Re: [TLS] Key agility issue in draft-ietf-tls-subcerts?

Ilari Liusvaara <ilariliusvaara@welho.com> Fri, 06 July 2018 05:35 UTC

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Date: Fri, 6 Jul 2018 08:34:09 +0300
From: Ilari Liusvaara <ilariliusvaara@welho.com>
To: "Patton,Christopher J" <cjpatton@ufl.edu>
Cc: "tls@ietf.org" <tls@ietf.org>
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Subject: Re: [TLS] Key agility issue in draft-ietf-tls-subcerts?
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On Fri, Jul 06, 2018 at 01:11:34AM +0000, Patton,Christopher J wrote:
> The string over which the delegation signature is computed contains
> the `SubjectPublicKeyInfo` of the DC public key. This in turn
> contains an `AlgorithmIdentifier`. Does an X.509 `AlgorithmIdentifier`
> determine a unique TLS `SignatureScheme`?

Unfortunately not. While the ECDSA and EdDSA keys indeed only have one
valid SignatureScheme, RSA keys have three (the difference being
using SHA-256 vs. SHA-384 vs. SHA-512). This holds for both genric
RSA keys and RSA-PSS keys (and the signatureschemes are different for
those two).

Also, relying on this even for ECDSA keys might not be a good idea,
as if another hash ever gets added, then there could be mulitple
SignatureSchemes even for ECDSA keys.


-Ilari