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[TLS] Re: Composite ML-DSA

Eric Rescorla <ekr@rtfm.com> Wed, 15 April 2026 16:21 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Wed, 15 Apr 2026 09:16:45 -0700
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To: "Salz, Rich" <rsalz=40akamai.com@dmarc.ietf.org>
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CC: Daniel Van Geest <daniel.vangeest=40cryptonext-security.com@dmarc.ietf.org>, "<tls@ietf.org>" <tls@ietf.org>
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Subject: [TLS] Re: Composite ML-DSA
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On Wed, Apr 15, 2026 at 9:03 AM Salz, Rich <rsalz=
40akamai.com@dmarc.ietf.org> wrote:

>
>
>    - TLDR; Any Composite ML-DSA combination presents as just another
>    signature algorithm. Yes, there are too many Composite ML-DSA
>    combinations.  That is independent of the complication objections and is
>    easily remedied in TLS by defining a subset.
>
>
> Okay, this API (C code) is all over the place:
> int EVP_SignFinal(EVP_MD_CTX *ctx,/* a context handle */
> unsigned char *sig,unsigned int *s, /* buffersize and buffer */
> EVP_PKEY *pkey /*the private key */
> );
>
> What do I change, compatibly, to allow multiple private keys?  A new API?
> For everything that takes “a” private key, it now takes “a set” of private
> keys?
>

I think this is a good example, because I would be expecting no changes
here.

Specifically, my expectation is that a composite signature algorithm would
appear as if it were a single signature algorithm with a single key. Of
course it would internally have multiple keys, but that's no different from
existing algorithms whose keys have multiple components (e.g., RSA).

I agree that you'll need to define new wire formats for the composite
signature
keys and the signatures, but I wouldn't expect the APIs to change at all.

Note that I'm not arguing here for composite signature, but I don't think
API
complexity is an obstacle.

-Ekr

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