Bitcoin Script ASM Format

Freddie Honohan ([e-mail])

Abstract

This proposal introduces a standardised method of dealing with the ASM representation of Bitcoin Script across the ecosystem language libraries (Py-SDK, TS-SDK, Go-SDK, Teranode etc).

Motivation

The purpose of this proposal is to provide a generalised standard in order for deterministic translation of Bitcoin Script to and from ASM for cross-language compatibility.

Currently, different implementations may produce inconsistent ASM representations for the same script. For example, the boolean value false might be rendered as OP_0, OP_FALSE depending on the library used. This lack of standardisation creates interoperability issues when scripts are shared across different tools and platforms.

Specification

1. It is important that ASM format scripts operate correctly regardless of where they were produced or processed.

2. The Hexadecimal and Binary representations are defined in BRC-14.

3. Since ASM format is primarily used for human-readability, op-codes with multiple representations like the boolean pair [OP_0, OP_1] are to be represented as their english full-name counterpart e.g. [OP_FALSE, OP_TRUE] Example:

   Hex:  0x00 0x51
   ASM:  OP_FALSE OP_TRUE
   Not:  OP_0 OP_1

1. Other discrepancies were found and documented here and rules are set out below to deal with them.

Implementation

1. The logic dealing with Bitcoin Script ASM is to be standardised across libraries and languages as described hereunder.

2. Op-Codes with several names must be parsed into the correct hex/binary byte despite the chosen ASM format name. Example Input Parsing:

   // All of these should parse to 0x00
   parseASM("OP_0")      // ✓ valid
   parseASM("OP_FALSE")  // ✓ valid
   
   // Both should output the same hex
   parseASM("OP_0") === parseASM("OP_FALSE")  // true

1. However op-codes with several names must be output into the most human-readable format (e.g. OP_0 will always output as OP_FALSE). Example Output Serialization:

   // Input hex, always outputs human-readable form
   toASM(0x00)  // "OP_FALSE" (not "OP_0")
   toASM(0x51)  // "OP_TRUE"  (not "OP_1")

1. Full Op-Code to Hex Table With Statuses

Index	Word	Hex	Legacy	Chronicles
0	OP_FALSE	0x00	✅	✅
1-75	OP_PUSHDATA	0x01-0x4b	✅	✅
76	OP_PUSHDATA1	0x4c	✅	✅
77	OP_PUSHDATA2	0x4d	✅	✅
78	OP_PUSHDATA4	0x4e	✅	✅
79	OP_1NEGATE	0x4f	✅	✅
80	OP_RESERVED	0x50	Reserved	Reserved
81	OP_TRUE	0x51	✅	✅
82	OP_2	0x52	✅	✅
83	OP_3	0x53	✅	✅
84	OP_4	0x54	✅	✅
85	OP_5	0x55	✅	✅
86	OP_6	0x56	✅	✅
87	OP_7	0x57	✅	✅
88	OP_8	0x58	✅	✅
89	OP_9	0x59	✅	✅
90	OP_10	0x5a	✅	✅
91	OP_11	0x5b	✅	✅
92	OP_12	0x5c	✅	✅
93	OP_13	0x5d	✅	✅
94	OP_14	0x5e	✅	✅
95	OP_15	0x5f	✅	✅
96	OP_16	0x60	✅	✅
97	OP_NOP	0x61	✅	✅
98	OP_VER	0x62	❌	✅
99	OP_IF	0x63	✅	✅
100	OP_NOTIF	0x64	✅	✅
101	OP_VERIF	0x65	❌	✅
102	OP_VERNOTIF	0x66	❌	✅
103	OP_ELSE	0x67	✅	✅
104	OP_ENDIF	0x68	✅	✅
105	OP_VERIFY	0x69	✅	✅
106	OP_RETURN	0x6a	✅	✅
107	OP_TOALTSTACK	0x6b	✅	✅
108	OP_FROMALTSTACK	0x6c	✅	✅
109	OP_2DROP	0x6d	✅	✅
110	OP_2DUP	0x6e	✅	✅
111	OP_3DUP	0x6f	✅	✅
112	OP_2OVER	0x70	✅	✅
113	OP_2ROT	0x71	✅	✅
114	OP_2SWAP	0x72	✅	✅
115	OP_IFDUP	0x73	✅	✅
116	OP_DEPTH	0x74	✅	✅
117	OP_DROP	0x75	✅	✅
118	OP_DUP	0x76	✅	✅
119	OP_NIP	0x77	✅	✅
120	OP_OVER	0x78	✅	✅
121	OP_PICK	0x79	✅	✅
122	OP_ROLL	0x7a	✅	✅
123	OP_ROT	0x7b	✅	✅
124	OP_SWAP	0x7c	✅	✅
125	OP_TUCK	0x7d	✅	✅
126	OP_CAT	0x7e	✅	✅
127	OP_SPLIT	0x7f	✅	✅
128	OP_NUM2BIN	0x80	✅	✅
129	OP_BIN2NUM	0x81	✅	✅
130	OP_SIZE	0x82	✅	✅
131	OP_INVERT	0x83	✅	✅
132	OP_AND	0x84	✅	✅
133	OP_OR	0x85	✅	✅
134	OP_XOR	0x86	✅	✅
135	OP_EQUAL	0x87	✅	✅
136	OP_EQUALVERIFY	0x88	✅	✅
137	OP_RESERVED1	0x89	Reserved	Reserved
138	OP_RESERVED2	0x8a	Reserved	Reserved
139	OP_1ADD	0x8b	✅	✅
140	OP_1SUB	0x8c	✅	✅
141	OP_2MUL	0x8d	❌	✅
142	OP_2DIV	0x8e	❌	✅
143	OP_NEGATE	0x8f	✅	✅
144	OP_ABS	0x90	✅	✅
145	OP_NOT	0x91	✅	✅
146	OP_0NOTEQUAL	0x92	✅	✅
147	OP_ADD	0x93	✅	✅
148	OP_SUB	0x94	✅	✅
149	OP_MUL	0x95	✅	✅
150	OP_DIV	0x96	✅	✅
151	OP_MOD	0x97	✅	✅
152	OP_LSHIFT	0x98	✅	✅
153	OP_RSHIFT	0x99	✅	✅
154	OP_BOOLAND	0x9a	✅	✅
155	OP_BOOLOR	0x9b	✅	✅
156	OP_NUMEQUAL	0x9c	✅	✅
157	OP_NUMEQUALVERIFY	0x9d	✅	✅
158	OP_NUMNOTEQUAL	0x9e	✅	✅
159	OP_LESSTHAN	0x9f	✅	✅
160	OP_GREATERTHAN	0xa0	✅	✅
161	OP_LESSTHANOREQUAL	0xa1	✅	✅
162	OP_GREATERTHANOREQUAL	0xa2	✅	✅
163	OP_MIN	0xa3	✅	✅
164	OP_MAX	0xa4	✅	✅
165	OP_WITHIN	0xa5	✅	✅
166	OP_RIPEMD160	0xa6	✅	✅
167	OP_SHA1	0xa7	✅	✅
168	OP_SHA256	0xa8	✅	✅
169	OP_HASH160	0xa9	✅	✅
170	OP_HASH256	0xaa	✅	✅
171	OP_CODESEPARATOR	0xab	✅	✅
172	OP_CHECKSIG	0xac	✅	✅
173	OP_CHECKSIGVERIFY	0xad	✅	✅
174	OP_CHECKMULTISIG	0xae	✅	✅
175	OP_CHECKMULTISIGVERIFY	0xaf	✅	✅
176	OP_NOP1	0xb0	✅	✅
177	OP_NOP2	0xb1	✅	✅
178	OP_NOP3	0xb2	✅	✅
179	OP_SUBSTR	0xb3	❌	✅
180	OP_LEFT	0xb4	❌	✅
181	OP_RIGHT	0xb5	❌	✅
182	OP_NOP4	0xb6	✅	✅
183	OP_NOP5	0xb7	✅	✅
184	OP_NOP6	0xb8	✅	✅
185	OP_NOP7	0xb9	✅	✅
186	OP_NOP8	0xba	✅	✅
187	OP_NOP9	0xbb	✅	✅
188	OP_NOP10	0xbc	✅	✅
253	OP_PUBKEYHASH	0xfd	Pseudo	Pseudo
254	OP_PUBKEY	0xfe	Pseudo	Pseudo
255	OP_INVALIDOPCODE	0xff	Invalid	Invalid

References

• BRC-14: Bitcoin Script Binary and Hex Formats - Ty Everett ([e-mail])