Hi,
I just did a test. Yes, you are right. If you encode 256 bytes (maximum size of signature with 2048 bit long key) then base64 encoding is 344 characters. Even for a signature with key length 1024 bits it's 172 characters. Now I am wondering how they can get output with only 128 characters. Hard yo really help without seeing data. Have you tried to encrypt file with OpenSSL and compare the results. COuld the bank provide code in some language that does what they want to do?
Cheers