Re: [AVTCORE] draft-ietf-avtcore-srtp-aes-gcm-15: Issue with short tags
Stephen Farrell <stephen.farrell@cs.tcd.ie> Sun, 07 June 2015 20:03 UTC
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Date: Sun, 07 Jun 2015 21:03:07 +0100
From: Stephen Farrell <stephen.farrell@cs.tcd.ie>
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To: Magnus Westerlund <magnus.westerlund@ericsson.com>, John Mattsson <john.mattsson@ericsson.com>, "avt@ietf.org" <avt@ietf.org>, "Igoe, Kevin M." <kmigoe@nsa.gov>, "mcgrew@cisco.com" <mcgrew@cisco.com>
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Cc: Tim Polk <tim.polk@nist.gov>
Subject: Re: [AVTCORE] draft-ietf-avtcore-srtp-aes-gcm-15: Issue with short tags
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Hi all, I currently have a discuss on this for a related reason that would otherwise be cleared by -16. I also support removal of the ciphersuite discussed below. So for Ben and chairs - I can either clear or keep my discuss and am happy to do whichever you prefer - just let me know. Lastly - thanks all for your forbearance in putting up with me trying to trim the list of ciphersuites at the last stage in the process. Cheers, S. On 02/06/15 14:42, Magnus Westerlund wrote: > WG, > > I have seen no reactions on this email from John. To my understanding > this appears to be a real issue and without anyone disputing his claims > I see the way forward is to request that the authors remove the ciphers > with short tags. > > Cheers > > Magnus Westerlund > (As WG chair) > > > John Mattsson skrev den 2015-05-21 17:35: >> Hi, >> >> My previous standpoint was that usage of GCM with short tags was >> acceptable if the NIST requirements were followed. Thinking more >> about the usage of GCM with short tags in general and the usage of >> GCM in SRTP in particular I have changed my mind. >> >> I do not think GCM with short tags (i.e. 64 bits) should be >> standardized by IETF even if the NIST requirements are followed, in >> fact I think that NIST should revise SP 800-38D. >> >> I strongly recommend that AEAD_AES_128_GCM_8 is removed from >> draft-ietf-avtcore-srtp-aes-gcm. >> >> (Note that this is only about GCM with short tags. I do fully >> recommend GCM for usage with 128-bit tags. I believe that with its >> excellent performance and proven security, it should be the first >> choice for everybody wanting an AEAD algorithm.) >> >> >> >> General usage of GCM with short tags: >> >> Regarding the general usage of GCM with short tags, I wrote a paper >> suggesting improvements to, and analyzing the complexity of, >> Ferguson’s method for authentication key recovery. In summary the >> security level (i.e. the effective key lengths) for GCM with 64-bit >> tags are 70–75 bits, far below not only the current NIST requirement >> of 112-bit security, but also the old NIST requirement of 80-bit >> security. >> >> https://eprint.iacr.org/2015/477 >> >> Note that draft-ietf-avtcore-srtp-aes-gcm-15 does not follow the NIST >> requirements, it choses deliberately to ignore them. This means that >> the security level for 64-bit tags against authentication key >> recovery is only 64 bits, down from the already low 70–75 bits >> offered by the NIST specification. >> >> >> >> Usage of GCM with short tags in SRTP: >> >> Regarding the usage of GCM in SRTP, Appendix C of SP 800-38D lists >> several guidelines for protocols using GCM with short tags. Two of >> these guidelines are that AAD should be limited to necessary header >> information and that protocols should not provide feedback regarding >> the integrity of individual packets. NIST then makes the statement: >> “An example of a protocol that meets these guidelines is Secure >> Real-time Transport Protocol carrying Voice over Internet Protocol, >> running over User Datagram Protocol”. This is not a correct statement >> and SRTP does in fact violate both of the guidelines mentioned >> above: >> >> - The AAD is not at all limited. In RTP, the associated data consists >> of the RTP header, which is not limited as e.g. the header in the TLS >> record layer. The RTP header is extensible with proprietary header >> extensions carrying any type of information. In RTCP, the scope of >> the AAD depends on the encryption flag E. If the encryption flag is >> '1', the AAD data is limited to necessary header information, but if >> the encryption flag is '0', the AAD consists of the entire RTCP >> packet. >> >> - RTCP receiver reports provide a wealth of information that can be >> used to determine the integrity of individual forged RTP packages, >> e.g. SSRC of the source, cumulative number of packets lost, extended >> highest sequence number received, last SR timestamp, and delay since >> last SR. The RTCP extension for port mapping [RFC6284] is even worse >> as it echoes back the 64-bit nonce received in the request. >> >> - RTP Rapid Synchronisation [RFC6051] is used, a forged Rapid >> Resynchronisation Request results in a RTP header extension with sync >> information sent from the sender. >> >> - If the RTP header extension Client-to-Mixer Audio Level Indication >> [RFC6464] is used, a forged RTP packet with a high audio level will >> result in the MCU forwarding the SSRC. As the SSRC is not encrypted, >> this is easily detected by the attacker. >> >> Even if encryption of RTCP is mandated and specific RTP header >> extensions and RTCP packets types are forbidden, an attacker may >> still in many cases determine whether a forgery was successful by >> looking at the length of packets. Either by looking at the length of >> RTCP packets from the sender or by looking at the length of RTP >> packets forwarded by an MCU. >> >> A further problem with SRTP and GCM is that SRTP is very often used >> in one-to-many scenarios. The maximum number of invocations of each >> instance of the authenticated decryption function would have to be >> restricted to q/r, where q is the maximum total number of invocations >> of the authenticated decryption function, and r is the total number >> of receivers, including any late joiners. >> >> All in all, SRTP does absolutely not meet the NIST guidelines for >> usage of GCM with short tags. >> >> Cheers, John >> > >
- [AVTCORE] draft-ietf-avtcore-srtp-aes-gcm-15 Igoe, Kevin M.
- Re: [AVTCORE] draft-ietf-avtcore-srtp-aes-gcm-15 Magnus Westerlund
- Re: [AVTCORE] draft-ietf-avtcore-srtp-aes-gcm-15 Ben Campbell
- Re: [AVTCORE] draft-ietf-avtcore-srtp-aes-gcm-15 Igoe, Kevin M.
- Re: [AVTCORE] draft-ietf-avtcore-srtp-aes-gcm-15 Ben Campbell
- Re: [AVTCORE] draft-ietf-avtcore-srtp-aes-gcm-15 John Mattsson
- [AVTCORE] draft-ietf-avtcore-srtp-aes-gcm-15: Iss… Magnus Westerlund
- Re: [AVTCORE] draft-ietf-avtcore-srtp-aes-gcm-15:… Stephen Farrell
- Re: [AVTCORE] draft-ietf-avtcore-srtp-aes-gcm-15:… Ben Campbell
- Re: [AVTCORE] draft-ietf-avtcore-srtp-aes-gcm-15:… Stephen Farrell
- Re: [AVTCORE] draft-ietf-avtcore-srtp-aes-gcm-15:… Ben Campbell
- Re: [AVTCORE] draft-ietf-avtcore-srtp-aes-gcm-15:… Stephen Farrell
- Re: [AVTCORE] draft-ietf-avtcore-srtp-aes-gcm-15:… Ben Campbell
- Re: [AVTCORE] draft-ietf-avtcore-srtp-aes-gcm-15:… Stephen Farrell
- Re: [AVTCORE] draft-ietf-avtcore-srtp-aes-gcm-15:… Ben Campbell