cdecode.c

/*
cdecoder.c - c source to a base64 decoding algorithm implementation

This is part of the libb64 project, and has been placed in the public domain.
For details, see http://sourceforge.net/projects/libb64
*/

#include "cdecode.h"

int base64_decode_value(char value_in)
{
    static const char decoding[] = {62,-1,-1,-1,63,52,53,54,55,56,57,58,59,60,61,-1,-1,-1,-2,-1,-1,-1,0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,-1,-1,-1,-1,-1,-1,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51};
    static const char decoding_size = sizeof(decoding);
    value_in -= 43;
    if (value_in < 0 || value_in > decoding_size) {
        return -1;
    }
    return decoding[(int)value_in];
}

void base64_init_decodestate(base64_decodestate* state_in)
{
    state_in->step = step_a;
    state_in->plainchar = 0;
}

int base64_decode_block(const char* code_in, const int length_in, char* plaintext_out, base64_decodestate* state_in)
{
    const char* codechar = code_in;
    char* plainchar = plaintext_out;
    char fragment;

    *plainchar = state_in->plainchar;

    switch (state_in->step) {
        while (1) {
        case step_a:
            do {
                if (codechar == code_in+length_in) {
                    state_in->step = step_a;
                    state_in->plainchar = *plainchar;
                    return plainchar - plaintext_out;
                }
                fragment = (char)base64_decode_value(*codechar++);
            } while (fragment < 0);
            *plainchar    = (fragment & 0x03f) << 2;
        case step_b:
            do {
                if (codechar == code_in+length_in) {
                    state_in->step = step_b;
                    state_in->plainchar = *plainchar;
                    return plainchar - plaintext_out;
                }
                fragment = (char)base64_decode_value(*codechar++);
            } while (fragment < 0);
            *plainchar++ |= (fragment & 0x030) >> 4;
            *plainchar    = (fragment & 0x00f) << 4;
        case step_c:
            do {
                if (codechar == code_in+length_in) {
                    state_in->step = step_c;
                    state_in->plainchar = *plainchar;
                    return plainchar - plaintext_out;
                }
                fragment = (char)base64_decode_value(*codechar++);
            } while (fragment < 0);
            *plainchar++ |= (fragment & 0x03c) >> 2;
            *plainchar    = (fragment & 0x003) << 6;
        case step_d:
            do {
                if (codechar == code_in+length_in) {
                    state_in->step = step_d;
                    state_in->plainchar = *plainchar;
                    return plainchar - plaintext_out;
                }
                fragment = (char)base64_decode_value(*codechar++);
            } while (fragment < 0);
            *plainchar++   |= (fragment & 0x03f);
        }
    }
    /* control should not reach here */
    return plainchar - plaintext_out;
}

int base64_decode_chars(const char* code_in, const int length_in, char* plaintext_out)
{
    base64_decodestate _state;
    base64_init_decodestate(&_state);
    int len = base64_decode_block(code_in, length_in, plaintext_out, &_state);
    if(len > 0) {
        plaintext_out[len] = 0;
    }
    return len;
}