A Key Recovery Attack on Discrete Log-based Schemes Using a Prime Order Subgroup

. Consider the well-known oracle attack: somehow one gets a certain computation result as a function of a secret key from the secret key owner and tries to extract some information on the secret key. This attacking scenario is well understood in the cryptographic community. However, there are many protocols based on the discrete logarithm problem that turn out to leak many of the secret key bits from this oracle attack, unless suitable checkings are carried out. In this paper we present a key recovery attack on various discrete log-based schemes working in a prime order subgroup. Our attack may reveal part of, or the whole secret key in most Diffie-Hellman-type key exchange protocols and some applications of ElGamal encryption and signature schemes. 1 Introduction Many cryptographic protocols have been developed based on the discrete logarithm problem. The main objective of developers is to design a protocol that is as difficult to break as the underlying discrete logarithm problem un..