Why is TxnSignature length not equal 64 bytes?

[G-Virus]

Hello everyone!

I'm implementing transaction serialization and sign operations in low-level, so trying to figure out how the protocol works. As far as I understood, the protocol uses either ECDSA (secp256k1) or ED25519 digital signatures for TxnSignature field. A digest of these algorithms takes 64 bytes (128 in hex representation), however libraries like xrpl-py generate a signature a little longer (in my case I'm getting at least 70 bytes).  It looks like it has some prefix that starts with 304... in hex. If I try to submit a transaction with a signature of 64 bytes the rippled server responds with "Transaction has a bad signature" or "invalid signature", despite the fact it is valid. Could you please tell me what is this prefix in TxnSignature and why the signature isn't equal 64 bytes? Thanks