2017 Volume E100.A Issue 1 Pages 73-90
In the ordinary security model for signature schemes, we consider an adversary that tries to forge a signature on a new message using only his knowledge of other valid message and signature pairs. To take into account side channel attacks such as tampering or fault-injection attacks, Bellare and Kohno (Eurocrypt 2003) formalized related-key attacks (RKA), where stronger adversaries are considered. In the RKA security model for signature schemes, we consider an adversary that can also manipulate the signing key and obtain signatures computed under the modified key. RKA security is defined with respect to the related-key deriving functions which are used by an adversary to manipulate the signing key. This paper considers RKA security of three established signature schemes: the Schnorr signature scheme, a variant of DSA, and a variant of ElGamal signature scheme. First, we show that these signature schemes are secure against a weak notion of RKA with respect to polynomial functions. Second, we demonstrate that, on the other hand, none of the Schnorr signature scheme, DSA, nor the ElGamal signature scheme achieves the standard notion of RKA security with respect to linear functions, by showing concrete attacks on these. Lastly, we show that slight modifications of the Schnorr signature scheme, (the considered variant of) DSA, and the variant of ElGamal signature scheme yield fully RKA secure schemes with respect to polynomial functions.
