1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
|
/* crypto/ec/ecp_nistputil.c */
/*
* Written by Bodo Moeller for the OpenSSL project.
*/
/* Copyright 2011 Google Inc.
*
* Licensed under the Apache License, Version 2.0 (the "License");
*
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
#include <openssl/opensslconf.h>
#ifndef OPENSSL_NO_EC_NISTP_64_GCC_128
/*
* Common utility functions for ecp_nistp224.c, ecp_nistp256.c, ecp_nistp521.c.
*/
# include <stddef.h>
# include "ec_lcl.h"
/*
* Convert an array of points into affine coordinates. (If the point at
* infinity is found (Z = 0), it remains unchanged.) This function is
* essentially an equivalent to EC_POINTs_make_affine(), but works with the
* internal representation of points as used by ecp_nistp###.c rather than
* with (BIGNUM-based) EC_POINT data structures. point_array is the
* input/output buffer ('num' points in projective form, i.e. three
* coordinates each), based on an internal representation of field elements
* of size 'felem_size'. tmp_felems needs to point to a temporary array of
* 'num'+1 field elements for storage of intermediate values.
*/
void ec_GFp_nistp_points_make_affine_internal(size_t num, void *point_array,
size_t felem_size,
void *tmp_felems,
void (*felem_one) (void *out),
int (*felem_is_zero) (const void
*in),
void (*felem_assign) (void *out,
const void
*in),
void (*felem_square) (void *out,
const void
*in),
void (*felem_mul) (void *out,
const void
*in1,
const void
*in2),
void (*felem_inv) (void *out,
const void
*in),
void (*felem_contract) (void
*out,
const
void
*in))
{
int i = 0;
# define tmp_felem(I) (&((char *)tmp_felems)[(I) * felem_size])
# define X(I) (&((char *)point_array)[3*(I) * felem_size])
# define Y(I) (&((char *)point_array)[(3*(I) + 1) * felem_size])
# define Z(I) (&((char *)point_array)[(3*(I) + 2) * felem_size])
if (!felem_is_zero(Z(0)))
felem_assign(tmp_felem(0), Z(0));
else
felem_one(tmp_felem(0));
for (i = 1; i < (int)num; i++) {
if (!felem_is_zero(Z(i)))
felem_mul(tmp_felem(i), tmp_felem(i - 1), Z(i));
else
felem_assign(tmp_felem(i), tmp_felem(i - 1));
}
/*
* Now each tmp_felem(i) is the product of Z(0) .. Z(i), skipping any
* zero-valued factors: if Z(i) = 0, we essentially pretend that Z(i) = 1
*/
felem_inv(tmp_felem(num - 1), tmp_felem(num - 1));
for (i = num - 1; i >= 0; i--) {
if (i > 0)
/*
* tmp_felem(i-1) is the product of Z(0) .. Z(i-1), tmp_felem(i)
* is the inverse of the product of Z(0) .. Z(i)
*/
/* 1/Z(i) */
felem_mul(tmp_felem(num), tmp_felem(i - 1), tmp_felem(i));
else
felem_assign(tmp_felem(num), tmp_felem(0)); /* 1/Z(0) */
if (!felem_is_zero(Z(i))) {
if (i > 0)
/*
* For next iteration, replace tmp_felem(i-1) by its inverse
*/
felem_mul(tmp_felem(i - 1), tmp_felem(i), Z(i));
/*
* Convert point (X, Y, Z) into affine form (X/(Z^2), Y/(Z^3), 1)
*/
felem_square(Z(i), tmp_felem(num)); /* 1/(Z^2) */
felem_mul(X(i), X(i), Z(i)); /* X/(Z^2) */
felem_mul(Z(i), Z(i), tmp_felem(num)); /* 1/(Z^3) */
felem_mul(Y(i), Y(i), Z(i)); /* Y/(Z^3) */
felem_contract(X(i), X(i));
felem_contract(Y(i), Y(i));
felem_one(Z(i));
} else {
if (i > 0)
/*
* For next iteration, replace tmp_felem(i-1) by its inverse
*/
felem_assign(tmp_felem(i - 1), tmp_felem(i));
}
}
}
/*-
* This function looks at 5+1 scalar bits (5 current, 1 adjacent less
* significant bit), and recodes them into a signed digit for use in fast point
* multiplication: the use of signed rather than unsigned digits means that
* fewer points need to be precomputed, given that point inversion is easy
* (a precomputed point dP makes -dP available as well).
*
* BACKGROUND:
*
* Signed digits for multiplication were introduced by Booth ("A signed binary
* multiplication technique", Quart. Journ. Mech. and Applied Math., vol. IV,
* pt. 2 (1951), pp. 236-240), in that case for multiplication of integers.
* Booth's original encoding did not generally improve the density of nonzero
* digits over the binary representation, and was merely meant to simplify the
* handling of signed factors given in two's complement; but it has since been
* shown to be the basis of various signed-digit representations that do have
* further advantages, including the wNAF, using the following general approach:
*
* (1) Given a binary representation
*
* b_k ... b_2 b_1 b_0,
*
* of a nonnegative integer (b_k in {0, 1}), rewrite it in digits 0, 1, -1
* by using bit-wise subtraction as follows:
*
* b_k b_(k-1) ... b_2 b_1 b_0
* - b_k ... b_3 b_2 b_1 b_0
* -------------------------------------
* s_k b_(k-1) ... s_3 s_2 s_1 s_0
*
* A left-shift followed by subtraction of the original value yields a new
* representation of the same value, using signed bits s_i = b_(i+1) - b_i.
* This representation from Booth's paper has since appeared in the
* literature under a variety of different names including "reversed binary
* form", "alternating greedy expansion", "mutual opposite form", and
* "sign-alternating {+-1}-representation".
*
* An interesting property is that among the nonzero bits, values 1 and -1
* strictly alternate.
*
* (2) Various window schemes can be applied to the Booth representation of
* integers: for example, right-to-left sliding windows yield the wNAF
* (a signed-digit encoding independently discovered by various researchers
* in the 1990s), and left-to-right sliding windows yield a left-to-right
* equivalent of the wNAF (independently discovered by various researchers
* around 2004).
*
* To prevent leaking information through side channels in point multiplication,
* we need to recode the given integer into a regular pattern: sliding windows
* as in wNAFs won't do, we need their fixed-window equivalent -- which is a few
* decades older: we'll be using the so-called "modified Booth encoding" due to
* MacSorley ("High-speed arithmetic in binary computers", Proc. IRE, vol. 49
* (1961), pp. 67-91), in a radix-2^5 setting. That is, we always combine five
* signed bits into a signed digit:
*
* s_(4j + 4) s_(4j + 3) s_(4j + 2) s_(4j + 1) s_(4j)
*
* The sign-alternating property implies that the resulting digit values are
* integers from -16 to 16.
*
* Of course, we don't actually need to compute the signed digits s_i as an
* intermediate step (that's just a nice way to see how this scheme relates
* to the wNAF): a direct computation obtains the recoded digit from the
* six bits b_(4j + 4) ... b_(4j - 1).
*
* This function takes those five bits as an integer (0 .. 63), writing the
* recoded digit to *sign (0 for positive, 1 for negative) and *digit (absolute
* value, in the range 0 .. 8). Note that this integer essentially provides the
* input bits "shifted to the left" by one position: for example, the input to
* compute the least significant recoded digit, given that there's no bit b_-1,
* has to be b_4 b_3 b_2 b_1 b_0 0.
*
*/
void ec_GFp_nistp_recode_scalar_bits(unsigned char *sign,
unsigned char *digit, unsigned char in)
{
unsigned char s, d;
s = ~((in >> 5) - 1); /* sets all bits to MSB(in), 'in' seen as
* 6-bit value */
d = (1 << 6) - in - 1;
d = (d & s) | (in & ~s);
d = (d >> 1) + (d & 1);
*sign = s & 1;
*digit = d;
}
#else
static void *dummy = &dummy;
#endif
|