Understanding Bitcoin PubKey Length Requirements

Bitcoin, a decentralized digital currency platform, has strict requirements for the format of a public key (or pubkey). The most commonly used format is a 256-bit or 32-byte key, but another requirement is often overlooked: the pubkey length.

In this article, we’ll dive into Bitcoin’s pubkey length requirements and why using the correct length is so important.

Pubkey Length Requirements

Bitcoin requires pubkeys to be a specific length. The most commonly used format is a 256-bit (32 byte) key, but alternative formats can also be used. However, these alternatives require different lengths:

• 33-byte pubkey: This format is commonly used in Bitcoin APIs and client implementations. It consists of two 16-bit words: the first word contains the length of the pubkey (in this case 32 bytes) and a flag indicating whether it is a public or private key.

• 65-byte pubkey: This format is often used on the Bitcoin testnet and some older clients. It requires three 16-bit words: the first word contains the length of the pubkey (in this case 64 bytes), followed by a flag and an additional byte indicating whether it is a public or private key.

Why length matters

The length of the pubkey is crucial because it determines how secure Bitcoin can be. The longer the pubkey, the more computationally expensive it becomes to verify transactions. In fact, solving a 256-bit pubkey would take over 1000 years using just one CPU core!

To put this into perspective, consider an example:

• A 33-byte pub key is equivalent to a 16-bit word (2^8) = about 65,536 possible combinations. This makes it theoretically impossible to recover the original key without some form of pre-computation or brute-force attack.

• A 65-byte pub key is equivalent to a 32-bit word (2^10) = about 1 trillion possible combinations.

Conclusion

Bitcoin’s pubkey length requirements are carefully designed to ensure the security and efficiency of the network. While alternative formats can be used in certain contexts, it is still recommended to use a 33- or 65-byte pub key for most applications. By understanding these requirements, developers can create secure and efficient Bitcoin clients that leverage the system’s strengths.

Example code

Here is a sample Python code snippet that shows how to generate a 33-byte and 65-byte pubkey:

import hashlib
def generate_pubkey(length=32):

Generate random bytes using SHA-256
public_key = hashlib.sha256().digest()
return public_key[:length];

Create a 33-byte pubkey
public_key_33 = generate_pubkey(33);
print(public_key_33)

Create a 65-byte pubkey
public_key_65 = generate_pubkey(65);
print (public_key_65)

Note: In this example, we use the hashlib library to generate random bytes using SHA-256, then split the resulting array to create the public key.