Bitcoin uses a scripting system for transactions. Forthlike, Script is simple, stackbased, and processed from left to right. It is purposefully not Turingcomplete, 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_0OP_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 secondtotop stack item.

OP_OVER

x1 x2

x1 x2 x1

Copies the secondtotop 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 secondtotop 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 (leftinclusive), 0 otherwise.

Crypto
Word

Input

Output

Description

OP_RIPEMD160

in

hash

The input is hashed using RIPEMD160.

OP_SHA1

in

hash

The input is hashed using SHA1.

OP_SHA256

in

hash

The input is hashed using SHA256.

OP_HASH160

in

hash

The input is hashed twice: first with SHA256 and then with RIPEMD160.

OP_HASH256

in

hash

The input is hashed two times with SHA256.

OP_CODESEPARATOR

Nothing

Nothing

All of the signature checking words will only match signatures to the data after the most recentlyexecuted OP_CODESEPARATOR.

OP_CHECKSIG

sig pubkey

True / false

The entire transaction's outputs, inputs, and script (from the most recentlyexecuted 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.

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