The Curious Case of the Half-Half Bitcoin ECDSA Nonces.

We report on a new class of ECDSA signature vulnerability observed in the wild on the Bitcoin blockchain that results from a signature nonce generated by concatenating half of the bits of the message hash together with half of the bits of the secret signing key. We give a lattice-based attack for efficiently recovering the secret key from a single signature of this form. We then search the entire Bitcoin blockchain for such signatures, and identify and track the activities of an apparently custom ECDSA/Bitcoin implementation that has been used to empty hundreds of compromised Bitcoin addresses for many years.