Z85 decoder, waaaaay too slow

gistfile1.txt

#include <stdlib.h>
#include <stdint.h>
#include <stdio.h>
#include <string.h>
#include <assert.h>

typedef unsigned char byte;

//  Maps base 256 to base 85
static char encoder [85 + 1] = {
    "0123456789"
    "abcdefghij"
    "klmnopqrst"
    "uvwxyzABCD"
    "EFGHIJKLMN"
    "OPQRSTUVWX"
    "YZ.-:+=^!/"
    "*?&<>()[]{"
    "}@%$#"
};

//  Maps base 85 to base 256
//  We chop off lower 32 and higher 128 ranges
static byte decoder [96] = {
    0x00, 0x44, 0x00, 0x54, 0x53, 0x52, 0x48, 0x00,
    0x4B, 0x4C, 0x46, 0x41, 0x00, 0x3F, 0x3E, 0x45,
    0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07,
    0x08, 0x09, 0x40, 0x00, 0x49, 0x42, 0x4A, 0x47,
    0x51, 0x24, 0x25, 0x26, 0x27, 0x28, 0x29, 0x2A,
    0x2B, 0x2C, 0x2D, 0x2E, 0x2F, 0x30, 0x31, 0x32,
    0x33, 0x34, 0x35, 0x36, 0x37, 0x38, 0x39, 0x3A,
    0x3B, 0x3C, 0x3D, 0x4D, 0x00, 0x4E, 0x43, 0x00,
    0x00, 0x0A, 0x0B, 0x0C, 0x0D, 0x0E, 0x0F, 0x10,
    0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17, 0x18,
    0x19, 0x1A, 0x1B, 0x1C, 0x1D, 0x1E, 0x1F, 0x20,
    0x21, 0x22, 0x23, 0x4F, 0x00, 0x50, 0x00, 0x00
};

//  --------------------------------------------------------------------------
//  Encode a byte array as a string

char *
Z85_encode (byte *data, size_t size)
{
    //  Accepts only byte arrays bounded to 4 bytes
    if (size % 4)
        return NULL;

    size_t encoded_size = size * 5 / 4;
    char *encoded = malloc (encoded_size + 1);
    uint char_nbr = 0;
    uint byte_nbr = 0;
    uint32_t value = 0;
    while (byte_nbr < size) {
        //  Accumulate value in base 256 (binary)
        value = value * 256 + data [byte_nbr++];
        if (byte_nbr % 4 == 0) {
            //  Output value in base 85
            uint divisor = 85 * 85 * 85 * 85;
            while (divisor) {
                encoded [char_nbr++] = encoder [value / divisor % 85];
                divisor /= 85;
            }
            value = 0;
        }
    }
    assert (char_nbr == encoded_size);
    encoded [char_nbr] = 0;
    return encoded;
}


//  --------------------------------------------------------------------------
//  Decode an encoded string into a byte array; size of array will be
//  strlen (string) * 4 / 5.

byte *
Z85_decode (char *string)
{
    //  Accepts only strings bounded to 5 bytes
    if (strlen (string) % 5)
        return NULL;

    size_t decoded_size = strlen (string) * 4 / 5;
    byte *decoded = malloc (decoded_size);

    uint byte_nbr = 0;
    uint char_nbr = 0;
    uint32_t value = 0;
    while (char_nbr < strlen (string)) {
        //  Accumulate value in base 85
        value = value * 85 + decoder [(byte) string [char_nbr++] - 32];
        if (char_nbr % 5 == 0) {
            //  Output value in base 256
            uint divisor = 256 * 256 * 256;
            while (divisor) {
                decoded [byte_nbr++] = value / divisor % 256;
                divisor /= 256;
            }
            value = 0;
        }
    }
    assert (byte_nbr == decoded_size);
    return decoded;
}

int main (void)
{
    //  We don't care what's in this buffer, it has no impact on the test
    const size_t test_size = 500000;
    byte test_data [test_size];

    printf ("\nEncoding binary to string...");
    fflush (stdout);
    char *encoded = Z85_encode (test_data, test_size);

    printf ("\nDecoding string to binary...");
    fflush (stdout);
    byte *decoded = Z85_decode (encoded);

    if (memcmp (test_data, decoded, test_size) == 0)
        puts (" OK");
    else
        puts (" Failed");

    free (encoded);
    free (decoded);
    return 0;
}