Base64 encoding and Base64 decoding in C

#include "lib.h"

/**
 * characters used for Base64 encoding
 */  
const char *BASE64_CHARS = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

/**
 * encode three bytes using base64 (RFC 3548)
 *
 * @param triple three bytes that should be encoded
 * @param result buffer of four characters where the result is stored
 */  
void _base64_encode_triple(unsigned char triple[3], char result[4])
 {
    int tripleValue, i;

    tripleValue = triple[0];
    tripleValue *= 256;
    tripleValue += triple[1];
    tripleValue *= 256;
    tripleValue += triple[2];

    for (i=0; i<4; i++)
    {
 result[3-i] = BASE64_CHARS[tripleValue%64];
 tripleValue /= 64;
    }
} 

/**
 * encode an array of bytes using Base64 (RFC 3548)
 *
 * @param source the source buffer
 * @param sourcelen the length of the source buffer
 * @param target the target buffer
 * @param targetlen the length of the target buffer
 * @return 1 on success, 0 otherwise
 */  
int base64_encode(unsigned char *source, size_t sourcelen, char *target, size_t targetlen)
 {
    /* check if the result will fit in the target buffer */
    if ((sourcelen+2)/3*4 > targetlen-1)
 return 0;

    /* encode all full triples */
    while (sourcelen >= 3)
    {
 _base64_encode_triple(source, target);
 sourcelen -= 3;
 source += 3;
 target += 4;
    }

    /* encode the last one or two characters */
    if (sourcelen > 0)
    {
 unsigned char temp[3];
 memset(temp, 0, sizeof(temp));
 memcpy(temp, source, sourcelen);
 _base64_encode_triple(temp, target);
 target[3] = '=';
 if (sourcelen == 1)
     target[2] = '=';

 target += 4;
    }

    /* terminate the string */
    target[0] = 0;

    return 1;
} 

/**
 * determine the value of a base64 encoding character
 *
 * @param base64char the character of which the value is searched
 * @return the value in case of success (0-63), -1 on failure
 */  
int _base64_char_value(char base64char)
 {
    if (base64char >= 'A' && base64char <= 'Z')
 return base64char-'A';
    if (base64char >= 'a' && base64char <= 'z')
 return base64char-'a'+26;
    if (base64char >= '0' && base64char <= '9')
 return base64char-'0'+2*26;
    if (base64char == '+')
 return 2*26+10;
    if (base64char == '/')
 return 2*26+11;
    return -1;
} 

/**
 * decode a 4 char base64 encoded byte triple
 *
 * @param quadruple the 4 characters that should be decoded
 * @param result the decoded data
 * @return lenth of the result (1, 2 or 3), 0 on failure
 */  
int _base64_decode_triple(char quadruple[4], unsigned char *result)
 {
    int i, triple_value, bytes_to_decode = 3, only_equals_yet = 1;
    int char_value[4];

    for (i=0; i<4; i++)
 char_value[i] = _base64_char_value(quadruple[i]);

    /* check if the characters are valid */
    for (i=3; i>=0; i--)
    {
 if (char_value[i]<0)
 {
     if (only_equals_yet && quadruple[i]=='=')
     {
  /* we will ignore this character anyway, make it something
   * that does not break our calculations */
  char_value[i]=0;
  bytes_to_decode--;
  continue;
     }
     return 0;
 }
 /* after we got a real character, no other '=' are allowed anymore */
 only_equals_yet = 0;
    }

    /* if we got "====" as input, bytes_to_decode is -1 */
    if (bytes_to_decode < 0)
 bytes_to_decode = 0;

    /* make one big value out of the partial values */
    triple_value = char_value[0];
    triple_value *= 64;
    triple_value += char_value[1];
    triple_value *= 64;
    triple_value += char_value[2];
    triple_value *= 64;
    triple_value += char_value[3];

    /* break the big value into bytes */
    for (i=bytes_to_decode; i<3; i++)
 triple_value /= 256;
    for (i=bytes_to_decode-1; i>=0; i--)
    {
 result[i] = triple_value%256;
 triple_value /= 256;
    }

    return bytes_to_decode;
} 

/**
 * decode base64 encoded data
 *
 * @param source the encoded data (zero terminated)
 * @param target pointer to the target buffer
 * @param targetlen length of the target buffer
 * @return length of converted data on success, -1 otherwise
 */  
size_t base64_decode(char *source, unsigned char *target, size_t targetlen)
 {
    char *src, *tmpptr;
    char quadruple[4], tmpresult[3];
    int i, tmplen = 3;
    size_t converted = 0;

    /* concatinate '===' to the source to handle unpadded base64 data */
    src = (char *)malloc(strlen(source)+5);
    if (src == NULL)
 return -1;
    strcpy(src, source);
    strcat(src, "====");
    tmpptr = src;

    /* convert as long as we get a full result */
    while (tmplen == 3)
    {
 /* get 4 characters to convert */
 for (i=0; i<4; i++)
 {
     /* skip invalid characters - we won't reach the end */
     while (*tmpptr != '=' && _base64_char_value(*tmpptr)<0)
  tmpptr++;

     quadruple[i] = *(tmpptr++);
 }

 /* convert the characters */
 tmplen = _base64_decode_triple(quadruple, tmpresult);

 /* check if the fit in the result buffer */
 if (targetlen < tmplen)
 {
     free(src);
     return -1;
 }

 /* put the partial result in the result buffer */
 memcpy(target, tmpresult, tmplen);
 target += tmplen;
 targetlen -= tmplen;
 converted += tmplen;
    }

    free(src);
    return converted;
}