Botan  2.8.0
Crypto and TLS for C++11
oaep.cpp
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1 /*
2 * OAEP
3 * (C) 1999-2010,2015,2018 Jack Lloyd
4 *
5 * Botan is released under the Simplified BSD License (see license.txt)
6 */
7 
8 #include <botan/oaep.h>
9 #include <botan/mgf1.h>
10 #include <botan/exceptn.h>
11 #include <botan/rng.h>
12 #include <botan/internal/ct_utils.h>
13 
14 namespace Botan {
15 
16 /*
17 * OAEP Pad Operation
18 */
19 secure_vector<uint8_t> OAEP::pad(const uint8_t in[], size_t in_length,
20  size_t key_length,
21  RandomNumberGenerator& rng) const
22  {
23  key_length /= 8;
24 
25  if(in_length > maximum_input_size(key_length * 8))
26  {
27  throw Invalid_Argument("OAEP: Input is too large");
28  }
29 
30  secure_vector<uint8_t> out(key_length);
31 
32  rng.randomize(out.data(), m_Phash.size());
33 
34  buffer_insert(out, m_Phash.size(), m_Phash.data(), m_Phash.size());
35  out[out.size() - in_length - 1] = 0x01;
36  buffer_insert(out, out.size() - in_length, in, in_length);
37 
38  mgf1_mask(*m_mgf1_hash,
39  out.data(), m_Phash.size(),
40  &out[m_Phash.size()], out.size() - m_Phash.size());
41 
42  mgf1_mask(*m_mgf1_hash,
43  &out[m_Phash.size()], out.size() - m_Phash.size(),
44  out.data(), m_Phash.size());
45 
46  return out;
47  }
48 
49 /*
50 * OAEP Unpad Operation
51 */
52 secure_vector<uint8_t> OAEP::unpad(uint8_t& valid_mask,
53  const uint8_t in[], size_t in_length) const
54  {
55  /*
56  Must be careful about error messages here; if an attacker can
57  distinguish them, it is easy to use the differences as an oracle to
58  find the secret key, as described in "A Chosen Ciphertext Attack on
59  RSA Optimal Asymmetric Encryption Padding (OAEP) as Standardized in
60  PKCS #1 v2.0", James Manger, Crypto 2001
61 
62  Also have to be careful about timing attacks! Pointed out by Falko
63  Strenzke.
64 
65  According to the standard (Section 7.1.1), the encryptor always
66  creates a message as follows:
67  i. Concatenate a single octet with hexadecimal value 0x00,
68  maskedSeed, and maskedDB to form an encoded message EM of
69  length k octets as
70  EM = 0x00 || maskedSeed || maskedDB.
71  where k is the length of the modulus N.
72  Therefore, the first byte can always be skipped safely.
73  */
74 
75  uint8_t skip_first = CT::is_zero<uint8_t>(in[0]) & 0x01;
76 
77  secure_vector<uint8_t> input(in + skip_first, in + in_length);
78 
79  const size_t hlen = m_Phash.size();
80 
81  mgf1_mask(*m_mgf1_hash,
82  &input[hlen], input.size() - hlen,
83  input.data(), hlen);
84 
85  mgf1_mask(*m_mgf1_hash,
86  input.data(), hlen,
87  &input[hlen], input.size() - hlen);
88 
89  return oaep_find_delim(valid_mask, input.data(), input.size(), m_Phash);
90  }
91 
92 secure_vector<uint8_t>
93 oaep_find_delim(uint8_t& valid_mask,
94  const uint8_t input[], size_t input_len,
95  const secure_vector<uint8_t>& Phash)
96  {
97  const size_t hlen = Phash.size();
98 
99  // Too short to be valid, reject immediately
100  if(input_len < 1 + 2*hlen)
101  {
102  return secure_vector<uint8_t>();
103  }
104 
105  CT::poison(input, input_len);
106 
107  size_t delim_idx = 2 * hlen;
108  uint8_t waiting_for_delim = 0xFF;
109  uint8_t bad_input = 0;
110 
111  for(size_t i = delim_idx; i < input_len; ++i)
112  {
113  const uint8_t zero_m = CT::is_zero<uint8_t>(input[i]);
114  const uint8_t one_m = CT::is_equal<uint8_t>(input[i], 1);
115 
116  const uint8_t add_m = waiting_for_delim & zero_m;
117 
118  bad_input |= waiting_for_delim & ~(zero_m | one_m);
119 
120  delim_idx += CT::select<uint8_t>(add_m, 1, 0);
121 
122  waiting_for_delim &= zero_m;
123  }
124 
125  // If we never saw any non-zero byte, then it's not valid input
126  bad_input |= waiting_for_delim;
127  bad_input |= CT::is_equal<uint8_t>(constant_time_compare(&input[hlen], Phash.data(), hlen), false);
128 
129  delim_idx &= ~CT::expand_mask<size_t>(bad_input);
130 
131  CT::unpoison(input, input_len);
132  CT::unpoison(&bad_input, 1);
133  CT::unpoison(&delim_idx, 1);
134 
135  valid_mask = ~bad_input;
136 
137  secure_vector<uint8_t> output(input + delim_idx + 1, input + input_len);
138  CT::cond_zero_mem(bad_input, output.data(), output.size());
139 
140  return output;
141  }
142 
143 /*
144 * Return the max input size for a given key size
145 */
146 size_t OAEP::maximum_input_size(size_t keybits) const
147  {
148  if(keybits / 8 > 2*m_Phash.size() + 1)
149  return ((keybits / 8) - 2*m_Phash.size() - 1);
150  else
151  return 0;
152  }
153 
154 /*
155 * OAEP Constructor
156 */
157 OAEP::OAEP(HashFunction* hash, const std::string& P) : m_mgf1_hash(hash)
158  {
159  m_Phash = m_mgf1_hash->process(P);
160  }
161 
163  HashFunction* mgf1_hash,
164  const std::string& P) : m_mgf1_hash(mgf1_hash)
165  {
166  std::unique_ptr<HashFunction> phash(hash); // takes ownership
167  m_Phash = phash->process(P);
168  }
169 
170 }
bool constant_time_compare(const uint8_t x[], const uint8_t y[], size_t len)
Definition: mem_ops.cpp:51
void poison(const T *p, size_t n)
Definition: ct_utils.h:47
void cond_zero_mem(T cond, T *array, size_t elems)
Definition: ct_utils.h:178
Definition: alg_id.cpp:13
size_t buffer_insert(std::vector< T, Alloc > &buf, size_t buf_offset, const T input[], size_t input_length)
Definition: secmem.h:103
OAEP(HashFunction *hash, const std::string &P="")
Definition: oaep.cpp:157
void unpoison(const T *p, size_t n)
Definition: ct_utils.h:58
std::vector< T, secure_allocator< T > > secure_vector
Definition: secmem.h:88
void mgf1_mask(HashFunction &hash, const uint8_t in[], size_t in_len, uint8_t out[], size_t out_len)
Definition: mgf1.cpp:14
size_t maximum_input_size(size_t) const override
Definition: oaep.cpp:146
secure_vector< uint8_t > oaep_find_delim(uint8_t &valid_mask, const uint8_t input[], size_t input_len, const secure_vector< uint8_t > &Phash)
Definition: oaep.cpp:93
MechanismType hash