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
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
// Copyright (c) 2019, Google Inc.
// Portions Copyright 2020 Brian Smith.
//
// Permission to use, copy, modify, and/or distribute this software for any
// purpose with or without fee is hereby granted, provided that the above
// copyright notice and this permission notice appear in all copies.
//
// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
// WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
// MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
// SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
// WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
// OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
// CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.

// This file is based on BoringSSL's gcm_nohw.c.

// This file contains a constant-time implementation of GHASH based on the notes
// in https://bearssl.org/constanttime.html#ghash-for-gcm and the reduction
// algorithm described in
// https://crypto.stanford.edu/RealWorldCrypto/slides/gueron.pdf.
//
// Unlike the BearSSL notes, we use u128 in the 64-bit implementation.

use super::{super::Block, Xi};
use crate::endian::BigEndian;
use core::convert::TryInto;

#[cfg(target_pointer_width = "64")]
fn gcm_mul64_nohw(a: u64, b: u64) -> (u64, u64) {
    #[inline(always)]
    fn lo(a: u128) -> u64 {
        a as u64
    }

    #[inline(always)]
    fn hi(a: u128) -> u64 {
        lo(a >> 64)
    }

    #[inline(always)]
    fn mul(a: u64, b: u64) -> u128 {
        u128::from(a) * u128::from(b)
    }

    // One term every four bits means the largest term is 64/4 = 16, which barely
    // overflows into the next term. Using one term every five bits would cost 25
    // multiplications instead of 16. It is faster to mask off the bottom four
    // bits of |a|, giving a largest term of 60/4 = 15, and apply the bottom bits
    // separately.
    let a0 = a & 0x1111111111111110;
    let a1 = a & 0x2222222222222220;
    let a2 = a & 0x4444444444444440;
    let a3 = a & 0x8888888888888880;

    let b0 = b & 0x1111111111111111;
    let b1 = b & 0x2222222222222222;
    let b2 = b & 0x4444444444444444;
    let b3 = b & 0x8888888888888888;

    let c0 = mul(a0, b0) ^ mul(a1, b3) ^ mul(a2, b2) ^ mul(a3, b1);
    let c1 = mul(a0, b1) ^ mul(a1, b0) ^ mul(a2, b3) ^ mul(a3, b2);
    let c2 = mul(a0, b2) ^ mul(a1, b1) ^ mul(a2, b0) ^ mul(a3, b3);
    let c3 = mul(a0, b3) ^ mul(a1, b2) ^ mul(a2, b1) ^ mul(a3, b0);

    // Multiply the bottom four bits of |a| with |b|.
    let a0_mask = 0u64.wrapping_sub(a & 1);
    let a1_mask = 0u64.wrapping_sub((a >> 1) & 1);
    let a2_mask = 0u64.wrapping_sub((a >> 2) & 1);
    let a3_mask = 0u64.wrapping_sub((a >> 3) & 1);
    let extra = u128::from(a0_mask & b)
        ^ (u128::from(a1_mask & b) << 1)
        ^ (u128::from(a2_mask & b) << 2)
        ^ (u128::from(a3_mask & b) << 3);

    let lo = (lo(c0) & 0x1111111111111111)
        ^ (lo(c1) & 0x2222222222222222)
        ^ (lo(c2) & 0x4444444444444444)
        ^ (lo(c3) & 0x8888888888888888)
        ^ lo(extra);
    let hi = (hi(c0) & 0x1111111111111111)
        ^ (hi(c1) & 0x2222222222222222)
        ^ (hi(c2) & 0x4444444444444444)
        ^ (hi(c3) & 0x8888888888888888)
        ^ hi(extra);
    (lo, hi)
}

#[cfg(not(target_pointer_width = "64"))]
fn gcm_mul32_nohw(a: u32, b: u32) -> u64 {
    #[inline(always)]
    fn mul(a: u32, b: u32) -> u64 {
        u64::from(a) * u64::from(b)
    }

    // One term every four bits means the largest term is 32/4 = 8, which does not
    // overflow into the next term.
    let a0 = a & 0x11111111;
    let a1 = a & 0x22222222;
    let a2 = a & 0x44444444;
    let a3 = a & 0x88888888;

    let b0 = b & 0x11111111;
    let b1 = b & 0x22222222;
    let b2 = b & 0x44444444;
    let b3 = b & 0x88888888;

    let c0 = mul(a0, b0) ^ mul(a1, b3) ^ mul(a2, b2) ^ mul(a3, b1);
    let c1 = mul(a0, b1) ^ mul(a1, b0) ^ mul(a2, b3) ^ mul(a3, b2);
    let c2 = mul(a0, b2) ^ mul(a1, b1) ^ mul(a2, b0) ^ mul(a3, b3);
    let c3 = mul(a0, b3) ^ mul(a1, b2) ^ mul(a2, b1) ^ mul(a3, b0);

    (c0 & 0x1111111111111111)
        | (c1 & 0x2222222222222222)
        | (c2 & 0x4444444444444444)
        | (c3 & 0x8888888888888888)
}

#[cfg(not(target_pointer_width = "64"))]
fn gcm_mul64_nohw(a: u64, b: u64) -> (u64, u64) {
    #[inline(always)]
    fn lo(a: u64) -> u32 {
        a as u32
    }
    #[inline(always)]
    fn hi(a: u64) -> u32 {
        lo(a >> 32)
    }

    let a0 = lo(a);
    let a1 = hi(a);
    let b0 = lo(b);
    let b1 = hi(b);
    // Karatsuba multiplication.
    let lo = gcm_mul32_nohw(a0, b0);
    let hi = gcm_mul32_nohw(a1, b1);
    let mid = gcm_mul32_nohw(a0 ^ a1, b0 ^ b1) ^ lo ^ hi;
    (lo ^ (mid << 32), hi ^ (mid >> 32))
}

pub(super) fn init(xi: [u64; 2]) -> super::u128 {
    // We implement GHASH in terms of POLYVAL, as described in RFC8452. This
    // avoids a shift by 1 in the multiplication, needed to account for bit
    // reversal losing a bit after multiplication, that is,
    // rev128(X) * rev128(Y) = rev255(X*Y).
    //
    // Per Appendix A, we run mulX_POLYVAL. Note this is the same transformation
    // applied by |gcm_init_clmul|, etc. Note |Xi| has already been byteswapped.
    //
    // See also slide 16 of
    // https://crypto.stanford.edu/RealWorldCrypto/slides/gueron.pdf
    let mut lo = xi[1];
    let mut hi = xi[0];

    let mut carry = hi >> 63;
    carry = 0u64.wrapping_sub(carry);

    hi <<= 1;
    hi |= lo >> 63;
    lo <<= 1;

    // The irreducible polynomial is 1 + x^121 + x^126 + x^127 + x^128, so we
    // conditionally add 0xc200...0001.
    lo ^= carry & 1;
    hi ^= carry & 0xc200000000000000;

    // This implementation does not use the rest of |Htable|.
    super::u128 { lo, hi }
}

fn gcm_polyval_nohw(xi: &mut [u64; 2], h: super::u128) {
    // Karatsuba multiplication. The product of |Xi| and |H| is stored in |r0|
    // through |r3|. Note there is no byte or bit reversal because we are
    // evaluating POLYVAL.
    let (r0, mut r1) = gcm_mul64_nohw(xi[0], h.lo);
    let (mut r2, mut r3) = gcm_mul64_nohw(xi[1], h.hi);
    let (mut mid0, mut mid1) = gcm_mul64_nohw(xi[0] ^ xi[1], h.hi ^ h.lo);
    mid0 ^= r0 ^ r2;
    mid1 ^= r1 ^ r3;
    r2 ^= mid1;
    r1 ^= mid0;

    // Now we multiply our 256-bit result by x^-128 and reduce. |r2| and
    // |r3| shifts into position and we must multiply |r0| and |r1| by x^-128. We
    // have:
    //
    //       1 = x^121 + x^126 + x^127 + x^128
    //  x^-128 = x^-7 + x^-2 + x^-1 + 1
    //
    // This is the GHASH reduction step, but with bits flowing in reverse.

    // The x^-7, x^-2, and x^-1 terms shift bits past x^0, which would require
    // another reduction steps. Instead, we gather the excess bits, incorporate
    // them into |r0| and |r1| and reduce once. See slides 17-19
    // of https://crypto.stanford.edu/RealWorldCrypto/slides/gueron.pdf.
    r1 ^= (r0 << 63) ^ (r0 << 62) ^ (r0 << 57);

    // 1
    r2 ^= r0;
    r3 ^= r1;

    // x^-1
    r2 ^= r0 >> 1;
    r2 ^= r1 << 63;
    r3 ^= r1 >> 1;

    // x^-2
    r2 ^= r0 >> 2;
    r2 ^= r1 << 62;
    r3 ^= r1 >> 2;

    // x^-7
    r2 ^= r0 >> 7;
    r2 ^= r1 << 57;
    r3 ^= r1 >> 7;

    *xi = [r2, r3];
}

pub(super) fn gmult(xi: &mut Xi, h: super::u128) {
    with_swapped_xi(xi, |swapped| {
        gcm_polyval_nohw(swapped, h);
    })
}

pub(super) fn ghash(xi: &mut Xi, h: super::u128, input: &[u8]) {
    with_swapped_xi(xi, |swapped| {
        input.chunks_exact(16).for_each(|inp| {
            swapped[0] ^= u64::from_be_bytes(inp[8..].try_into().unwrap());
            swapped[1] ^= u64::from_be_bytes(inp[..8].try_into().unwrap());
            gcm_polyval_nohw(swapped, h);
        });
    });
}

#[inline]
fn with_swapped_xi(Xi(xi): &mut Xi, f: impl FnOnce(&mut [u64; 2])) {
    let unswapped = xi.u64s_be_to_native();
    let mut swapped: [u64; 2] = [unswapped[1], unswapped[0]];
    f(&mut swapped);
    *xi = Block::from_u64_be(BigEndian::from(swapped[1]), BigEndian::from(swapped[0]))
}