The Anatomy of Bitcoin Transactions
About Me
Information Security Consultant @ Blit-Tech
Software Developer @ Shelter Insurance
Email: nabil@blit.tech
Blockchains a Quick Refresher
What is a Bitcoin Transaction
A Bitcoin Transaction is a record assigning the outputs of a previous transaction to new outputs based on the Transaction Script
Glossary
- Bitcoin Address
- The hash of a public key
- TxId
- Transaction ID, hash of the transaction
- UTxO
- Unspent Transaction Outputs
Glossary Continued
- Public Key Script
- Transaction output script that allows the holder of the corresponding private key to spend the output
- Signature Script
- Script proving the creator is the owner of the corresponding private key
- Coinbase Transaction
- Special transaction that creates a new Bitcoin
The Transaction Structure
| Size | Name | Type | Description |
|---|---|---|---|
| 4 | version | uint32 | Version Number |
| 0 or 2 | flags | uint8[2] | Optional and always 0001 |
| 1 to 9 | tx_in count | var_int | Number of Input Transactions |
| Varies | tx_in | Input Transactions | Specifies prev. output and signature script |
| 1 to 9 | tx_out count | var_int | Number of Outputs |
| Varies | tx_out | Output Transaction | The Output Transactions |
| 4 | lock_time | uint32 | Currently always 0. |
The tx_in Structure
| Size | Name | Type | Description |
|---|---|---|---|
| 36 | prev_output | outpoint | 32 byte ID of the previous transaction + 4 byte index of specific output |
| 1 to 9 | script length | var_int | The length of the Signature Script |
| Varies | signature script | uchar[] | Script that confirms the specified output's script |
| 4 | sequence | uint32 | Currently not used |
The tx_out Structure
| Size | Name | Type | Description |
|---|---|---|---|
| 8 | value | int64 | The output's value |
| 1 to 9 | script length | var_int | The length of the pk_script |
| Varies | pk_script | uchar[] | Script setting up the conditions that allow the coin to spent |
Implications of the Structure
- The transaction must consume (spend) the full value of all it's inputs
- A transaction might have an output that is spent and another that isn't, thus UTxO
- Two transactions might attempt to spend the same output of a transaction.
Scripts
- Where the magic happens
- Non Turing Complete
- Stack Based
- Stateless
- Accepted Scripts are Standardized
- Signature Script is prepended to PubKey Script
The Standard Scripts
- P2PKH: Pay to public key hash
- P2SH: Pay to script hash
- Multisig: m-of-n signatures
"Other" Standard Scripts
- Pubkey: Simplified version of P2PKH. Not used in new transactions
- Null Data: Standard script for storing arbitrary data on the blockchain
P2PKH
Signature Script
<Sig> <PubKey>
PubKey Script
OP_DUP OP_HASH160 <PubkeyHash> OP_EQUALVERIFY OP_CHECKSIG
P2PKH Evaluation
<Sig> <PubKey> OP_DUP OP_HASH160 <PubkeyHash> OP_EQUALVERIFY OP_CHECKSIG
P2PK Evaluation
<Sig> <PubKey> <PubKey> OP_HASH160 <PubkeyHash> OP_EQUALVERIFY OP_CHECKSIG
P2PK Evaluation
<Sig> <PubKey> <PubkeyHash> <PubkeyHash> OP_EQUALVERIFY OP_CHECKSIG
P2PK Evaluation
<Sig> <PubKey> OP_CHECKSIG
P2PK Evaluation
TRUE
P2SH
Signaure Script
<sig> <redeemScript>
PubKey Script
OP_HASH160 <Hash160(redeemScript)> OP_EQAL
Any standard script can be used as the redeemScript
MultiSig
Signature Script
OP_0 <sig A> [sig B] [sig C...]
PubKey Script
<m> <A pk> [B pk] [C pk..] <n> OP_CHECKMULTISIG
Pubkey
Signature Script
<sig>
PubKey Script
<pubkey> OP_CHECKSIG
Null Data
No Signature Script since the transaction is not spendable
PubKey Script
OP_RETURN <0 to 40 bytes of data>
Coinbase Transaction
- The first transaction in a new block
- Has no input transactions
- Outputs define how the block reward (fees + subsidy) can be spend
The True Nature of Bitcois
A wallet's Bitcoins are just the UTxO in the block chain that the wallet is allowed to spend