Abstract. We present Binohash, a collision-resistant hash function for Bitcoin Script that enables limited transaction introspection without consensus changes. By exploiting the FindAndDelete quirk in legacy OP_CHECKMULTISIG combined with proof-of-work signature grinding, Binohash creates a transaction digest directly readable in Script. This enables covenant-like functionality for trustless chain introspection for protocols like BitVM bridges.
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OP_SIZE <60 - n > OP_EQUALVERIFY < pubkey > OP_CHECKSIGVERIFY
Instead of OP_EQUALVERIFY, OP_LESSTHAN OP_VERIFY can be used. Then, smaller signatures are also accepted. Also, OP_CHECKSEQUENCEVERIFY or OP_CHECKLOCKTIMEVERIFY can provide an incentive, to produce the smallest signature in a given time, and then, broadcast a valid transaction earlier, than other competing miners.