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Improvements to the Merkle signature scheme ( extended abstract )

We propose two efficient and secure versions of the Merkle signature scheme. One of them is forward secure and its security is based on that of its primitives: a collision resistant and one way hash function and a cryptographic secure pseudorandom bit generator. In the other version; the private key is composed of two separated parts: a user-key and a home-key. These parts of the private key evolve during several periods of the lifetime of the instance of the scheme, while the public key remains the same. The goal of the second version is to prevent forged signatures in all periods except for those where only the user-key is compromised. In those periods where only the home-key is compromised, the intruder cannot forge any signature at all. In case that the home-key and the user-key are compromised, the scheme result in a forward secure one. These two versions are quite efficient, existentially unforgeable under adaptive chosen message attack and have forward security.