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cli/vendor/ring/src/arithmetic/bigint.rs

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chore: checkpoint before Python removal
2026-03-26 22:33:59 +00:00
// Copyright 2015-2023 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.
//! Multi-precision integers.
//!
//! # Modular Arithmetic.
//!
//! Modular arithmetic is done in finite commutative rings ℤ/mℤ for some
//! modulus *m*. We work in finite commutative rings instead of finite fields
//! because the RSA public modulus *n* is not prime, which means ℤ/nℤ contains
//! nonzero elements that have no multiplicative inverse, so ℤ/nℤ is not a
//! finite field.
//!
//! In some calculations we need to deal with multiple rings at once. For
//! example, RSA private key operations operate in the rings ℤ/nℤ, ℤ/pℤ, and
//! ℤ/qℤ. Types and functions dealing with such rings are all parameterized
//! over a type `M` to ensure that we don't wrongly mix up the math, e.g. by
//! multiplying an element of ℤ/pℤ by an element of ℤ/qℤ modulo q. This follows
//! the "unit" pattern described in [Static checking of units in Servo].
//!
//! `Elem` also uses the static unit checking pattern to statically track the
//! Montgomery factors that need to be canceled out in each value using it's
//! `E` parameter.
//!
//! [Static checking of units in Servo]:
//! https://blog.mozilla.org/research/2014/06/23/static-checking-of-units-in-servo/
use self::boxed_limbs::BoxedLimbs;
pub(crate) use self::{
modulus::{Modulus, OwnedModulus},
modulusvalue::OwnedModulusValue,
private_exponent::PrivateExponent,
};
use super::{inout::AliasingSlices3, limbs512, montgomery::*, LimbSliceError, MAX_LIMBS};
use crate::{
bits::BitLength,
c,
error::{self, LenMismatchError},
limb::{self, Limb, LIMB_BITS},
polyfill::slice::{self, AsChunks},
};
use core::{
marker::PhantomData,
num::{NonZeroU64, NonZeroUsize},
};
mod boxed_limbs;
mod modulus;
mod modulusvalue;
mod private_exponent;
pub trait PublicModulus {}
// When we need to create a new `Elem`, first we create a `Storage` and then
// move its `limbs` into the new element. When we want to recylce an `Elem`'s
// memory allocation, we convert it back into a `Storage`.
pub struct Storage<M> {
limbs: BoxedLimbs<M>,
}
impl<M, E> From<Elem<M, E>> for Storage<M> {
fn from(elem: Elem<M, E>) -> Self {
Self { limbs: elem.limbs }
}
}
/// Elements of ℤ/mℤ for some modulus *m*.
//
// Defaulting `E` to `Unencoded` is a convenience for callers from outside this
// submodule. However, for maximum clarity, we always explicitly use
// `Unencoded` within the `bigint` submodule.
pub struct Elem<M, E = Unencoded> {
limbs: BoxedLimbs<M>,
/// The number of Montgomery factors that need to be canceled out from
/// `value` to get the actual value.
encoding: PhantomData<E>,
}
impl<M, E> Elem<M, E> {
pub fn clone_into(&self, mut out: Storage<M>) -> Self {
out.limbs.copy_from_slice(&self.limbs);
Self {
limbs: out.limbs,
encoding: self.encoding,
}
}
}
impl<M, E> Elem<M, E> {
#[inline]
pub fn is_zero(&self) -> bool {
limb::limbs_are_zero(&self.limbs).leak()
}
}
/// Does a Montgomery reduction on `limbs` assuming they are Montgomery-encoded ('R') and assuming
/// they are the same size as `m`, but perhaps not reduced mod `m`. The result will be
/// fully reduced mod `m`.
///
/// WARNING: Takes a `Storage` as an in/out value.
fn from_montgomery_amm<M>(mut in_out: Storage<M>, m: &Modulus<M>) -> Elem<M, Unencoded> {
let mut one = [0; MAX_LIMBS];
one[0] = 1;
let one = &one[..m.limbs().len()];
limbs_mul_mont(
(&mut in_out.limbs[..], one),
m.limbs(),
m.n0(),
m.cpu_features(),
)
.unwrap_or_else(unwrap_impossible_limb_slice_error);
Elem {
limbs: in_out.limbs,
encoding: PhantomData,
}
}
#[cfg(any(test, not(target_arch = "x86_64")))]
impl<M> Elem<M, R> {
#[inline]
pub fn into_unencoded(self, m: &Modulus<M>) -> Elem<M, Unencoded> {
from_montgomery_amm(Storage::from(self), m)
}
}
impl<M> Elem<M, Unencoded> {
pub fn from_be_bytes_padded(
input: untrusted::Input,
m: &Modulus<M>,
) -> Result<Self, error::Unspecified> {
Ok(Self {
limbs: BoxedLimbs::from_be_bytes_padded_less_than(input, m)?,
encoding: PhantomData,
})
}
#[inline]
pub fn fill_be_bytes(&self, out: &mut [u8]) {
// See Falko Strenzke, "Manger's Attack revisited", ICICS 2010.
limb::big_endian_from_limbs(&self.limbs, out)
}
}
pub fn elem_mul_into<M, AF, BF>(
mut out: Storage<M>,
a: &Elem<M, AF>,
b: &Elem<M, BF>,
m: &Modulus<M>,
) -> Elem<M, <(AF, BF) as ProductEncoding>::Output>
where
(AF, BF): ProductEncoding,
{
limbs_mul_mont(
(out.limbs.as_mut(), b.limbs.as_ref(), a.limbs.as_ref()),
m.limbs(),
m.n0(),
m.cpu_features(),
)
.unwrap_or_else(unwrap_impossible_limb_slice_error);
Elem {
limbs: out.limbs,
encoding: PhantomData,
}
}
pub fn elem_mul<M, AF, BF>(
a: &Elem<M, AF>,
mut b: Elem<M, BF>,
m: &Modulus<M>,
) -> Elem<M, <(AF, BF) as ProductEncoding>::Output>
where
(AF, BF): ProductEncoding,
{
limbs_mul_mont(
(&mut b.limbs[..], &a.limbs[..]),
m.limbs(),
m.n0(),
m.cpu_features(),
)
.unwrap_or_else(unwrap_impossible_limb_slice_error);
Elem {
limbs: b.limbs,
encoding: PhantomData,
}
}
// r *= 2.
fn elem_double<M, AF>(r: &mut Elem<M, AF>, m: &Modulus<M>) {
limb::limbs_double_mod(&mut r.limbs, m.limbs())
.unwrap_or_else(unwrap_impossible_len_mismatch_error)
}
// TODO: This is currently unused, but we intend to eventually use this to
// reduce elements (x mod q) mod p in the RSA CRT. If/when we do so, we
// should update the testing so it is reflective of that usage, instead of
// the old usage.
pub fn elem_reduced_once<A, M>(
mut r: Storage<M>,
a: &Elem<A, Unencoded>,
m: &Modulus<M>,
other_modulus_len_bits: BitLength,
) -> Elem<M, Unencoded> {
assert_eq!(m.len_bits(), other_modulus_len_bits);
r.limbs.copy_from_slice(&a.limbs);
limb::limbs_reduce_once(&mut r.limbs, m.limbs())
.unwrap_or_else(unwrap_impossible_len_mismatch_error);
Elem {
limbs: r.limbs,
encoding: PhantomData,
}
}
#[inline]
pub fn elem_reduced<Larger, Smaller>(
mut r: Storage<Smaller>,
a: &Elem<Larger, Unencoded>,
m: &Modulus<Smaller>,
other_prime_len_bits: BitLength,
) -> Elem<Smaller, RInverse> {
// This is stricter than required mathematically but this is what we
// guarantee and this is easier to check. The real requirement is that
// that `a < m*R` where `R` is the Montgomery `R` for `m`.
assert_eq!(other_prime_len_bits, m.len_bits());
// `limbs_from_mont_in_place` requires this.
assert_eq!(a.limbs.len(), m.limbs().len() * 2);
let mut tmp = [0; MAX_LIMBS];
let tmp = &mut tmp[..a.limbs.len()];
tmp.copy_from_slice(&a.limbs);
limbs_from_mont_in_place(&mut r.limbs, tmp, m.limbs(), m.n0());
Elem {
limbs: r.limbs,
encoding: PhantomData,
}
}
#[inline]
fn elem_squared<M, E>(
mut a: Elem<M, E>,
m: &Modulus<M>,
) -> Elem<M, <(E, E) as ProductEncoding>::Output>
where
(E, E): ProductEncoding,
{
limbs_square_mont(&mut a.limbs, m.limbs(), m.n0(), m.cpu_features())
.unwrap_or_else(unwrap_impossible_limb_slice_error);
Elem {
limbs: a.limbs,
encoding: PhantomData,
}
}
pub fn elem_widen<Larger, Smaller>(
mut r: Storage<Larger>,
a: Elem<Smaller, Unencoded>,
m: &Modulus<Larger>,
smaller_modulus_bits: BitLength,
) -> Result<Elem<Larger, Unencoded>, error::Unspecified> {
if smaller_modulus_bits >= m.len_bits() {
return Err(error::Unspecified);
}
let (to_copy, to_zero) = r.limbs.split_at_mut(a.limbs.len());
to_copy.copy_from_slice(&a.limbs);
to_zero.fill(0);
Ok(Elem {
limbs: r.limbs,
encoding: PhantomData,
})
}
// TODO: Document why this works for all Montgomery factors.
pub fn elem_add<M, E>(mut a: Elem<M, E>, b: Elem<M, E>, m: &Modulus<M>) -> Elem<M, E> {
limb::limbs_add_assign_mod(&mut a.limbs, &b.limbs, m.limbs())
.unwrap_or_else(unwrap_impossible_len_mismatch_error);
a
}
// TODO: Document why this works for all Montgomery factors.
pub fn elem_sub<M, E>(mut a: Elem<M, E>, b: &Elem<M, E>, m: &Modulus<M>) -> Elem<M, E> {
prefixed_extern! {
// `r` and `a` may alias.
fn LIMBS_sub_mod(
r: *mut Limb,
a: *const Limb,
b: *const Limb,
m: *const Limb,
num_limbs: c::NonZero_size_t,
);
}
let num_limbs = NonZeroUsize::new(m.limbs().len()).unwrap();
(a.limbs.as_mut(), b.limbs.as_ref())
.with_non_dangling_non_null_pointers_rab(num_limbs, |r, a, b| {
let m = m.limbs().as_ptr(); // Also non-dangling because num_limbs is non-zero.
unsafe { LIMBS_sub_mod(r, a, b, m, num_limbs) }
})
.unwrap_or_else(unwrap_impossible_len_mismatch_error);
a
}
// The value 1, Montgomery-encoded some number of times.
pub struct One<M, E>(Elem<M, E>);
impl<M> One<M, RR> {
// Returns RR = = R**2 (mod n) where R = 2**r is the smallest power of
// 2**LIMB_BITS such that R > m.
//
// Even though the assembly on some 32-bit platforms works with 64-bit
// values, using `LIMB_BITS` here, rather than `N0::LIMBS_USED * LIMB_BITS`,
// is correct because R**2 will still be a multiple of the latter as
// `N0::LIMBS_USED` is either one or two.
pub(crate) fn newRR(mut out: Storage<M>, m: &Modulus<M>) -> Self {
// The number of limbs in the numbers involved.
let w = m.limbs().len();
// The length of the numbers involved, in bits. R = 2**r.
let r = w * LIMB_BITS;
m.oneR(&mut out.limbs);
let mut acc: Elem<M, R> = Elem {
limbs: out.limbs,
encoding: PhantomData,
};
// 2**t * R can be calculated by t doublings starting with R.
//
// Choose a t that divides r and where t doublings are cheaper than 1 squaring.
//
// We could choose other values of t than w. But if t < d then the exponentiation that
// follows would require multiplications. Normally d is 1 (i.e. the modulus length is a
// power of two: RSA 1024, 2048, 4097, 8192) or 3 (RSA 1536, 3072).
//
// XXX(perf): Currently t = w / 2 is slightly faster. TODO(perf): Optimize `elem_double`
// and re-run benchmarks to rebalance this.
let t = w;
let z = w.trailing_zeros();
let d = w >> z;
debug_assert_eq!(w, d * (1 << z));
debug_assert!(d <= t);
debug_assert!(t < r);
for _ in 0..t {
elem_double(&mut acc, m);
}
// Because t | r:
//
// MontExp(2**t * R, r / t)
// = (2**t)**(r / t) * R (mod m) by definition of MontExp.
// = (2**t)**(1/t * r) * R (mod m)
// = (2**(t * 1/t))**r * R (mod m)
// = (2**1)**r * R (mod m)
// = 2**r * R (mod m)
// = R * R (mod m)
// = RR
//
// Like BoringSSL, use t = w (`m.limbs.len()`) which ensures that the exponent is a power
// of two. Consequently, there will be no multiplications in the Montgomery exponentiation;
// there will only be lg(r / t) squarings.
//
// lg(r / t)
// = lg((w * 2**b) / t)
// = lg((t * 2**b) / t)
// = lg(2**b)
// = b
// TODO(MSRV:1.67): const B: u32 = LIMB_BITS.ilog2();
const B: u32 = if cfg!(target_pointer_width = "64") {
6
} else if cfg!(target_pointer_width = "32") {
5
} else {
panic!("unsupported target_pointer_width")
};
#[allow(clippy::assertions_on_constants)]
const _LIMB_BITS_IS_2_POW_B: () = assert!(LIMB_BITS == 1 << B);
debug_assert_eq!(r, t * (1 << B));
for _ in 0..B {
acc = elem_squared(acc, m);
}
Self(Elem {
limbs: acc.limbs,
encoding: PhantomData, // PhantomData<RR>
})
}
}
impl<M> One<M, RRR> {
pub(crate) fn newRRR(One(oneRR): One<M, RR>, m: &Modulus<M>) -> Self {
Self(elem_squared(oneRR, m))
}
}
impl<M, E> AsRef<Elem<M, E>> for One<M, E> {
fn as_ref(&self) -> &Elem<M, E> {
&self.0
}
}
impl<M: PublicModulus, E> One<M, E> {
pub fn clone_into(&self, out: Storage<M>) -> Self {
Self(self.0.clone_into(out))
}
}
/// Calculates base**exponent (mod m).
///
/// The run time is a function of the number of limbs in `m` and the bit
/// length and Hamming Weight of `exponent`. The bounds on `m` are pretty
/// obvious but the bounds on `exponent` are less obvious. Callers should
/// document the bounds they place on the maximum value and maximum Hamming
/// weight of `exponent`.
// TODO: The test coverage needs to be expanded, e.g. test with the largest
// accepted exponent and with the most common values of 65537 and 3.
pub(crate) fn elem_exp_vartime<M>(
out: Storage<M>,
base: Elem<M, R>,
exponent: NonZeroU64,
m: &Modulus<M>,
) -> Elem<M, R> {
// Use what [Knuth] calls the "S-and-X binary method", i.e. variable-time
// square-and-multiply that scans the exponent from the most significant
// bit to the least significant bit (left-to-right). Left-to-right requires
// less storage compared to right-to-left scanning, at the cost of needing
// to compute `exponent.leading_zeros()`, which we assume to be cheap.
//
// As explained in [Knuth], exponentiation by squaring is the most
// efficient algorithm when the Hamming weight is 2 or less. It isn't the
// most efficient for all other, uncommon, exponent values but any
// suboptimality is bounded at least by the small bit length of `exponent`
// as enforced by its type.
//
// This implementation is slightly simplified by taking advantage of the
// fact that we require the exponent to be a positive integer.
//
// [Knuth]: The Art of Computer Programming, Volume 2: Seminumerical
// Algorithms (3rd Edition), Section 4.6.3.
let exponent = exponent.get();
let mut acc = base.clone_into(out);
let mut bit = 1 << (64 - 1 - exponent.leading_zeros());
debug_assert!((exponent & bit) != 0);
while bit > 1 {
bit >>= 1;
acc = elem_squared(acc, m);
if (exponent & bit) != 0 {
acc = elem_mul(&base, acc, m);
}
}
acc
}
pub fn elem_exp_consttime<N, P>(
out: Storage<P>,
base: &Elem<N>,
oneRRR: &One<P, RRR>,
exponent: &PrivateExponent,
p: &Modulus<P>,
other_prime_len_bits: BitLength,
) -> Result<Elem<P, Unencoded>, LimbSliceError> {
// `elem_exp_consttime_inner` is parameterized on `STORAGE_LIMBS` only so
// we can run tests with larger-than-supported-in-operation test vectors.
elem_exp_consttime_inner::<N, P, { ELEM_EXP_CONSTTIME_MAX_MODULUS_LIMBS * STORAGE_ENTRIES }>(
out,
base,
oneRRR,
exponent,
p,
other_prime_len_bits,
)
}
// The maximum modulus size supported for `elem_exp_consttime` in normal
// operation.
const ELEM_EXP_CONSTTIME_MAX_MODULUS_LIMBS: usize = 2048 / LIMB_BITS;
const _LIMBS_PER_CHUNK_DIVIDES_ELEM_EXP_CONSTTIME_MAX_MODULUS_LIMBS: () =
assert!(ELEM_EXP_CONSTTIME_MAX_MODULUS_LIMBS % limbs512::LIMBS_PER_CHUNK == 0);
const WINDOW_BITS: u32 = 5;
const TABLE_ENTRIES: usize = 1 << WINDOW_BITS;
const STORAGE_ENTRIES: usize = TABLE_ENTRIES + if cfg!(target_arch = "x86_64") { 3 } else { 0 };
#[cfg(not(target_arch = "x86_64"))]
fn elem_exp_consttime_inner<N, M, const STORAGE_LIMBS: usize>(
out: Storage<M>,
base_mod_n: &Elem<N>,
oneRRR: &One<M, RRR>,
exponent: &PrivateExponent,
m: &Modulus<M>,
other_prime_len_bits: BitLength,
) -> Result<Elem<M, Unencoded>, LimbSliceError> {
use crate::{bssl, limb::Window};
let base_rinverse: Elem<M, RInverse> = elem_reduced(out, base_mod_n, m, other_prime_len_bits);
let num_limbs = m.limbs().len();
let m_chunked: AsChunks<Limb, { limbs512::LIMBS_PER_CHUNK }> = match slice::as_chunks(m.limbs())
{
(m, []) => m,
_ => {
return Err(LimbSliceError::len_mismatch(LenMismatchError::new(
num_limbs,
)))
}
};
let cpe = m_chunked.len(); // 512-bit chunks per entry.
// This code doesn't have the strict alignment requirements that the x86_64
// version does, but uses the same aligned storage for convenience.
assert!(STORAGE_LIMBS % (STORAGE_ENTRIES * limbs512::LIMBS_PER_CHUNK) == 0); // TODO: `const`
let mut table = limbs512::AlignedStorage::<STORAGE_LIMBS>::zeroed();
let mut table = table
.aligned_chunks_mut(TABLE_ENTRIES, cpe)
.map_err(LimbSliceError::len_mismatch)?;
// TODO: Rewrite the below in terms of `AsChunks`.
let table = table.as_flattened_mut();
fn gather<M>(table: &[Limb], acc: &mut Elem<M, R>, i: Window) {
prefixed_extern! {
fn LIMBS_select_512_32(
r: *mut Limb,
table: *const Limb,
num_limbs: c::size_t,
i: Window,
) -> bssl::Result;
}
Result::from(unsafe {
LIMBS_select_512_32(acc.limbs.as_mut_ptr(), table.as_ptr(), acc.limbs.len(), i)
})
.unwrap();
}
fn power<M>(
table: &[Limb],
mut acc: Elem<M, R>,
m: &Modulus<M>,
i: Window,
mut tmp: Elem<M, R>,
) -> (Elem<M, R>, Elem<M, R>) {
for _ in 0..WINDOW_BITS {
acc = elem_squared(acc, m);
}
gather(table, &mut tmp, i);
let acc = elem_mul(&tmp, acc, m);
(acc, tmp)
}
fn entry(table: &[Limb], i: usize, num_limbs: usize) -> &[Limb] {
&table[(i * num_limbs)..][..num_limbs]
}
fn entry_mut(table: &mut [Limb], i: usize, num_limbs: usize) -> &mut [Limb] {
&mut table[(i * num_limbs)..][..num_limbs]
}
// table[0] = base**0 (i.e. 1).
m.oneR(entry_mut(table, 0, num_limbs));
// table[1] = base*R == (base/R * RRR)/R
limbs_mul_mont(
(
entry_mut(table, 1, num_limbs),
base_rinverse.limbs.as_ref(),
oneRRR.as_ref().limbs.as_ref(),
),
m.limbs(),
m.n0(),
m.cpu_features(),
)?;
for i in 2..TABLE_ENTRIES {
let (src1, src2) = if i % 2 == 0 {
(i / 2, i / 2)
} else {
(i - 1, 1)
};
let (previous, rest) = table.split_at_mut(num_limbs * i);
let src1 = entry(previous, src1, num_limbs);
let src2 = entry(previous, src2, num_limbs);
let dst = entry_mut(rest, 0, num_limbs);
limbs_mul_mont((dst, src1, src2), m.limbs(), m.n0(), m.cpu_features())?;
}
let mut acc = Elem {
limbs: base_rinverse.limbs,
encoding: PhantomData,
};
let tmp = m.alloc_zero();
let tmp = Elem {
limbs: tmp.limbs,
encoding: PhantomData,
};
let (acc, _) = limb::fold_5_bit_windows(
exponent.limbs(),
|initial_window| {
gather(&table, &mut acc, initial_window);
(acc, tmp)
},
|(acc, tmp), window| power(&table, acc, m, window, tmp),
);
Ok(acc.into_unencoded(m))
}
#[cfg(target_arch = "x86_64")]
fn elem_exp_consttime_inner<N, M, const STORAGE_LIMBS: usize>(
out: Storage<M>,
base_mod_n: &Elem<N>,
oneRRR: &One<M, RRR>,
exponent: &PrivateExponent,
m: &Modulus<M>,
other_prime_len_bits: BitLength,
) -> Result<Elem<M, Unencoded>, LimbSliceError> {
use super::limbs::x86_64::mont::{
gather5, mul_mont5, mul_mont_gather5_amm, power5_amm, scatter5, sqr_mont5,
};
use crate::{
cpu::{
intel::{Adx, Bmi2},
GetFeature as _,
},
limb::{LeakyWindow, Window},
polyfill::slice::AsChunksMut,
};
let n0 = m.n0();
let cpu2 = m.cpu_features().get_feature();
let cpu3 = m.cpu_features().get_feature();
if base_mod_n.limbs.len() != m.limbs().len() * 2 {
return Err(LimbSliceError::len_mismatch(LenMismatchError::new(
base_mod_n.limbs.len(),
)));
}
let m_original: AsChunks<Limb, 8> = match slice::as_chunks(m.limbs()) {
(m, []) => m,
_ => return Err(LimbSliceError::len_mismatch(LenMismatchError::new(8))),
};
let cpe = m_original.len(); // 512-bit chunks per entry
let oneRRR = &oneRRR.as_ref().limbs;
let oneRRR = match slice::as_chunks(oneRRR) {
(c, []) => c,
_ => {
return Err(LimbSliceError::len_mismatch(LenMismatchError::new(
oneRRR.len(),
)))
}
};
// The x86_64 assembly was written under the assumption that the input data
// is aligned to `MOD_EXP_CTIME_ALIGN` bytes, which was/is 64 in OpenSSL.
// Subsequently, it was changed such that, according to BoringSSL, they
// only require 16 byte alignment. We enforce the old, stronger, alignment
// unless/until we can see a benefit to reducing it.
//
// Similarly, OpenSSL uses the x86_64 assembly functions by giving it only
// inputs `tmp`, `am`, and `np` that immediately follow the table.
// According to BoringSSL, in older versions of the OpenSSL code, this
// extra space was required for memory safety because the assembly code
// would over-read the table; according to BoringSSL, this is no longer the
// case. Regardless, the upstream code also contained comments implying
// that this was also important for performance. For now, we do as OpenSSL
// did/does.
const MOD_EXP_CTIME_ALIGN: usize = 64;
// Required by
const _TABLE_ENTRIES_IS_32: () = assert!(TABLE_ENTRIES == 32);
const _STORAGE_ENTRIES_HAS_3_EXTRA: () = assert!(STORAGE_ENTRIES == TABLE_ENTRIES + 3);
assert!(STORAGE_LIMBS % (STORAGE_ENTRIES * limbs512::LIMBS_PER_CHUNK) == 0); // TODO: `const`
let mut table = limbs512::AlignedStorage::<STORAGE_LIMBS>::zeroed();
let mut table = table
.aligned_chunks_mut(STORAGE_ENTRIES, cpe)
.map_err(LimbSliceError::len_mismatch)?;
let (mut table, mut state) = table.split_at_mut(TABLE_ENTRIES * cpe);
assert_eq!((table.as_ptr() as usize) % MOD_EXP_CTIME_ALIGN, 0);
// These are named `(tmp, am, np)` in BoringSSL.
let (mut acc, mut rest) = state.split_at_mut(cpe);
let (mut base_cached, mut m_cached) = rest.split_at_mut(cpe);
// "To improve cache locality" according to upstream.
m_cached
.as_flattened_mut()
.copy_from_slice(m_original.as_flattened());
let m_cached = m_cached.as_ref();
let out: Elem<M, RInverse> = elem_reduced(out, base_mod_n, m, other_prime_len_bits);
let base_rinverse = match slice::as_chunks(&out.limbs) {
(c, []) => c,
_ => {
return Err(LimbSliceError::len_mismatch(LenMismatchError::new(
out.limbs.len(),
)))
}
};
// base_cached = base*R == (base/R * RRR)/R
mul_mont5(
base_cached.as_mut(),
base_rinverse,
oneRRR,
m_cached,
n0,
cpu2,
)?;
let base_cached = base_cached.as_ref();
let mut out = Storage::from(out); // recycle.
// Fill in all the powers of 2 of `acc` into the table using only squaring and without any
// gathering, storing the last calculated power into `acc`.
fn scatter_powers_of_2(
mut table: AsChunksMut<Limb, 8>,
mut acc: AsChunksMut<Limb, 8>,
m_cached: AsChunks<Limb, 8>,
n0: &N0,
mut i: LeakyWindow,
cpu: Option<(Adx, Bmi2)>,
) -> Result<(), LimbSliceError> {
loop {
scatter5(acc.as_ref(), table.as_mut(), i)?;
i *= 2;
if i >= TABLE_ENTRIES as LeakyWindow {
break;
}
sqr_mont5(acc.as_mut(), m_cached, n0, cpu)?;
}
Ok(())
}
// All entries in `table` will be Montgomery encoded.
// acc = table[0] = base**0 (i.e. 1).
m.oneR(acc.as_flattened_mut());
scatter5(acc.as_ref(), table.as_mut(), 0)?;
// acc = base**1 (i.e. base).
acc.as_flattened_mut()
.copy_from_slice(base_cached.as_flattened());
// Fill in entries 1, 2, 4, 8, 16.
scatter_powers_of_2(table.as_mut(), acc.as_mut(), m_cached, n0, 1, cpu2)?;
// Fill in entries 3, 6, 12, 24; 5, 10, 20, 30; 7, 14, 28; 9, 18; 11, 22; 13, 26; 15, 30;
// 17; 19; 21; 23; 25; 27; 29; 31.
for i in (3..(TABLE_ENTRIES as LeakyWindow)).step_by(2) {
let power = Window::from(i - 1);
assert!(power < 32); // Not secret,
unsafe {
mul_mont_gather5_amm(
acc.as_mut(),
base_cached,
table.as_ref(),
m_cached,
n0,
power,
cpu3,
)
}?;
scatter_powers_of_2(table.as_mut(), acc.as_mut(), m_cached, n0, i, cpu2)?;
}
let table = table.as_ref();
let acc = limb::fold_5_bit_windows(
exponent.limbs(),
|initial_window| {
unsafe { gather5(acc.as_mut(), table, initial_window) }
.unwrap_or_else(unwrap_impossible_limb_slice_error);
acc
},
|mut acc, window| {
unsafe { power5_amm(acc.as_mut(), table, m_cached, n0, window, cpu3) }
.unwrap_or_else(unwrap_impossible_limb_slice_error);
acc
},
);
// Reuse `base_rinverse`'s limbs to save an allocation.
out.limbs.copy_from_slice(acc.as_flattened());
Ok(from_montgomery_amm(out, m))
}
/// Verified a == b**-1 (mod m), i.e. a**-1 == b (mod m).
pub fn verify_inverses_consttime<M>(
a: &Elem<M, R>,
b: Elem<M, Unencoded>,
m: &Modulus<M>,
) -> Result<(), error::Unspecified> {
let r = elem_mul(a, b, m);
limb::verify_limbs_equal_1_leak_bit(&r.limbs)
}
#[inline]
pub fn elem_verify_equal_consttime<M, E>(
a: &Elem<M, E>,
b: &Elem<M, E>,
) -> Result<(), error::Unspecified> {
let equal = limb::limbs_equal_limbs_consttime(&a.limbs, &b.limbs)
.unwrap_or_else(unwrap_impossible_len_mismatch_error);
if !equal.leak() {
return Err(error::Unspecified);
}
Ok(())
}
#[cold]
#[inline(never)]
fn unwrap_impossible_len_mismatch_error<T>(LenMismatchError { .. }: LenMismatchError) -> T {
unreachable!()
}
#[cold]
#[inline(never)]
fn unwrap_impossible_limb_slice_error(err: LimbSliceError) {
match err {
LimbSliceError::LenMismatch(_) => unreachable!(),
LimbSliceError::TooShort(_) => unreachable!(),
LimbSliceError::TooLong(_) => unreachable!(),
}
}
#[cfg(test)]
mod tests {
use super::*;
use crate::cpu;
use crate::testutil as test;
// Type-level representation of an arbitrary modulus.
struct M {}
impl PublicModulus for M {}
#[test]
fn test_elem_exp_consttime() {
let cpu_features = cpu::features();
test::run(
test_vector_file!("../../crypto/fipsmodule/bn/test/mod_exp_tests.txt"),
|section, test_case| {
assert_eq!(section, "");
let m = consume_modulus::<M>(test_case, "M");
let m = m.modulus(cpu_features);
let expected_result = consume_elem(test_case, "ModExp", &m);
let base = consume_elem(test_case, "A", &m);
let e = {
let bytes = test_case.consume_bytes("E");
PrivateExponent::from_be_bytes_for_test_only(untrusted::Input::from(&bytes), &m)
.expect("valid exponent")
};
let oneRR = One::newRR(m.alloc_zero(), &m);
let oneRRR = One::newRRR(oneRR, &m);
// `base` in the test vectors is reduced (mod M) already but
// the API expects the bsae to be (mod N) where N = M * P for
// some other prime of the same length. Fake that here.
// Pretend there's another prime of equal length.
struct N {}
let other_modulus_len_bits = m.len_bits();
let base: Elem<N> = {
let mut limbs = BoxedLimbs::zero(base.limbs.len() * 2);
limbs[..base.limbs.len()].copy_from_slice(&base.limbs);
Elem {
limbs,
encoding: PhantomData,
}
};
let too_big = m.limbs().len() > ELEM_EXP_CONSTTIME_MAX_MODULUS_LIMBS;
let actual_result = if !too_big {
elem_exp_consttime(
m.alloc_zero(),
&base,
&oneRRR,
&e,
&m,
other_modulus_len_bits,
)
} else {
let actual_result = elem_exp_consttime(
m.alloc_zero(),
&base,
&oneRRR,
&e,
&m,
other_modulus_len_bits,
);
// TODO: Be more specific with which error we expect?
assert!(actual_result.is_err());
// Try again with a larger-than-normally-supported limit
elem_exp_consttime_inner::<_, _, { (4096 / LIMB_BITS) * STORAGE_ENTRIES }>(
m.alloc_zero(),
&base,
&oneRRR,
&e,
&m,
other_modulus_len_bits,
)
};
match actual_result {
Ok(r) => assert_elem_eq(&r, &expected_result),
Err(LimbSliceError::LenMismatch { .. }) => panic!(),
Err(LimbSliceError::TooLong { .. }) => panic!(),
Err(LimbSliceError::TooShort { .. }) => panic!(),
};
Ok(())
},
)
}
// TODO: fn test_elem_exp_vartime() using
// "src/rsa/bigint_elem_exp_vartime_tests.txt". See that file for details.
// In the meantime, the function is tested indirectly via the RSA
// verification and signing tests.
#[test]
fn test_elem_mul() {
let cpu_features = cpu::features();
test::run(
test_vector_file!("../../crypto/fipsmodule/bn/test/mod_mul_tests.txt"),
|section, test_case| {
assert_eq!(section, "");
let m = consume_modulus::<M>(test_case, "M");
let m = m.modulus(cpu_features);
let expected_result = consume_elem(test_case, "ModMul", &m);
let a = consume_elem(test_case, "A", &m);
let b = consume_elem(test_case, "B", &m);
let b = into_encoded(m.alloc_zero(), b, &m);
let a = into_encoded(m.alloc_zero(), a, &m);
let actual_result = elem_mul(&a, b, &m);
let actual_result = actual_result.into_unencoded(&m);
assert_elem_eq(&actual_result, &expected_result);
Ok(())
},
)
}
#[test]
fn test_elem_squared() {
let cpu_features = cpu::features();
test::run(
test_vector_file!("bigint_elem_squared_tests.txt"),
|section, test_case| {
assert_eq!(section, "");
let m = consume_modulus::<M>(test_case, "M");
let m = m.modulus(cpu_features);
let expected_result = consume_elem(test_case, "ModSquare", &m);
let a = consume_elem(test_case, "A", &m);
let a = into_encoded(m.alloc_zero(), a, &m);
let actual_result = elem_squared(a, &m);
let actual_result = actual_result.into_unencoded(&m);
assert_elem_eq(&actual_result, &expected_result);
Ok(())
},
)
}
#[test]
fn test_elem_reduced() {
let cpu_features = cpu::features();
test::run(
test_vector_file!("bigint_elem_reduced_tests.txt"),
|section, test_case| {
assert_eq!(section, "");
struct M {}
let m_ = consume_modulus::<M>(test_case, "M");
let m = m_.modulus(cpu_features);
let expected_result = consume_elem(test_case, "R", &m);
let a =
consume_elem_unchecked::<M>(test_case, "A", expected_result.limbs.len() * 2);
let other_modulus_len_bits = m_.len_bits();
let actual_result = elem_reduced(m.alloc_zero(), &a, &m, other_modulus_len_bits);
let oneRR = One::newRR(m.alloc_zero(), &m);
let actual_result = elem_mul(oneRR.as_ref(), actual_result, &m);
assert_elem_eq(&actual_result, &expected_result);
Ok(())
},
)
}
#[test]
fn test_elem_reduced_once() {
let cpu_features = cpu::features();
test::run(
test_vector_file!("bigint_elem_reduced_once_tests.txt"),
|section, test_case| {
assert_eq!(section, "");
struct M {}
struct O {}
let m = consume_modulus::<M>(test_case, "m");
let m = m.modulus(cpu_features);
let a = consume_elem_unchecked::<O>(test_case, "a", m.limbs().len());
let expected_result = consume_elem::<M>(test_case, "r", &m);
let other_modulus_len_bits = m.len_bits();
let actual_result =
elem_reduced_once(m.alloc_zero(), &a, &m, other_modulus_len_bits);
assert_elem_eq(&actual_result, &expected_result);
Ok(())
},
)
}
fn consume_elem<M>(
test_case: &mut test::TestCase,
name: &str,
m: &Modulus<M>,
) -> Elem<M, Unencoded> {
let value = test_case.consume_bytes(name);
Elem::from_be_bytes_padded(untrusted::Input::from(&value), m).unwrap()
}
fn consume_elem_unchecked<M>(
test_case: &mut test::TestCase,
name: &str,
num_limbs: usize,
) -> Elem<M, Unencoded> {
let bytes = test_case.consume_bytes(name);
let mut limbs = BoxedLimbs::zero(num_limbs);
limb::parse_big_endian_and_pad_consttime(untrusted::Input::from(&bytes), &mut limbs)
.unwrap();
Elem {
limbs,
encoding: PhantomData,
}
}
fn consume_modulus<M>(test_case: &mut test::TestCase, name: &str) -> OwnedModulus<M> {
let value = test_case.consume_bytes(name);
OwnedModulus::from(
OwnedModulusValue::from_be_bytes(untrusted::Input::from(&value)).unwrap(),
)
}
fn assert_elem_eq<M, E>(a: &Elem<M, E>, b: &Elem<M, E>) {
if elem_verify_equal_consttime(a, b).is_err() {
panic!("{:x?} != {:x?}", &*a.limbs, &*b.limbs);
}
}
fn into_encoded<M>(out: Storage<M>, a: Elem<M, Unencoded>, m: &Modulus<M>) -> Elem<M, R> {
let oneRR = One::newRR(out, m);
elem_mul(oneRR.as_ref(), a, m)
}
}
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