Script
Bitcoin uses a scripting system for transactions. Forth-like, Script is simple, stack-based, and processed from left to right. It is purposefully not Turing-complete, with no loops.
A script is essentially a list of instructions recorded with each transaction that describe how the next person wanting to spend the Bitcoins being transferred can gain access to them. The script for a typical Bitcoin transfer to destination Bitcoin address D simply encumbers future spending of the bitcoins with two things: the spender must provide
- a public key that, when hashed, yields destination address D embedded in the script, and
- a signature to show evidence of the private key corresponding to the public key just provided.
Scripting provides the flexibility to change the parameters of what's needed to spend transferred Bitcoins. For example, the scripting system could be used to require two private keys, or a combination of several, or even no keys at all.
A transaction is valid if nothing in the combined script triggers failure and the top stack item is true (non-zero). The party who originally sent the Bitcoins now being spent, dictates the script operations that will occur last in order to release them for use in another transaction. The party wanting to spend them must provide the input(s) to the previously recorded script that results in those operations occurring last leaving behind true (non-zero).
The stacks hold byte vectors. Byte vectors are interpreted as little-endian variable-length integers with the most significant bit determining the sign of the integer. Thus 0x81 represents -1. 0x80 is another representation of zero (so called negative 0). Byte vectors are interpreted as Booleans where False is represented by any representation of zero, and True is represented by any representation of non-zero.
Words
This is a list of all Script words (commands/functions). Some of the more complicated opcodes are disabled out of concern that the client might have a bug in their implementation; if a transaction using such an opcode were to be included in the chain any fix would risk forking the chain.
True=1 and False=0.
Constants
When talking about scripts, these value-pushing words are usually omitted.
| Word | Opcode | Hex | Input | Output | Description |
|---|---|---|---|---|---|
| OP_0, OP_FALSE | 0 | 0x00 | Nothing. | (empty value) | An empty array of bytes is pushed onto the stack. (This is not a no-op: an item is added to the stack.) |
| N/A | 1-75 | 0x01-0x4b | (special) | data | The next opcode bytes is data to be pushed onto the stack |
| OP_PUSHDATA1 | 76 | 0x4c | (special) | data | The next byte contains the number of bytes to be pushed onto the stack. |
| OP_PUSHDATA2 | 77 | 0x4d | (special) | data | The next two bytes contain the number of bytes to be pushed onto the stack. |
| OP_PUSHDATA4 | 78 | 0x4e | (special) | data | The next four bytes contain the number of bytes to be pushed onto the stack. |
| OP_1NEGATE | 79 | 0x4f | Nothing. | -1 | The number -1 is pushed onto the stack. |
| OP_1, OP_TRUE | 81 | 0x51 | Nothing. | 1 | The number 1 is pushed onto the stack. |
| OP_2-OP_16 | 82-96 | 0x52-0x60 | Nothing. | 2-16 | The number in the word name (2-16) is pushed onto the stack. |
Flow control
| Word | Opcode | Hex | Input | Output | Description |
|---|---|---|---|---|---|
| OP_NOP | 97 | 0x61 | Nothing | Nothing | Does nothing. |
| OP_IF | 99 | 0x63 | <expression> if [statements] [else [statements]]* endif | If the top stack value is not 0, the statements are executed. The top stack value is removed. | |
| OP_NOTIF | 100 | 0x64 | <expression> if [statements] [else [statements]]* endif | If the top stack value is 0, the statements are executed. The top stack value is removed. | |
| OP_ELSE | 103 | 0x67 | <expression> 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 | <expression> if [statements] [else [statements]]* endif | Ends an if/else block. | |
| OP_VERIFY | 105 | 0x69 | True / false | Nothing / False | Marks transaction as invalid if top stack value is not true. True is removed, but false is not. |
| OP_RETURN | 106 | 0x6a | Nothing | Nothing | Marks transaction as invalid. |
Stack
| Word | Opcode | 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 | <Stack size> | 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 <n> | 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 <n> | ... 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 before 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
If any opcode marked as disabled is present in a script, it must abort and fail.
| Word | Opcode | Hex | Input | Output | Description |
|---|---|---|---|---|---|
| OP_CAT | 126 | 0x7e | x1 x2 | out | Concatenates two strings. disabled. |
| OP_SUBSTR | 127 | 0x7f | in begin size | out | Returns a section of a string. disabled. |
| OP_LEFT | 128 | 0x80 | in size | out | Keeps only characters left of the specified point in a string. disabled. |
| OP_RIGHT | 129 | 0x81 | in size | out | Keeps only characters right of the specified point in a string. disabled. |
| OP_SIZE | 130 | 0x82 | in | in size | Returns the length of the input string. |
Bitwise logic
If any opcode marked as disabled is present in a script, it must abort and fail.
| Word | Opcode | Hex | Input | Output | Description |
|---|---|---|---|---|---|
| OP_INVERT | 131 | 0x83 | in | out | Flips all of the bits in the input. disabled. |
| OP_AND | 132 | 0x84 | x1 x2 | out | Boolean and between each bit in the inputs. disabled. |
| OP_OR | 133 | 0x85 | x1 x2 | out | Boolean or between each bit in the inputs. disabled. |
| OP_XOR | 134 | 0x86 | x1 x2 | out | Boolean exclusive or between each bit in the inputs. disabled. |
| OP_EQUAL | 135 | 0x87 | x1 x2 | True / false | Returns 1 if the inputs are exactly equal, 0 otherwise. |
| OP_EQUALVERIFY | 136 | 0x88 | x1 x2 | True / false | Same as OP_EQUAL, but runs OP_VERIFY afterward. |
Arithmetic
Note: Arithmetic inputs are limited to signed 32-bit integers, but may overflow their output.
If any input value for any of these commands is longer than 4 bytes, the script must abort and fail. If any opcode marked as disabled is present in a script - it must also abort and fail.
| Word | Opcode | 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 | out | If the input is 0 or 1, it is flipped. Otherwise the output will be 0. |
| OP_0NOTEQUAL | 146 | 0x92 | in | out | 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. disabled. |
| OP_MOD | 151 | 0x97 | a b | out | Returns the remainder after dividing a by b. disabled. |
| 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 | out | If both a and b are not 0, the output is 1. Otherwise 0. |
| OP_BOOLOR | 155 | 0x9b | a b | out | If a or b is not 0, the output is 1. Otherwise 0. |
| OP_NUMEQUAL | 156 | 0x9c | a b | out | Returns 1 if the numbers are equal, 0 otherwise. |
| OP_NUMEQUALVERIFY | 157 | 0x9d | a b | out | Same as OP_NUMEQUAL, but runs OP_VERIFY afterward. |
| OP_NUMNOTEQUAL | 158 | 0x9e | a b | out | Returns 1 if the numbers are not equal, 0 otherwise. |
| OP_LESSTHAN | 159 | 0x9f | a b | out | Returns 1 if a is less than b, 0 otherwise. |
| OP_GREATERTHAN | 160 | 0xa0 | a b | out | Returns 1 if a is greater than b, 0 otherwise. |
| OP_LESSTHANOREQUAL | 161 | 0xa1 | a b | out | Returns 1 if a is less than or equal to b, 0 otherwise. |
| OP_GREATERTHANOREQUAL | 162 | 0xa2 | a b | out | 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 | out | Returns 1 if x is within the specified range (left-inclusive), 0 otherwise. |
Crypto
| Word | Opcode | Hex | Input | Output | Description |
|---|---|---|---|---|---|
| OP_RIPEMD160 | 166 | 0xa6 | in | hash | The input is hashed using RIPEMD-160. |
| OP_SHA1 | 167 | 0xa7 | in | hash | The input is hashed using SHA-1. |
| OP_SHA256 | 168 | 0xa8 | in | hash | The input is hashed using SHA-256. |
| OP_HASH160 | 169 | 0xa9 | in | hash | The input is hashed twice: first with SHA-256 and then with RIPEMD-160. |
| OP_HASH256 | 170 | 0xaa | in | hash | The input is hashed two times with SHA-256. |
| OP_CODESEPARATOR | 171 | 0xab | Nothing | Nothing | All of the signature checking words will only match signatures to the data after the most recently-executed OP_CODESEPARATOR. |
| OP_CHECKSIG | 172 | 0xac | sig pubkey | True / false | The entire transaction's outputs, inputs, and script (from the most recently-executed OP_CODESEPARATOR to the end) are hashed. The signature used by OP_CHECKSIG must be a valid signature for this hash and public key. If it is, 1 is returned, 0 otherwise. |
| OP_CHECKSIGVERIFY | 173 | 0xad | sig pubkey | True / false | Same as OP_CHECKSIG, but OP_VERIFY is executed afterward. |
| OP_CHECKMULTISIG | 174 | 0xae | x sig1 sig2 ... <number of signatures> pub1 pub2 <number of public keys> | True / False | For each signature and public key pair, OP_CHECKSIG is executed. If more public keys than signatures are listed, some key/sig pairs can fail. All signatures need to match a public key. If all signatures are valid, 1 is returned, 0 otherwise. Due to a bug, one extra unused value is removed from the stack. |
| OP_CHECKMULTISIGVERIFY | 175 | 0xaf | x sig1 sig2 ... <number of signatures> pub1 pub2 ... <number of public keys> | True / False | Same as OP_CHECKMULTISIG, but OP_VERIFY is executed afterward. |
Pseudo-words
These words are used internally for assisting with transaction matching. They are invalid if used in actual scripts.
| Word | Opcode | Hex | Description |
|---|---|---|---|
| OP_PUBKEYHASH | 253 | 0xfd | Represents a public key hashed with OP_HASH160. |
| OP_PUBKEY | 254 | 0xfe | Represents a public key compatible with OP_CHECKSIG. |
| OP_INVALIDOPCODE | 255 | 0xff | Matches any opcode that is not yet assigned. |
Reserved words
Any opcode not assigned is also reserved. Using an unassigned opcode makes the transaction invalid.
| Word | Opcode | Hex | When used... |
|---|---|---|---|
| OP_RESERVED | 80 | 0x50 | Transaction is invalid unless occuring in an unexecuted OP_IF branch |
| OP_VER | 98 | 0x62 | Transaction is invalid unless occuring in an unexecuted OP_IF branch |
| OP_VERIF | 101 | 0x65 | Transaction is invalid even when occuring in an unexecuted OP_IF branch |
| OP_VERNOTIF | 102 | 0x66 | Transaction is invalid even when occuring in an unexecuted OP_IF branch |
| OP_RESERVED1 | 137 | 0x89 | Transaction is invalid unless occuring in an unexecuted OP_IF branch |
| OP_RESERVED2 | 138 | 0x8a | Transaction is invalid unless occuring in an unexecuted OP_IF branch |
| OP_NOP1-OP_NOP10 | 176-185 | 0xb0-0xb9 | The word is ignored. |
Scripts
This is a list of interesting scripts. Keep in mind that all constants actually use the data-pushing commands above. Note that there is a small number of standard script forms that are relayed from node to node; non-standard scripts are accepted if they are in a block, but nodes will not relay them.
Standard Transaction to Bitcoin address (pay-to-pubkey-hash)
scriptPubKey: OP_DUP OP_HASH160 <pubKeyHash> OP_EQUALVERIFY OP_CHECKSIG scriptSig: <sig> <pubKey>
To demonstrate how scripts look on the wire, here is a raw scriptPubKey:
76 A9 14
OP_DUP OP_HASH160 Bytes to push
89 AB CD EF AB BA AB BA AB BA AB BA AB BA AB BA AB BA AB BA 88 AC
Data to push OP_EQUALVERIFY OP_CHECKSIG
Note: scriptSig is in the input of the spending transaction and scriptPubKey is in the output of the previously unspent i.e. "available" transaction.
Here is how each word is processed:
| Stack | Script | Description |
|---|---|---|
| Empty. | <sig> <pubKey> OP_DUP OP_HASH160 <pubKeyHash> OP_EQUALVERIFY OP_CHECKSIG | scriptSig and scriptPubKey are combined. |
| <sig> <pubKey> | OP_DUP OP_HASH160 <pubKeyHash> OP_EQUALVERIFY OP_CHECKSIG | Constants are added to the stack. |
| <sig> <pubKey> <pubKey> | OP_HASH160 <pubKeyHash> OP_EQUALVERIFY OP_CHECKSIG | Top stack item is duplicated. |
| <sig> <pubKey> <pubHashA> | <pubKeyHash> OP_EQUALVERIFY OP_CHECKSIG | Top stack item is hashed. |
| <sig> <pubKey> <pubHashA> <pubKeyHash> | OP_EQUALVERIFY OP_CHECKSIG | Constant added. |
| <sig> <pubKey> | OP_CHECKSIG | Equality is checked between the top two stack items. |
| true | Empty. | Signature is checked for top two stack items. |
Standard Generation Transaction (pay-to-pubkey)
OP_CHECKSIG is used directly without first hashing the public key. By default the reference implementation uses this form for coinbase payment, and scriptPubKeys of this transaction form are recognized as payments to user. The disadvantage of this transaction form is that the whole public key needs to be known in advance, implying longer payment addresses, and that it provides less protection in the event of a break in the ECDSA signature algorithm.
scriptPubKey: <pubKey> OP_CHECKSIG scriptSig: <sig>
Checking process:
| Stack | Script | Description |
|---|---|---|
| Empty. | <sig> <pubKey> OP_CHECKSIG | scriptSig and scriptPubKey are combined. |
| <sig> <pubKey> | OP_CHECKSIG | Constants are added to the stack. |
| true | Empty. | Signature is checked for top two stack items. |
Provably Unspendable/Prunable Outputs
The standard way to mark a transaction as provably unspendable is with a scriptPubKey of the following form:
scriptPubKey: OP_RETURN {zero or more ops}
OP_RETURN immediately marks the script as invalid, guaranteeing that no scriptSig exists that could possibly spend that output. Thus the output can be immediately pruned from the UTXO set even if it has not been spent. eb31ca1a4cbd97c2770983164d7560d2d03276ae1aee26f12d7c2c6424252f29 is an example: it has a single output of zero value, thus giving the full 0.125BTC fee to the miner who mined the transaction without adding an entry to the UTXO set. You can also use OP_RETURN to add data to a transaction without the data ever appearing in the UTXO set, as seen in 1a2e22a717d626fc5db363582007c46924ae6b28319f07cb1b907776bd8293fc; P2Pool does this with the share chain hash txout in the coinbase of blocks it creates.
Note that this mechanism is not yet a standard transaction type, and thus will not be relayed by nodes on mainnet.
Anyone-Can-Spend Outputs
Conversely a transaction can be made spendable by anyone at all:
scriptPubKey: (empty) scriptSig: OP_TRUE
With some software changes such transactions can be used as a way to donate funds to miners in addition to transaction fees: any miner who mines such a transaction can also include an additional one after it sending the funds to an address they control. This mechanism may be used in the future for fidelity bonds to sacrifice funds in a provable way.
Anyone-Can-Spend outputs are currently considered non-standard, and are not relayed on the P2P network.
Transaction puzzle
Transaction a4bfa8ab6435ae5f25dae9d89e4eb67dfa94283ca751f393c1ddc5a837bbc31b is an interesting puzzle.
scriptPubKey: OP_HASH256 6fe28c0ab6f1b372c1a6a246ae63f74f931e8365e15a089c68d6190000000000 OP_EQUAL scriptSig:
To spend the transaction you need to come up with some data such that hashing the data twice results in the given hash.
| Stack | Script | Description |
|---|---|---|
| Empty. | OP_HASH256 <given_hash> OP_EQUAL | |
| OP_HASH256 <given_hash> OP_EQUAL | scriptSig added to the stack. | |
| <data_hash> | <given_hash> OP_EQUAL | The data is hashed. |
| <data_hash> <given_hash> | OP_EQUAL | The given hash is pushed to the stack. |
| true | Empty. | The hashes are compared, leaving true on the stack. |
This transaction was successfully spent by 09f691b2263260e71f363d1db51ff3100d285956a40cc0e4f8c8c2c4a80559b1. The required data happened to be the Genesis block, and the given hash was the genesis block hash. Note that while transactions like this are fun, they are not secure, because they do not contain any signatures and thus any transaction attempting to spend them can be replaced with a different transaction sending the funds somewhere else.