/src/botan/src/lib/math/numbertheory/mod_inv.cpp
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1 | | /* |
2 | | * (C) 1999-2011,2016,2018,2019,2020 Jack Lloyd |
3 | | * |
4 | | * Botan is released under the Simplified BSD License (see license.txt) |
5 | | */ |
6 | | |
7 | | #include <botan/numthry.h> |
8 | | |
9 | | #include <botan/internal/ct_utils.h> |
10 | | #include <botan/internal/divide.h> |
11 | | #include <botan/internal/mp_core.h> |
12 | | #include <botan/internal/rounding.h> |
13 | | |
14 | | namespace Botan { |
15 | | |
16 | | namespace { |
17 | | |
18 | 0 | BigInt inverse_mod_odd_modulus(const BigInt& n, const BigInt& mod) { |
19 | | // Caller should assure these preconditions: |
20 | 0 | BOTAN_DEBUG_ASSERT(n.is_positive()); |
21 | 0 | BOTAN_DEBUG_ASSERT(mod.is_positive()); |
22 | 0 | BOTAN_DEBUG_ASSERT(n < mod); |
23 | 0 | BOTAN_DEBUG_ASSERT(mod >= 3 && mod.is_odd()); |
24 | | |
25 | | /* |
26 | | This uses a modular inversion algorithm designed by Niels Möller |
27 | | and implemented in Nettle. The same algorithm was later also |
28 | | adapted to GMP in mpn_sec_invert. |
29 | | |
30 | | It can be easily implemented in a way that does not depend on |
31 | | secret branches or memory lookups, providing resistance against |
32 | | some forms of side channel attack. |
33 | | |
34 | | There is also a description of the algorithm in Appendix 5 of "Fast |
35 | | Software Polynomial Multiplication on ARM Processors using the NEON Engine" |
36 | | by Danilo Câmara, Conrado P. L. Gouvêa, Julio López, and Ricardo |
37 | | Dahab in LNCS 8182 |
38 | | https://conradoplg.cryptoland.net/files/2010/12/mocrysen13.pdf |
39 | | |
40 | | Thanks to Niels for creating the algorithm, explaining some things |
41 | | about it, and the reference to the paper. |
42 | | */ |
43 | |
|
44 | 0 | const size_t mod_words = mod.sig_words(); |
45 | 0 | BOTAN_ASSERT(mod_words > 0, "Not empty"); |
46 | |
|
47 | 0 | secure_vector<word> tmp_mem(5 * mod_words); |
48 | |
|
49 | 0 | word* v_w = &tmp_mem[0]; |
50 | 0 | word* u_w = &tmp_mem[1 * mod_words]; |
51 | 0 | word* b_w = &tmp_mem[2 * mod_words]; |
52 | 0 | word* a_w = &tmp_mem[3 * mod_words]; |
53 | 0 | word* mp1o2 = &tmp_mem[4 * mod_words]; |
54 | |
|
55 | 0 | CT::poison(tmp_mem.data(), tmp_mem.size()); |
56 | |
|
57 | 0 | copy_mem(a_w, n.data(), std::min(n.size(), mod_words)); |
58 | 0 | copy_mem(b_w, mod.data(), std::min(mod.size(), mod_words)); |
59 | 0 | u_w[0] = 1; |
60 | | // v_w = 0 |
61 | | |
62 | | // compute (mod + 1) / 2 which [because mod is odd] is equal to |
63 | | // (mod / 2) + 1 |
64 | 0 | copy_mem(mp1o2, mod.data(), std::min(mod.size(), mod_words)); |
65 | 0 | bigint_shr1(mp1o2, mod_words, 0, 1); |
66 | 0 | word carry = bigint_add2_nc(mp1o2, mod_words, u_w, 1); |
67 | 0 | BOTAN_ASSERT_NOMSG(carry == 0); |
68 | | |
69 | | // Only n.bits() + mod.bits() iterations are required, but avoid leaking the size of n |
70 | 0 | const size_t execs = 2 * mod.bits(); |
71 | |
|
72 | 0 | for(size_t i = 0; i != execs; ++i) { |
73 | 0 | const word odd_a = a_w[0] & 1; |
74 | | |
75 | | //if(odd_a) a -= b |
76 | 0 | word underflow = bigint_cnd_sub(odd_a, a_w, b_w, mod_words); |
77 | | |
78 | | //if(underflow) { b -= a; a = abs(a); swap(u, v); } |
79 | 0 | bigint_cnd_add(underflow, b_w, a_w, mod_words); |
80 | 0 | bigint_cnd_abs(underflow, a_w, mod_words); |
81 | 0 | bigint_cnd_swap(underflow, u_w, v_w, mod_words); |
82 | | |
83 | | // a >>= 1 |
84 | 0 | bigint_shr1(a_w, mod_words, 0, 1); |
85 | | |
86 | | //if(odd_a) u -= v; |
87 | 0 | word borrow = bigint_cnd_sub(odd_a, u_w, v_w, mod_words); |
88 | | |
89 | | // if(borrow) u += p |
90 | 0 | bigint_cnd_add(borrow, u_w, mod.data(), mod_words); |
91 | |
|
92 | 0 | const word odd_u = u_w[0] & 1; |
93 | | |
94 | | // u >>= 1 |
95 | 0 | bigint_shr1(u_w, mod_words, 0, 1); |
96 | | |
97 | | //if(odd_u) u += mp1o2; |
98 | 0 | bigint_cnd_add(odd_u, u_w, mp1o2, mod_words); |
99 | 0 | } |
100 | |
|
101 | 0 | auto a_is_0 = CT::Mask<word>::set(); |
102 | 0 | for(size_t i = 0; i != mod_words; ++i) { |
103 | 0 | a_is_0 &= CT::Mask<word>::is_zero(a_w[i]); |
104 | 0 | } |
105 | |
|
106 | 0 | auto b_is_1 = CT::Mask<word>::is_equal(b_w[0], 1); |
107 | 0 | for(size_t i = 1; i != mod_words; ++i) { |
108 | 0 | b_is_1 &= CT::Mask<word>::is_zero(b_w[i]); |
109 | 0 | } |
110 | |
|
111 | 0 | BOTAN_ASSERT(a_is_0.is_set(), "A is zero"); |
112 | | |
113 | | // if b != 1 then gcd(n,mod) > 1 and inverse does not exist |
114 | | // in which case zero out the result to indicate this |
115 | 0 | (~b_is_1).if_set_zero_out(v_w, mod_words); |
116 | | |
117 | | /* |
118 | | * We've placed the result in the lowest words of the temp buffer. |
119 | | * So just clear out the other values and then give that buffer to a |
120 | | * BigInt. |
121 | | */ |
122 | 0 | clear_mem(&tmp_mem[mod_words], 4 * mod_words); |
123 | |
|
124 | 0 | CT::unpoison(tmp_mem.data(), tmp_mem.size()); |
125 | |
|
126 | 0 | BigInt r; |
127 | 0 | r.swap_reg(tmp_mem); |
128 | 0 | return r; |
129 | 0 | } |
130 | | |
131 | 0 | BigInt inverse_mod_pow2(const BigInt& a1, size_t k) { |
132 | | /* |
133 | | * From "A New Algorithm for Inversion mod p^k" by Çetin Kaya Koç |
134 | | * https://eprint.iacr.org/2017/411.pdf sections 5 and 7. |
135 | | */ |
136 | |
|
137 | 0 | if(a1.is_even() || k == 0) { |
138 | 0 | return BigInt::zero(); |
139 | 0 | } |
140 | 0 | if(k == 1) { |
141 | 0 | return BigInt::one(); |
142 | 0 | } |
143 | | |
144 | 0 | BigInt a = a1; |
145 | 0 | a.mask_bits(k); |
146 | |
|
147 | 0 | BigInt b = BigInt::one(); |
148 | 0 | BigInt X = BigInt::zero(); |
149 | 0 | BigInt newb; |
150 | |
|
151 | 0 | const size_t a_words = a.sig_words(); |
152 | |
|
153 | 0 | X.grow_to(round_up(k, BOTAN_MP_WORD_BITS) / BOTAN_MP_WORD_BITS); |
154 | 0 | b.grow_to(a_words); |
155 | | |
156 | | /* |
157 | | Hide the exact value of k. k is anyway known to word length |
158 | | granularity because of the length of a, so no point in doing more |
159 | | than this. |
160 | | */ |
161 | 0 | const size_t iter = round_up(k, BOTAN_MP_WORD_BITS); |
162 | |
|
163 | 0 | for(size_t i = 0; i != iter; ++i) { |
164 | 0 | const bool b0 = b.get_bit(0); |
165 | 0 | X.conditionally_set_bit(i, b0); |
166 | 0 | newb = b - a; |
167 | 0 | b.ct_cond_assign(b0, newb); |
168 | 0 | b >>= 1; |
169 | 0 | } |
170 | |
|
171 | 0 | X.mask_bits(k); |
172 | 0 | X.const_time_unpoison(); |
173 | 0 | return X; |
174 | 0 | } |
175 | | |
176 | | } // namespace |
177 | | |
178 | 0 | BigInt inverse_mod(const BigInt& n, const BigInt& mod) { |
179 | 0 | if(mod.is_zero()) { |
180 | 0 | throw Invalid_Argument("inverse_mod modulus cannot be zero"); |
181 | 0 | } |
182 | 0 | if(mod.is_negative() || n.is_negative()) { |
183 | 0 | throw Invalid_Argument("inverse_mod: arguments must be non-negative"); |
184 | 0 | } |
185 | 0 | if(n.is_zero() || (n.is_even() && mod.is_even())) { |
186 | 0 | return BigInt::zero(); |
187 | 0 | } |
188 | | |
189 | 0 | if(mod.is_odd()) { |
190 | | /* |
191 | | Fastpath for common case. This leaks if n is greater than mod or |
192 | | not, but we don't guarantee const time behavior in that case. |
193 | | */ |
194 | 0 | if(n < mod) { |
195 | 0 | return inverse_mod_odd_modulus(n, mod); |
196 | 0 | } else { |
197 | 0 | return inverse_mod_odd_modulus(ct_modulo(n, mod), mod); |
198 | 0 | } |
199 | 0 | } |
200 | | |
201 | | // If n is even and mod is even we already returned 0 |
202 | | // If n is even and mod is odd we jumped directly to odd-modulus algo |
203 | 0 | BOTAN_DEBUG_ASSERT(n.is_odd()); |
204 | |
|
205 | 0 | const size_t mod_lz = low_zero_bits(mod); |
206 | 0 | BOTAN_ASSERT_NOMSG(mod_lz > 0); |
207 | 0 | const size_t mod_bits = mod.bits(); |
208 | 0 | BOTAN_ASSERT_NOMSG(mod_bits > mod_lz); |
209 | |
|
210 | 0 | if(mod_lz == mod_bits - 1) { |
211 | | // In this case we are performing an inversion modulo 2^k |
212 | 0 | return inverse_mod_pow2(n, mod_lz); |
213 | 0 | } |
214 | | |
215 | 0 | if(mod_lz == 1) { |
216 | | /* |
217 | | Inversion modulo 2*o is an easier special case of CRT |
218 | | |
219 | | This is exactly the main CRT flow below but taking advantage of |
220 | | the fact that any odd number ^-1 modulo 2 is 1. As a result both |
221 | | inv_2k and c can be taken to be 1, m2k is 2, and h is always |
222 | | either 0 or 1, and its value depends only on the low bit of inv_o. |
223 | | |
224 | | This is worth special casing because we generate RSA primes such |
225 | | that phi(n) is of this form. However this only works for keys |
226 | | that we generated in this way; pre-existing keys will typically |
227 | | fall back to the general algorithm below. |
228 | | */ |
229 | |
|
230 | 0 | const BigInt o = mod >> 1; |
231 | 0 | const BigInt n_redc = ct_modulo(n, o); |
232 | 0 | const BigInt inv_o = inverse_mod_odd_modulus(n_redc, o); |
233 | | |
234 | | // No modular inverse in this case: |
235 | 0 | if(inv_o == 0) { |
236 | 0 | return BigInt::zero(); |
237 | 0 | } |
238 | | |
239 | 0 | BigInt h = inv_o; |
240 | 0 | h.ct_cond_add(!inv_o.get_bit(0), o); |
241 | 0 | return h; |
242 | 0 | } |
243 | | |
244 | | /* |
245 | | * In this case we are performing an inversion modulo 2^k*o for |
246 | | * some k >= 2 and some odd (not necessarily prime) integer. |
247 | | * Compute the inversions modulo 2^k and modulo o, then combine them |
248 | | * using CRT, which is possible because 2^k and o are relatively prime. |
249 | | */ |
250 | | |
251 | 0 | const BigInt o = mod >> mod_lz; |
252 | 0 | const BigInt n_redc = ct_modulo(n, o); |
253 | 0 | const BigInt inv_o = inverse_mod_odd_modulus(n_redc, o); |
254 | 0 | const BigInt inv_2k = inverse_mod_pow2(n, mod_lz); |
255 | | |
256 | | // No modular inverse in this case: |
257 | 0 | if(inv_o == 0 || inv_2k == 0) { |
258 | 0 | return BigInt::zero(); |
259 | 0 | } |
260 | | |
261 | 0 | const BigInt m2k = BigInt::power_of_2(mod_lz); |
262 | | // Compute the CRT parameter |
263 | 0 | const BigInt c = inverse_mod_pow2(o, mod_lz); |
264 | | |
265 | | // Compute h = c*(inv_2k-inv_o) mod 2^k |
266 | 0 | BigInt h = c * (inv_2k - inv_o); |
267 | 0 | const bool h_neg = h.is_negative(); |
268 | 0 | h.set_sign(BigInt::Positive); |
269 | 0 | h.mask_bits(mod_lz); |
270 | 0 | const bool h_nonzero = h.is_nonzero(); |
271 | 0 | h.ct_cond_assign(h_nonzero && h_neg, m2k - h); |
272 | | |
273 | | // Return result inv_o + h * o |
274 | 0 | h *= o; |
275 | 0 | h += inv_o; |
276 | 0 | return h; |
277 | 0 | } |
278 | | |
279 | | } // namespace Botan |