Digital signature with secretly embedded warning

We present a digital signature scheme with secretly embedded warning. The embedded warning is a protection mechanism in case of restraint or blackmail. Extending ordinary digital signatures we propose schemes where a signer, approached by a powerful adversary that demands handing over a signing key, can disclose his private key. In our solution the signer is able to generate a feigned key indistinguishable from the genuine one. Then such a key can be used to embed a special warning message within a signature to indicate coercion. Such warnings can be transferred via subliminal channel to some trusted authority.