cdecode.c

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/*
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 <b64/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;
        
        if(length_in == 0) {
    return 0;
  }

        *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 = 0; // no state to save; use default value
                                        return plainchar - plaintext_out;
                                }
                                fragment = (char)base64_decode_value(*codechar++);
                        } while (fragment < 0);
                        *plainchar    = (fragment & 0x03f) << 2;
                        //lint -fallthrough
        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;
                        //lint -fallthrough
        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;
                        //lint -fallthrough
        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;
}