which would be easier to
perform.
It must also be noted that if the ROC is modified by an attacker and
no integrity protection is used, the output of the decryption will
not be useful to the upper layers, and these must be able to cope
with data that appears random. In the case integrity protection is
used on the packets containing the ROC, and the ROC is modified by an
attacker (and the receiver already has an approximation of the ROC,
e.g., by getting it previously), the packet will be discarded and the
receiver will not be able to decrypt correctly. Note, however, that
the situation is better in the latter case, since the receiver now
can try different ROC values in a neighborhood around the approximate
value he already has.
As RCC is expected to be used in a broadcast setting where group
membership will be based on access to a symmetric group key, it is
important to point out the following. With symmetric-key-based
integrity protection, it may be as easy, if not easier, to get access
to the integrity key (often a combination of a low-cost activity of
purchasing a subscription and breaking the security of a terminal to
extract the integrity key) as being able to transmit.
A word of warning regarding the choice of length of the
authentication tag: Note that, in contrast to common MAC tags, there
is a clear distinction made between the RCC authentication tag and
the RCC MAC. The tag is the container holding the MAC (and for some
packets also the ROC), and the MAC is the output from the MAC-
algorithm (i.e., HMAC-SHA1). The length of the authentication tag
Lehtovirta, et al. Standards Track PAGE 8
RFC 4771 Roll-Over Counter Carrying Transform January 2007
with the RCC transform includes the four-octet ROC in some packets.
This means that for a tag-length of n octets, there is only room for
a MAC of length n - 4, i.e., a tag-length of n octets does not
provide a full n-octet integrity protection on all packets. There
are five cases:
1. RCCm1 is used and tag-length is n. For those packets that
SEQ = 0 mod R, the ROC is carried in the tag and occupies four
octets. This leaves n - 4 octets for the MAC.
2. RCCm1 is used and tag-length is n. For those packets that
SEQ != 0 mod R, there is no ROC carried in the tag. For RCCm1
there is no MAC on packets not carrying the ROC, so neither the
length of the MAC nor the length of the tag has any relevance.
3. RCCm2 is used and tag-length is n. For those packets that
SEQ = 0 mod R, the ROC is carried in the tag and occupies four
octets. This leaves n - 4 octets for the MAC (this is
equivalent to case 1).
4. RCCm2 is used and tag-length is n. For those packets that
SEQ != 0 mod R, there is no ROC carried in the tag. This
leaves n octets for the MAC.
5. RCCm3 is used. RCCm3 does not use any MAC, but the ROC still
occupies four octets in the tag for packets with SEQ = 0 mod R,
so the tag-length MUST be set to four. For packets with
SEQ != 0 mod R, neither the length of the MAC nor the length of
the tag has any relevance.
The conclusion is that in cases 1 and 3, the length of the MAC is
shorter than the length of the authentication tag. To achieve the
same (or less) MAC forgery success probability on all packets when
using RCCm1 or RCCm2, as with the default integrity transform in RFC
3711, the tag-length must be set to 14 octets, which means that the
length of MAC_tr is 10 octets.
It is recommended to set the tag-length to 14 octets when RCCm1 or
RCCm2 is used, and the tag-length MUST be set to four octets when
RCCm3 is used.
Lehtovirta, et al. Standards Track PAGE 9
RFC 4771 Roll-Over Counter Carrying Transform January 2007
6. IANA Considerations
According to Section 10 of RFC 3830, IETF consensus is required to
register values in the range 0-240 in the SRTP auth alg namespace and
the SRTP Type namespace.
The value 2 for RCCm1, the value 3 for RCCm2, and the value 4 for
RCCm3 have been registered in the SRTP auth alg namespace as
specified in Table 1 in Section 4.
The value 13 for ROC transmission rate has been registered in the
SRTP Type namespace as specified in Table 2 in Section 4.
The values 14 to 19 have been registered in the SRTP Type namespace
according to Table 3 in Section 4.
7. Acknowledgements
We would like to thank Nigel Dallard, Lakshminath Dondeti, and David
McGrew for fruitful comments and discussions.
8. References
8.1. Normative References
[RFC 3830] Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K.
Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830,
August 2004.
[RFC 3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
8.2. Informative References
[MBMS] 3GPP TS 33.246, "3G Security; Security of Multimedia
Broadcast/ Multicast Service (MBMS)", October 2006.
[BCMCS] 3GPP2 X.S0022-0, "Broadcast and Multicast Service in
cdma2000 Wireless IP Network", February 2005.
Lehtovirta, et al. Standards Track PAGE 10
RFC 4771 Roll-Over Counter Carrying Transform January 2007
Authors' Addresses
Vesa Lehtovirta
Ericsson Research
02420 Jorvas
Finland
Phone: +358 9 2993314
EMail: vesa.lehtovirta@ericsson.com
Mats Naslund
Ericsson Research
SE-16480 Stockholm
Sweden
Phone: +46 8 58533739
EMail: mats.naslund@ericsson.com
Karl Norrman
Ericsson Research
SE-16480 Stockholm
Sweden
Phone: +46 8 4044502
EMail: karl.norrman@ericsson.com
Lehtovirta, et al. Standards Track PAGE 11
RFC 4771 Roll-Over Counter Carrying Transform January 2007
Full Copyright Statement
Copyright © The IETF Trust (2007).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
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Lehtovirta, et al. Standards Track PAGE 12
Integrity Transform Carrying Roll-Over Counter for the Secure Real-time Transport Protocol (SRTP)
RFC TOTAL SIZE: 27945 bytes
PUBLICATION DATE: Tuesday, January 16th, 2007
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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