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/*
* OAEP
* (C) 1999-2010,2015 Jack Lloyd
*
* Botan is released under the Simplified BSD License (see license.txt)
*/
#include <botan/oaep.h>
#include <botan/mgf1.h>
#include <botan/internal/ct_utils.h>
namespace Botan {
/*
* OAEP Pad Operation
*/
secure_vector<byte> OAEP::pad(const byte in[], size_t in_length,
size_t key_length,
RandomNumberGenerator& rng) const
{
key_length /= 8;
if(in_length > maximum_input_size(key_length * 8))
{
throw Invalid_Argument("OAEP: Input is too large");
}
secure_vector<byte> out(key_length);
rng.randomize(out.data(), m_Phash.size());
buffer_insert(out, m_Phash.size(), m_Phash.data(), m_Phash.size());
out[out.size() - in_length - 1] = 0x01;
buffer_insert(out, out.size() - in_length, in, in_length);
mgf1_mask(*m_hash,
out.data(), m_Phash.size(),
&out[m_Phash.size()], out.size() - m_Phash.size());
mgf1_mask(*m_hash,
&out[m_Phash.size()], out.size() - m_Phash.size(),
out.data(), m_Phash.size());
return out;
}
/*
* OAEP Unpad Operation
*/
secure_vector<byte> OAEP::unpad(byte& valid_mask,
const byte in[], size_t in_length) const
{
/*
Must be careful about error messages here; if an attacker can
distinguish them, it is easy to use the differences as an oracle to
find the secret key, as described in "A Chosen Ciphertext Attack on
RSA Optimal Asymmetric Encryption Padding (OAEP) as Standardized in
PKCS #1 v2.0", James Manger, Crypto 2001
Also have to be careful about timing attacks! Pointed out by Falko
Strenzke.
According to the standard (Section 7.1.1), the encryptor always
creates a message as follows:
i. Concatenate a single octet with hexadecimal value 0x00,
maskedSeed, and maskedDB to form an encoded message EM of
length k octets as
EM = 0x00 || maskedSeed || maskedDB.
where k is the length of the modulus N.
Therefore, the first byte can always be skipped safely.
*/
byte skip_first = CT::is_zero<byte>(in[0]) & 0x01;
secure_vector<byte> input(in + skip_first, in + in_length);
CT::poison(input.data(), input.size());
const size_t hlen = m_Phash.size();
mgf1_mask(*m_hash,
&input[hlen], input.size() - hlen,
input.data(), hlen);
mgf1_mask(*m_hash,
input.data(), hlen,
&input[hlen], input.size() - hlen);
size_t delim_idx = 2 * hlen;
byte waiting_for_delim = 0xFF;
byte bad_input = 0;
for(size_t i = delim_idx; i < input.size(); ++i)
{
const byte zero_m = CT::is_zero<byte>(input[i]);
const byte one_m = CT::is_equal<byte>(input[i], 1);
const byte add_m = waiting_for_delim & zero_m;
bad_input |= waiting_for_delim & ~(zero_m | one_m);
delim_idx += CT::select<byte>(add_m, 1, 0);
waiting_for_delim &= zero_m;
}
// If we never saw any non-zero byte, then it's not valid input
bad_input |= waiting_for_delim;
bad_input |= CT::is_equal<byte>(same_mem(&input[hlen], m_Phash.data(), hlen), false);
CT::unpoison(input.data(), input.size());
CT::unpoison(&bad_input, 1);
CT::unpoison(&delim_idx, 1);
valid_mask = ~bad_input;
secure_vector<byte> output(input.begin() + delim_idx + 1, input.end());
CT::cond_zero_mem(bad_input, output.data(), output.size());
return output;
}
/*
* Return the max input size for a given key size
*/
size_t OAEP::maximum_input_size(size_t keybits) const
{
if(keybits / 8 > 2*m_Phash.size() + 1)
return ((keybits / 8) - 2*m_Phash.size() - 1);
else
return 0;
}
/*
* OAEP Constructor
*/
OAEP::OAEP(HashFunction* hash, const std::string& P) : m_hash(hash)
{
m_Phash = m_hash->process(P);
}
}
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