APO lets a Bitcoin signature authorize any compatible UTXO rather than one fixed outpoint. It allows for rebindable pre-signed transactions for Lightning, vaults, and layer-2 protocols without new key management overhead. This is Part 3 in the technical article series about Bitcoin covenants by Cointelegraph Research. To read the previous article click here. SIGHASH_ANYPREVOUT, as proposed in BIP 118, builds on the earlier SIGHASH_NOINPUT concept mentioned in the 2015 Lightning Network paper by Joseph Poon and Thaddeus Dryja, and later formally proposed by Joseph Poon on the bitcoin-dev mailing list in February 2016. SIGHASH_ANYPREVOUT is not a new opcode but a proposed new value for the SIGHASH flag, designed to be deployed as a soft-fork upgrade to Bitcoin. The SIGHASH flag is appended to a signature and determines which parts of a transaction are signed and will be checked by the CHECKSIG opcode. The selected flag is chosen by the signer, not enforced by the scriptPubKey. Due to the technical details rela...