Ladder Script introduces transaction version 4 (RUNG_TX). Spending conditions are declared as named, typed blocks organised into rungs — a model borrowed from industrial PLC ladder diagrams used in safety-critical factory control. Every byte in every block has a declared type with enforced size constraints. Evaluation is deterministic with bounded execution time. There are 59 block types across 10 families covering signatures, timelocks, hash verification, covenants, recursion, anchors, programmable logic, compound patterns, governance, and legacy Bitcoin transaction types.
Anti-spam hardened. Three coordinated defenses close all practical data embedding surfaces. merkle_pub_key: public keys are not stored in condition fields at all — they are folded into the Merkle leaf hash at fund time and verified positionally at spend time. There is no writable pubkey slot in conditions for an attacker to fill with arbitrary data. Selective inversion: key-consuming blocks (SIG, MULTISIG, HTLC, and 14 others) cannot be inverted. This prevents the garbage-pubkey attack where an attacker uses a junk key, lets the signature fail, inverts the result to SATISFIED, and lands arbitrary data in the block witness. Hash lock deprecation: HASH_PREIMAGE and HASH160_PREIMAGE are removed. These were invertible non-key blocks with writable hash fields — an inverted hash lock with a garbage hash and arbitrary preimage data could embed up to 252 bytes per block. All legitimate hash-lock use cases are covered by HTLC and HASH_SIG compound blocks, which combine hash verification with a mandatory signature. With MLSC (0xC2), the on-chain output is a fixed 33-byte scriptPubKey regardless of how many spending paths or conditions exist.
Post-quantum ready. FALCON-512, FALCON-1024, Dilithium3, and SPHINCS+ are native signature schemes. A single SCHEME field on any signature block routes verification to classical Schnorr or any post-quantum algorithm without changing the wire format or condition structure. With merkle_pub_key, condition size is constant regardless of key size — a 33-byte Schnorr key and a 1,952-byte Dilithium3 key produce the same Merkle leaf. The full key is provided only in the witness at spend time, where its hash is verified against the committed leaf. COSIGN lets a single PQ-secured UTXO serve as a co-spending guardian for unlimited classical UTXOs (theoretical max depth ~4.3 billion spends) — incremental PQ migration without a flag day.
Programmable money. Covenants, recursive conditions, rate limiters, state machines, sequenced approvals, fee-band gating, adaptor signatures, and protocol anchoring — all as composable typed blocks within the same wire format. The PLC family alone brings 14 block types drawn from decades of proven industrial automation patterns. Legacy Bitcoin transaction types (P2PKH through P2TR script-path) are wrapped as typed blocks, significantly reducing data-embedding surfaces including the taproot inscription vector.