ring/aead/gcm/
fallback.rs

1// Copyright (c) 2019, Google Inc.
2// Portions Copyright 2020-2024 Brian Smith.
3//
4// Permission to use, copy, modify, and/or distribute this software for any
5// purpose with or without fee is hereby granted, provided that the above
6// copyright notice and this permission notice appear in all copies.
7//
8// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
9// WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
10// MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
11// SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
12// WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
13// OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
14// CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
15
16// This file is based on BoringSSL's gcm_nohw.c.
17
18// This file contains a implementation of GHASH based on the notes
19// in https://bearssl.org/constanttime.html#ghash-for-gcm and the reduction
20// algorithm described in
21// https://crypto.stanford.edu/RealWorldCrypto/slides/gueron.pdf.
22//
23// Unlike the BearSSL notes, we use u128 in the 64-bit implementation.
24
25use super::{ffi::U128, KeyValue, UpdateBlock, UpdateBlocks, Xi, BLOCK_LEN};
26use crate::polyfill::{slice::AsChunks, ArraySplitMap as _};
27
28#[derive(Clone)]
29pub struct Key {
30    h: U128,
31}
32
33impl Key {
34    pub(in super::super) fn new(value: KeyValue) -> Self {
35        Self { h: init(value) }
36    }
37}
38
39impl UpdateBlock for Key {
40    fn update_block(&self, xi: &mut Xi, a: [u8; BLOCK_LEN]) {
41        xi.bitxor_assign(a);
42        gmult(xi, self.h);
43    }
44}
45
46impl UpdateBlocks for Key {
47    fn update_blocks(&self, xi: &mut Xi, input: AsChunks<u8, BLOCK_LEN>) {
48        ghash(xi, self.h, input);
49    }
50}
51
52#[cfg(target_pointer_width = "64")]
53fn gcm_mul64_nohw(a: u64, b: u64) -> (u64, u64) {
54    #[allow(clippy::cast_possible_truncation)]
55    #[inline(always)]
56    fn lo(a: u128) -> u64 {
57        a as u64
58    }
59
60    #[inline(always)]
61    fn hi(a: u128) -> u64 {
62        lo(a >> 64)
63    }
64
65    #[inline(always)]
66    fn mul(a: u64, b: u64) -> u128 {
67        u128::from(a) * u128::from(b)
68    }
69
70    // One term every four bits means the largest term is 64/4 = 16, which barely
71    // overflows into the next term. Using one term every five bits would cost 25
72    // multiplications instead of 16. It is faster to mask off the bottom four
73    // bits of |a|, giving a largest term of 60/4 = 15, and apply the bottom bits
74    // separately.
75    let a0 = a & 0x1111111111111110;
76    let a1 = a & 0x2222222222222220;
77    let a2 = a & 0x4444444444444440;
78    let a3 = a & 0x8888888888888880;
79
80    let b0 = b & 0x1111111111111111;
81    let b1 = b & 0x2222222222222222;
82    let b2 = b & 0x4444444444444444;
83    let b3 = b & 0x8888888888888888;
84
85    let c0 = mul(a0, b0) ^ mul(a1, b3) ^ mul(a2, b2) ^ mul(a3, b1);
86    let c1 = mul(a0, b1) ^ mul(a1, b0) ^ mul(a2, b3) ^ mul(a3, b2);
87    let c2 = mul(a0, b2) ^ mul(a1, b1) ^ mul(a2, b0) ^ mul(a3, b3);
88    let c3 = mul(a0, b3) ^ mul(a1, b2) ^ mul(a2, b1) ^ mul(a3, b0);
89
90    // Multiply the bottom four bits of |a| with |b|.
91    let a0_mask = 0u64.wrapping_sub(a & 1);
92    let a1_mask = 0u64.wrapping_sub((a >> 1) & 1);
93    let a2_mask = 0u64.wrapping_sub((a >> 2) & 1);
94    let a3_mask = 0u64.wrapping_sub((a >> 3) & 1);
95    let extra = u128::from(a0_mask & b)
96        ^ (u128::from(a1_mask & b) << 1)
97        ^ (u128::from(a2_mask & b) << 2)
98        ^ (u128::from(a3_mask & b) << 3);
99
100    let lo = (lo(c0) & 0x1111111111111111)
101        ^ (lo(c1) & 0x2222222222222222)
102        ^ (lo(c2) & 0x4444444444444444)
103        ^ (lo(c3) & 0x8888888888888888)
104        ^ lo(extra);
105    let hi = (hi(c0) & 0x1111111111111111)
106        ^ (hi(c1) & 0x2222222222222222)
107        ^ (hi(c2) & 0x4444444444444444)
108        ^ (hi(c3) & 0x8888888888888888)
109        ^ hi(extra);
110    (lo, hi)
111}
112
113#[cfg(not(target_pointer_width = "64"))]
114fn gcm_mul32_nohw(a: u32, b: u32) -> u64 {
115    #[inline(always)]
116    fn mul(a: u32, b: u32) -> u64 {
117        u64::from(a) * u64::from(b)
118    }
119
120    // One term every four bits means the largest term is 32/4 = 8, which does not
121    // overflow into the next term.
122    let a0 = a & 0x11111111;
123    let a1 = a & 0x22222222;
124    let a2 = a & 0x44444444;
125    let a3 = a & 0x88888888;
126
127    let b0 = b & 0x11111111;
128    let b1 = b & 0x22222222;
129    let b2 = b & 0x44444444;
130    let b3 = b & 0x88888888;
131
132    let c0 = mul(a0, b0) ^ mul(a1, b3) ^ mul(a2, b2) ^ mul(a3, b1);
133    let c1 = mul(a0, b1) ^ mul(a1, b0) ^ mul(a2, b3) ^ mul(a3, b2);
134    let c2 = mul(a0, b2) ^ mul(a1, b1) ^ mul(a2, b0) ^ mul(a3, b3);
135    let c3 = mul(a0, b3) ^ mul(a1, b2) ^ mul(a2, b1) ^ mul(a3, b0);
136
137    (c0 & 0x1111111111111111)
138        | (c1 & 0x2222222222222222)
139        | (c2 & 0x4444444444444444)
140        | (c3 & 0x8888888888888888)
141}
142
143#[cfg(not(target_pointer_width = "64"))]
144fn gcm_mul64_nohw(a: u64, b: u64) -> (u64, u64) {
145    #[inline(always)]
146    fn lo(a: u64) -> u32 {
147        a as u32
148    }
149    #[inline(always)]
150    fn hi(a: u64) -> u32 {
151        lo(a >> 32)
152    }
153
154    let a0 = lo(a);
155    let a1 = hi(a);
156    let b0 = lo(b);
157    let b1 = hi(b);
158    // Karatsuba multiplication.
159    let lo = gcm_mul32_nohw(a0, b0);
160    let hi = gcm_mul32_nohw(a1, b1);
161    let mid = gcm_mul32_nohw(a0 ^ a1, b0 ^ b1) ^ lo ^ hi;
162    (lo ^ (mid << 32), hi ^ (mid >> 32))
163}
164
165fn init(value: KeyValue) -> U128 {
166    let xi = value.into_inner();
167
168    // We implement GHASH in terms of POLYVAL, as described in RFC 8452. This
169    // avoids a shift by 1 in the multiplication, needed to account for bit
170    // reversal losing a bit after multiplication, that is,
171    // rev128(X) * rev128(Y) = rev255(X*Y).
172    //
173    // Per Appendix A, we run mulX_POLYVAL. Note this is the same transformation
174    // applied by |gcm_init_clmul|, etc. Note |Xi| has already been byteswapped.
175    //
176    // See also slide 16 of
177    // https://crypto.stanford.edu/RealWorldCrypto/slides/gueron.pdf
178    let mut lo = xi[1];
179    let mut hi = xi[0];
180
181    let mut carry = hi >> 63;
182    carry = 0u64.wrapping_sub(carry);
183
184    hi <<= 1;
185    hi |= lo >> 63;
186    lo <<= 1;
187
188    // The irreducible polynomial is 1 + x^121 + x^126 + x^127 + x^128, so we
189    // conditionally add 0xc200...0001.
190    lo ^= carry & 1;
191    hi ^= carry & 0xc200000000000000;
192
193    // This implementation does not use the rest of |Htable|.
194    U128 { hi, lo }
195}
196
197fn gcm_polyval_nohw(xi: &mut [u64; 2], h: U128) {
198    // Karatsuba multiplication. The product of |Xi| and |H| is stored in |r0|
199    // through |r3|. Note there is no byte or bit reversal because we are
200    // evaluating POLYVAL.
201    let (r0, mut r1) = gcm_mul64_nohw(xi[0], h.lo);
202    let (mut r2, mut r3) = gcm_mul64_nohw(xi[1], h.hi);
203    let (mut mid0, mut mid1) = gcm_mul64_nohw(xi[0] ^ xi[1], h.hi ^ h.lo);
204    mid0 ^= r0 ^ r2;
205    mid1 ^= r1 ^ r3;
206    r2 ^= mid1;
207    r1 ^= mid0;
208
209    // Now we multiply our 256-bit result by x^-128 and reduce. |r2| and
210    // |r3| shifts into position and we must multiply |r0| and |r1| by x^-128. We
211    // have:
212    //
213    //       1 = x^121 + x^126 + x^127 + x^128
214    //  x^-128 = x^-7 + x^-2 + x^-1 + 1
215    //
216    // This is the GHASH reduction step, but with bits flowing in reverse.
217
218    // The x^-7, x^-2, and x^-1 terms shift bits past x^0, which would require
219    // another reduction steps. Instead, we gather the excess bits, incorporate
220    // them into |r0| and |r1| and reduce once. See slides 17-19
221    // of https://crypto.stanford.edu/RealWorldCrypto/slides/gueron.pdf.
222    r1 ^= (r0 << 63) ^ (r0 << 62) ^ (r0 << 57);
223
224    // 1
225    r2 ^= r0;
226    r3 ^= r1;
227
228    // x^-1
229    r2 ^= r0 >> 1;
230    r2 ^= r1 << 63;
231    r3 ^= r1 >> 1;
232
233    // x^-2
234    r2 ^= r0 >> 2;
235    r2 ^= r1 << 62;
236    r3 ^= r1 >> 2;
237
238    // x^-7
239    r2 ^= r0 >> 7;
240    r2 ^= r1 << 57;
241    r3 ^= r1 >> 7;
242
243    *xi = [r2, r3];
244}
245
246fn gmult(xi: &mut Xi, h: U128) {
247    with_swapped_xi(xi, |swapped| {
248        gcm_polyval_nohw(swapped, h);
249    })
250}
251
252fn ghash(xi: &mut Xi, h: U128, input: AsChunks<u8, BLOCK_LEN>) {
253    with_swapped_xi(xi, |swapped| {
254        input.into_iter().for_each(|&input| {
255            let input = input.array_split_map(u64::from_be_bytes);
256            swapped[0] ^= input[1];
257            swapped[1] ^= input[0];
258            gcm_polyval_nohw(swapped, h);
259        });
260    });
261}
262
263#[inline]
264fn with_swapped_xi(Xi(xi): &mut Xi, f: impl FnOnce(&mut [u64; 2])) {
265    let unswapped: [u64; 2] = xi.array_split_map(u64::from_be_bytes);
266    let mut swapped: [u64; 2] = [unswapped[1], unswapped[0]];
267    f(&mut swapped);
268    let (xi_0, xi_1) = xi.split_at_mut(BLOCK_LEN / 2);
269    xi_0.copy_from_slice(&u64::to_be_bytes(swapped[1]));
270    xi_1.copy_from_slice(&u64::to_be_bytes(swapped[0]));
271}
ring/aead/gcm/fallback.rs

ring/aead/gcm/
fallback.rs
