base64_dump.c

#include <stdio.h>
#include <stdint.h>

void base64_dump(const uint8_t *data, size_t len)
{
  static const char lut[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
  size_t pos;

  for (pos = 0; pos + 3 <= len; pos += 3) {
    putchar(lut[  data[pos  ] >> 2]);
    putchar(lut[((data[pos  ] & 0x03) << 4) | (data[pos+1] >> 4)]);
    putchar(lut[((data[pos+1] & 0x0f) << 2) | (data[pos+2] >> 6)]);
    putchar(lut[  data[pos+2] & 0x3f]);
  }
  if (pos < len) {
    putchar(lut[  data[pos  ] >> 2]);
    if (pos + 1 < len) {
      putchar(lut[((data[pos  ] & 0x03) << 4) | (data[pos+1] >> 4)]);
      putchar(lut[ (data[pos+1] & 0x0f) << 2]);
    } else {
      putchar(lut[ (data[pos  ] & 0x03) << 4]);
      putchar('=');
    }
    putchar('=');
  }
}

CC0 license.
based on http://tools.ietf.org/html/rfc3548