Passive SSH Key Compromise Via Lattices

This whitepaper demonstrates that a passive network attacker can opportunistically obtain private RSA host keys from an SSH server that experiences a naturally arising fault during signature computation. In prior work, this was not believed to be possible for the SSH protocol because the signature included information like the shared Diffie-Hellman secret that would not be available to a passive network observer. The paper shows that for the signature parameters commonly in use for SSH, there is an efficient lattice attack to recover the private key in case of a signature fault. The authors provide a security analysis of the SSH, IKEv1, and IKEv2 protocols in this scenario, and use their attack to discover hundreds of compromised keys in the wild from several independently vulnerable implementations.