ring 0.17.8

Safe, fast, small crypto using Rust.
Documentation 
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// 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 AUTHORS DISCLAIM ALL WARRANTIES
// WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
// MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHORS 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.

use super::{limb, BoxedLimbs, Limb, LimbMask, Modulus};
use crate::error;
use alloc::boxed::Box;

pub struct PrivateExponent {
    limbs: Box<[Limb]>,
}

impl PrivateExponent {
    // `p` is the modulus for which the exponent is in the interval [1, `p` - 1).
    pub fn from_be_bytes_padded<M>(
        input: untrusted::Input,
        p: &Modulus<M>,
    ) -> Result<Self, error::Unspecified> {
        let dP = BoxedLimbs::from_be_bytes_padded_less_than(input, p)?;

        // Proof that `dP < p - 1`:
        //
        // If `dP < p` then either `dP == p - 1` or `dP < p - 1`. Since `p` is
        // odd, `p - 1` is even. `d` is odd, and an odd number modulo an even
        // number is odd. Therefore `dP` must be odd. But then it cannot be
        // `p - 1` and so we know `dP < p - 1`.
        //
        // Further we know `dP != 0` because `dP` is not even.
        if limb::limbs_are_even_constant_time(&dP) != LimbMask::False {
            return Err(error::Unspecified);
        }

        Ok(Self {
            limbs: dP.into_limbs(),
        })
    }

    #[cfg(test)]
    pub fn from_be_bytes_for_test_only<M>(
        input: untrusted::Input,
        p: &Modulus<M>,
    ) -> Result<Self, error::Unspecified> {
        // Do exactly what `from_be_bytes_padded` does for any inputs it accepts.
        if let r @ Ok(_) = Self::from_be_bytes_padded(input, p) {
            return r;
        }

        let dP = BoxedLimbs::<M>::positive_minimal_width_from_be_bytes(input)?;

        Ok(Self {
            limbs: dP.into_limbs(),
        })
    }

    #[inline]
    pub(super) fn limbs(&self) -> &[Limb] {
        &self.limbs
    }
}