Re: [AVTCORE] draft-ietf-avtcore-srtp-aes-gcm-15: Issue with short tags
Stephen Farrell <stephen.farrell@cs.tcd.ie> Mon, 29 June 2015 16:33 UTC
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Date: Mon, 29 Jun 2015 17:33:18 +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" <avt@ietf.org>, "mcgrew@cisco.com" <mcgrew@cisco.com>
Subject: Re: [AVTCORE] draft-ietf-avtcore-srtp-aes-gcm-15: Issue with short tags
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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