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Transactions

The following subsections briefly document core transaction details.

OpCodes

The opcodes used in the pubkey scripts of standard transactions are:

  • Various data pushing opcodes from 0x00 to 0x4e (1–78). These aren’t typically shown in examples, but they must be used to push signatures and public keys onto the stack. See the link below this list for a description.

  • OP_TRUE/OP_1 (0x51) and OP_2 through OP_16 (0x52–0x60), which push the values 1 through 16 to the stack.

  • “OP_CHECKSIG” consumes a signature and a full public key, and pushes true onto the stack if the transaction data specified by the SIGHASH flag was converted into the signature using the same ECDSA private key that generated the public key. Otherwise, it pushes false onto the stack.

  • “OP_DUP” pushes a copy of the topmost stack item on to the stack.

  • “OP_HASH160” consumes the topmost item on the stack, computes the RIPEMD160(SHA256()) hash of that item, and pushes that hash onto the stack.

  • “OP_EQUAL” consumes the top two items on the stack, compares them, and pushes true onto the stack if they are the same, false if not.

  • “OP_VERIFY” consumes the topmost item on the stack. If that item is zero (false) it terminates the script in failure.

  • “OP_EQUALVERIFY” runs “OP_EQUAL” and then “OP_VERIFY” in sequence.

  • “OP_CHECKMULTISIG” consumes the value (n) at the top of the stack, consumes that many of the next stack levels (public keys), consumes the value (m) now at the top of the stack, and consumes that many of the next values (signatures) plus one extra value.

    The “one extra value” it consumes is the result of an off-by-one error in the Bitcoin Core implementation. This value is not used, so signature scripts prefix the list of secp256k1 signatures with a single OP_0 (0x00).

    “OP_CHECKMULTISIG” compares the first signature against each public key until it finds an ECDSA match. Starting with the subsequent public key, it compares the second signature against each remaining public key until it finds an ECDSA match. The process is repeated until all signatures have been checked or not enough public keys remain to produce a successful result.

    Because public keys are not checked again if they fail any signature comparison, signatures must be placed in the signature script using the same order as their corresponding public keys were placed in the pubkey script or redeem script. See the “OP_CHECKMULTISIG” warning below for more details.

  • “OP_RETURN” terminates the script in failure when executed.

A complete list of opcodes can be found on the Bitcoin Wiki Script Page, with an authoritative list in the opcodetype enum of the Bitcoin Core script header file

Warning icon Signature script modification warning: Signature scripts are not signed, so anyone can modify them. This means signature scripts should only contain data and data-pushing opcodes which can’t be modified without causing the pubkey script to fail. Placing non-data-pushing opcodes in the signature script currently makes a transaction non-standard, and future consensus rules may forbid such transactions altogether. (Non-data-pushing opcodes are already forbidden in signature scripts when spending a P2SH pubkey script.)

Warning icon “OP_CHECKMULTISIG”warning: The multisig verification process described above requires that signatures in the signature script be provided in the same order as their corresponding public keys in the pubkey script or redeem script. For example, the following combined signature and pubkey script will produce the stack and comparisons shown:

OP_0 <A sig> <B sig> OP_2 <A pubkey> <B pubkey> <C pubkey> OP_3

Sig Stack       Pubkey Stack  (Actually a single stack)
---------       ------------
B sig           C pubkey
A sig           B pubkey
OP_0            A pubkey

1. B sig compared to C pubkey (no match)
2. B sig compared to B pubkey (match #1)
3. A sig compared to A pubkey (match #2)

Success: two matches found

But reversing the order of the signatures with everything else the same will fail, as shown below:

OP_0 <B sig> <A sig> OP_2 <A pubkey> <B pubkey> <C pubkey> OP_3

Sig Stack       Pubkey Stack  (Actually a single stack)
---------       ------------
A sig           C pubkey
B sig           B pubkey
OP_0            A pubkey

1. A sig compared to C pubkey (no match)
2. A sig compared to B pubkey (no match)

Failure, aborted: two signature matches required but none found so far, and there's only one pubkey remaining

Address Conversion

The hashes used in P2PKH and P2SH outputs are commonly encoded as Bitcoin addresses. This is the procedure to encode those hashes and decode the addresses.

First, get your hash. For P2PKH, you RIPEMD-160(SHA256()) hash a ECDSA public key derived from your 256-bit ECDSA private key (random data). For P2SH, you RIPEMD-160(SHA256()) hash a redeem script serialized in the format used in raw transactions (described in a following sub-section). Taking the resulting hash:

  1. Add an address version byte in front of the hash. The version bytes commonly used by Bitcoin are:
    • 0x00 for P2PKH addresses on the main Bitcoin network (mainnet)
    • 0x6f for P2PKH addresses on the Bitcoin testing network (testnet)
    • 0x05 for P2SH addresses on mainnet
    • 0xc4 for P2SH addresses on testnet
  2. Create a copy of the version and hash; then hash that twice with SHA256: SHA256(SHA256(version . hash))
  3. Extract the first four bytes from the double-hashed copy. These are used as a checksum to ensure the base hash gets transmitted correctly.
  4. Append the checksum to the version and hash, and encode it as a base58 string: BASE58(version . hash . checksum)

Bitcoin’s base58 encoding, called Base58Check may not match other implementations. Tier Nolan provided the following example encoding algorithm to the Bitcoin Wiki Base58Check encoding page under the Creative Commons Attribution 3.0 license:

code_string = "123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz"
x = convert_bytes_to_big_integer(hash_result)

output_string = ""

while(x > 0)
   {
       (x, remainder) = divide(x, 58)
       output_string.append(code_string[remainder])
   }

repeat(number_of_leading_zero_bytes_in_hash)
   {
   output_string.append(code_string[0]);
   }

output_string.reverse();

Bitcoin’s own code can be traced using the base58 header file.

To convert addresses back into hashes, reverse the base58 encoding, extract the checksum, repeat the steps to create the checksum and compare it against the extracted checksum, and then remove the version byte.

Raw Transaction Format

Bitcoin transactions are broadcast between peers in a serialized byte format, called raw format. It is this form of a transaction which is SHA256(SHA256()) hashed to create the TXID and, ultimately, the merkle root of a block containing the transaction—making the transaction format part of the consensus rules.

Bitcoin Core and many other tools print and accept raw transactions encoded as hex.

As of Bitcoin Core 0.9.3 (October 2014), all transactions use the version 1 format described below. (Note: transactions in the block chain are allowed to list a higher version number to permit soft forks, but they are treated as version 1 transactions by current software.)

A raw transaction has the following top-level format:

Bytes Name Data Type Description
4 version uint32_t Transaction version number; currently version 1 or 2. Programs creating transactions using newer consensus rules may use higher version numbers. Version 2 means that BIP 68 applies.
Varies tx_in count compactSize uint Number of inputs in this transaction.
Varies tx_in txIn Transaction inputs. See description of txIn below.
Varies tx_out count compactSize uint Number of outputs in this transaction.
Varies tx_out txOut Transaction outputs. See description of txOut below.
4 lock_time uint32_t A time (Unix epoch time) or block number. See the locktime parsing rules.

A transaction may have multiple inputs and outputs, so the txIn and txOut structures may recur within a transaction. CompactSize unsigned integers are a form of variable-length integers; they are described in the CompactSize section.

TxIn: A Transaction Input (Non-Coinbase)

Each non-coinbase input spends an outpoint from a previous transaction. (Coinbase inputs are described separately after the example section below.)

Bytes Name Data Type Description
36 previous_output outpoint The previous outpoint being spent. See description of outpoint below.
Varies script bytes compactSize uint The number of bytes in the signature script. Maximum is 10,000 bytes.
Varies signature script char[] A script-language script which satisfies the conditions placed in the outpoint’s pubkey script. Should only contain data pushes; see the signature script modification warning.
4 sequence uint32_t Sequence number. Default for Bitcoin Core and almost all other programs is 0xffffffff.

Outpoint: The Specific Part Of A Specific Output

Because a single transaction can include multiple outputs, the outpoint structure includes both a TXID and an output index number to refer to specific output.

Bytes Name Data Type Description
32 hash char[32] The TXID of the transaction holding the output to spend. The TXID is a hash provided here in internal byte order.
4 index uint32_t The output index number of the specific output to spend from the transaction. The first output is 0x00000000.

TxOut: A Transaction Output

Each output spends a certain number of satoshis, placing them under control of anyone who can satisfy the provided pubkey script.

Bytes Name Data Type Description
8 value int64_t Number of satoshis to spend. May be zero; the sum of all outputs may not exceed the sum of satoshis previously spent to the outpoints provided in the input section. (Exception: coinbase transactions spend the block subsidy and collected transaction fees.)
1+ pk_script bytes compactSize uint Number of bytes in the pubkey script. Maximum is 10,000 bytes.
Varies pk_script char[] Defines the conditions which must be satisfied to spend this output.

Example

The sample raw transaction itemized below is the one created in the Simple Raw Transaction section of the Developer Examples. It spends a previous pay-to-pubkey output by paying to a new pay-to-pubkey-hash (P2PKH) output.

01000000 ................................... Version

01 ......................................... Number of inputs
|
| 7b1eabe0209b1fe794124575ef807057
| c77ada2138ae4fa8d6c4de0398a14f3f ......... Outpoint TXID
| 00000000 ................................. Outpoint index number
|
| 49 ....................................... Bytes in sig. script: 73
| | 48 ..................................... Push 72 bytes as data
| | | 30450221008949f0cb400094ad2b5eb3
| | | 99d59d01c14d73d8fe6e96df1a7150de
| | | b388ab8935022079656090d7f6bac4c9
| | | a94e0aad311a4268e082a725f8aeae05
| | | 73fb12ff866a5f01 ..................... [Secp256k1][secp256k1] signature
|
| ffffffff ................................. Sequence number: UINT32_MAX

01 ......................................... Number of outputs
| f0ca052a01000000 ......................... Satoshis (49.99990000 BTC)
|
| 19 ....................................... Bytes in pubkey script: 25
| | 76 ..................................... OP_DUP
| | a9 ..................................... OP_HASH160
| | 14 ..................................... Push 20 bytes as data
| | | cbc20a7664f2f69e5355aa427045bc15
| | | e7c6c772 ............................. PubKey hash
| | 88 ..................................... OP_EQUALVERIFY
| | ac ..................................... OP_CHECKSIG

00000000 ................................... locktime: 0 (a block height)

Coinbase Input: The Input Of The First Transaction In A Block

The first transaction in a block, called the coinbase transaction, must have exactly one input, called a coinbase. The coinbase input currently has the following format.

Bytes Name Data Type Description
32 hash (null) char[32] A 32-byte null, as a coinbase has no previous outpoint.
4 index (UINT32_MAX) uint32_t 0xffffffff, as a coinbase has no previous outpoint.
Varies script bytes compactSize uint The number of bytes in the coinbase script, up to a maximum of 100 bytes.
Varies (4) height script The block height of this block as required by BIP34. Uses script language: starts with a data-pushing opcode that indicates how many bytes to push to the stack followed by the block height as a little-endian unsigned integer. This script must be as short as possible, otherwise it may be rejected. The data-pushing opcode will be 0x03 and the total size four bytes until block 16,777,216 about 300 years from now.
Varies coinbase script None The coinbase field: Arbitrary data not exceeding 100 bytes minus the (4) height bytes. Miners commonly place an extra nonce in this field to update the block header merkle root during hashing.
4 sequence uint32_t Sequence number.

Most (but not all) blocks prior to block height 227,836 used block version 1 which did not require the height parameter to be prefixed to the coinbase script. The block height parameter is now required.

Although the coinbase script is arbitrary data, if it includes the bytes used by any signature-checking operations such as “OP_CHECKSIG”, those signature checks will be counted as signature operations (sigops) towards the block’s sigop limit. To avoid this, you can prefix all data with the appropriate push operation.

An itemized coinbase transaction:

01000000 .............................. Version

01 .................................... Number of inputs
| 00000000000000000000000000000000
| 00000000000000000000000000000000 ...  Previous outpoint TXID
| ffffffff ............................ Previous outpoint index
|
| 29 .................................. Bytes in coinbase
| |
| | 03 ................................ Bytes in height
| | | 4e0105 .......................... Height: 328014
| |
| | 062f503253482f0472d35454085fffed
| | f2400000f90f54696d65202620486561
| | 6c74682021 ........................ Arbitrary data
| 00000000 ............................ Sequence

01 .................................... Output count
| 2c37449500000000 .................... Satoshis (25.04275756 BTC)
| 1976a914a09be8040cbf399926aeb1f4
| 70c37d1341f3b46588ac ................ P2PKH script
| 00000000 ............................ Locktime

CompactSize Unsigned Integers

The raw transaction format and several peer-to-peer network messages use a type of variable-length integer to indicate the number of bytes in a following piece of data.

Bitcoin Core code and this document refers to these variable length integers as compactSize. Many other documents refer to them as var_int or varInt, but this risks conflation with other variable-length integer encodings—such as the CVarInt class used in Bitcoin Core for serializing data to disk. Because it’s used in the transaction format, the format of compactSize unsigned integers is part of the consensus rules.

For numbers from 0 to 252, compactSize unsigned integers look like regular unsigned integers. For other numbers up to 0xffffffffffffffff, a byte is prefixed to the number to indicate its length—but otherwise the numbers look like regular unsigned integers in little-endian order.

Value Bytes Used Format
>= 0 && <= 252 1 uint8_t
>= 253 && <= 0xffff 3 0xfd followed by the number as uint16_t
>= 0x10000 && <= 0xffffffff 5 0xfe followed by the number as uint32_t
>= 0x100000000 && <= 0xffffffffffffffff 9 0xff followed by the number as uint64_t

For example, the number 515 is encoded as 0xfd0302.