Internet Engineering Task Force Lehtovirta, Naslund, Norrman
(Ericsson)
INTERNET-DRAFT
EXPIRES: August 2006 February 2006
Integrity Transform Carrying Roll-over Counter
<draft-lehtovirta-srtp-rcc-01.txt>
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Abstract
This document defines an integrity transform for SRTP [RFC3711],
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 get into
synchronization. The mechanism also enhances SRTP operation in cases
where there is a risk of loosing sender-receiver synchronization.
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TABLE OF CONTENTS
1. Introduction...................................................2
2. The transform..................................................3
3. Transform versions.............................................4
4. Parameter negotiation..........................................5
5. Security Considerations........................................6
6. IANA Considerations............................................7
8. Author's Addresses.............................................7
9. References.....................................................8
1. Introduction
When a user joins an ongoing SRTP session, he must be given, using
out of band signalling, the value of the ROC the sender is currently
using. For instance, it can be transferred in the Security Payload
of a MIKEY [RFC3830] 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,
however, not join the service until after a few hours, at which
point the sender's sequence number (SEQ) has wrapped around, and
the sender hence has meanwhile 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. 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 signalling.
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 has 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 is close after
wraparound, say SEQ = 0x0001. The sender generates a MIKEY
message, and includes the current value of ROC, say ROC = 1, in
the security policy payload. The MIKEY message reaches the
receiver, who reads the ROC value and initializes its local ROC
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to 1. Now, if a 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 will be
out of synch.
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 synch.
These problems have been recognized in 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.
To avoid problems, 3GPP2 have chosen to carry the ROC in the MKI
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, and hence care must be taken
to avoid obvious attacks against the synchronization.
3GPP has agreed on a solution carrying the ROC in selected SRTP
packets. In this solution the ROC is carried in the authentication
tag of a special integrity transform.
The benefit of this approach is that the functionality of fast and
robust synchronization can be standardized as a separate integrity
transform in IETF, using the hooks existing in SRTP. This way,
there will not be interoperability problems, and other solutions
than the 3GPP multicast/broadcast multimedia service can make use of
the functionality as desired. Furthermore, when the ROC is
transmitted to the receiver it needs to be integrity protected, to
avoid DoS attacks or transmission errors bringing the receiver out
of synch. Hence, it makes sense to carry the ROC inside the
authentication tag of an integrity transform.
2. The transform
The transform, hereafter called Roll-over Counter Carrying Transform
(or RCC for short), works as follows.
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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 default integrity transform
over the authenticated portion of the packet(i.e. packet || ROC
sender), obtaining the value MAC. Next the sender constructs the
tag as TAG = ROC_sender || MAC, where ROC_sender is the value of his
local ROC, and appends the tag to the packet.
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. In the step where integrity protection is to
be verified, if the SEQ is equal to 0 modulo R, the receiver
verifies the MAC using the default integrity transform, but does not
include the four octets at the end of the packet containing the
sender's ROC value. According to RFC 3711, the receiver shall
include his own ROC value in the MAC calculation. In RCC, however,
the receiver replaces his local ROC value by the value found in the
packet in the MAC calculation. 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 packets 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, and the receiver will not correct his ROC
because of this.
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 SRTCP already carries the entire index inband, 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.
3. Transform versions
The above given transform only provides integrity protection for the
packets that carry the ROC (this will be referred to version 1). In
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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 version 2).
Thus, note the following difference. Using version 2 will integrity
protect all RTP packets, but only add ROC to those having SEQ
divisible by R. Using version 1 and setting R equal to one, will
also integrity protect all packets, but will in addition add ROC to
each packet.
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 version and the rate, R, at which the ROC
will be transmitted. This can be done using, e.g., MIKEY [RFC3830].
To perform the parameter negotiation using MIKEY, there is a need to
register two integrity transforms, RCCv1 and RCCv2 in Table 6.10.1.c
of [RFC3830].
Table 1. Integrity transforms
SRTP auth alg | Value
--------------+------
RCCv1 | 2
RCCv2 | 3
Furthermore, the parameter R, must be registered in Table 6.10.1.a
of [RFC3830].
Table 2. Integrity transform parameter
Type | Meaning | Possible values
-----+-----------------------------+----------------
13 | ROC transmission rate | 16-bit integer
The ROC transmission rate, R, is given with the leftmost bit being
the most significant. 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.
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To be able to use different integrity transforms for SRTP and SRTCP,
which is needed in connection to the use of RCC, the following
additional parameters must be registered in Table 6.10.1.a of
[RFC3830]:
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 (type 14 and 15)
are the same as for the "Authentication algorithm" parameter (type
2) in Table 6.10.1.a of RFC3830 with the addition of the values
found in Table 1 above.
The possible values for session authentication key lengths (type 16
and 17) are the same as for the "Session Auth. key length" parameter
(type 3) in Table 6.10.1.a of RFC3830.
The possible values for authentication tag lengths (type 18 and 19)
are the same as for the "Authentication tag length" parameter (type
11) in Table 6.10.1.a of RFC3830.
To avoid ambiguities when introducing these new parameters that have
overlapping functionality to existing parameters in Table 6.10.1.a
of RFC3830, 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 RCCV1, SRTP will use the RCCV1 algorithm, but since
there is no specific algorithm chosen for SRTCP, the more generally
specified one (HMAC-SHA-1) is used for SRTCP.
5. Security Considerations
An analogous method already exists in SRTCP (the SRTCP index is
carried in each packet under integrity protection) and 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
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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 R:th packet is integrity
protected in version 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.
6. IANA Considerations
Please add the following to the IANA registry at
http://www.iana.org/assignments/mikey-payloads (This paragraph to be
removed after IANA processing).
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.
It is requested to register the value 2 for RCCv1 and the value 3
for RCCv2 in the SRTP auth alg namespace as specified in Table 1 in
Section 4.
It is also requested to register the value 13 for ROC transmission
rate in the SRTP Type namespace as specified in Table 2 in Section
4.
It is also requested to register the values 14 to 19 according to
Table 3 in Section 4 to the SRTP Type namespace.
8. Author's Addresses
Questions and comments should be directed to the authors:
Vesa Lehtovirta
Ericsson Research
02420 Jorvas Phone: +358 9 2993314
Finland EMail: vesa.lehtovirta@ericsson.com
Mats Naslund
Ericsson Research
SE-16480 Stockholm Phone: +46 8 58533739
Sweden EMail: mats.naslund@ericsson.com
Karl Norrman
Ericsson Research
SE-16480 Stockholm Phone: +46 8 4044502
Sweden EMail: karl.norrman@ericsson.com
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9. References
Normative
[RFC3830] Arkko et al., "MIKEY: Multimedia Internet KEYing", RFC
3830, August 2004.
[RFC3711] Baugher et al., "The Secure Real-time Transport Protocol
(SRTP)", RFC3711, March 2004.
Informative
[MBMS] 3GPP TS 33.246, "Technical Specification 3rd Generation
Partnership Project; Technical Specification Group Services and
System Aspects; Security; Security of Multimedia Broadcast/Multicast
Service."
[BCMCS] 3GPP2 X.S0022-0, "Broadcast and Multicast Service in
cdma2000 Wireless IP network"
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