How do I base64 encode (decode) in C?

Question

I have binary data in an unsigned char variable. I need to convert them to PEM base64 in c. I looked in openssl library but i could not find any function. Does any body have

Answer 1

I fix @ryyst answer's bug and this is a url safe version:

    #include 
#include 
#include 
#include 
 
static char encoding_table[] = {'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H',
                                'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P',
                                'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X',
                                'Y', 'Z', 'a', 'b', 'c', 'd', 'e', 'f',
                                'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n',
                                'o', 'p', 'q', 'r', 's', 't', 'u', 'v',
                                'w', 'x', 'y', 'z', '0', '1', '2', '3',
                                '4', '5', '6', '7', '8', '9', '-', '_'};
static char *decoding_table = NULL;
static int mod_table[] = {0, 2, 1};
 
void build_decoding_table() {
 
    decoding_table = malloc(256);
 
    for (int i = 0; i < 64; i++)
        decoding_table[(unsigned char) encoding_table[i]] = i;
}
 
 
void base64_cleanup() {
    free(decoding_table);
} 
 
char *base64_encode(const char *data,
                    size_t input_length,
                    size_t *output_length) {
 
    *output_length = 4 * ((input_length + 2) / 3);
 
    char *encoded_data = malloc(*output_length);
    if (encoded_data == NULL) return NULL;
 
    for (int i = 0, j = 0; i < input_length;) {
 
        uint32_t octet_a = i < input_length ? (unsigned char)data[i++] : 0;
        uint32_t octet_b = i < input_length ? (unsigned char)data[i++] : 0;
        uint32_t octet_c = i < input_length ? (unsigned char)data[i++] : 0;
 
        uint32_t triple = (octet_a << 0x10) + (octet_b << 0x08) + octet_c;
 
        encoded_data[j++] = encoding_table[(triple >> 3 * 6) & 0x3F];
        encoded_data[j++] = encoding_table[(triple >> 2 * 6) & 0x3F];
        encoded_data[j++] = encoding_table[(triple >> 1 * 6) & 0x3F];
        encoded_data[j++] = encoding_table[(triple >> 0 * 6) & 0x3F];
    }
 
    //int i=0;
    for (int i = 0; i < mod_table[input_length % 3]; i++)
        encoded_data[*output_length - 1 - i] = '=';
    
    *output_length  = *output_length -2 + mod_table[input_length % 3];
    encoded_data[*output_length] =0;

    return encoded_data;
}
 
 
unsigned char *base64_decode(const char *data,
                             size_t input_length,
                             size_t *output_length) {
 
    if (decoding_table == NULL) build_decoding_table();
 
    if (input_length % 4 != 0) return NULL;
 
    *output_length = input_length / 4 * 3;
    if (data[input_length - 1] == '=') (*output_length)--;
    if (data[input_length - 2] == '=') (*output_length)--;
 
    unsigned char *decoded_data = malloc(*output_length);
    if (decoded_data == NULL) return NULL;
 
    for (int i = 0, j = 0; i < input_length;) {
 
        uint32_t sextet_a = data[i] == '=' ? 0 & i++ : decoding_table[data[i++]];
        uint32_t sextet_b = data[i] == '=' ? 0 & i++ : decoding_table[data[i++]];
        uint32_t sextet_c = data[i] == '=' ? 0 & i++ : decoding_table[data[i++]];
        uint32_t sextet_d = data[i] == '=' ? 0 & i++ : decoding_table[data[i++]];
 
        uint32_t triple = (sextet_a << 3 * 6)
        + (sextet_b << 2 * 6)
        + (sextet_c << 1 * 6)
        + (sextet_d << 0 * 6);
 
        if (j < *output_length) decoded_data[j++] = (triple >> 2 * 8) & 0xFF;
        if (j < *output_length) decoded_data[j++] = (triple >> 1 * 8) & 0xFF;
        if (j < *output_length) decoded_data[j++] = (triple >> 0 * 8) & 0xFF;
    }
 
    return decoded_data;
}
 
int main(){
    
    const char * data = "Hello World! 您好！世界！";
    size_t input_size = strlen(data);
    printf("Input size: %ld \n",input_size);
    char * encoded_data = base64_encode(data, input_size, &input_size);
    printf("After size: %ld \n",input_size);
    printf("Encoded Data is: %s \n",encoded_data);
    
    size_t decode_size = strlen(encoded_data);
    printf("Output size: %ld \n",decode_size);
    unsigned char * decoded_data = base64_decode(encoded_data, decode_size, &decode_size);
    printf("After size: %ld \n",decode_size);
    printf("Decoded Data is: %s \n",decoded_data);
    return 0;
}