ring/rsa/public_key.rs
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// Copyright 2015-2021 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::{PublicExponent, PublicModulus, N, PUBLIC_KEY_PUBLIC_MODULUS_MAX_LEN};
use crate::{
arithmetic::bigint,
bits, cpu, error,
io::{self, der, der_writer},
limb::LIMB_BYTES,
};
use alloc::boxed::Box;
/// An RSA Public Key.
#[derive(Clone)]
pub struct PublicKey {
n: PublicModulus,
e: PublicExponent,
serialized: Box<[u8]>,
}
derive_debug_self_as_ref_hex_bytes!(PublicKey);
impl PublicKey {
pub(super) fn from_modulus_and_exponent(
n: untrusted::Input,
e: untrusted::Input,
n_min_bits: bits::BitLength,
n_max_bits: bits::BitLength,
e_min_value: PublicExponent,
cpu_features: cpu::Features,
) -> Result<Self, error::KeyRejected> {
let n_bytes = n;
let e_bytes = e;
// This is an incomplete implementation of NIST SP800-56Br1 Section
// 6.4.2.2, "Partial Public-Key Validation for RSA." That spec defers
// to NIST SP800-89 Section 5.3.3, "(Explicit) Partial Public Key
// Validation for RSA," "with the caveat that the length of the modulus
// shall be a length that is specified in this Recommendation." In
// SP800-89, two different sets of steps are given, one set numbered,
// and one set lettered. TODO: Document this in the end-user
// documentation for RSA keys.
let n = PublicModulus::from_be_bytes(n, n_min_bits..=n_max_bits, cpu_features)?;
let e = PublicExponent::from_be_bytes(e, e_min_value)?;
// If `n` is less than `e` then somebody has probably accidentally swapped
// them. The largest acceptable `e` is smaller than the smallest acceptable
// `n`, so no additional checks need to be done.
// XXX: Steps 4 & 5 / Steps d, e, & f are not implemented. This is also the
// case in most other commonly-used crypto libraries.
// TODO: Remove this re-parsing, and stop allocating this here.
// Instead we should serialize on demand without allocation, from
// `Modulus::be_bytes()` and `Exponent::be_bytes()`.
let n_bytes = io::Positive::from_be_bytes(n_bytes)
.map_err(|_: error::Unspecified| error::KeyRejected::unexpected_error())?;
let e_bytes = io::Positive::from_be_bytes(e_bytes)
.map_err(|_: error::Unspecified| error::KeyRejected::unexpected_error())?;
let serialized = der_writer::write_all(der::Tag::Sequence, &|output| {
der_writer::write_positive_integer(output, &n_bytes);
der_writer::write_positive_integer(output, &e_bytes);
});
Ok(Self { n, e, serialized })
}
/// The length, in bytes, of the public modulus.
///
/// The modulus length is rounded up to a whole number of bytes if its
/// bit length isn't a multiple of 8.
pub fn modulus_len(&self) -> usize {
self.n().len_bits().as_usize_bytes_rounded_up()
}
/// The public modulus.
#[inline]
pub(super) fn n(&self) -> &PublicModulus {
&self.n
}
/// The public exponent.
#[inline]
pub(super) fn e(&self) -> PublicExponent {
self.e
}
/// Calculates base**e (mod n), filling the first part of `out_buffer` with
/// the result.
///
/// This is constant-time with respect to the value in `base` (only).
///
/// The result will be a slice of the encoded bytes of the result within
/// `out_buffer`, if successful.
pub(super) fn exponentiate<'out>(
&self,
base: untrusted::Input,
out_buffer: &'out mut [u8; PUBLIC_KEY_PUBLIC_MODULUS_MAX_LEN],
) -> Result<&'out [u8], error::Unspecified> {
let n = &self.n.value();
// The encoded value of the base must be the same length as the modulus,
// in bytes.
if base.len() != self.n.len_bits().as_usize_bytes_rounded_up() {
return Err(error::Unspecified);
}
// RFC 8017 Section 5.2.2: RSAVP1.
// Step 1.
let s = bigint::Elem::from_be_bytes_padded(base, n)?;
if s.is_zero() {
return Err(error::Unspecified);
}
// Step 2.
let m = self.exponentiate_elem(s);
// Step 3.
Ok(fill_be_bytes_n(m, self.n.len_bits(), out_buffer))
}
/// Calculates base**e (mod n).
///
/// This is constant-time with respect to `base` only.
pub(super) fn exponentiate_elem(&self, base: bigint::Elem<N>) -> bigint::Elem<N> {
let n = self.n.value();
let base = bigint::elem_mul(n.oneRR().as_ref(), base, n);
// During RSA public key operations the exponent is almost always either
// 65537 (0b10000000000000001) or 3 (0b11), both of which have a Hamming
// weight of 2. The maximum bit length and maximum Hamming weight of the
// exponent is bounded by the value of `PublicExponent::MAX`.
bigint::elem_exp_vartime(base, self.e.value(), &n.as_partial()).into_unencoded(n)
}
}
// XXX: Refactor `signature::KeyPair` to get rid of this.
impl AsRef<[u8]> for PublicKey {
fn as_ref(&self) -> &[u8] {
&self.serialized
}
}
/// Returns the big-endian representation of `elem` that is
/// the same length as the minimal-length big-endian representation of
/// the modulus `n`.
///
/// `n_bits` must be the bit length of the public modulus `n`.
fn fill_be_bytes_n(
elem: bigint::Elem<N>,
n_bits: bits::BitLength,
out: &mut [u8; PUBLIC_KEY_PUBLIC_MODULUS_MAX_LEN],
) -> &[u8] {
let n_bytes = n_bits.as_usize_bytes_rounded_up();
let n_bytes_padded = ((n_bytes + (LIMB_BYTES - 1)) / LIMB_BYTES) * LIMB_BYTES;
let out = &mut out[..n_bytes_padded];
elem.fill_be_bytes(out);
let (padding, out) = out.split_at(n_bytes_padded - n_bytes);
assert!(padding.iter().all(|&b| b == 0));
out
}