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 or nesting if statements.

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 (1). 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 (1).

Scripts are big-endian. (Is all data, also?)

Words

This is a list of all Script words (commands/functions). Some are currently disabled for security reasons.

True=1 and False=0.

Constants

When talking about scripts, these value-pushing words are usually omitted.

Word	Opcode	Input	Output	Description
OP_0, OP_FALSE	0	Nothing.	0	The number 0 is pushed onto the stack.
N/A	1-75	(special)	data	The next opcode bytes is data to be pushed onto the stack
OP_PUSHDATA1	76	(special)	data	The next byte contains the number of bytes to be pushed onto the stack.
OP_PUSHDATA2	77	(special)	data	The next two bytes contain the number of bytes to be pushed onto the stack.
OP_PUSHDATA4	78	(special)	data	The next four bytes contain the number of bytes to be pushed onto the stack.
OP_1NEGATE	79	Nothing.	-1	The number -1 is pushed onto the stack.
OP_1, OP_TRUE	81	Nothing.	1	The number 1 is pushed onto the stack.
OP_2-OP_16	82-96	Nothing.	2-16	The number in the word name (2-16) is pushed onto the stack.

Flow control

Word	Opcode	Input	Output	Description
OP_NOP	97	Nothing	Nothing	Does nothing.
OP_IF	99	<expression> if [statements] [else [statements]] endif		If the top stack value is 1, the statements are executed. The top stack value is removed.
OP_NOTIF	100	<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	<expression> if [statements] [else [statements]] endif		If the preceding OP_IF or OP_NOTIF was not executed then these statements are.
OP_ENDIF	104	<expression> if [statements] [else [statements]] endif		Ends an if/else block.
OP_VERIFY	105	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	Nothing	Nothing	Marks transaction as invalid.

Stack

Word	Opcode	Input	Output	Description
OP_TOALTSTACK	107	x1	(alt)x1	Puts the input onto the top of the alt stack. Removes it from the main stack.
OP_FROMALTSTACK	108	(alt)x1	x1	Puts the input onto the top of the main stack. Removes it from the alt stack.
OP_IFDUP	115	x	x / x x	If the input is true or false, duplicate it.
OP_DEPTH	116	Nothing	<Stack size>	Puts the number of stack items onto the stack.
OP_DROP	117	x	Nothing	Removes the top stack item.
OP_DUP	118	x	x x	Duplicates the top stack item.
OP_NIP	119	x1 x2	x2	Removes the second-to-top stack item.
OP_OVER	120	x1 x2	x1 x2 x1	Copies the second-to-top stack item to the top.
OP_PICK	121	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	xn ... x2 x1 x0 <n>	... x2 x1 x0 xn	The item n back in the stack is moved to the top.
OP_ROT	123	x1 x2 x3	x2 x3 x1	The top three items on the stack are rotated to the left.
OP_SWAP	124	x1 x2	x2 x1	The top two items on the stack are swapped.
OP_TUCK	125	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	x1 x2	Nothing	Removes the top two stack items.
OP_2DUP	110	x1 x2	x1 x2 x1 x2	Duplicates the top two stack items.
OP_3DUP	111	x1 x2 x3	x1 x2 x3 x1 x2 x3	Duplicates the top three stack items.
OP_2OVER	112	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	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	x1 x2 x3 x4	x3 x4 x1 x2	Swaps the top two pairs of items.

Splice

Word	Opcode	Input	Output	Description
OP_CAT	126	x1 x2	out	Concatenates two strings. Currently disabled.
OP_SUBSTR	127	in begin size	out	Returns a section of a string. Currently disabled.
OP_LEFT	128	in size	out	Keeps only characters left of the specified point in a string. Currently disabled.
OP_RIGHT	129	in size	out	Keeps only characters right of the specified point in a string. Currently disabled.
OP_SIZE	130	in	in size	Returns the length of the input string.

Bitwise logic

Word	Input	Output	Description
OP_INVERT	131	in	out	Flips all of the bits in the input. Currently disabled.
OP_AND	132	x1 x2	out	Boolean and between each bit in the inputs. Currently disabled.
OP_OR	133	x1 x2	out	Boolean or between each bit in the inputs. Currently disabled.
OP_XOR	134	x1 x2	out	Boolean exclusive or between each bit in the inputs. Currently disabled.
OP_EQUAL	135	x1 x2	True / false	Returns 1 if the inputs are exactly equal, 0 otherwise.
OP_EQUALVERIFY	136	x1 x2	True / false	Same as OP_EQUAL, but runs OP_VERIFY afterward.

Arithmetic

Word	Opcode	Input	Output	Description
OP_1ADD	139	in	out	1 is added to the input.
OP_1SUB	140	in	out	1 is subtracted from the input.
OP_2MUL	141	in	out	The input is multiplied by 2. Currently disabled.
OP_2DIV	142	in	out	The input is divided by 2. Currently disabled.
OP_NEGATE	143	in	out	The sign of the input is flipped.
OP_ABS	144	in	out	The input is made positive.
OP_NOT	145	in	out	If the input is 0 or 1, it is flipped. Otherwise the output will be 0.
OP_0NOTEQUAL	146	in	out	Returns 1 if the input is 0. 0 otherwise.
OP_ADD	147	a b	out	a is added to b.
OP_SUB	148	a b	out	b is subtracted from a.
OP_MUL	149	a b	out	a is multiplied by b. Currently disabled.
OP_DIV	150	a b	out	a is divided by b. Currently disabled.
OP_MOD	151	a b	out	Returns the remainder after dividing a by b. Currently disabled.
OP_LSHIFT	152	a b	out	Shifts a left b bits, preserving sign. Currently disabled.
OP_RSHIFT	153	a b	out	Shifts a right b bits, preserving sign. Currently disabled.
OP_BOOLAND	154	a b	out	If both a and b are not 0, the output is 1. Otherwise 0.
OP_BOOLOR	155	a b	out	If a or b is not 0, the output is 1. Otherwise 0.
OP_NUMEQUAL	156	a b	out	Returns 1 if the numbers are equal, 0 otherwise.
OP_NUMEQUALVERIFY	157	a b	out	Same as OP_NUMEQUAL, but runs OP_VERIFY afterward.
OP_NUMNOTEQUAL	158	a b	out	Returns 1 if the numbers are not equal, 0 otherwise.
OP_LESSTHAN	159	a b	out	Returns 1 if a is less than b, 0 otherwise.
OP_GREATERTHAN	160	a b	out	Returns 1 if a is greater than b, 0 otherwise.
OP_LESSTHANOREQUAL	161	a b	out	Returns 1 if a is less than or equal to b, 0 otherwise.
OP_GREATERTHANOREQUAL	162	a b	out	Returns 1 if a is greater than or equal to b, 0 otherwise.
OP_MIN	163	a b	out	Returns the smaller of a and b.
OP_MAX	164	a b	out	Returns the larger of a and b.
OP_WITHIN	165	x min max	out	Returns 1 if x is within the specified range (left-inclusive), 0 otherwise.

Crypto

Word	Opcode	Input	Output	Description
OP_RIPEMD160	166	in	hash	The input is hashed using RIPEMD-160.
OP_SHA1	167	in	hash	The input is hashed using SHA-1.
OP_SHA256	168	in	hash	The input is hashed using SHA-256.
OP_HASH160	169	in	hash	The input is hashed twice: first with SHA-256 and then with RIPEMD-160.
OP_HASH256	170	in	hash	The input is hashed two times with SHA-256.
OP_CODESEPARATOR	171	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	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	sig pubkey	True / false	Same as OP_CHECKSIG, but OP_VERIFY is executed afterward.
OP_CHECKMULTISIG	174	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.
OP_CHECKMULTISIGVERIFY	175	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	Description
OP_PUBKEYHASH	253	Represents a public key hashed with OP_HASH160.
OP_PUBKEY	254	Represents a public key compatible with OP_CHECKSIG.
OP_INVALIDOPCODE	255	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	When used...
OP_RESERVED	80	Transaction is invalid
OP_VER	98	Transaction is invalid
OP_VERIF	101	Transaction is invalid
OP_VERNOTIF	102	Transaction is invalid
OP_RESERVED1	137	Transaction is invalid
OP_RESERVED2	138	Transaction is invalid
OP_NOP1-OP_NOP10	176-185	The word is ignored.

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

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 (or ommitting OP_DROP and allowing the value to sit on the stack unused). Scripts are limited to 10,000 bytes and 201 instructions/values, and each individual instruction/value is limited to 520 bytes.

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

Stack	Script	Description
Empty.	<sig> <pubKey> OP_CHECKSIG	scriptSig and scriptPubKey are combined.
<sig> <pubKey> <message>	OP_DROP OP_CHECKSIG	Constants are added to the stack.
<sig> <pubKey>	OP_CHECKSIG	Top item in the stack is removed.
true	Empty.	Signature is checked for top two stack items.