/**
* base32 (de)coder implementation as specified by RFC4648.
*
* Copyright (c) 2010 Adrien Kunysz
*
* Permission is hereby granted, free of charge, to any person obtaining a copy
* of this software and associated documentation files (the "Software"), to deal
* in the Software without restriction, including without limitation the rights
* to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
* copies of the Software, and to permit persons to whom the Software is
* furnished to do so, subject to the following conditions:
*
* The above copyright notice and this permission notice shall be included in
* all copies or substantial portions of the Software.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
* AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
* THE SOFTWARE.
**/
#include <assert.h> // assert()
#include <limits.h> // CHAR_BIT
#include "base32.h"
/**
* Let this be a sequence of plain data before encoding:
*
* 01234567 01234567 01234567 01234567 01234567
* +--------+--------+--------+--------+--------+
* |< 0 >< 1| >< 2 ><|.3 >< 4.|>< 5 ><.|6 >< 7 >|
* +--------+--------+--------+--------+--------+
*
* There are 5 octets of 8 bits each in each sequence.
* There are 8 blocks of 5 bits each in each sequence.
*
* You probably want to refer to that graph when reading the algorithms in this
* file. We use "octet" instead of "byte" intentionnaly as we really work with
* 8 bits quantities. This implementation will probably not work properly on
* systems that don't have exactly 8 bits per (unsigned) char.
**/
static size_t min(size_t x, size_t y)
{
return x < y ? x : y;
}
static const unsigned char PADDING_CHAR = '=';
/**
* Pad the given buffer with len padding characters.
*/
static void pad(unsigned char *buf, int len)
{
for (int i = 0; i < len; i++)
buf[i] = PADDING_CHAR;
}
/**
* This convert a 5 bits value into a base32 character.
* Only the 5 least significant bits are used.
*/
static unsigned char encode_char(unsigned char c)
{
static unsigned char base32[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZ234567";
return base32[c & 0x1F]; // 0001 1111
}
/**
* Decode given character into a 5 bits value.
* Returns -1 iff the argument given was an invalid base32 character
* or a padding character.
*/
static int decode_char(unsigned char c)
{
int retval = -1;
if (c >= 'A' && c <= 'Z')
retval = c - 'A';
if (c >= '2' && c <= '7')
retval = c - '2' + 26;
assert(retval == -1 || ((retval & 0x1F) == retval));
return retval;
}
/**
* Given a block id between 0 and 7 inclusive, this will return the index of
* the octet in which this block starts. For example, given 3 it will return 1
* because block 3 starts in octet 1:
*
* +--------+--------+
* | ......<|.3 >....|
* +--------+--------+
* octet 1 | octet 2
*/
static int get_octet(int block)
{
assert(block >= 0 && block < 8);
return (block*5) / 8;
}
/**
* Given a block id between 0 and 7 inclusive, this will return how many bits
* we can drop at the end of the octet in which this block starts.
* For example, given block 0 it will return 3 because there are 3 bits
* we don't care about at the end:
*
* +--------+-
* |< 0 >...|
* +--------+-
*
* Given block 1, it will return -2 because there
* are actually two bits missing to have a complete block:
*
* +--------+-
* |.....< 1|..
* +--------+-
**/
static int get_offset(int block)
{
assert(block >= 0 && block < 8);
return (8 - 5 - (5*block) % 8);
}
/**
* Like "b >> offset" but it will do the right thing with negative offset.
* We need this as bitwise shifting by a negative offset is undefined
* behavior.
*/
static unsigned char shift_right(unsigned char byte, int offset)
{
if (offset > 0)
return byte >> offset;
else
return byte << -offset;
}
static unsigned char shift_left(unsigned char byte, int offset)
{
return shift_right(byte, - offset);
}
/**
* Encode a sequence. A sequence is no longer than 5 octets by definition.
* Thus passing a length greater than 5 to this function is an error. Encoding
* sequences shorter than 5 octets is supported and padding will be added to the
* output as per the specification.
*/
static void encode_sequence(const unsigned char *plain, int len, unsigned char *coded)
{
assert(CHAR_BIT == 8); // not sure this would work otherwise
assert(len >= 0 && len <= 5);
for (int block = 0; block < 8; block++) {
int octet = get_octet(block); // figure out which octet this block starts in
int junk = get_offset(block); // how many bits do we drop from this octet?
if (octet >= len) { // we hit the end of the buffer
pad(&coded[block], 8 - block);
return;
}
unsigned char c = shift_right(plain[octet], junk); // first part
if (junk < 0 // is there a second part?
&& octet < len - 1) // is there still something to read?
{
c |= shift_right(plain[octet+1], 8 + junk);
}
coded[block] = encode_char(c);
}
}
void base32_encode(const unsigned char *plain, size_t len, unsigned char *coded)
{
// All the hard work is done in encode_sequence(),
// here we just need to feed it the data sequence by sequence.
for (size_t i = 0, j = 0; i < len; i += 5, j += 8) {
encode_sequence(&plain[i], min(len - i, 5), &coded[j]);
}
}
static int decode_sequence(const unsigned char *coded, unsigned char *plain)
{
assert(CHAR_BIT == 8);
assert(coded && plain);
plain[0] = 0;
for (int block = 0; block < 8; block++) {
int offset = get_offset(block);
int octet = get_octet(block);
int c = decode_char(coded[block]);
if (c < 0) // invalid char, stop here
return octet;
plain[octet] |= shift_left(c, offset);
if (offset < 0) { // does this block overflows to next octet?
assert(octet < 4);
plain[octet+1] = shift_left(c, 8 + offset);
}
}
return 5;
}
size_t base32_decode(const unsigned char *coded, unsigned char *plain)
{
size_t written = 0;
for (size_t i = 0, j = 0; ; i += 8, j += 5) {
int n = decode_sequence(&coded[i], &plain[j]);
written += n;
if (n < 5)
return written;
}
}