WIF Prefixes: 5, K, and L

By looking at the first character of a WIF key, a seasoned Bitcoin user can immediately tell whether the key is for a legacy uncompressed address or a modern compressed address.

1. The Prefix "5" (Legacy)

If a WIF key starts with the number 5, it is an Uncompressed Private Key.

• Structure: 0x80 + [32-byte key] + [Checksum].

• Public Key: This key will derive an uncompressed 65-byte public key.

• Address: This will result in a legacy 1... address.

• Example: 5HueCGU...

2. The Prefixes "K" and "L" (Modern)

If a WIF key starts with K or L, it is a Compressed Private Key.

• Structure: 0x80 + [32-byte key] + 0x01 + [Checksum].

• Public Key: This key will derive a compressed 33-byte public key.

• Address: This will result in a modern 1... (P2PKH), 3... (P2SH), or bc1... address.

• Example: Kz7f2Kz... or L1h8pW...

3. Why two letters (K and L)?

Because the private key is 32 bytes (256 bits), the total range of possible numbers is very large.

• When we add the 0x01 flag and encode it to Base58, some keys fall into the range that starts with K, and some fall into the range that starts with L.

• Both are mathematically equivalent in terms of "Compression Status."

4. Why this matters

If you import a 5... key into a wallet, the wallet will look for funds at the uncompressed address. If you import a K... key, it will look at the compressed address.

• Critical Fact: The same private key will show a Zero Balance if you use the wrong WIF prefix, because the funds are sitting at a different address!

First Char	Compression	Added Byte	Public Key Size
5	None	None	65 Bytes
K	Compressed	0x01	33 Bytes
L	Compressed	0x01	33 Bytes

In the next section, we will explore the Compression Flag Quirk.