Pay-to-Pubkey Hash

Pay-to-PubKey-Hash (Pay-to-Public-Key-Hash, P2PKH) is the basic form of making a transaction and is the most common form of transaction on the Bitcoin network. Transactions that pay to a Bitcoin address contain P2PKH scripts that are resolved by sending the public key and a digital signature created by the corresponding private key.

The ScriptPubKey and ScriptSig for a transaction is shown below:

Pay-to-PubKey-Hash Review

Two types of payment are referred as P2PK (pay to public key) and P2PKH (pay to public key hash).

Satoshi later decided to use P2PKH instead of P2PK for two reasons:

  • Elliptic Curve Cryptography (the cryptography used by your public key and private key) is vulnerable to a modified Shor’s algorithm for solving the discrete logarithm problem on elliptic curves. In plain English, it means that in the future a quantum computer might be able to retrieve a private key from a public key. By publishing the public key only when the coins are spent (and assuming that addresses are not reused), such an attack is rendered ineffective.
  • With the hash being smaller (20 bytes) it is easier to print and easier to embed into small storage mediums like QR codes.

A Bitcoin address is only a hash, so the sender can’t provide a full public key in scriptPubKey. When redeeming coins that have been sent to a Bitcoin address, the recipient provides both the signature and the public key. The script verifies that the provided public key does hash to the hash in scriptPubKey, and then it also checks the signature against the public key.

Checking process:

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.

Pay-to-PublicKey Hash Example

The figures below help illustrate how these features are used by showing the workflow Alice uses to send Bob a transaction and which Bob later uses to spend that transaction. Both Alice and Bob will use the most common form of the standard Pay-To-Public-Key-Hash (P2PKH) transaction type. P2PKH lets Alice spend satoshis to a typical Bitcoin address, and then lets Bob further spend those satoshis using a simple cryptographic key pair.

Bob must first generate a private/public key pair before Alice can create the first transaction. Bitcoin uses the Elliptic Curve Digital Signature Algorithm (ECDSA) with the secp256k1 curve; secp256k1 private keys are 256 bits of random data. A copy of that data is deterministically transformed into an secp256k1 public key. Because the transformation can be reliably repeated later, the public key does not need to be stored.

The public key (pubkey) is then cryptographically hashed. This pubkey hash can also be reliably repeated later, so it also does not need to be stored. The hash shortens and obfuscates the public key, making manual transcription easier and providing security against unanticipated problems which might allow reconstruction of private keys from public key data at some later point.

Bob provides the pubkey hash to Alice. Pubkey hashes are almost always sent encoded as Bitcoin addresses, which are base58-encoded strings containing an address version number, the hash, and an error-detection checksum to catch typos. The address can be transmitted through any medium, including one-way mediums which prevent the spender from communicating with the receiver, and it can be further encoded into another format, such as a QR code containing a bitcoin: URI. Once Alice has the address and decodes it back into a standard hash, she can create the first transaction. She creates a standard P2PKH transaction output containing instructions which allow anyone to spend that output if they can prove they control the private key corresponding to Bob’s hashed public key. These instructions are called the pubkey script or scriptPubKey.

Alice broadcasts the transaction and it is added to the block chain. The network categorizes it as an Unspent Transaction Output (UTXO), and Bob’s wallet software displays it as a spendable balance. When, some time later, Bob decides to spend the UTXO, he must create an input which references the transaction Alice created by its hash, called a Transaction Identifier (txid), and the specific output she used by its index number (output index). He must then create a signature script—a collection of data parameters which satisfy the conditions Alice placed in the previous output’s pubkey script. Signature scripts are also called scriptSigs.

See also