Re: [Dcrup] DKIM-Signing hashes with Ed25519
Viktor Dukhovni <ietf-dane@dukhovni.org> Wed, 14 November 2018 23:39 UTC
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From: Viktor Dukhovni <ietf-dane@dukhovni.org>
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Subject: Re: [Dcrup] DKIM-Signing hashes with Ed25519
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> On Nov 14, 2018, at 4:44 PM, Дилян Палаузов <dilyan.palauzov@aegee.org> wrote: > > At https://www.ietf.org/mail-archive/web/dcrup/current/msg00501.html is > stated, that GnuTLS does ED25519 signing of data, but not of hashed — > ED25519 is not meant for the latter. The answer at > https://www.ietf.org/mail-archive/web/dcrup/current/msg00502.html says, > that “if the spec says there’s a pure version that doesn’t hash its > input, the library would implement it.” Suppose a + b = c, then we also say: a = c - b. This just boils down hair-splitting as to what is the message, and what is the signature function. Traditionally, you take a message M and sign it with a signature function that employs a signing algorithm S combined with a hash function H: let f = Sign(S, H, keys) signature = f(metadata || M) Now with Ed25519 the signature primitive signs data directly, it is not parametrized by a hash algorithm. So the RFC boils down to: let f = Ed25519(keys) signature = f(metadata || H(M)) i.e. instead of signing the "message" M, you sign the message H(M). That's it. All that changes is whether the hash H is part of the signature function, or is used separately to prepare the message. In general uses of Ed25519, whether to pre-hash the message or not is an implementation choice. The pure construction with no pre-hash has better collision-resistance than you get with pre-hashing, but it requires the whole message to be in memory at the same time, so is only well-suited to sufficiently short messages: https://tools.ietf.org/html/rfc8032#section-4 -- Viktor.
- [Dcrup] DKIM-Signing hashes with Ed25519 Дилян Палаузов
- Re: [Dcrup] DKIM-Signing hashes with Ed25519 Jeremy Harris
- Re: [Dcrup] DKIM-Signing hashes with Ed25519 Viktor Dukhovni
- Re: [Dcrup] DKIM-Signing hashes with Ed25519 John Levine
- Re: [Dcrup] DKIM-Signing hashes with Ed25519 Jeremy Harris
- Re: [Dcrup] DKIM-Signing hashes with Ed25519 Salz, Rich