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Re: [Cfrg] A little room for AES-192 in TLS?

Ted Krovetz <ted@krovetz.net> Tue, 17 January 2017 16:20 UTC

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From: Ted Krovetz <ted@krovetz.net>
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Date: Tue, 17 Jan 2017 08:20:39 -0800
References: <20170115205926.853FB60A6D@jupiter.mumble.net> <1484577818.5104.1.camel@quad> <D4A2A7CE.57FDF%john.mattsson@ericsson.com> <CABcZeBPGxT=9iiChy4PxD_zMHWcHU=AhCLoe7wEHHtryw2rfwg@mail.gmail.com> <D4A2B50D.7E040%kenny.paterson@rhul.ac.uk> <CAHOTMVJrHBn4AR7PCJ14xKYCVjdxF7SiswiOABX_g6A5gsQGDg@mail.gmail.com> <1484593651.5104.49.camel@quad> <1df3ba4212e44f9d8e3e6fabf8610cc0@usma1ex-dag1mb1.msg.corp.akamai.com> <1484662079.5135.49.camel@quad> <9d54608c721c465788a38e5cc8e8cac6@usma1ex-dag1mb1.msg.corp.akamai.com> <CACz1E9rZrso0184wiiK04UJnv4sBWZwtM2yYumha08Z-4n0=KQ@mail.gmail.com> <CAHOTMVLGoj7RPFQBTRu_d+kOoBfrKmi+CG+ityyW=x3G4t_AaQ@mail.gmail.com>
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Subject: Re: [Cfrg] A little room for AES-192 in TLS?
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> ChaCha20 uses a 256-bit key:
> 
> https://tools.ietf.org/html/rfc7539 

If you model Chacha's internal hash function as a 48-byte-to-64-byte PRF, then it is secure against related keys too.

Chacha works by supplying 48 byte (Key || Nonce || Counter) to Chacha's internal hash function repeatedly, each time with a counter increment. If Chacha's internal hash function is a PRF, then clearly *any* change in the key produces independent output.

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