For signing serialized transaction it needs make sha256 one time, but EOS returns signature longer than 64 symbols. My question is - what is additional info added to the signature?
A transaction signature is formed of a
SIG_ prefix, a part that defines which curve algorithm was used to create the signature e.g;
R1, and the base58 encoded SHA256 hash. e.g;
A signature being constructed in swift - https://github.com/memtrip/eos-swift/blob/master/eosswift/eos-core/encode/Base58.swift#L19