Heilman's Lamport scheme exploits the variability of ECDSA signature lengths and uses the OP_SIZE opcode to measure the size of the signature. The length of the signature is then signed using Lamport signatures. Even if a powerful quantum computer allows an attacker to create signatures without the private key, DER-encoded ECDSA signatures will still vary in length and must commit to the transactions that contain them. These transactions will also need to include any additional data necessary to make them valid, such as hash preimages.