Cryptology ePrint Archive: Report 2002/129

Key-collisions in (EC)DSA: Attacking Non-repudiation

Tomas Rosa

Abstract: A new kind of attack on the non-repudiation property of digital signature schemes is presented. We introduce a notion of key-collisions, which may allow an attacker to claim that the message (presented to a judge) has been signed by someone else. We show how to compute key-collisions for the DSA and ECDSA signature schemes effectively. The main idea of these attacks has been inspired by the well-known notion of message-collisions, where an attacker claims that the signature presented at the court belongs to a different message. Both of these collision-based attacks significantly weaken the non-repudiation property of signature schemes. Moreover, they weaken the non-repudiation of protocols based on these schemes. It is shown that key-collision resistance of the (EC)DSA schemes requires the incorporation of a mechanism ensuring honest generation of (EC)DSA instances. The usage of such a mechanism shall be verifiable by an independent third party without revealing any secret information. We propose and discuss basic general countermeasures against key-collision attacks on the (EC)DSA schemes.

Category / Keywords: digital signatures, non-repudiation

Publication Info: Paper supports a talk given at CRYPTO 2002 Rump Session (was: On Key-collisions in (EC)DSA Schemes)

Date: received 26 Aug 2002, last revised 10 Jan 2004

Contact author: t_rosa at volny cz

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Version: 20040110:132836 (All versions of this report)

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