Re: [AVTCORE] draft-ietf-avtcore-srtp-aes-gcm-15: Issue with short tags
Stephen Farrell <stephen.farrell@cs.tcd.ie> Wed, 22 July 2015 11:22 UTC
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Date: Wed, 22 Jul 2015 12:22:08 +0100
From: Stephen Farrell <stephen.farrell@cs.tcd.ie>
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To: Ben Campbell <ben@nostrum.com>
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Cc: Magnus Westerlund <magnus.westerlund@ericsson.com>, Tim Polk <tim.polk@nist.gov>, avt@ietf.org, mcgrew@cisco.com
Subject: Re: [AVTCORE] draft-ietf-avtcore-srtp-aes-gcm-15: Issue with short tags
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On 22/07/15 12:20, Ben Campbell wrote: > Hi Stephen, > > It looks like version 17 removed the short tag. Do you still expect to > re-review? Nope, will clear soon's I get a chance S > > Thanks! > > Ben. > > On 29 Jun 2015, at 18:33, Stephen Farrell wrote: > >> Hi Ben, >> >> Back from vacation... and I've cleared that. Happy to review >> when the short tag stuff is taken out of course (and I support >> taking that out). >> >> Cheers, >> S. >> >> On 16/06/15 23:12, Ben Campbell wrote: >>> Hi Stephen, >>> >>> If you are satisfied, please feel free to clear, unless you want to hold >>> a discuss on the short tag issue itself. I will not approve until that >>> (and some other things) are resolved. >>> >>> Thanks! >>> >>> Ben. >>> On 7 Jun 2015, at 15:03, Stephen Farrell wrote: >>> >>>> 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 >>>>>> >>>>> >>>>> >>>> >>>> _______________________________________________ >>>> Audio/Video Transport Core Maintenance >>>> avt@ietf.org >>>> https://www.ietf.org/mailman/listinfo/avt
- [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