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// base64.cpp

#include <stdio.h> // for size_t

// function prototypes

size_t base64_encode (char *dest, const char *source, size_t source_len);

size_t base64_decode (unsigned char *dest, const unsigned char *source, size_t source_len);

size_t base64_encode (char *dest, const char *source, size_t source_len)

{

   // encode a string in its base64 equivalent. It is up to the caller to ensure

   // that dest is large enough to contain the base64 representation of source.

   // Typically, a 4/3 size ratio is needed (i.e. when source is 30 bytes long,

   // dest should be large enough to contain 40 bytes).

   // When finished, return the length of the decoded data.

   static const char char_values[64 + 1] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

   unsigned char bytes[3];

   char *ptr;

   ptr = dest; // save where the destination buffer starts

   // as long as there are more than 3 characters left...

   while (source_len >= 3)

   {

      // get 3 bytes from source string

      bytes[0] = *(source++);

      bytes[1] = *(source++);

      bytes[2] = *(source++);

      // split these 3 bytes in 4 chunks of 6 bit each and replace each of them

      // with the corresponding character

      *(ptr++) = char_values[bytes[0] >> 2];

      *(ptr++) = char_values[((bytes[0] & 0x03) << 4) | (bytes[1] >> 4)];

      *(ptr++) = char_values[((bytes[1] & 0x0F) << 2) | (bytes[2] >> 6)];

      *(ptr++) = char_values[bytes[2] & 0x3F];

      source_len -= 3; // 3 characters less to process

   }

   // are there any characters left ?

   if (source_len > 0)

   {

      // encode the 6 most significant bits of the first character

      bytes[0] = *(source++);

      *(ptr++) = char_values[bytes[0] >> 2];

      // is there a second character to encode ?

      if (source_len > 1)

      {

         // encode the 2 least significant bytes of the first character

         // and the 8 bytes of the second character over 2 characters

         bytes[1] = *source;

         *(ptr++) = char_values [((bytes[0] & 0x03) << 4) | (bytes[1] >> 4)];

         *(ptr++) = char_values [(bytes[1] & 0x0F) << 2];

      }

      // else it was the only character left

      else

      {

         // encode the 2 least significant bytes of that character

         *(ptr++) = char_values[(bytes[0] & 0x03) << 4];

         *(ptr++) = '='; // padding (no character left)

      }

      *(ptr++) = '='; // padding (no character left)

   }

   *ptr = 0; // terminate the string

   return (ptr - dest); // finished, source has been base64-encoded in dest

}

size_t base64_decode (unsigned char *dest, const unsigned char *source, size_t source_len)

{

   // decodes a base64 string into its 8-bit equivalent. It is up to the caller

   // to ensure that dest is large enough to contain the decoded representation

   // of source. Typically, a 3/4 size ratio is needed (i.e. when source is 40

   // bytes long, dest should be large enough to contain 30 bytes).

   // When finished, return the length of the decoded data.

   #define __ 0xFF

   static const unsigned char dtable[256] =

   {

      __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

      __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

      __, __, __, __, __, __, __, __, __, __, __, 62, __, __, __, 63, // _ _ _ _ _ _ _ _ _ _ _ + _ _ _ /

      52, 53, 54, 55, 56, 57, 58, 59, 60, 61, __, __, __,  0, __, __, // 0 1 2 3 4 5 6 7 8 9 _ _ _ = _ _

      __,  0,  1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12, 13, 14, // _ A B C D E F G H I J K L M N O

      15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, __, __, __, __, __, // P Q R S T U V W X Y Z _ _ _ _ _

      __, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, // _ a b c d e f g h i j k l m n o

      41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, __, __, __, __, __, // p q r s t u v w x y z _ _ _ _ _

      __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

      __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

      __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

      __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

      __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

      __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

      __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

      __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __  // _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

   };

   #undef __

   unsigned char rawin[4];

   unsigned char block[4];

   unsigned char *pos;

   unsigned char tmp;

   size_t char_index;

   size_t count;

   pos = dest; // initialize output stream writer at the beginning of the stream

   count = 0; // reset the block character counter

   // for each base64 character in source...

   for (char_index = 0; char_index < source_len; char_index++)

   {

      tmp = dtable[source[char_index]]; // peek at the translation of this character in the table

      if (tmp == 0xFF)

         continue; // skip (ignore) all non-base64 characters

      rawin[count] = source[char_index]; // keep track of the character we're translating

      block[count] = tmp; // store the translation in the current block

      count++; // there's one character more in the current 4-character block

      // each time we've filled a 4-character block, decode it in 3 bytes in the output stream

      if (count == 4)

      {

         *pos++ = (block[0] << 2) | (block[1] >> 4);

         *pos++ = (block[1] << 4) | (block[2] >> 2);

         *pos++ = (block[2] << 6) | (block[3] >> 0);

         count = 0; // and reset the character counter for the next block

      }

   }

   // handle stream end: have we decoded anything ?

   if (pos > dest)

   {

      if (rawin[2] == '=')

         pos -= 2;

      else if (rawin[3] == '=')

         pos--;

   }

   *pos = 0; // drop a string terminator where the decoded stream ends (but don't take it in account in the length)

   return (pos - dest); // source has been base64-decoded in dest, return decoded length

}