SEC Format

The Standards for Efficient Cryptography (SEC) is the standard method for encoding a Bitcoin public key. A Bitcoin public key is a point on the elliptic curve, called secp256k1, and thus has an _x_ and _y_ coordinate. However, each _x_ value only has two possible _y_ values, and due to the nature of secp256k1, one of these _y_ values is odd and the other is even for every _x_ value. Thus, the _x_ value and the parity of the _y_ value are sufficient to identify the public key. The parity of the _y_ value is displayed by either a 0x02 or an 0x03 byte, indicating even or odd respectively. This is followed by the _x_ value, which is a 32 byte number. This format is called compressed, because it only takes up 33 bytes, compared with the uncompressed SEC format, which begins with an 0x04 prefix followed by the full _x_ and _y_ values and takes up 65 bytes. Learn more about Bitcoin public keys.