Re: attack against ElGamal (and related algorithms)

From: Paul Rubin (//phr.cx_at_NOSPAM.invalid)
Date: 12/21/03 

Date: 21 Dec 2003 03:06:17 -0800

Atom 'Smasher' <ngbz@fhfcvpvbhf.bet> writes:
> 1) Eve has Bob's public key (it is, after all, public).
> 2) Eve's copy of PGP (or GnuPG, etc) is modified to use the same
> "k" every time.
> 3) Eve can encrypt as many messages as she wants, using Bob's public key
> and a fixed "k". (Eve knows the plain-text, the cipher-text, "k",
> and the public-key)
> 4) Eve can then recover Bob's private key.

No no, that's a misreading. To get Bob's private key, she'd have to
modify Bob's copy of PGP to re-use k, and then get Bob to sign two
different messages with the same k, not modify her own copy. If she
modifies her own copy as you suggest and then signs multiple messages,
then she can recover her own private key, but she already knows her
own private key so doesn't need to recover it.

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