Re: attack against ElGamal (and related algorithms)

From: Atom 'Smasher' (ngbz_at_fhfcvpvbhf.bet)
Date: 12/21/03 

Date: Sun, 21 Dec 2003 06:28:21 -0500

> 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.
=====================

the point isn't for Eve to sign anything... the point is that Eve can use a
fixed "k" to encrypt a message using Bob's public key.

the protocol seems to require that a unique "k" is used for each encryption,
or else there's a problem. that problem seems easy to create under
controlled conditions.

atom

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