More on Bitcoin Transactions
Transaction Outputs and Inputs
Transaction serialization – inputs
When transactions are serialized for transmission on the network, their inputs are encoded into a byte stream as shown in Transaction input serialization.
Table 2. Transaction input serialization
Size | Field | Description |
---|---|---|
32 bytes |
Transaction Hash |
Pointer to the transaction containing the UTXO to be spent |
4 bytes |
Output Index |
The index number of the UTXO to be spent; first one is 0 |
1–9 bytes (VarInt) |
Unlocking-Script Size |
Unlocking-Script length in bytes, to follow |
Variable |
Unlocking-Script |
A script that fulfills the conditions of the UTXO locking script |
4 bytes |
Sequence Number |
Used for locktime or disabled (0xFFFFFFFF) |
As with the outputs, let's see if we can find the inputs from Alice's transaction in the serialized format. First, the inputs decoded:
"vin": [ { "txid": "7957a35fe64f80d234d76d83a2a8f1a0d8149a41d81de548f0a65a8a999f6f18", "vout": 0, "scriptSig" : "3045022100884d142d86652a3f47ba4746ec719bbfbd040a570b1deccb b6498c75c4ae24cb02204b9f039ff08df09cbe9f6addac960298cad530a863ea8f53982c0 9db8f6e3813[ALL] 0484ecc0d46f1918b30928fa0e4ed99f16a0fb4fde0735e7ade8416a b9fe423cc5412336376789d172787ec3457eee41c04f4938de5cc17b4a10fa336a8d752adf", "sequence": 4294967295 } ],
Now, let's see if we can identify these fields in the serialized hex encoding in Alice's transaction, serialized and presented in hexadecimal notation:
Example 2. Alice’s transaction, serialized and presented in hexadecimal notation
0100000001186f9f998a5aa6f048e51dd8419a14d8a0f1a8a2836dd734d2804fe65fa35779000000008b483045022100884d142d86652a3f47
ba4746ec719bbfbd040a570b1deccbb6498c75c4ae24cb02204b9f039ff08df09cbe9f6addac960298cad530a863ea8f53982c09db8f6e3813
01410484ecc0d46f1918b30928fa0e4ed99f16a0fb4fde0735e7ade8416ab9fe423cc5412336376789d172787ec3457eee41c04f4938de5cc1
7b4a10fa336a8d752adfffffffff0260e31600000000001976a914ab6 8025513c3dbd2f7b92a94e0581f5d50f654e788acd0ef800000000000
1976a9147f9b1a7fb68d60c536c2fd8aeaa53a8f3cc025a888ac00000 000
Hints:
- The transaction ID is serialized in reversed byte order, so it starts with (hex) 18 and ends with 79
- The output index is a 4-byte group of zeros, easy to identify
- The length of the scriptSig is 139 bytes, or 8b in hex
- The sequence number is set to FFFFFFFF, again easy to identify
ScriptSig is a specific type of unlocking script that when serialized for transmission on the network, inputs are encoded into a byte stream as shown in ScriptSig input serialization. The serialization of the signature field is detailed in Serialization of signatures (DER). The signature field also includes a Signature Hash Type (SIGHASH), which is detailed in ignature Hash Types (SIGHASH).
Table 3. ScriptSig input serialization
Size | Field | Description |
---|---|---|
1–9 bytes (VarInt) |
Signature Size |
Signature length in bytes, to follow |
Variable |
Signature |
A signature that is produced by the user's wallet from his or her private key, which includes a SIGHASH |
1–9 bytes (VarInt) |
Public Key Size |
Public key length in bytes, to follow |
Variable |
Public Key |
The public key, unhashed |