Public Key Hash (PKH) Overview

A Public Key Hash (PKH) is a compact, 160-bit "Fingerprint" of your public key. In the Bitcoin hierarchy, it sits between your Public Key and your Base58 Address. It is the core data used in P2PKH (Pay-to-Public-Key-Hash) transactions.

1. The Compact Identifier

A raw public key is either 33 bytes or 65 bytes. By hashing it, we reduce it to exactly 20 bytes (160 bits).

• Shorter Addresses: 20 bytes is much easier to convert into a short, human-readable address.

• Uniformity: Whether you use a compressed or uncompressed key, the resulting hash is always the same size.

2. The HASH160 Process

In Bitcoin technical documentation, this process is often called HASH160. It is a "Double Hash" consisting of:

1. SHA256: For high-collision resistance.

2. RIPEMD160: To shorten the result to 20 bytes.

3. The Obfuscation Layer

When you give someone your Bitcoin address, you are NOT giving them your public key. You are giving them a Hash of your public key.

• The network only learns your actual public key when you Spend your Bitcoin.

• This provides a layer of privacy and security, as your public key is hidden behind a cryptographic wall until the moment it is needed.

4. The Standard for Legacy Addresses

Most addresses starting with the number 1 (Legacy) or the number 3 (P2SH) are built around a 20-byte HASH160 value. Even modern SegWit addresses use this hash, though they wrap it in a different format.

Attribute	Public Key	Public Key Hash
Size (Bytes)	33 or 65	20
Visibility	Public (at spend)	Public (always)
Security	Elliptic Curve	SHA256 + RIPEMD160
Role	Signature Verification	Address Derivation

In the next section, we will analyze the Double-Hash Algorithm in detail.