Uncompressed Public Keys (The Legacy)

In the earliest versions of Bitcoin, public keys were stored in their full, "Uncompressed" form. While simple to understand, this format is highly inefficient and is rarely used in modern Bitcoin transactions.

1. The 65-Byte Structure

An uncompressed public key consists of three parts joined together:

1. Prefix (1 byte): Always 0x04. This signals "Uncompressed."

2. X-Coordinate (32 bytes): The horizontal position.

3. Y-Coordinate (32 bytes): The vertical position.

Total Size: 65 Bytes.

2. Why was it used?

When Satoshi Nakamoto first released Bitcoin, he used the OpenSSL library, which defaulted to uncompressed keys. At the time, the blockchain was small, and saving 32 bytes per transaction wasn't a major priority.

• Simplicity: To verify a signature, you need both X and Y. Storing them both means the computer doesn't have to do any extra math to find them.

3. The "Double Address" Problem

One of the strangest quirks of legacy Bitcoin is that the same private key can generate two different addresses:

• One address from the Uncompressed public key (starting with 1).

• One address from the Compressed public key (also starting with 1). This led to confusion in early wallets, where users might see a zero balance because their wallet was looking for the "Wrong" version of their key.

4. Why we moved away

In a blockchain, Space is Money.

• Every transaction in a block costs fees based on its size in bytes.

• By storing the Y-coordinate, we are effectively wasting 32 bytes of space, because the Y-coordinate can be mathematically calculated from the X-coordinate.

Format	Prefix	Size	Status
Uncompressed	04	65 Bytes	Legacy / Deprecated

In the next section, we will explore the Compressed format that saved the network Gigabytes of data.