Opcodes used in Bitcoin Script
Bitcoin uses a scripting system for transactions. Forth-like, Script is simple, stack-based, and processed from left to right. The script inside transaction outputs is intentionally not Turing-complete and has no jump instructions to prevent the formation of loops, however with the use of an off-chain agent turing-complete processes can be built using the ledger as a ticker tape to store computational results.
A transaction output script is a predicate formed by a list of instructions that describe how the next person wanting to transfer the tokens locked in the script must unlock them. The script for a typical P2PKH script to Bitcoin address D simply encumbers future spending of the bitcoins with the provision of two things:
- a public key that, when hashed, yields destination address D embedded in the script, and
- a signature to prove ownership 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 keys, or even no keys at all. The tokens are unlocked if the solution provided by the spending party leaves a non-zero value on the top of the stack when the script terminates.
De facto, Bitcoin script is defined by the code run by the nodes building the Block chain. Nodes collectively agree on the opcode set that is available for use, and how to process those opcodes. Throughout the history of Bitcoin there have been numerous changes to the way script is processed including the addition of new opcodes and disablement or outright removal of opcodes from the set.
The nodes checking Bitcoin script process transaction inputs in a script evaluation engine. The engine is comprised of three stacks which are:
- The main stack
- The alt stack
- The For-loop stack
The main and alt stacks hold byte vectors which can be used by Bitcoin opcodes to process script outcomes. When used as numbers, 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). Positive 0 is represented by a null-length vector. 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.
Currently byte vectors on the stack are not allowed to be more than 520 bytes long however in the unbounded bitcoin protocol while pushdata opcodes are limited to pushing 4.3GB onto the stack it is theoretically possible to concatenate multiple objects on the stack to form larger singular data items for processing.
Currently Opcodes which take integers and bools off the stack require that they be no more than 4 bytes long, but addition and subtraction can overflow and result in a 5 byte integer being put on the stack. After the Genesis upgrade in early 2020, miners will be free to mine transactions with data items of any size possible within protocol rules. These will be usable with mathematical functions within script. At this time, Miners will collectively agree on appropriate data limits rather than allowing a centralised committee to form a set of default constraints.
Opcodes
This is a list of all Script words, also known as opcodes, commands, or functions.
OP_NOP1-OP_NOP10 were originally set aside to be used when HASH and other security functions become insecure due to improvements in computing.
False is zero or negative zero (using any number of bytes) or an empty array, and True is anything else.
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 in little endian order. |
| OP_PUSHDATA4 | 78 | 0x4e | (special) | data | The next four bytes contain the number of bytes to be pushed onto the stack in little endian order. |
| 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_VER | 98 | 0x62 | Nothing | Transaction version | Puts the version of the transaction onto the stack DISABLED |
| OP_IF | 99 | 0x63 | <expression> 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 | <expression> notif [statements] [else [statements]]* endif | If the top stack value is False, the statements are executed. The top stack value is removed. | |
| OP_VERIF | 101 | 0x65 | Version | <version> verif [statements] [else [statements]]* endif | If the top stack value is EQUAL to the version of the transaction, the statements are executed. The top stack value is removed. DISABLED |
| OP_VERNOTIF | 102 | 0x66 | Version | <version> vernotif [statements] [else [statements]]* endif | If the top stack value is NOT EQUAL to the version of the transaction, the statements are executed. The top stack value is removed. DISABLED |
| 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. All blocks must end, or the transaction is 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 | Nothing | Ends script with top value on stack as final result | OP_RETURN can also be used to create "False Return" outputs with a scriptPubKey consisting of OP_FALSE OP_RETURN followed by data. Such outputs are provably unspendable and should be given a value of zero Satoshis. These outputs can be pruned from storage in the UTXO set, reducing its size. Currently the BitcoinSV network supports multiple FALSE RETURN outputs in a given transaction with each one capable of holding up to 100kB of data. After the Genesis upgrade in 2020 miners will be free to mine transactions containing FALSE RETURN outputs of any size. |
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. |
| Word | Opcode | Hex | Input | Output | Description |
|---|---|---|---|---|---|
| OP_CAT | 126 | 0x7e | x1 x2 | out | Concatenates two strings. |
| OP_SPLIT | 127 | 0x7f | in size | x1 x2 | Breaks a string into two sections of length 'size' and the remainder. |
| OP_SIZE | 130 | 0x82 | in | in size | Pushes the string length of the top element of the stack (without popping it). |
Bitwise logic
| Word | Opcode | Hex | Input | Output | Description |
|---|---|---|---|---|---|
| OP_INVERT | 131 | 0x83 | in | out | Flips all of the bits in the input. |
| OP_AND | 132 | 0x84 | x1 x2 | out | Boolean and between each bit in the inputs. |
| OP_OR | 133 | 0x85 | x1 x2 | out | Boolean or between each bit in the inputs. |
| OP_XOR | 134 | 0x86 | x1 x2 | out | Boolean exclusive or between each bit in 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
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.
| 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. |
| OP_2DIV | 142 | 0x8e | in | out | The input is divided by 2. |
| 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. |
| OP_DIV | 150 | 0x96 | a b | out | a is divided by b. |
| OP_MOD | 151 | 0x97 | a b | out | Returns the remainder after dividing a by b. |
| OP_LSHIFT | 152 | 0x98 | a b | out | Shifts a left b bits, preserving sign. |
| OP_RSHIFT | 153 | 0x99 | a b | out | Shifts a right b bits, preserving sign. |
| 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 | Nothing / fail | 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 | Nothing / fail | 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 | Compares the first signature against each public key until it finds an ECDSA match. Starting with the subsequent public key, it compares the second signature against each remaining public key until it finds an ECDSA match. The process is repeated until all signatures have been checked or not enough public keys remain to produce a successful result. All signatures need to match a public key. Because public keys are not checked again if they fail any signature comparison, signatures must be placed in the scriptSig using the same order as their corresponding public keys were placed in the scriptPubKey or redeemScript. 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> | Nothing / fail | Same as OP_CHECKMULTISIG, but OP_VERIFY is executed afterward. |
Locktime
| Word | Opcode | Hex | Input | Output | Description |
|---|---|---|---|---|---|
| OP_CHECKLOCKTIMEVERIFY (previously OP_NOP2) | 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 BIP 0065. This opcode will be deprecated post activation of the Genesis upgrade. Any UTXOs that incorporate it into their locking script will remain spendable however if it appears in new transactions it will be treated as OP_NOP2 |
| OP_CHECKSEQUENCEVERIFY (previously OP_NOP3) | 178 | 0xb2 | x | x / fail | Marks transaction as invalid if the relative lock time of the input (enforced by BIP 0068 with nSequence) is not equal to or longer than the value of the top stack item. The precise semantics are described in BIP 0112. This opcode will be deprecated post activation of the Genesis upgrade. Any UTXOs that incorporate it into their locking script will remain spendable however if it appears in new transactions it will be treated as OP_NOP3 |
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_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_NOP4-OP_NOP10 | 176, 179-185 | 0xb0, 0xb3-0xb9 | The word is ignored. Does not mark transaction as invalid. |
Script examples
The following is a list of interesting scripts. When notating scripts, data to be pushed to the stack is generally enclosed in angle brackets and data push commands are omitted. Non-bracketed words are opcodes. These examples include the “OP_” prefix, but it is permissible to omit it. Thus “<pubkey1> <pubkey2> OP_2 OP_CHECKMULTISIG” may be abbreviated to “<pubkey1> <pubkey2> 2 CHECKMULTISIG”. 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. |
Obsolete pay-to-pubkey transaction
OP_CHECKSIG is used directly without first hashing the public key. This was used by early versions of Bitcoin where people paid directly to IP addresses, before Bitcoin addresses were introduced. scriptPubKeys of this transaction form are still recognized as payments to user by Bitcoin Core. 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 'False Return' scriptPubKey of the following form:
scriptPubKey: OP_FALSE OP_RETURN {zero or more ops}
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 in the script was the genesis block header hashed twice with SHA-256. 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.
Incentivized finding of hash collisions
In 2013 Peter Todd created scripts that result in true if a hash collision is found. Bitcoin addresses resulting from these scripts can have money sent to them. If someone finds a hash collision they can spend the bitcoins on that address, so this setup acts as an incentive for somebody to do so.
For example the SHA1 script:
scriptPubKey: OP_2DUP OP_EQUAL OP_NOT OP_VERIFY OP_SHA1 OP_SWAP OP_SHA1 OP_EQUAL scriptSig: <preimage1> <preimage2>
See the bitcointalk thread <ref>bitcointalk forum thread on the hash collision bounties</ref> and reddit thread<ref>https://www.reddit.com/r/Bitcoin/comments/1mavh9/trustless_bitcoin_bounty_for_sha1_sha256_etc/</ref> for more details.
In February 2017 the SHA1 bounty worth 2.48 bitcoins was claimed.