Botan  2.11.0
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  const uint8_t skip_first = CT::Mask<uint8_t>::is_zero(in[0]).if_set_return(1);
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  CT::Mask<uint8_t> waiting_for_delim = CT::Mask<uint8_t>::set();
110 
111  for(size_t i = delim_idx; i < input_len; ++i)
112  {
113  const auto zero_m = CT::Mask<uint8_t>::is_zero(input[i]);
114  const auto one_m = CT::Mask<uint8_t>::is_equal(input[i], 1);
115 
116  const auto add_m = waiting_for_delim & zero_m;
117 
118  bad_input_m |= waiting_for_delim & ~(zero_m | one_m);
119 
120  delim_idx += add_m.if_set_return(1);
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_m |= waiting_for_delim;
127  bad_input_m |= CT::Mask<uint8_t>::is_zero(ct_compare_u8(&input[hlen], Phash.data(), hlen));
128 
129  delim_idx += 1;
130 
131  valid_mask = (~bad_input_m).unpoisoned_value();
132  const secure_vector<uint8_t> output = CT::copy_output(bad_input_m, input, input_len, delim_idx);
133 
134  CT::unpoison(input, input_len);
135 
136  return output;
137  }
138 
139 /*
140 * Return the max input size for a given key size
141 */
142 size_t OAEP::maximum_input_size(size_t keybits) const
143  {
144  if(keybits / 8 > 2*m_Phash.size() + 1)
145  return ((keybits / 8) - 2*m_Phash.size() - 1);
146  else
147  return 0;
148  }
149 
150 /*
151 * OAEP Constructor
152 */
153 OAEP::OAEP(HashFunction* hash, const std::string& P) : m_mgf1_hash(hash)
154  {
155  m_Phash = m_mgf1_hash->process(P);
156  }
157 
159  HashFunction* mgf1_hash,
160  const std::string& P) : m_mgf1_hash(mgf1_hash)
161  {
162  std::unique_ptr<HashFunction> phash(hash); // takes ownership
163  m_Phash = phash->process(P);
164  }
165 
166 }
static Mask< T > cleared()
Definition: ct_utils.h:115
void poison(const T *p, size_t n)
Definition: ct_utils.h:48
uint8_t ct_compare_u8(const uint8_t x[], const uint8_t y[], size_t len)
Definition: mem_ops.cpp:53
secure_vector< uint8_t > copy_output(CT::Mask< uint8_t > bad_input, const uint8_t input[], size_t input_length, size_t offset)
Definition: ct_utils.cpp:13
Definition: alg_id.cpp:13
T if_set_return(T x) const
Definition: ct_utils.h:248
size_t buffer_insert(std::vector< T, Alloc > &buf, size_t buf_offset, const T input[], size_t input_length)
Definition: secmem.h:80
OAEP(HashFunction *hash, const std::string &P="")
Definition: oaep.cpp:153
void unpoison(const T *p, size_t n)
Definition: ct_utils.h:59
static Mask< T > set()
Definition: ct_utils.h:107
std::vector< T, secure_allocator< T > > secure_vector
Definition: secmem.h:65
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:142
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
static Mask< T > is_zero(T x)
Definition: ct_utils.h:141
static Mask< T > is_equal(T x, T y)
Definition: ct_utils.h:149
MechanismType hash