Re: [TLS] Deployment ... Re: This working group has failed

mrex@sap.com (Martin Rex) Tue, 19 November 2013 19:49 UTC

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To: Marsh Ray <maray@microsoft.com>
Date: Tue, 19 Nov 2013 20:49:48 +0100 (CET)
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From: mrex@sap.com (Martin Rex)
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Cc: "tls@ietf.org" <tls@ietf.org>, Michael Staubermann <Michael.Staubermann@webolution.de>
Subject: Re: [TLS] Deployment ... Re: This working group has failed
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Marsh Ray wrote:
> Martin Rex wrote:
> > 
> > The TLSv1.1 PRF should be OK for 128-bit security strength.
> 
> TLS 1.1 https://tools.ietf.org/html/rfc4346#page-13
> "   TLS's PRF is created by splitting the secret into two halves and
>    using one half to generate data with P_MD5 and the other half to
>    generate data with P_SHA-1, then exclusive-ORing the outputs of these
>    two expansion functions together."
> 
> Good for 160 bits I figure?
> 
> TLS 1.2  https://tools.ietf.org/html/rfc5246#page-14
> "   In this section, we define one PRF, based on HMAC.  This PRF with the
>    SHA-256 hash function is used for all cipher suites defined in this
>    document and in TLS documents published prior to this document when
>    TLS 1.2 is negotiated.  New cipher suites MUST explicitly specify a
>    PRF and, in general, SHOULD use the TLS PRF with SHA-256 or a
>    stronger standard hash function.
> "
> Sounds stronger to me.

The weakest link in the chain determines the strength, not the
strongest link in the chain.

NIST SP800-57 provides the following guidance on strength:

Bits of   symm.    FFC       IFC     ECC     Hash     Hash     Hash
security cipher  (DSA/DH) (e.g.RSA)         DigSig    HMAC   KeyDeriv

  80     2TDES    1024/160   1024  160-223   SHA-1    SHA-1    SHA-1
 112     3TDES    2048/224   2048  224-255  SHA-224   SHA-1    SHA-1
 128    AES-128   3072/256   3072  256-383  SHA-256   SHA-1    SHA-1
 192    AES-192   7680/384   7680  384-511  SHA-384  SHA-224  SHA-224
 256    AES-256  15360/512  15360   512+    SHA-512  SHA-256  SHA-256

So for use with RSA-2048 certs and AES128 encryption, the
TLSv1.0/v1.1 PRF should be OK.  You may want to look at how
many signatures in certificates, CRLs, OCSP, etc. still use
SHA-1, and where this gets you in the above table.


-Martin