xref: /freebsd/contrib/bearssl/src/symcipher/poly1305_i15.c (revision 031beb4e239bfce798af17f5fe8dba8bcaf13d99)
1 /*
2  * Copyright (c) 2017 Thomas Pornin <pornin@bolet.org>
3  *
4  * Permission is hereby granted, free of charge, to any person obtaining
5  * a copy of this software and associated documentation files (the
6  * "Software"), to deal in the Software without restriction, including
7  * without limitation the rights to use, copy, modify, merge, publish,
8  * distribute, sublicense, and/or sell copies of the Software, and to
9  * permit persons to whom the Software is furnished to do so, subject to
10  * the following conditions:
11  *
12  * The above copyright notice and this permission notice shall be
13  * included in all copies or substantial portions of the Software.
14  *
15  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
16  * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
17  * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
18  * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
19  * BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
20  * ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
21  * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
22  * SOFTWARE.
23  */
24 
25 #include "inner.h"
26 
27 /*
28  * This is a "reference" implementation of Poly1305 that uses the
29  * generic "i15" code for big integers. It is slow, but it handles all
30  * big-integer operations with generic code, thereby avoiding most
31  * tricky situations with carry propagation and modular reduction.
32  */
33 
34 /*
35  * Modulus: 2^130-5.
36  */
37 static const uint16_t P1305[] = {
38 	0x008A,
39 	0x7FFB, 0x7FFF, 0x7FFF, 0x7FFF, 0x7FFF, 0x7FFF, 0x7FFF, 0x7FFF, 0x03FF
40 };
41 
42 /*
43  * -p mod 2^15.
44  */
45 #define P0I   0x4CCD
46 
47 /*
48  * R^2 mod p, for conversion to Montgomery representation (R = 2^135,
49  * since we use 9 words of 15 bits each, and 15*9 = 135).
50  */
51 static const uint16_t R2[] = {
52 	0x008A,
53 	0x6400, 0x0000, 0x0000, 0x0000, 0x0000, 0x0000, 0x0000, 0x0000, 0x0000
54 };
55 
56 /*
57  * Perform the inner processing of blocks for Poly1305. The "r" array
58  * is in Montgomery representation, while the "a" array is not.
59  */
60 static void
61 poly1305_inner(uint16_t *a, const uint16_t *r, const void *data, size_t len)
62 {
63 	const unsigned char *buf;
64 
65 	buf = data;
66 	while (len > 0) {
67 		unsigned char tmp[16], rev[16];
68 		uint16_t b[10];
69 		uint32_t ctl;
70 		int i;
71 
72 		/*
73 		 * If there is a partial block, right-pad it with zeros.
74 		 */
75 		if (len < 16) {
76 			memset(tmp, 0, sizeof tmp);
77 			memcpy(tmp, buf, len);
78 			buf = tmp;
79 			len = 16;
80 		}
81 
82 		/*
83 		 * Decode next block and apply the "high bit". Since
84 		 * decoding is little-endian, we must byte-swap the buffer.
85 		 */
86 		for (i = 0; i < 16; i ++) {
87 			rev[i] = buf[15 - i];
88 		}
89 		br_i15_decode_mod(b, rev, sizeof rev, P1305);
90 		b[9] |= 0x0100;
91 
92 		/*
93 		 * Add the accumulator to the decoded block (modular
94 		 * addition).
95 		 */
96 		ctl = br_i15_add(b, a, 1);
97 		ctl |= NOT(br_i15_sub(b, P1305, 0));
98 		br_i15_sub(b, P1305, ctl);
99 
100 		/*
101 		 * Multiply by r, result is the new accumulator value.
102 		 */
103 		br_i15_montymul(a, b, r, P1305, P0I);
104 
105 		buf += 16;
106 		len -= 16;
107 	}
108 }
109 
110 /*
111  * Byteswap a 16-byte value.
112  */
113 static void
114 byteswap16(unsigned char *buf)
115 {
116 	int i;
117 
118 	for (i = 0; i < 8; i ++) {
119 		unsigned x;
120 
121 		x = buf[i];
122 		buf[i] = buf[15 - i];
123 		buf[15 - i] = x;
124 	}
125 }
126 
127 /* see bearssl_block.h */
128 void
129 br_poly1305_i15_run(const void *key, const void *iv,
130 	void *data, size_t len, const void *aad, size_t aad_len,
131 	void *tag, br_chacha20_run ichacha, int encrypt)
132 {
133 	unsigned char pkey[32], foot[16];
134 	uint16_t t[10], r[10], acc[10];
135 
136 	/*
137 	 * Compute the MAC key. The 'r' value is the first 16 bytes of
138 	 * pkey[].
139 	 */
140 	memset(pkey, 0, sizeof pkey);
141 	ichacha(key, iv, 0, pkey, sizeof pkey);
142 
143 	/*
144 	 * If encrypting, ChaCha20 must run first, followed by Poly1305.
145 	 * When decrypting, the operations are reversed.
146 	 */
147 	if (encrypt) {
148 		ichacha(key, iv, 1, data, len);
149 	}
150 
151 	/*
152 	 * Run Poly1305. We must process the AAD, then ciphertext, then
153 	 * the footer (with the lengths). Note that the AAD and ciphertext
154 	 * are meant to be padded with zeros up to the next multiple of 16,
155 	 * and the length of the footer is 16 bytes as well.
156 	 */
157 
158 	/*
159 	 * Apply the "clamping" operation on the encoded 'r' value.
160 	 */
161 	pkey[ 3] &= 0x0F;
162 	pkey[ 7] &= 0x0F;
163 	pkey[11] &= 0x0F;
164 	pkey[15] &= 0x0F;
165 	pkey[ 4] &= 0xFC;
166 	pkey[ 8] &= 0xFC;
167 	pkey[12] &= 0xFC;
168 
169 	/*
170 	 * Decode the clamped 'r' value. Decoding should use little-endian
171 	 * so we must byteswap the value first.
172 	 */
173 	byteswap16(pkey);
174 	br_i15_decode_mod(t, pkey, 16, P1305);
175 
176 	/*
177 	 * Convert 'r' to Montgomery representation.
178 	 */
179 	br_i15_montymul(r, t, R2, P1305, P0I);
180 
181 	/*
182 	 * Accumulator is 0.
183 	 */
184 	br_i15_zero(acc, 0x8A);
185 
186 	/*
187 	 * Process the additional authenticated data, ciphertext, and
188 	 * footer in due order.
189 	 */
190 	br_enc64le(foot, (uint64_t)aad_len);
191 	br_enc64le(foot + 8, (uint64_t)len);
192 	poly1305_inner(acc, r, aad, aad_len);
193 	poly1305_inner(acc, r, data, len);
194 	poly1305_inner(acc, r, foot, sizeof foot);
195 
196 	/*
197 	 * Decode the value 's'. Again, a byteswap is needed.
198 	 */
199 	byteswap16(pkey + 16);
200 	br_i15_decode_mod(t, pkey + 16, 16, P1305);
201 
202 	/*
203 	 * Add the value 's' to the accumulator. That addition is done
204 	 * modulo 2^128, so we just ignore the carry.
205 	 */
206 	br_i15_add(acc, t, 1);
207 
208 	/*
209 	 * Encode the result (128 low bits) to the tag. Encoding should
210 	 * be little-endian.
211 	 */
212 	br_i15_encode(tag, 16, acc);
213 	byteswap16(tag);
214 
215 	/*
216 	 * If decrypting, then ChaCha20 runs _after_ Poly1305.
217 	 */
218 	if (!encrypt) {
219 		ichacha(key, iv, 1, data, len);
220 	}
221 }
222