# 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.

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