Re: [Cfrg] Curve25519 lacks private/public homomorphism?

Tony Arcieri <bascule@gmail.com> Tue, 05 February 2019 17:27 UTC

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From: Tony Arcieri <bascule@gmail.com>
Date: Tue, 5 Feb 2019 09:27:00 -0800
Message-ID: <CAHOTMVJCecMBeXfag6h9Ag2ZhkF43HC5Sa3k6J+TGmSCFWj9nw@mail.gmail.com>
To: Dan Brown <danibrown@blackberry.com>
Cc: Richard Barnes <rlb@ipv.sx>, CFRG <cfrg@irtf.org>
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Subject: Re: [Cfrg] Curve25519 lacks private/public homomorphism?
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On Tue, Feb 5, 2019 at 9:20 AM Dan Brown <danibrown@blackberry.com> wrote:

> Could you not just add delta to the private key (instead of multiplying
> the private key)?  (Add [delta]G to the public key.)
>

Using addition instead of multiplication is the BIP32 method, and it has
the same problems. See the Ed25519-BIP32 key recovery attack here:

https://forum.web3.foundation/t/key-recovery-attack-on-bip32-ed25519/44

-- 
Tony Arcieri