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Re: [Cfrg] Ed25519 - X25519 keypair equivalences

chris - <chrispatton@gmail.com> Fri, 05 June 2020 18:35 UTC

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From: chris - <chrispatton@gmail.com>
Date: Fri, 05 Jun 2020 14:35:17 -0400
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To: Robert Moskowitz <rgm-sec@htt-consult.com>
Cc: cfrg@ietf.org
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Subject: Re: [Cfrg] Ed25519 - X25519 keypair equivalences
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Hi Robert,

Using the same secret for two different applications is not generally
secure. Whether it's secure depends intrinsically on what the applications
are.

Best,
Chris P.

On Fri, Jun 5, 2020 at 1:55 PM Robert Moskowitz <rgm-sec@htt-consult.com>
wrote:

> I am trying to understand rfc7748 sec 4.1 last paragraph:
>
>     The Montgomery curve defined here is equal to the one defined in
>     [curve25519], and the equivalent twisted Edwards curve is equal to
>     the one defined in [ed25519].
>
> It seems that if I have an Ed25519 key in an X.509 certificate, I can
> use that equally for an EdDSA signature and an ECDH key derivation.
>
> Are there limits on this?  Like a particular group of keypairs (like
> what parameters).
>
> Are there crypto risks/attacks if 2 keypairs are used between two
> entities both for signing (Ed25519) and encryption (via X25519)?
>
> This is potentially powerful...
>
> thanks
>
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