RFC 4648 (https://tools.ietf.org/html/rfc4648) standardizes two Base-64 alphabets. Nettle currently only supports the traditional base-64 alphabet from section 4.
There is growing use amongst new protocol definitions and extensions, particularly in the HTTP area for the URL-safe extension alphabet instead of the classical Base-64 alphabet.
The attached patch implements a proposed API/ABI extension adding support for RFC 4648 section 5 "Base 64 Encoding with URL and Filename Safe Alphabet"
For the standardized alphabets external code simply calls the init() function relevant to the alphabet it is needing to encode/decode with. The library internally uses the context to select which lookup table to use for later base64 function calls.
For custom or non-standard alphabets a pointer to the alphabet lookup table is included in the encode/decode contexts. External code can memset() a context to empty and provide the alphabet lookup table pointer.
Addtionally this patch adds a simple fuzz unit test for both Base-64 and Base-64 URL extended alphabet encoders and decoders.
Amos Jeffries Treehouse Networks Ltd.
PS, I have also removed the dead code wrappend in "#if 0" rather than updating it.