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.

Scripts are big-endian and their sequences of bytes is interpreted as a sequence of opcodes with operands. Only opcodes with byte values less than 0x4f have additional operands.

Scripts and Bitcoin transactions

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 exists and is true (non-zero). The party who originally stored 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) on the stack.
Only the very first transaction in each bitcoin block has no input script (the entry there is called "coinbase"), because the Bitcoins created with the block are created from hot air and not released by a previous recorded script.

Stack

The stacks hold byte vectors (there is the (main) stack and an alternative stack). 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 overview

For each opcode is given: its nemonic without the leading OP_, its hexadecimal byte value, the number of stack items needed (poped) during execution, a semicolon and the number of stack items which has been pushed (after execution). A * after the needed stack items indicates that further bytes from the script following the current opcode are needed. A # indicates that items on the alternative stack are poped/pushed. A @ indicates that further (global) data from the transaction is needed. A / indicates alternatives of a value.
Different colors indicate different semantic execution groups of opcodes:

copy a fixed number of bytes but at most 75 onto stack	copy bytes onto stack which number is given by a 1, 2 or 4 byte value.	copy 1 specific byte onto stack
undefined opcode	a pseudo-opcode, unassigned for scripts
NOP or opcode is part of a IF-flow-control construction
(stack conditionally) invalidation of the script this opcode is part of
stack item is moved between main stack and alternate stack	stack item(s) is/are moved, copied or deleted
byte vectors are appended or divided or their size is determined	byte vectors are interpreted bit-position-independently
byte vectors are interpreted and processed as signed integers
a hash value is computed or a signature checked

Opcode byte-matrix

_FALSE 0x00 0 ; 1(0)

- 0x01 0 *; 1(1)

- 0x02 0 *; 1(2)

- 0x03 0 *; 1(3)

- 0x04 0 *; 1(4)

- 0x04 0 *; 1(5)

- 0x06 0 *; 1(6)

- 0x07 0 *; 1(7)

- 0x08 0 *; 1(8)

- 0x09 0 *; 1(9)

- 0x0a 0 *; 1(10)

- 0x0b 0 *; 1(11)

- 0x0c 0 *; 1(12)

- 0x0d 0 *; 1(13)

- 0x0e 0 *; 1(14)

- 0x0f 0 *; 1(15)

- 0x10 0 *; 1(16)

- 0x11 0 *; 1(17)

- 0x12 0 *; 1(18)

- 0x13 0 *; 1(19)

- 0x14 0 *; 1(20)

- 0x15 0 *; 1(21)

- 0x16 0 *; 1(22)

- 0x17 0 *; 1(23)

- 0x18 0 *; 1(24)

- 0x19 0 *; 1(25)

- 0x1a 0 *; 1(26)

- 0x1b 0 *; 1(27)

- 0x1c 0 *; 1(28)

- 0x1d 0 *; 1(29)

- 0x1e 0 *; 1(30)

- 0x1f 0 *; 1(31)

- 0x20 0 *; 1(32)

- 0x21 0 *; 1(33)

- 0x22 0 *; 1(34)

- 0x23 0 *; 1(35)

- 0x24 0 *; 1(36)

- 0x25 0 *; 1(37)

- 0x26 0 *; 1(38)

- 0x27 0 *; 1(39)

- 0x28 0 *; 1(40)

- 0x29 0 *; 1(41)

- 0x2a 0 *; 1(42)

- 0x2b 0 *; 1(43)

- 0x2c 0 *; 1(44)

- 0x2d 0 *; 1(45)

- 0x2e 0 *; 1(46)

- 0x2f 0 *; 1(47)

- 0x30 0 *; 1(48)

- 0x31 0 *; 1(49)

- 0x32 0 *; 1(50)

- 0x33 0 *; 1(51)

- 0x34 0 *; 1(52)

- 0x35 0 *; 1(53)

- 0x36 0 *; 1(54)

- 0x37 0 *; 1(55)

- 0x38 0 *; 1(56)

- 0x39 0 *; 1(57)

- 0x3a 0 *; 1(58)

- 0x3b 0 *; 1(59)

- 0x3c 0 *; 1(60)

- 0x3d 0 *; 1(61)

- 0x3e 0 *; 1(62)

- 0x3f 0 *; 1(63)

- 0x40 0 *; 1(64)

- 0x41 0 *; 1(65)

- 0x42 0 *; 1(66)

- 0x43 0 *; 1(67)

- 0x44 0 *; 1(68)

- 0x45 0 *; 1(69)

- 0x46 0 *; 1(70)

- 0x47 0 *; 1(71)

- 0x48 0 *; 1(72)

- 0x49 0 *; 1(73)

- 0x4a 0 *; 1(74)

- 0x4b 0 *; 1(75)

_PUSHDATA1 0x4c 0 *; 1(0 - 255)

_PUSHDATA2 0x4d 0 *; 1(0 - 2^16-1)

_PUSHDATA4 0x4e 0 *; 1(0 - 2^32-1)

_1NEGATE 0x4f 0 ; 1(1)

_RESERVED 0x50

_1 0x51 0 ; 1(1)

_2 0x52 0 ; 1(1)

_3 0x53 0 ; 1(1)

_4 0x54 0 ; 1(1)

_5 0x55 0 ; 1(1)

_6 0x56 0 ; 1(1)

_7 0x57 0 ; 1(1)

_8 0x58 0 ; 1(1)

_9 0x59 0 ; 1(1)

_10 0x5a 0 ; 1(1)

_11 0x5b 0 ; 1(1)

_12 0x5c 0 ; 1(1)

_13 0x5d 0 ; 1(1)

_14 0x5e 0 ; 1(1)

_15 0x5f 0 ; 1(1)

_16 0x60 0 ; 1(1)

_NOP 0x61 0 ; 0

_VER 0x62

_IF 0x63 1 ; 0

_NOTIF 0x64 1 ; 0

_VERIF 0x65

_VERNOTIF 0x66

_ELSE 0x67 0 ; 0

_ENDIF 0x68 0 ; 0

_VERIFY 0x69 1 ; 0/1

_RETURN 0x6a 0 ; 0

_TOALTSTACK 0x6b 1 ; 0

_FROMALTSTACK 0x6c 0 #; 1

_2DROP 0x6d 2 ; 0

_2DUP 0x6e 2 ; 4

_3DUP 0x6f 3 ; 6

_2OVER 0x70 4 ; 6

_2ROT 0x71 6 ; 6

_2SWAP 0x72 4 ; 4

_IFDUP 0x73 1 ; 1/2

_DEPTH 0x74 0 ; 1

_DROP 0x75 1 ; 0

_DUP 0x76 1 ; 2

_NIP 0x77 2 ; 1

_OVER 0x78 2 ; 3

_PICK 0x79 1+n+1 ; n+2

_ROLL 0x7a 1+n+1 ; n+1

_ROT 0x7b 3 ; 3

_SWAP 0x7c 2 ; 2

_TUCK 0x7d 2 ; 3

_CAT 0x7e 2 ; 1

_SUBSTR 0x7f 3 ; 1

_LEFT 0x80 2 ; 1

_RIGHT 0x81 2 ; 1

_SIZE 0x82 1 ; 2

_INVERT 0x83 1 ; 1

_AND 0x84 2 ; 1

_OR 0x85 2 ; 1

_XOR 0x86 2 ; 1

_EQUAL 0x87 2 ; 1

_EQUALVERIFY 0x88 2 ; 0/1

_RESERVED1 0x89

_RESERVED2 0x8a

_1ADD 0x8b 1 ; 1

_1SUB 0x8c 1 ; 1

_2MUL 0x8d 1 ; 1

_2DIV 0x8e 1 ; 1

_NEGATE 0x8f 1 ; 1

_ABS 0x90 1 ; 1

_NOT 0x91 1 ; 1

_0NOTEQUAL 0x92 1 ; 1

_ADD 0x93 2 ; 1

_SUB 0x94 2 ; 1

_MUL 0x95 2 ; 1

_DIV 0x96 2 ; 1

_MOD 0x97 2 ; 1

_LSHIFT 0x98 2 ; 1

_RSHIFT 0x99 2 ; 1

_BOOLAND 0x9a 2 ; 1

_BOOLOR 0x9b 2 ; 1

_NUMEQUAL 0x9c 2 ; 1

_NUMEQUALVERIFY 0x9d 2 ; 0/1

_NUMNOTEQUAL 0x9e 2 ; 1

_LESSTHAN 0x9f 2 ; 1

_GREATERTHAN 0xa0 2 ; 1

_LESSTHANOREQUAL 0xa1 2 ; 1

_GREATERTHANOREQUAL 0xa2 2 ; 1

_MIN 0xa3 2 ; 1

_MAX 0xa4 2 ; 1

_WITHIN 0xa5 3 ; 1

_RIPEMD160 0xa6 1 ; 1

_SHA1 0xa7 1 ; 1

_SHA256 0xa8 1 ; 1

_HASH160 0xa9 1 ; 1

_HASH256 0xaa 1 ; 1

_CODESEPARATOR 0xab 0 ; 0

_CHECKSIG 0xac 2 @; 1

_CHECKSIGVERIFY 0xad 2 @; 0/1

_CHECKMULTISIG 0xae 2n+2 @; 1

_CHECKMULTISIGVERIFY 0xaf 2n+2 @; 0/1

_NOP1 0xb0 0 ; 0

_NOP2 0xb1 0 ; 0

_NOP3 0xb2 0 ; 0

_NOP4 0xb3 0 ; 0

_NOP5 0xb4 0 ; 0

_NOP6 0xb5 0 ; 0

_NOP7 0xb6 0 ; 0

_NOP8 0xb7 0 ; 0

_NOP9 0xb8 0 ; 0

_NOP10 0xb9 0 ; 0

- 0xba

- 0xbb

- 0xbc

- 0xbd

- 0xbe

- 0xbf

- 0xc0

- 0xc1

- 0xc2

- 0xc3

- 0xc4

- 0xc5

- 0xc6

- 0xc7

- 0xc8

- 0xc9

- 0xca

- 0xcb

- 0xcc

- 0xcd

- 0xce

- 0xcf

- 0xd0

- 0xd1

- 0xd2

- 0xd3

- 0xd4

- 0xd5

- 0xd6

- 0xd7

- 0xd8

- 0xd9

- 0xda

- 0xdb

- 0xdc

- 0xdd

- 0xde

- 0xdf

- 0xe0

- 0xe1

- 0xe2

- 0xe3

- 0xe4

- 0xe5

- 0xe6

- 0xe7

- 0xe8

- 0xe9

- 0xea

- 0xeb

- 0xec

- 0xed

- 0xee

- 0xef

- 0xf0

- 0xf1

- 0xf2

- 0xf3

- 0xf4

- 0xf5

- 0xf6

- 0xf7

- 0xf8

- 0xf9

- 0xfa

_SMALLINTEGER 0xfb

_PUBKEYS 0xfc

_PUBKEYHASH 0xfd

_PUBKEY 0xfe

_INVALIDOPCODE 0xff

The opcode value 0x00 with nemonic OP_FALSE is also named OP_0 and the opcode value 0x51 with nemonic OP_1 as also named OP_TRUE.
Except pseudo-ops and the definition of OP_NOP1 - OP_NOP10 since bitcoin version 0.3.6, no further opcode interpretation seems to have changed since bitcoin version 0.2.0 of 17th december 2009.

Opcode descriptions

It follows for each opcode less than decimal 185 (hexa 0xba) a description for its usage. The columns entitled 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).

Push data from instruction onto stack

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	(operand)	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 byte with value -1 is pushed onto the stack.
OP_1, OP_TRUE	81	0x51	Nothing	1	The byte with value 1 is pushed onto the stack.
OP_2 - OP_16	82 - 96	0x52 - 0x60	Nothing	2-16	The byte with 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 preceeding 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 item organization

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. Numerically the value is multiplied by -1
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. Test whether its value equals 0
OP_0NOTEQUAL	146	0x92	a	Boolean	Returns 0 if the input is 0. 1 otherwise. Test whether its value is unequal 0
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.

Version opcodes

Version opcodes are disabled since bitcoin 0.2.0 and should not be used (maybe reassigned in the future).

Nemonic	Decimal	Hex	Description
OP_VER	98	0x62	Script evalution is triggered invalid (Formerly pushed the bitcoin-serialize version onto stack).
OP_VERIF	101	0x65	Script evalution is triggered invalid (Formerly top stack item was popped and =< compared to present version)
OP_VERNOTIF	102	0x66	Script evalution is triggered invalid (Formerly top stack item was popped and > compared to present version)

Reserved opcodes

Each opcode not assigned (currently also every opcode value > 185) is also reserved. Using any unassigned opcode triggers the script evaluation to be invalid.

Nemonic	Decimal	Hex	Description
OP_RESERVED	80	0x50	Script evalution is triggered invalid-
OP_RESERVED1	137	0x89	Script evalution is triggered invalid.
OP_RESERVED2	138	0x8a	Script evalution is triggered invalid.

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.

Scripts in bitcoin transactions

This is a list of important scripts to see op-codes "at work". Keep in mind that all constants actually use the data-pushing commands above. At start of a (transaction input) script evaluation, the stack is always empty.

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.

Transaction to IP address (generation transaction)

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.

In the case of a Coin generation transaction (the first transaction of each block), there is no txin-script, resp. this code (coinbase) is completle ignored for the transaction. Only the txout-script / scriptPubKey is used.

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

Script validation

A script evaluation is considered invalid, if any of these conditions meets:

• the total size of the script exceeds (currently) 10000 bytes
• there are more than (currently) 201 opcodes of opcode value > 0x60 in the script
• each instruction is (currently) limited to maximal 520 bytes. This effects only the opcodes OP_PUSHDATA2 and OP_PUSHDATA4
• the executed opcode of the script has insufficient input (from stack, from script or from anywhere)
• the opcode is not defined (for execution) - indicated by white or light gray background color in the table
• the opcode is disabled (currently 15 opcodes) - indicated by white font color in the opcode overview matrix
• stack overflow occurs during execution of the opcode (currently the number of items on the stack and on the alternate stack is limited to 1000)
• the opcodes OP_VERIF and OP_NOTVERIF invalidate a script which contains this opcode (even if this opcode is not executed!)
• the opcodes OP_ELSE or OP_ENDIF has no matching OP_IF
  
• the script execution has been finished but there is (at least) an unmatched OP_IF
  
• the opcodes OP_VERIFY, OP_EQUALVERIFY, OP_NUMEQUALVERIFY, OP_CHECKSIGVERIFY and OP_CHECKMULTISIGVERIFY invalidate the script if the stack-top item is false
• the execution of OP_RETURN
• the numerical value of the top-stack item is negative or greater than the number of items - 2 on the stack if OP_PICK or OP_ROLL shall be executed
• an item used as public key or signature for each of the opcodes OP_CHECKSIG, OP_CHECKSIGVERIFY, OP_CHECKMULTISIG or OP_CHECKMULTISIGVERIFY can be neither a public key or signature (for the given hash)
• the number of signature items or the number of public key items on the stack is negative for OP_CHECKMULTISIG or OP_CHECKMULTISIGVERIFY
• there or more signatures items than public key items on the stack for OP_CHECKMULTISIG or OP_CHECKMULTISIGVERIFY
• there are (currently) more than 20 public key items on the stack for OP_CHECKMULTISIG or CHECKMULTISIGVERIFY
• the sum of opcodes > 0x60 and the number of public keys in all executed OP_CHECKMULTISIG or CHECKMULTISIGVERIFY exceeds (currently) 201
• the script execution has been finished and the stack is empty or the top-stack item is false (numerical value 0)

else the script evaluation is considered to be valid. If invalidation of a script is triggered, the script is immediately finished. These arbitrary looking limitations of 10000, 201, 520, 1000 and 20 might be increased in the future -- they serve to prevent strong slow down of the evaluation of the total script while not restricting its practical usage really.

See Also

• Transactions
• Contracts

References

• src/script.cpp in bitcoin-0.1.0