Convert base64 encoded string to java byte array

Question

I am writing a decryption class (AES/CBC/PKCS7Padding) where the encrypted data is coming from C#. I want to take the following string (which is base64 encoded):

usiTyri3/gPJJ0F6Kj9qYL0w/zXiUAEcslUH6/zVIjs=

and convert it to a byte array in java to pass into the SecretKeySpec as the key. I know there is the issue of C# having unsigned bytes and java only having signed bytes. How then can I pass this sting which has values greater than 127 within it and have java accept the key and initialization vectors?

Answer 1

You don't have to worry about byte signedness because base64 encoded data never uses more than 6 bits in each byte (that's why it's called base 64, because you only use 64 characters which is 6 bits, to represent part of a data byte).

If your concern is the resulting data (3 data bytes for every 4 base64 characters), don't worry about that, either. An unsigned byte 255 in C# is the same as the signed byte -1 in Java.

To encode data, you can bitwise-and each byte with 0xff and store it in an int, then encode the least significant 8 bits. Or just bitwise-or each byte with 0x80 and store it in ant int and decode the least significant 8 bits.

But I think you would be better off using Bouncy Castle or the standard JCE to deal with all that stuff. The 'S' in PKCS7 means Standard so data encrypted in C# should decrypt fine in Java and vice versa.