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Opcodes

The complete txscript language includes over 100 operation codes, or opcodes, however only a small number of these are used in the vast majority of the transactions on the Decred blockchain.

Data Push

These opcodes are used to push commonly used constants onto the stack.

Word Opcode Hex Description
OP_FALSE/OP_0 0 0x00 An empty array of bytes is pushed onto the stack.
OP_DATA_1 - OP_DATA_75 1-75 0x01-0x4b The next 1-75 bytes is data to be pushed onto the stack.
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 The number -1 is pushed onto the stack.
OP_TRUE/OP_1 81 0x51 The number 1 is pushed onto the stack.
OP_2 - OP-16 82-96 0x52-0x60 The number 2-16 is pushed onto the stack.

Flow Control

These opcodes enable conditional logic as well as allowing scripts to stop execution.

Word Opcode Hex Description
OP_IF 99 0x63 If the top stack value is not False, the statements are executed. The top stack value is removed.
OP_NOTIF 100 0x64 If the top stack value is False, the statements are executed. The top stack value is removed.
OP_ELSE 103 0x67 If the preceding OP_IF or OP_NOTIF or OP_ELSE was not executed then these statements are executed, and if the preceding OP_IF or OP_NOTIF or OP_ELSE was executed then these statements are not executed.
OP_ENDIF 104 0x68 Ends an if/else block. All blocks must end, or script execution fails. An OP_ENDIF without OP_IF earlier is also invalid.
OP_VERIFY 105 0x69 Script execution fails if top stack value is not true. The top stack value is removed.
OP_RETURN 106 0x6a Script execution fails if executed.

Stack

These opcodes allow scripts to modify the state of the stack.

Word Opcode Hex Description
OP_TOALTSTACK 107 0x6b Puts the input onto the top of the alt stack. Removes it from the main stack.
OP_FROMALTSTACK 108 0x6c Puts the input onto the top of the main stack. Removes it from the alt stack.
OP_2DROP 109 0x6d Removes the top two stack items.
OP_2DUP 110 0x6e Duplicates the top two stack items.
OP_3DUP 111 0x6f Duplicates the top three stack items.
OP_2OVER 112 0x70 Copies the pair of items two spaces back in the stack to the front.
OP_2ROT 113 0x71 The fifth and sixth items back are moved to the top of the stack.
OP_2SWAP 114 0x72 Swaps the top two pairs of items.
OP_IFDUP 115 0x73 If the top stack value is not 0, duplicate it.
OP_DEPTH 116 0x74 Puts the number of stack items onto the stack.
OP_DROP 117 0x75 Removes the top stack item.
OP_DUP 118 0x76 Duplicates the top stack item.
OP_NIP 119 0x77 Removes the second-to-top stack item.
OP_OVER 120 0x78 Copies the second-to-top stack item to the top.
OP_PICK 121 0x79 The item n back in the stack is copied to the top.
OP_ROLL 122 0x7a The item n back in the stack is moved to the top.
OP_ROT 123 0x7b The top three items on the stack are rotated to the left.
OP_SWAP 124 0x7c The top two items on the stack are swapped.
OP_TUCK 125 0x7d The item at the top of the stack is copied and inserted before the second-to-top item.

Splice

These opcodes allow a script to perform a limited set of string manipulations.

Word Opcode Hex Description
OP_CAT 126 0x7e Concatenates two strings.
OP_SUBSTR 127 0x7f Returns a section of a string.
OP_LEFT 128 0x80 Keeps only characters left of the specified point in a string.
OP_RIGHT 129 0x81 Keeps only characters right of the specified point in a string.
OP_SIZE 130 0x82 Pushes the string length of the top element of the stack (without popping it).

Bitwise Logic

These opcodes allow scripts to manipulate data by performing bit-level logic operations.

Inputs are limited to signed 32-bit integers.

Word Opcode Hex Description
OP_INVERT 131 0x83 Flips all of the bits in the input.
OP_AND 132 0x84 Boolean and between each bit in the inputs.
OP_OR 133 0x85 Boolean or between each bit in the inputs.
OP_XOR 134 0x86 Boolean exclusive or between each bit in the inputs.

Bytewise Comparison

These opcodes compare the top two items on the stack as raw bytes.

Word Opcode Hex Description
OP_EQUAL 135 0x87 Returns 1 if the inputs are exactly equal, 0 otherwise.
OP_EQUALVERIFY 136 0x88 Same as OP_EQUAL, but runs OP_VERIFY afterward.

Bitwise Rotation

These opcodes enable circular shifting of bits. They treat the top two items on the stack as 32-bit integers, the first respresents the number of bits to rotate to the left or right, and the second is the value to be rotated.

Word Opcode Hex Description
OP_ROTR 137 0x89 Shift bits right, sign is not preserved.
OP_ROTL 138 0x8a Shift bits left, sign is not preserved.

Arithmetic

These opcodes implement common arithmetic operations to be used on integer values.

Arithmetic inputs are limited to signed 32-bit integers, but may overflow their output. For example, it is possible for OP_ADD to have 2^31 - 1 for its two operands resulting 2^32 - 2, which overflows, but is still pushed to the stack as the result of the addition.

Word Opcode Hex Description
OP_1ADD 139 0x8b 1 is added to the input.
OP_1SUB 140 0x8c 1 is subtracted from the input.
OP_2MUL 141 0x8d The input is multiplied by 2.
OP_2DIV 142 0x8e The input is divided by 2.
OP_NEGATE 143 0x8f The sign of the input is flipped.
OP_ABS 144 0x90 The input is made positive.
OP_NOT 145 0x91 If the input is 0 or 1, it is flipped. Otherwise the output will be 0.
OP_0NOTEQUAL 146 0x92 Returns 0 if the input is 0. 1 otherwise.
OP_ADD 147 0x93 Performs addition on the inputs.
OP_SUB 148 0x94 Performs subtraction on the inputs.
OP_MUL 149 0x95 Performs multiplication on the inputs.
OP_DIV 150 0x96 Performs division on the inputs.
OP_MOD 151 0x97 Performs modulo on the inputs.
OP_LSHIFT 152 0x98 Shifts bits left. Zeros are shifted in on the right. Does not preserve sign.
OP_RSHIFT 153 0x99 Shifts bits right. Uses sign extension to preserve sign.
OP_BOOLAND 154 0x9a If both inputs are not 0, the output is 1. Otherwise 0.
OP_BOOLOR 155 0x9b If either input is not 0, the output is 1. Otherwise 0.
OP_NUMEQUAL 156 0x9c Returns 1 if the input numbers are equal, 0 otherwise.
OP_NUMEQUALVERIFY 157 0x9d Same as OP_NUMEQUAL, but runs OP_VERIFY afterward.
OP_NUMNOTEQUAL 158 0x9e Returns 1 if the input numbers are not equal, 0 otherwise.
OP_LESSTHAN 159 0x9f Returns 1 if first input is less than second input, 0 otherwise.
OP_GREATERTHAN 160 0xa0 Returns 1 if first input is greater than second input, 0 otherwise.
OP_LESSTHANOREQUAL 161 0xa1 Returns 1 if first input is less than or equal to second input, 0 otherwise.
OP_GREATERTHANOREQUAL 162 0xa2 Returns 1 if first input is greater than or equal to second input, 0 otherwise.
OP_MIN 163 0xa3 Returns the smaller of two inputs.
OP_MAX 164 0xa4 Returns the larger of two inputs.
OP_WITHIN 165 0xa5 Returns 1 if x is within the specified range (left-inclusive), 0 otherwise.

Cryptography

These opcodes implement cryptographic primitives commonly used in transaction scripts.

The OP_SHA256 opcode is activated by DCP-0002. On mainnet, this agenda was activated on block 189,568 (November 2017). Prior to this agenda activation, OP_SHA256 is treated as OP_UNKNOWN192.

Note that OP_CODESEPARATOR is disabled. Any script including OP_CODESEPARATOR will abort and fail, even if it is within an unexecuted OP_IF branch.

Word Opcode Hex Description
OP_RIPEMD160 166 0xa6 The input is hashed using RIPEMD-160.
OP_SHA1 167 0xa7 The input is hashed using SHA-1.
OP_BLAKE256 168 0xa8 The input is hashed using BLAKE-256.
OP_HASH160 169 0xa9 The input is hashed twice: first with BLAKE-256 and then with RIPEMD-160.
OP_HASH256 170 0xaa The input is hashed two times with BLAKE-256.
OP_CODESEPARATOR 171 0xab All of the signature checking words will only match signatures to the data after the most recently-executed OP_CODESEPARATOR. Disabled.
OP_CHECKSIG 172 0xac Consumes a signature and a full public key, and pushes 1 onto the stack if the transaction data specified by the SIGHASH flag was converted into the signature using the same private key that generated the public key for the given signature type. Otherwise, it pushes 0 onto the stack.
OP_CHECKSIGVERIFY 173 0xad Same as OP_CHECKSIG, but OP_VERIFY is executed afterward.
OP_CHECKMULTISIG 174 0xae 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. If all signatures are valid, 1 is returned, 0 otherwise.
OP_CHECKMULTISIGVERIFY 175 0xaf Same as OP_CHECKMULTISIG, but OP_VERIFY is executed afterward.
OP_CHECKSIGALT 190 0xbe Consumes a signature type, signature and a full public key, and pushes 1 onto the stack if the transaction data specified by the SIGHASH flag was converted into the signature using the same ECDSA private key that generated the public key. Otherwise, it pushes 0 onto the stack.
OP_CHECKSIGALTVERIFY 191 0xbf Same as OP_CHECKSIGALT, but OP_VERIFY is executed afterward.
OP_SHA256 192 0xc0 The input is hashed using SHA-256. Previously OP_UNKNOWN192.

Lock time

These opcodes are activated by DCP-0003. On mainnet, this agenda was activated on block 189,568 (November 2017). Prior to this agenda activation, these opcodes are treated as OP_NOP2 and OP_NOP3 respectively.

Word Opcode Hex Description
OP_CHECKLOCKTIMEVERIFY 177 0xb1 Script execution fails if the top stack item is greater than the transaction’s nLockTime field, otherwise script evaluation continues as though an OP_NOP was executed. Previously OP_NOP2.
OP_CHECKSEQUENCEVERIFY 178 0xb2 Script execution fails if the relative lock time of the input is not equal to or longer than the value of the top stack item. Previously OP_NOP3.

Proof-of-Stake

These opcodes are used to create Proof-of-Stake transactions.

They are all considered NOPs within the scripting system. Their semantics are enforced outside of the scripting system by the staking consensus rules.

Word Opcode Hex Description
OP_SSTX 186 0xba Tags an output as a ticket purchase.
OP_SSGEN 187 0xbb Tags an output as a ticket vote.
OP_SSRTX 188 0xbc Tags an output as a ticket revocation.
OP_SSTXCHANGE 189 0xbd Tags an output as ticket purchase change.

No op

As the name implies, these opcodes do nothing, they are reserved for future use. The txscript engine can be passed a flag which will cause usage of these opcodes to return an error.

Following the activation of DCP-0002, OP_UNKNOWN192 is redefined as OP_SHA256.

Following the activation of DCP-0003, OP_NOP2 and OP_NOP3 are redefined as OP_CHECKLOCKTIMEVERIFY and OP_CHECKSEQUENCEVERIFY respectively.

Word Opcode Hex Description
OP_NOP 97 0x61 Does nothing. Does not cause script execution to fail.
OP_NOP1 176 0xb0 Does nothing. Does not cause script execution to fail.
OP_NOP4 - OP_NOP10 179 - 185 0xb3 - 0xb9 Does nothing. Does not cause script execution to fail.
OP_UNKNOWN193 - OP_UNKNOWN248 193 - 248 0xc1 - 0xf8 Does nothing. Does not cause script execution to fail.

Reserved

Word Opcode Hex Description
OP_RESERVED 80 0x50 Script execution fails unless occuring in an unexecuted OP_IF branch.
OP_VER 98 0x62 Script execution fails unless occuring in an unexecuted OP_IF branch.
OP_VERIF 101 0x65 Script execution fails even when occuring in an unexecuted OP_IF branch.
OP_VERNOTIF 102 0x66 Script execution fails even when occuring in an unexecuted OP_IF branch.