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Two Bitcoin script opcodes, OPCHECKSIGFROMSTACK and OPCAT, can be combined to let Bitcoin scripts verify transaction structure at the moment of spending, according to an analysis published as the fourth installment in a series on Bitcoin covenant proposals.
The pairing requires no pre-signed key management and introduces no new consensus rules beyond the two opcodes themselves.
How each opcode functions OPCHECKSIGFROMSTACK, abbreviated OPCSFS, authenticates signatures over arbitrary data.
That single capability sets it apart from conventional Bitcoin signature opcodes, which check signatures against fixed transaction fields.
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