Re: [TLS] Last Call: <draft-kanno-tls-camellia-00.txt> (Additionx
Martin Rex <mrex@sap.com> Tue, 08 March 2011 20:18 UTC
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From: Martin Rex <mrex@sap.com>
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To: mrex@sap.com
Date: Tue, 08 Mar 2011 21:19:41 +0100
In-Reply-To: <201103082015.p28KFo12012459@fs4113.wdf.sap.corp> from "Martin Rex" at Mar 8, 11 09:15:50 pm
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Subject: Re: [TLS] Last Call: <draft-kanno-tls-camellia-00.txt> (Additionx
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resend (Sorry, for the typos.) Martin Rex wrote: > > The truncation of hashes/PRFs/HMACs is a trade-off. > > A trade-off between collision-resistance and how much clue > is provided about the input. TLSv1.0 (rfc2246) references RFC-2104 (HMAC) TLSv1.1 (rfc4346) contains a normative reference to RFC-2104 (HMAC) TLSv1.2 (rfc5246) contains a normative reference to RFC-2104 (HMAC) http://tools.ietf.org/html/rfc2104#section-5 5. Truncated output Applications of HMAC can choose to truncate the output of HMAC by outputting the t leftmost bits of the HMAC computation for some parameter t (namely, the computation is carried in the normal way as defined in section 2 ! above but the end result is truncated to t bits). We recommend that ! the output length t be not less than half the length of the hash ! output (to match the birthday attack bound) and not less than 80 bits (a suitable lower bound on the number of bits that need to be predicted by an attacker). That rfc-5246 did not increase the finished messages to at least 16 octets (128) when using a SHA-256 based PRF, without providing any rationale, seems like a defect to me. It is clearly against the recommendation in the normative reference. The difference is not so much about TLSv1.2 itself, where truncation to 96 bits is used with SHA-256 instead of the recommended minimum length of 128, but the bad precedent for successor specs using longer hashes, suggesting a truncation to 96 bits for a SHA-384 based PRF, which is extremely far off the recommended minimum HMAC truncation length of 192 bits for SHA-384. I think that a TLS cipher suite definition that uses a PRF based on SHA-384 ought to be required to provide a rationale when truncating the finished messages to 96 bits, instead of the minimum "half the length of the hash" recommended by rfc-2104. -Martin
- [TLS] Last Call: <draft-kanno-tls-camellia-00.txt… The IESG
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Nikos Mavrogiannopoulos
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Satoru Kanno
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Nikos Mavrogiannopoulos
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Sean Turner
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Martin Rex
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Eric Rescorla
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Marsh Ray
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Martin Rex
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Eric Rescorla
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Martin Rex
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Eric Rescorla
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Marsh Ray
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Eric Rescorla
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Martin Rex
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Eric Rescorla
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Martin Rex
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Eric Rescorla
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Martin Rex
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Martin Rex
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Marsh Ray
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Blumenthal, Uri - 0668 - MITLL
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Martin Rex
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Martin Rex
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Hovav Shacham
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Martin Rex
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Marsh Ray
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Blumenthal, Uri - 0668 - MITLL
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Marsh Ray
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Eric Rescorla
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Hovav Shacham
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Peter Gutmann
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Peter Gutmann
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Blumenthal, Uri - 0668 - MITLL
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Eric Rescorla
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Nikos Mavrogiannopoulos
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Peter Gutmann
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Martin Rex
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Yoav Nir
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Nikos Mavrogiannopoulos
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Martin Rex
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Eric Rescorla
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Eric Rescorla
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Eric Rescorla
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Eric Rescorla
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Martin Rex
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Nikos Mavrogiannopoulos
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Nikos Mavrogiannopoulos
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Eric Rescorla
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Eric Rescorla
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Martin Rex
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Eric Rescorla
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Peter Gutmann
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Stephen Kent
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Nikos Mavrogiannopoulos
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Eric Rescorla
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Martin Rex
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Eric Rescorla
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Stephen Kent
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Martin Rex
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Nikos Mavrogiannopoulos
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Martin Rex
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Marsh Ray
- Re: [TLS] Last Call: <draft-kanno-tls-camellia-00… Nikos Mavrogiannopoulos