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

1. a public key that, when hashed, yields destination address D embedded in the script, and
2. 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).

Scripts are big-endian.

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.

opcode descriptions

It follows for each opcode less than decimal 185 (hexa 0xba) a description for its usage. The columns entiteld nemonic, decimal and Hex should be evident. The column input gives the needed items of the stack (and the alternate stack) and the column output indicates the resulting items on the stack (and the alternate stack). The naming of the entries in these two columns is chosen to reflect their interpretation. x,x0,x1,... means arbitrary or no interpretation, a, b and c as a signed integer value, n,index,size and depth non-negative integer values, Boolean as either a true or false.
Some of the more complicated opcodes are disabled out of concern that the client might have (and has) a bug in the current implementation due to the historically not as 2-complement interpretations of the byte vectors as numerical values (the most significant byte holds the sign of the byte vectors).

Constants

Nemonic	Decimal	Hex	Input	Output	Description
OP_FALSE, OP_0	0	0x00	Nothing	Empty string	A byte vector of length 0 is pushed onto the stack. (Thus, it is not a no-op!)
(no official nemonics)	1 - 75	0x01 - 0x4b	(operands)	x	The next <opcode>-many bytes are to be pushed onto the stack.
OP_PUSHDATA1	76	0x4c	(operands)	x	The next byte contains the number of bytes to be pushed onto the stack which follow this byte.
OP_PUSHDATA2	77	0x4d	(operands)	x	The next two bytes contain the number of bytes to be pushed onto the stack which follow theses 2 bytes.
OP_PUSHDATA4	78	0x4e	(operands)	x	The next four bytes contain the number of bytes to be pushed onto the stack which follow theses 4 bytes.
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 value <opcode>-80 (thus, one of 2 - 16) is pushed onto the stack.

Flow control

Nemonic	Decimal	Hex	Input	Output	Description
OP_NOP	97	0x61	Nothing	Nothing	Does nothing.
OP_IF	99	0x63	Boolean	Nothing	If the top stack value is not 0, the statements are executed. The top stack value is removed. Lead in a logcial <value> then [statements] [else [statements]] endif expression
OP_NOTIF	100	0x64	Boolean	Nothing	If the top stack value is 0, the statements are executed. The top stack value is removed. Lead in a logcial <value> then [statements] [else [statements]] endif expression
OP_ELSE	103	0x67	Nothing	Nothing	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	Nothing	Nothing	Ends a logcial If <value> then [statements] [else [statements]] expression
OP_VERIFY	105	0x69	Boolean	Nothing / False	If top stack value is not true then marks transaction as invalid . A value true is removed, but false is not.
OP_RETURN	106	0x6a	Nothing	Nothing	Marks transaction as invalid.

Stack

Nemonic	Decimal	Hex	Input	Output	Description
OP_TOALTSTACK	107	0x6b	x1 (alt)	(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 (alt)	Puts the input onto the top of the (main) stack. Removes it from the alt stack.
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.
OP_IFDUP	115	0x73	x	x / x x	If the top stack value is not 0, duplicate it.
OP_DEPTH	116	0x74	Nothing	depth	Puts the number of stack items onto the stack as one little-endian coded byte-vector
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.

Splice

Nemonic	Decimal	Hex	Input	Output	Description
OP_CAT	126	0x7e	x1 x2	out	Concatenates two strings resp. byte vectors. Currently disabled.
OP_SUBSTR	127	0x7f	x index size	out	Returns the section started at position <index> and of length <size> of a string resp. byte vector. Currently disabled.
OP_LEFT	128	0x80	x index	out	Keeps only characters left of the specified point in a string. Currently disabled.
OP_RIGHT	129	0x81	x index	out	Keeps only characters right of the specified point in a string. Currently disabled.
OP_SIZE	130	0x82	x	x size	Returns the length of the input string resp. byte vector resp. stack item.

Bitwise logic

Nemonic	Decimal	Hex	Input	Output	Description
OP_INVERT	131	0x83	x1	x0	Flips all of the bits in the input. Currently disabled.
OP_AND	132	0x84	x1 x2	x0	Boolean and between each bit in the inputs. Currently disabled.
OP_OR	133	0x85	x1 x2	x0	Boolean or between each bit in the inputs. Currently disabled.
OP_XOR	134	0x86	x1 x2	x0	Boolean exclusive or between each bit in the inputs. Currently disabled.
OP_EQUAL	135	0x87	x1 x2	Boolean	Returns 1 if the inputs are byte-wise equal, 0 otherwise.
OP_EQUALVERIFY	136	0x88	x1 x2	Nothing / false	Same as OP_EQUAL, but runs OP_VERIFY afterward.

Arithmetic

(In very early versions, the implemented arithmetic opcodes were limited to maximal 4 byte vectors.)

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

Crypto

Nemonic	Decimal	Hex	Input	Output	Description
OP_RIPEMD160	166	0xa6	x	hash20	The input is hashed using RIPEMD-160.
OP_SHA1	167	0xa7	x	hash20	The input is hashed using SHA-1.
OP_SHA256	168	0xa8	x	hash32	The input is hashed using SHA-256.
OP_HASH160	169	0xa9	x	hash20	The input is hashed twice: first with SHA-256 and then with RIPEMD-160.
OP_HASH256	170	0xaa	x	hash32	The input is hashed two times with SHA-256.
OP_CODESEPARATOR	171	0xab	Nothing	Nothing	All of the signature checking opcodes will only match signatures to the data after the most recently-executed OP_CODESEPARATOR.
OP_CHECKSIG	172	0xac	sig pubkey	Boolean	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 a further, extern specified hash and the given public key. If it is, 1 is returned, 0 otherwise.
OP_CHECKSIGVERIFY	173	0xad	sig pubkey	Nothing / False	Same as OP_CHECKSIG, but OP_VERIFY is executed afterward.
OP_CHECKMULTISIG	174	0xae	sig1 sig2 ... <number of signatures> pub1 pub2 <number of public keys>	Boolean	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	sig1 sig2 ... <number of signatures> pub1 pub2 ... <number of public keys>	Nothing / False	Same as OP_CHECKMULTISIG, but OP_VERIFY is executed afterward.

Transparent opcodes

Nemonic	Decimal	Hex	Description
OP_NOP1-OP_NOP10	176-185	0xb0-0xb9	The opcode has no effect.

Pseudo-opcodes

These nemonics are used internally for assisting with transaction matching. They are invalid if used in actual scripts.

Nemonic	Decimal	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 opcodes

Nemonic	Decimal	Hex	Description
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

Each opcode not assigned (currently also every opcode value > 185) is also reserved. Using an unassigned opcode makes the transaction invalid.

Scripts

This is a list of interesting scripts. Keep in mind that all constants actually use the data-pushing commands above.

Standard Transaction to Bitcoin address

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 to IP address

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.

Transaction with a message

It's possible to add arbitrary data to any transaction by just adding some data along with OP_DROP.

scriptPubKey: <message> OP_DROP <pubKey> OP_CHECKSIG
scriptSig: <sig>

Stack	Script	Description
Empty.	<sig> <message> OP_DROP <pubKey> OP_CHECKSIG
<sig>	<message> OP_DROP <pubKey> OP_CHECKSIG	scriptSig added to the stack.
<sig> <message>	OP_DROP <pubKey> OP_CHECKSIG	The message has been put.
<sig>	<pubKey> OP_CHECKSIG	Top stack item has been removed.
<sig> <pubKey>	OP_CHECKSIG	Checking signature against the public key.
true	Empty.	Stack holds the value of signature check now.

Example non standard transaction on Testnet

These 2 links below show a non standard transaction. It just prepends the hex of "bob" and the operation OP_DROP which just removes it. As you can see they can be spent as normal.

Input non-std transaction: http://blockexplorer.com/testnet/t/6ttfeb55B1

Spent by: http://blockexplorer.com/testnet/t/AFdRB1CHS3

See Also

• Transactions
• Contracts