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algos_enc: padding for hiding message length

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This commit is contained in:
Mirek Kratochvil 2013-04-22 14:56:20 +02:00
parent 5633358195
commit e19ba185b7
1 changed files with 47 additions and 13 deletions
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60
src/algos_enc.cpp
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@ -67,7 +67,8 @@ int algo_mceqd256::create_keypair (sencode**pub, sencode**priv, prng&rng)
* randombits = randomness that pads the message bits to whole byte.
* nrbits = 1 byte with number of random bits applied
*
* Then we are padding stuff in 256-byte blocks this way:
* Then we are padding stuff with a padding of length at most 255 blocks, to
* format:
*
* messagemessage [randomrandomrandom] 1bytesize
*
@ -81,8 +82,38 @@ int algo_mceqd256::create_keypair (sencode**pub, sencode**priv, prng&rng)
* (even if there are no message bytes in it, there still must be the zero
* 1byte number that is telling us.)
* - stuff in bytes is always thought about as big-endian
*
* 1bytesize is determined from message and message length in bits, as:
*
* size = h1(msg) + h2(|msg|)
*
* where h1 and h2 are hash functions to [0..127]
*/
#include "rmd_hash.h"
static byte msg_pad_length (const std::vector<byte>& msg)
{
uint64_t len = msg.size();
std::vector<byte> lenbytes;
lenbytes.resize (sizeof (uint64_t), 0);
for (uint i = 0; i < sizeof (uint64_t); ++i) {
lenbytes[i] = len & 0xff;
len >>= 8;
}
std::vector<byte> tmp;
rmd128hash hf;
byte res = 0;
tmp = hf (lenbytes);
res += tmp[0] & 0x7f;
tmp = hf (msg);
res += tmp[0] & 0x7f;
return res;
}
static void message_pad (const bvector&in, std::vector<byte>&out, prng&rng)
{
out.clear();
@ -106,33 +137,36 @@ static void message_pad (const bvector&in, std::vector<byte>&out, prng&rng)
out[i >> 3] = in.size() & 0x7;
//byte stage
uint overflow = out.size() & 0xff;
uint pad_block_start = out.size() >> 8;
uint pad_block_start_byte = pad_block_start << 8;
byte padsize = msg_pad_length (out);
uint out_end = out.size();
//make space for the bytes
out.resize ( (pad_block_start + 1) << 8, 0);
out.resize (out_end + padsize + 1, 0);
//fill random bytes
for (i = overflow; i < 0xff; ++i)
out[i + pad_block_start_byte] = rng.random (256);
for (i = 0; i < padsize; ++i)
out[out_end + i] = rng.random (256);
//fill the overflow size byte
out[pad_block_start_byte + 0xff] = overflow;
out[out_end + padsize] = padsize;
}
static bool message_unpad (const std::vector<byte>&in, bvector&out)
static bool message_unpad (std::vector<byte> in, bvector&out)
{
//check byte padding sizes
if (!in.size() ) return false;
if (in.size() & 0xff) return false;
//get rid of the byte padding
uint byte_overflow = in[in.size() - 1];
uint in_end = in.size() + byte_overflow - 256;
uint padsize = in[in.size() - 1];
if (padsize + 1 > in.size() ) return false;
uint in_end = in.size() - padsize - 1;
if (!in_end) return false; //we need the last byte for bit padding
in.resize (in_end);
if (padsize != msg_pad_length (in) ) return false;
//get bit padding information (now it's the last byte)
if (!in_end) return false;
uint bit_overflow = in[in_end - 1];
//there must not be more than one byte of overflown bits