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 transaction is valid if nothing in the combined script triggers failure and the top stack item is true (1).

FIXME: How is Script represented on the network, and how are constants expressed?

Words

This is a list of most Script words (commands/functions). A few (like OP_0-OP_16) are omitted. Some are currently disabled for security reasons.

True=1 and False=0.

Flow control

Word	Input	Output	Description
OP_NOP	Nothing	Nothing	Does nothing.
OP_IF	<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	<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	<expression> if [statements] [else [statements]] endif		If the preceding OP_IF was not executed, these statements are.
OP_ENDIF	<expression> if [statements] [else [statements]] endif		Ends an if/else block.
OP_VERIFY	True / false	Nothing / False	Marks transaction as invalid if top stack value is not true. True is removed, but false is not.
OP_RETURN	Nothing	Nothing	Marks transaction as invalid.

Stack

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

Splice

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

Bitwise logic

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

Arithmetic

Word	Input	Output	Description
OP_1ADD	in	out	1 is added to the input.
OP_1SUB	in	out	1 is subtracted from the input.
OP_2MUL	in	out	The input is multiplied by 2. Currently disabled.
OP_2DIV	in	out	The input is divided by 2. Currently disabled.
OP_NEGATE	in	out	The sign of the input is flipped.
OP_ABS	in	out	The input is made positive.
OP_NOT	in	out	If the input is 0 or 1, it is flipped. Otherwise the output will be 0.
OP_0NOTEQUAL	in	out	Returns 1 if the input is 0. 0 otherwise.
OP_ADD	a b	out	a is added to b.
OP_SUB	a b	out	b is subtracted from a.
OP_MUL	a b	out	a is multiplied by b. Currently disabled.
OP_DIV	a b	out	a is divided by b. Currently disabled.
OP_MOD	a b	out	Returns the remainder after dividing a by b. Currently disabled.
OP_LSHIFT	a b	out	Shifts a left b bits, preserving sign. Currently disabled.
OP_RSHIFT	a b	out	Shifts a right b bits, preserving sign. Currently disabled.
OP_BOOLAND	a b	out	If both a and b are not 0, the output is 1. Otherwise 0.
OP_BOOLOR	a b	out	If a or b is not 0, the output is 1. Otherwise 0.
OP_NUMEQUAL	a b	out	Returns 1 if the numbers are equal, 0 otherwise.
OP_NUMEQUALVERIFY	a b	out	Same as OP_NUMEQUAL, but runs OP_VERIFY afterward.
OP_NUMNOTEQUAL	a b	out	Returns 1 if the numbers are not equal, 0 otherwise.
OP_LESSTHAN	a b	out	Returns 1 if a is less than b, 0 otherwise.
OP_GREATERTHAN	a b	out	Returns 1 if a is greater than b, 0 otherwise.
OP_LESSTHANOREQUAL	a b	out	Returns 1 if a is less than or equal to b, 0 otherwise.
OP_GREATERTHANOREQUAL	a b	out	Returns 1 if a is greater than or equal to b, 0 otherwise.
OP_MIN	a b	out	Returns the smaller of a and b.
OP_MAX	a b	out	Returns the larger of a and b.
OP_WITHIN	x min max	out	Returns 1 if x is within the specified range (left-inclusive), 0 otherwise.

Crypto

Word	Input	Output	Description
OP_RIPEMD160	in	hash	The input is hashed using RIPEMD-160.
OP_SHA1	in	hash	The input is hashed using SHA-1.
OP_SHA256	in	hash	The input is hashed using SHA-256.
OP_HASH160	in	hash	The input is hashed twice: first with SHA-256 and then with RIPEMD-160.
OP_HASH256	in	hash	The input is hashed two times with SHA-256.
OP_CODESEPARATOR	Nothing	Nothing	All of the signature checking words will only match signatures to the data after the most recently-executed OP_CODESEPARATOR.
OP_CHECKSIG	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	sig pubkey	True / false	Same as OP_CHECKSIG, but OP_VERIFY is executed afterward.
OP_CHECKMULTISIG	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	sig1 sig2 ... <number of signatures> pub1 pub2 ... <number of public keys>	True / False	Same as OP_CHECKMULTISIG, but OP_VERIFY is executed afterward.

Scripts

Standard Transaction to Bitcoin address

scriptPubKey: OP_DUP OP_HASH160 <pubKeyHash> OP_EQUALVERIFY OP_CHECKSIG
scriptSig: <sig> <pubKey>

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>

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.