I found information, that it is possible to construct a hash and signature that look valid if the hash is not computed within the contract itself (we are talking about ECDSA/ecrecover here).
So, the task is to construct a hash + v, r, s that would resolve to a particular address using ecrecover(). I have access to several signed hashes.
Can anybody provide an additional clue how it would be possible? Unfortunately, there is very little information on this topic.
============ I was adviced to clarify the issue. So, this is the correct question: given (hash,r,s) known to verify against an ECDSA public key that can be recovered, construct (hash′,r′,s′) with hash′≠hash that verifies against the same public key.