Address

Bitmessage adresses are Base58 encoded public key hashes. An address looks like BM-BcbRqcFFSQUUmXFKsPJgVQPSiFA3Xash. All Addresses start with BM-, however clients should accept addresses without the prefix. PyBitmessage does this. The reason behind this idea is the fact, that when double clicking on an address for copy and paste, the prefix is usually not selected due to the dash being a common separator.

Public Key usage

Addresses may look complicated but they fulfill the purpose of verifying the sender. A Message claiming to be from a specific address can simply be checked by decoding a special field in the data packet with the public key, that represents the address. If the decryption succeeds, the message is from the address it claims to be.

Length

Without the BM- prefix, an address is usually 32-34 chars long. Since an address is a hash it can be calculated by the client in a way, that the first bytes are zero (\0) and bitmessage strips these. This causes the client to do much more work to be lucky and find such an address. This is an optional checkbox in PyBitmessage.