Script

Bitcoin Cash transactions make use of a scripting language to authorize and secure transfers. While, colloquially, there is a tendency to refer to transactions as "sending" Bitcoin Cash to "an address", that is merely an abstraction. In fact, the only thing that permits the spending of existing UTXOs is the successful execution of a script. The only thing preventing the spending of newly created UTXOs is the difficulty of producing a successfully executing script. Through the use of cryptographic signatures and hash functions, such scripts are often designed specifically to be difficult to produce unless you are the intended spender of a given UTXO, though that need not necessarily be the case.

For more information on how Transactions are commonly secured, see Locking Script.

This page will focus on how the scripts are run, what they are capable of, and what limitations they have.

Script Execution

Scripts are executed using a stack-based memory model and have an intentionally restricted set of available operations. Unlike the common general-purpose programming languages your are probably aware of, Script (the term for the language itself) does not allow for loops, persistent state/memory across script executions, or the definition of functions. Instead, scripts are expected to contain whatever data they need and use the available operations to prove transaction validity.

Features

In addition to the primary stack ("the stack"), there is a secondary stack, referred to as the "alt-stack", which data can be moved to temporarily. Any data left on the alt-stack is lost when a given sub-script finishes execution. In effect, any data moved to the alt-stack by an unlocking script is not present when the locking script runs.

There are a large number of op-codes that support everything from simple stack-manipulation, to mathematical calculations, to complex cryptographic processes. In terms of control structures there are only basic conditional branching (IF/ELSE) operations available.

Transaction Validation

Scripts are run when validating transactions, and successful execution of all of the scripts defined by the transaction is a necessary, but not sufficient, condition for transaction validity. See Transaction Validation for more details.

As a part of validating a transaction, a script is built for each input spent by the transaction. Each script is the sequential execution (carrying over the same stack, but not alt-stack) of the unlocking script provided with the input definition (which is used that the beginning of the script) and the locking script provided by the previous output being referenced. The exception to this is pay to script hash, which has an altered execution workflow. In general, though, this combined unlocking/locking script is then executed and considered successful if and only if the following conditions are met:

Additionally, in order for the combined script to be valid, the following must be true:

Operation codes (opcodes)

The table below lists the currently allocated op codes. Op codes marked with (ignored) are permitted but will do nothing when executed. Op codes marked with (disabled) are permitted in scripts so long as they are not executed. Op codes marked with (do not use) are disallowed and will make a transaction invalid merely be being present.

Constants

Word Value Hex Input Output Description
OP_0, OP_FALSE 0 0x00  0 An empty array of bytes is pushed onto the stack. See also OP_X
N/A 1-75 0x01-0x4b   The next value bytes is data to be pushed onto the stack. See also OP_DATA_X
OP_PUSHDATA1 76 0x4c   The next byte contains the number of bytes to be pushed onto the stack.
OP_PUSHDATA2 77 0x4d   The next two bytes contain the number of bytes to be pushed onto the stack in little endian order.
OP_PUSHDATA4 78 0x4e   The next four bytes contain the number of bytes to be pushed onto the stack in little endian order.
OP_1NEGATE 79 0x4f  -1 The number -1 is pushed onto the stack.
OP_1, OP_TRUE 81 0x51  1 The number 1 is pushed onto the stack.
OP_2-OP_16 82-96 0x52-0x60 2-16 The number (2-16) is pushed onto the stack.

Flow control

Word Value Hex Input Output Description
OP_NOP 97 0x61 Does nothing.
OP_IF 99 0x63 IF [statements] [ELSE [statements]] ENDIF If the top stack value is not False, the statements are executed. The top stack value is removed.
OP_NOTIF 100 0x64 NOTIF [statements] [ELSE [statements]] ENDIF If the top stack value is False, the statements are executed. The top stack value is removed.
OP_ELSE 103 0x67 IF [statements] [ELSE [statements]] ENDIF If the preceding OP_IF or OP_NOTIF or OP_ELSE was not executed then these statements are and if the preceding OP_IF or OP_NOTIF or OP_ELSE was executed then these statements are not.
OP_ENDIF 104 0x68 IF [statements] [ELSE [statements]] ENDIF Ends an if/else block. All blocks must end, or the transaction is marked as invalid. An OP_ENDIF without OP_IF earlier is also invalid.
OP_VERIFY 105 0x69 true / false Nothing / fail Marks transaction as invalid if top stack value is not true. The top stack value is removed.
OP_RETURN 106 0x6a fail Marks the output as unspendable. Since Bitcoin Core 0.9, a standard way of attaching extra data to transactions is to add a zero-value output with a scriptPubKey consisting of OP_RETURN followed by data. Such outputs are provably unspendable and specially discarded from storage in the UTXO set, reducing their cost to the network. Current standard relay rules on the Bitcoin Cash network allow a single output with OP_RETURN, that contains any sequence of push statements (or OP_RESERVED) after the OP_RETURN provided the total scriptPubKey length is at most 223 bytes.

Stack

Word Value Hex Input Output Description
OP_TOALTSTACK 107 0x6b x1 (alt) x1 Puts the input onto the top of the alt stack. Removes it from the main stack.
OP_FROMALTSTACK 108 0x6c (alt) x1 x1 Puts the input onto the top of the main stack. Removes it from the alt stack.
OP_IFDUP 115 0x73 x x / x x If the top stack value is not 0, duplicate it.
OP_DEPTH 116 0x74 Nothing Puts the number of stack items onto the stack.
OP_DROP 117 0x75 x Nothing Removes the top stack item.
OP_DUP 118 0x76 x x x Duplicates the top stack item.
OP_NIP 119 0x77 x1 x2 x2 Removes the second-to-top stack item.
OP_OVER 120 0x78 x1 x2 x1 x2 x1 Copies the second-to-top stack item to the top.
OP_PICK 121 0x79 xn ... x2 x1 x0 xn ... x2 x1 x0 xn The item n back in the stack is copied to the top.
OP_ROLL 122 0x7a xn ... x2 x1 x0 x(n-1) ... x2 x1 x0 xn The item n back in the stack is moved to the top.
OP_ROT 123 0x7b x1 x2 x3 x2 x3 x1 The top three items on the stack are rotated to the left.
OP_SWAP 124 0x7c x1 x2 x2 x1 The top two items on the stack are swapped.
OP_TUCK 125 0x7d x1 x2 x2 x1 x2 The item at the top of the stack is copied and inserted below the second-to-top item.
OP_2DROP 109 0x6d x1 x2 Nothing Removes the top two stack items.
OP_2DUP 110 0x6e x1 x2 x1 x2 x1 x2 Duplicates the top two stack items.
OP_3DUP 111 0x6f x1 x2 x3 x1 x2 x3 x1 x2 x3 Duplicates the top three stack items.
OP_2OVER 112 0x70 x1 x2 x3 x4 x1 x2 x3 x4 x1 x2 Copies the pair of items two spaces back in the stack to the front.
OP_2ROT 113 0x71 x1 x2 x3 x4 x5 x6 x3 x4 x5 x6 x1 x2 The fifth and sixth items back are moved to the top of the stack.
OP_2SWAP 114 0x72 x1 x2 x3 x4 x3 x4 x1 x2 Swaps the top two pairs of items.

Splice

Word Value Hex Input Output Description
OP_CAT 126 0x7e x1 x2 out Concatenates two byte sequences
OP_SPLIT 127 0x7f x n x1 x2 Splits byte sequence x at position n. Known as OP_SUBSTR before 2018-05-15.
OP_NUM2BIN 128 0x80 a b out Converts numeric value a into byte sequence of length b. Known as OP_LEFT before 2018-05-15.
OP_BIN2NUM 129 0x81 x out Converts byte sequence x into a numeric value. Known as OP_RIGHT before 2018-05-15.
OP_SIZE 130 0x82 x x size Pushes the string length of the top element of the stack (without popping it).

Bitwise logic

Word Value Hex Input Output Description
OP_INVERT 131 0x83 N/A N/A DISABLED
OP_AND 132 0x84 x1 x2 out Boolean AND between each bit of the inputs
OP_OR 133 0x85 x1 x2 out Boolean OR between each bit of the inputs.
OP_XOR 134 0x86 x1 x2 out Boolean EXCLUSIVE OR between each bit of the inputs.
OP_EQUAL 135 0x87 x1 x2 true / false Returns 1 if the inputs are exactly equal, 0 otherwise.
OP_EQUALVERIFY 136 0x88 x1 x2 Nothing / fail Same as OP_EQUAL, but runs OP_VERIFY afterward.

Arithmetic

Word Value Hex Input Output Description
OP_1ADD 139 0x8b in out 1 is added to the input.
OP_1SUB 140 0x8c in out 1 is subtracted from the input.
OP_2MUL 141 0x8d in out The input is multiplied by 2. DISABLED
OP_2DIV 142 0x8e in out The input is divided by 2. DISABLED
OP_NEGATE 143 0x8f in out The sign of the input is flipped.
OP_ABS 144 0x90 in out The input is made positive.
OP_NOT 145 0x91 in true / false If the input is 0 or 1, it is flipped. Otherwise the output will be 0.
OP_0NOTEQUAL 146 0x92 in true / false Returns 0 if the input is 0. 1 otherwise.
OP_ADD 147 0x93 a b out a is added to b.
OP_SUB 148 0x94 a b out b is subtracted from a.
OP_MUL 149 0x95 a b out a is multiplied by b. DISABLED
OP_DIV 150 0x96 a b out a is divided by b.
OP_MOD 151 0x97 a b out Returns the remainder after a is divided by b.
OP_LSHIFT 152 0x98 a b out Shifts a left b bits, preserving sign. DISABLED
OP_RSHIFT 153 0x99 a b out Shifts a right b bits, preserving sign. DISABLED
OP_BOOLAND 154 0x9a a b true / false If both a and b are not 0, the output is 1. Otherwise 0.
OP_BOOLOR 155 0x9b a b true / false If a or b is not 0, the output is 1. Otherwise 0.
OP_NUMEQUAL 156 0x9c a b true / false Returns 1 if the numbers are equal, 0 otherwise.
OP_NUMEQUALVERIFY 157 0x9d a b Nothing / fail Same as OP_NUMEQUAL, but runs OP_VERIFY afterward.
OP_NUMNOTEQUAL 158 0x9e a b true / false Returns 1 if the numbers are not equal, 0 otherwise.
OP_LESSTHAN 159 0x9f a b true / false Returns 1 if a is less than b, 0 otherwise.
OP_GREATERTHAN 160 0xa0 a b true / false Returns 1 if a is greater than b, 0 otherwise.
OP_LESSTHANOREQUAL 161 0xa1 a b true / false Returns 1 if a is less than or equal to b, 0 otherwise.
OP_GREATERTHANOREQUAL 162 0xa2 a b true / false Returns 1 if a is greater than or equal to b, 0 otherwise.
OP_MIN 163 0xa3 a b out Returns the smaller of a and b.
OP_MAX 164 0xa4 a b out Returns the larger of a and b.
OP_WITHIN 165 0xa5 x min max true / false Returns 1 if x is within the specified range (left-inclusive), 0 otherwise.

Cryptography

Word Value Hex Input Output Description
OP_RIPEMD160 166 0xa6 in hash Hashes input with RIPEMD-160.
OP_SHA1 167 0xa7 in hash Hashes input with SHA-1.
OP_SHA256 168 0xa8 in hash Hashes input with SHA-256.
OP_HASH160 169 0xa9 in hash Hashes input with SHA-256 and then with RIPEMD-160.
OP_HASH256 170 0xaa in hash Hashes input twice with SHA-256.
OP_CODESEPARATOR 171 0xab Nothing Nothing Makes OP_CHECK(MULTI)SIG(VERIFY) use the subset of the script of everything after the most recently-executed OP_CODESEPARATOR when computing the sighash.
OP_CHECKSIG 172 0xac sig pubkey true / false The last byte (=sighash type) of the signature is removed. The sighash for this input is calculated based on the sighash type. The truncated signature used by OP_CHECKSIG must be a valid ECDSA or Schnorr signature for this hash and public key. If it is valid, 1 is returned, if it is empty, 0 is returned, otherwise the operation fails.
OP_CHECKSIGVERIFY 173 0xad sig pubkey Nothing / fail Same as OP_CHECKSIG, but OP_VERIFY is executed afterward.
OP_CHECKMULTISIG 174 0xae dummy sig1 sig2 ... <# of sigs> pub1 pub2 ... <# of pubkeys> true / false Signatures are checked against public keys. Signatures must be placed in the unlocking script using the same order as their corresponding public keys were placed in the locking script or redeem script. If all signatures are valid, 1 is returned, 0 otherwise. All elements are removed from the stack. For more information on the execution of this opcode, see Multisignature.
OP_CHECKMULTISIGVERIFY 175 0xaf dummy sig1 sig2 ... <# of sigs> pub1 pub2 ... <# of pubkeys> Nothing / fail Same as OP_CHECKMULTISIG, but OP_VERIFY is executed afterward.
OP_CHECKDATASIG 186 0xba sig msg pubkey true / false Check if signature is valid for message and a public key. See spec 
OP_CHECKDATASIGVERIFY 187 0xbb sig msg pubkey nothing / fail  Same as OP_CHECKDATASIG, but runs OP_VERIFY afterward.

Locktime

Word Value Hex Input Output Description
OP_CHECKLOCKTIMEVERIFY 177 0xb1 x x / fail Marks transaction as invalid if the top stack item is greater than the transaction's nLockTime field, otherwise script evaluation continues as though an OP_NOP was executed. Transaction is also invalid if 1. the stack is empty; or 2. the top stack item is negative; or 3. the top stack item is greater than or equal to 500000000 while the transaction's nLockTime field is less than 500000000, or vice versa; or 4. the input's nSequence field is equal to 0xffffffff. The precise semantics are described in BIP65.
OP_CHECKSEQUENCEVERIFY 178 0xb2 x x / fail Marks transaction as invalid if the relative lock time of the input (enforced by BIP68 with nSequence) is not equal to or longer than the value of the top stack item. The precise semantics are described in BIP112.

Reserved

Word Value Hex Description
OP_NOP1 176 0xb0 Previously reserved for OP_EVAL (BIP12).
OP_NOP4-OP_NOP10 179-185 0b3-0xb9 Ignored. Does not mark transaction as invalid.

Uncategorized

Please help improve this article by catigorizing and describing the following up codes.

Hex Word
0x50 OP_RESERVED (disabled)
0x62 OP_VER (disabled)
0x65 OP_VERIF (do not use)
0x66 OP_VERNOTIF (do not use)
0x89 OP_RESERVED1 (do not use)
0x8A OP_RESERVED2 (do not use)
0xBC OP_REVERSEBYTES (disabled)
0xBD - 0xFF Unused (disabled)

Node-Specific Behavior

Some node implementations define custom op codes.

bchd

Op Code Range Name
0xFA OP_SMALLINTEGER
0xFB OP_PUBKEYS
0xFD OP_PUBKEYHASH
0xFE OP_PUBKEY
0xFF OP_INVALIDOPCODE

Bitcoin ABC

Op Code Range Name
0xF0 OP_PREFIX_BEGIN
0xF7 OP_PREFIX_END
0xFF OP_INVALIDOPCODE

Bitcoin Unlimited

Op Code Range Name
0xF0 OP_BIGINTEGER
0xF1 OP_DATA
0xFA OP_SMALLINTEGER
0xFB OP_PUBKEYS
0xFD OP_PUBKEYHASH
0xFE OP_PUBKEY
0xFF OP_INVALIDOPCODE

Flowee the Hub

Op Code Range Name
0xFA OP_SMALLINTEGER
0xFB OP_PUBKEYS
0xFD OP_PUBKEYHASH
0xFE OP_PUBKEY
0xFF OP_INVALIDOPCODE