Inputs: Previous txid

Transaction→Previous TXID

Structure

Version

4 bytes

Segwit (Optional)

Marker

1 byte

Flag

1 byte

Structure

Input Count

variable

Input Structure

Previous TXID

32 bytes

Output Index

4 bytes

ScriptSig Size

variable

ScriptSig

variable

Sequence

4 bytes

Structure

Output Count

variable

Output Structure

Amount

8 bytes

ScriptPubKey Size

variable

ScriptPubKey

variable

Structure

Witness

variable

Structure

Locktime

4 bytes

Size

32 bytes

Format

Natural Byte Order

Description

The TXID of the transaction containing the output you want to spend.

Example

7967a5185e907a25225574544c31f7b059c1a191d65b53dcc1554d339c4f9efc

Byte Visualization

Byte 1

Byte 2

Byte 3

Byte 4

Byte 5

Byte 6

Byte 7

Byte 8

Byte 9

Byte 10

Byte 11

Byte 12

Byte 13

Byte 14

Byte 15

Byte 16

Byte 17

Byte 18

Byte 19

Byte 20

Byte 21

Byte 22

Byte 23

Byte 24

Byte 25

Byte 26

Byte 27

Byte 28

Byte 29

Byte 30

Byte 31

Byte 32

The previous transaction ID (txid) is a 32-byte hash that uniquely identifies the transaction containing the output we want to spend.

Let's examine our transaction to understand this better:

1- Byte Order

The txid uses Bitcoin's internal byte order (little-endian). Let's examine this transaction's previous txid:

python

# Display order (what you see on block explorers)
display_txid = "5e6e1a9b4ce3f9f39467d7129e3ecfbe6c81c08dd377aac666fedc9a758fe614"

# Internal order (how it appears in raw transaction)
internal_txid = "14e68f759adcfe66c6aa77d38dc0816cbecf3e9e12d76794f3f9e34c9b1a6e5e"

You can view this transaction in a block explorer using the display order:

-> https://mempool.space/tx/5e6e1a9b4ce3f9f39467d7129e3ecfbe...

Converting between byte orders

Here's a Python function to convert between display and internal byte orders:

python

def reverse_bytes(hex_string):
    """Convert between internal and display byte order"""
    return bytes.fromhex(hex_string)[::-1].hex()

2- Implementation Example

Here's how you might parse a transaction input's txid:

python

def parse_txid(raw_tx: bytes, offset: int = 0) -> tuple[str, int]:
    """
    Parse a transaction ID from raw transaction bytes
    
    Args:
        raw_tx: Raw transaction bytes
        offset: Starting position in bytes
        
    Returns:
        (txid, new_offset)
    """
    # Read 32 bytes for txid
    txid_bytes = raw_tx[offset:offset + 32]
    
    # Convert to hex string (internal byte order)
    txid = txid_bytes.hex()
    
    return txid, offset + 32

3- Inspecting with Bitcoin Core RPC Commands

You can use Bitcoin Core's RPC commands to look up and inspect transactions by their txid. Use getrawtransaction <txid> true to view the full transaction details:

bash

$ bitcoin-cli getrawtransaction \
    5e6e1a9b4ce3f9f39467d7129e3ecfbe6c81c08dd377aac666fedc9a758fe614 true

{
"txid": "5e6e1a9b4ce3f9f39467d7129e3ecfbe6c81c08dd377aac666fedc9a758fe614",
"hash": "5e6e1a9b4ce3f9f39467d7129e3ecfbe6c81c08dd377aac666fedc9a758fe614",
"version": 1,
"size": 225,
"vsize": 225,
"weight": 900,
"locktime": 0,
"vin": [
{
"txid": "14e68f759adcfe66c6aa77d38dc0816cbecf3e9e12d76794f3f9e34c9b1a6e5e",
"vout": 0,
"scriptSig": {
"asm": "3045022100b18dd5124cf2fcd31af3747a55ed941867f781c340007d9d86dae9e04fa8933f02200bdf817b2e6122ba97b8de40541728ead3988df7462241d6ce70e0f6788e126c[ALL] 02cd3dfbe89ebca9ee2cfeb5d36234c7d33bce1c329f8a0a318271a48f",
"hex": "483045022100b18dd5124cf2fcd31af3747a55ed941867f781c340007d9d86dae9e04fa8933f02200bdf817b2e6122ba97b8de40541728ead3988df7462241d6ce70e0f6788e126c012102cd3dfbe89ebca9ee2cfeb5d36234c7d33bce1c329f8a0a318271a48f"
},
"sequence": 4294967295
}
],
"vout": [...]
}

Note

Remember to use the display order of the txid when querying through RPC commands. The internal byte order used in raw transactions must be reversed.

Creating a TXID

The TXID is created differently depending on the transaction type:

Legacy transactions: HASH256 (double SHA-256) of all transaction data
Segwit transactions: HASH256 of transaction data, excluding the marker, flag, and witness fields

This difference in segwit transactions was intentionally designed to prevent transaction malleability, as signatures are no longer part of the TXID calculation.

Historical Note

Duplicate TXID: e3bf3d07d4b0375638d5f1db5255fe07ba2c4cb067cd81b84ee974b6585fb468

There was a unique situation in Bitcoin's history where the same TXID (above) occurred in multiple blocks: 91,722 and 91,880. As a result, BIP 30 was implemented to prevent blocks from containing duplicate TXIDs, and BIP 34 was introduced to require coinbase transactions to include block height in their data.

Implementation Details

Here's a detailed look at how transaction IDs are calculated in code:

python

class Tx:
    def id(self):
        """Human-readable hexadecimal of the transaction hash"""
        return self.hash().hex()

    def hash(self):
        """Binary hash of the legacy serialization"""
        return hash256(self.serialize())[::-1]

The transaction ID is created by:

Serializing the entire transaction
Performing a double SHA256 hash
Reversing the byte order
Converting to hexadecimal (for display)

Note

The transaction ID calculation differs between legacy and segwit transactions. For segwit, the witness data is excluded from the ID calculation to prevent transaction malleability.

Suggest Edits

Previous: Input count
Next: Inputs: Output Index (vout)

Inputs: Previous txid

Transaction→Previous TXID

Structure

Version

4 bytes

Segwit (Optional)

Marker

1 byte

Flag

1 byte

Structure

Input Count

variable

Input Structure

Previous TXID

32 bytes

Output Index

4 bytes

ScriptSig Size

variable

ScriptSig

variable

Sequence

4 bytes

Structure

Output Count

variable

Output Structure

Amount

8 bytes

ScriptPubKey Size

variable

ScriptPubKey

variable

Structure

Witness

variable

Structure

Locktime

4 bytes

Size

32 bytes

Format

Natural Byte Order

Description

The TXID of the transaction containing the output you want to spend.

Example

7967a5185e907a25225574544c31f7b059c1a191d65b53dcc1554d339c4f9efc

Byte Visualization

Byte 1

Byte 2

Byte 3

Byte 4

Byte 5

Byte 6

Byte 7

Byte 8

Byte 9

Byte 10

Byte 11

Byte 12

Byte 13

Byte 14

Byte 15

Byte 16

Byte 17

Byte 18

Byte 19

Byte 20

Byte 21

Byte 22

Byte 23

Byte 24

Byte 25

Byte 26

Byte 27

Byte 28

Byte 29

Byte 30

Byte 31

Byte 32

The previous transaction ID (txid) is a 32-byte hash that uniquely identifies the transaction containing the output we want to spend.

Let's examine our transaction to understand this better:

1- Byte Order

The txid uses Bitcoin's internal byte order (little-endian). Let's examine this transaction's previous txid:

python

# Display order (what you see on block explorers)
display_txid = "5e6e1a9b4ce3f9f39467d7129e3ecfbe6c81c08dd377aac666fedc9a758fe614"

# Internal order (how it appears in raw transaction)
internal_txid = "14e68f759adcfe66c6aa77d38dc0816cbecf3e9e12d76794f3f9e34c9b1a6e5e"

You can view this transaction in a block explorer using the display order:

-> https://mempool.space/tx/5e6e1a9b4ce3f9f39467d7129e3ecfbe...

Converting between byte orders

Here's a Python function to convert between display and internal byte orders:

python

def reverse_bytes(hex_string):
    """Convert between internal and display byte order"""
    return bytes.fromhex(hex_string)[::-1].hex()

2- Implementation Example

Here's how you might parse a transaction input's txid:

python

def parse_txid(raw_tx: bytes, offset: int = 0) -> tuple[str, int]:
    """
    Parse a transaction ID from raw transaction bytes
    
    Args:
        raw_tx: Raw transaction bytes
        offset: Starting position in bytes
        
    Returns:
        (txid, new_offset)
    """
    # Read 32 bytes for txid
    txid_bytes = raw_tx[offset:offset + 32]
    
    # Convert to hex string (internal byte order)
    txid = txid_bytes.hex()
    
    return txid, offset + 32

3- Inspecting with Bitcoin Core RPC Commands

You can use Bitcoin Core's RPC commands to look up and inspect transactions by their txid. Use getrawtransaction <txid> true to view the full transaction details:

bash

$ bitcoin-cli getrawtransaction \
    5e6e1a9b4ce3f9f39467d7129e3ecfbe6c81c08dd377aac666fedc9a758fe614 true

{
"txid": "5e6e1a9b4ce3f9f39467d7129e3ecfbe6c81c08dd377aac666fedc9a758fe614",
"hash": "5e6e1a9b4ce3f9f39467d7129e3ecfbe6c81c08dd377aac666fedc9a758fe614",
"version": 1,
"size": 225,
"vsize": 225,
"weight": 900,
"locktime": 0,
"vin": [
{
"txid": "14e68f759adcfe66c6aa77d38dc0816cbecf3e9e12d76794f3f9e34c9b1a6e5e",
"vout": 0,
"scriptSig": {
"asm": "3045022100b18dd5124cf2fcd31af3747a55ed941867f781c340007d9d86dae9e04fa8933f02200bdf817b2e6122ba97b8de40541728ead3988df7462241d6ce70e0f6788e126c[ALL] 02cd3dfbe89ebca9ee2cfeb5d36234c7d33bce1c329f8a0a318271a48f",
"hex": "483045022100b18dd5124cf2fcd31af3747a55ed941867f781c340007d9d86dae9e04fa8933f02200bdf817b2e6122ba97b8de40541728ead3988df7462241d6ce70e0f6788e126c012102cd3dfbe89ebca9ee2cfeb5d36234c7d33bce1c329f8a0a318271a48f"
},
"sequence": 4294967295
}
],
"vout": [...]
}

Note

Remember to use the display order of the txid when querying through RPC commands. The internal byte order used in raw transactions must be reversed.

Creating a TXID

The TXID is created differently depending on the transaction type:

Legacy transactions: HASH256 (double SHA-256) of all transaction data
Segwit transactions: HASH256 of transaction data, excluding the marker, flag, and witness fields

This difference in segwit transactions was intentionally designed to prevent transaction malleability, as signatures are no longer part of the TXID calculation.

Historical Note

Duplicate TXID: e3bf3d07d4b0375638d5f1db5255fe07ba2c4cb067cd81b84ee974b6585fb468

Implementation Details

Here's a detailed look at how transaction IDs are calculated in code:

python

class Tx:
    def id(self):
        """Human-readable hexadecimal of the transaction hash"""
        return self.hash().hex()

    def hash(self):
        """Binary hash of the legacy serialization"""
        return hash256(self.serialize())[::-1]

The transaction ID is created by:

Serializing the entire transaction
Performing a double SHA256 hash
Reversing the byte order
Converting to hexadecimal (for display)

Note

The transaction ID calculation differs between legacy and segwit transactions. For segwit, the witness data is excluded from the ID calculation to prevent transaction malleability.

Suggest Edits

Previous: Input count
Next: Inputs: Output Index (vout)