Implement Encoding for all Uints

Author: fjarri

Cargo.lock

@@ -18,6 +18,7 @@ rust-version = "1.85"
 
 [dependencies]
 subtle = { version = "2.6", default-features = false }
+bytemuck = { version = "1.24", default-features = false, features = ["must_cast", "must_cast_extra"] }
 
 # optional dependencies
 der = { version = "0.8.0-rc.9", optional = true, default-features = false }

Cargo.toml

@@ -167,6 +167,8 @@
 #[macro_use]
 extern crate alloc;
 
+pub use uint::encoding::{EncodedUint, TryFromSliceError};
+
 #[cfg(feature = "rand_core")]
 pub use rand_core;
 #[cfg(feature = "rlp")]

src/lib.rs

@@ -231,6 +231,14 @@ impl Serialize for Limb {
 #[cfg(feature = "zeroize")]
 impl zeroize::DefaultIsZeroes for Limb {}
 
+// SAFETY: `Limb` is a newtype of an integer POD type
+#[allow(unsafe_code)]
+unsafe impl bytemuck::Zeroable for Limb {}
+
+// SAFETY: `Limb` is a newtype of an integer POD type
+#[allow(unsafe_code)]
+unsafe impl bytemuck::Pod for Limb {}
+
 #[cfg(test)]
 mod tests {
     #[cfg(feature = "alloc")]

src/limb.rs

@@ -616,7 +616,7 @@ pub trait Encoding: Sized {
         + Copy
         + Clone
         + Sized
-        + for<'a> TryFrom<&'a [u8], Error = core::array::TryFromSliceError>;
+        + for<'a> TryFrom<&'a [u8], Error: core::error::Error>;
 
     /// Decode from big endian bytes.
     fn from_be_bytes(bytes: Self::Repr) -> Self;

src/traits.rs

@@ -16,10 +16,13 @@ pub use extra_sizes::*;
 pub(crate) use ref_type::UintRef;
 
 use crate::{
-    Bounded, ConstChoice, ConstCtOption, ConstOne, ConstZero, Constants, Encoding, FixedInteger,
-    Int, Integer, Limb, NonZero, Odd, One, Unsigned, Word, Zero, modular::MontyForm,
+    Bounded, ConstChoice, ConstCtOption, ConstOne, ConstZero, Constants, FixedInteger, Int,
+    Integer, Limb, NonZero, Odd, One, Unsigned, Word, Zero, modular::MontyForm,
 };
 
+#[cfg(feature = "serde")]
+use crate::Encoding;
+
 #[macro_use]
 mod macros;
 
@@ -413,10 +416,10 @@ where
     where
         D: Deserializer<'de>,
     {
-        let mut buffer = Self::ZERO.to_le_bytes();
+        let mut buffer = Encoding::to_le_bytes(&Self::ZERO);
         serdect::array::deserialize_hex_or_bin(buffer.as_mut(), deserializer)?;
 
-        Ok(Self::from_le_bytes(buffer))
+        Ok(Encoding::from_le_bytes(buffer))
     }
 }
 
@@ -429,7 +432,7 @@ where
     where
         S: Serializer,
     {
-        serdect::array::serialize_hex_lower_or_bin(&Encoding::to_le_bytes(self), serializer)
+        serdect::slice::serialize_hex_lower_or_bin(&Encoding::to_le_bytes(self), serializer)
     }
 }
 
@@ -603,7 +606,7 @@ mod tests {
         let be_bytes = a.to_be_bytes();
         let le_bytes = a.to_le_bytes();
         for i in 0..16 {
-            assert_eq!(le_bytes[i], be_bytes[15 - i]);
+            assert_eq!(le_bytes.as_ref()[i], be_bytes.as_ref()[15 - i]);
         }
 
         let a_from_be = U128::from_be_bytes(be_bytes);

src/uint.rs

@@ -872,10 +872,16 @@ mod tests {
         );
         let rem = U256::rem_wide(lo_hi, &modulus);
         // Lower half is zero
-        assert_eq!(rem.to_be_bytes()[0..16], U128::ZERO.to_be_bytes());
+        assert_eq!(
+            &rem.to_be_bytes().as_ref()[0..16],
+            U128::ZERO.to_be_bytes().as_ref()
+        );
         // Upper half
         let expected = U128::from_be_hex("203F80FE03F80FE03F80FE03F80FE041");
-        assert_eq!(rem.to_be_bytes()[16..], expected.to_be_bytes());
+        assert_eq!(
+            &rem.to_be_bytes().as_ref()[16..],
+            expected.to_be_bytes().as_ref()
+        );
 
         let remv = U256::rem_wide_vartime(lo_hi, &modulus);
         assert_eq!(rem, remv);

src/uint/div.rs

@@ -6,18 +6,17 @@ mod der;
 #[cfg(feature = "rlp")]
 mod rlp;
 
+use core::fmt;
+
 #[cfg(feature = "alloc")]
 use alloc::{string::String, vec::Vec};
 
 use super::Uint;
-use crate::{DecodeError, Limb, Word};
+use crate::{DecodeError, Encoding, Limb, Word};
 
 #[cfg(feature = "alloc")]
 use crate::{ConstChoice, NonZero, Reciprocal, UintRef, WideWord};
 
-#[cfg(feature = "hybrid-array")]
-use crate::Encoding;
-
 #[cfg(feature = "alloc")]
 const RADIX_ENCODING_LIMBS_LARGE: usize = 16;
 #[cfg(feature = "alloc")]
@@ -204,58 +203,140 @@ impl<const LIMBS: usize> Uint<LIMBS> {
         let mut buf = *self;
         radix_encode_limbs_mut_to_string(radix, buf.as_mut_uint_ref())
     }
+
+    /// Serialize as big endian bytes.
+    pub const fn to_be_bytes(&self) -> EncodedUint<LIMBS> {
+        EncodedUint::new_be(self)
+    }
+
+    /// Serialize as little endian bytes.
+    pub const fn to_le_bytes(&self) -> EncodedUint<LIMBS> {
+        EncodedUint::new_le(self)
+    }
 }
 
-/// Encode a [`Uint`] to a big endian byte array of the given size.
-pub(crate) const fn uint_to_be_bytes<const LIMBS: usize, const BYTES: usize>(
-    uint: &Uint<LIMBS>,
-) -> [u8; BYTES] {
-    if BYTES != LIMBS * Limb::BYTES {
-        panic!("BYTES != LIMBS * Limb::BYTES");
+/// [`Uint`] encoded as bytes.
+// Until const generic expressions are stable, we cannot statically declare a `u8` array
+// of the size `LIMBS * Limb::BYTES`.
+// So instead we use the array of words, and treat it as an array of bytes.
+// It's a little hacky, but it works, because the array is guaranteed to be contiguous.
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+pub struct EncodedUint<const LIMBS: usize>([Word; LIMBS]);
+
+impl<const LIMBS: usize> Default for EncodedUint<LIMBS> {
+    fn default() -> Self {
+        Self([0; LIMBS])
     }
+}
 
-    let mut ret = [0u8; BYTES];
-    let mut i = 0;
+impl<const LIMBS: usize> EncodedUint<LIMBS> {
+    const fn new_le(value: &Uint<LIMBS>) -> Self {
+        let mut buffer = [0; LIMBS];
+        let mut i = 0;
 
-    while i < LIMBS {
-        let limb_bytes = uint.limbs[LIMBS - i - 1].0.to_be_bytes();
-        let mut j = 0;
+        while i < LIMBS {
+            let src_bytes = &value.limbs[i].0.to_le_bytes();
 
-        while j < Limb::BYTES {
-            ret[i * Limb::BYTES + j] = limb_bytes[j];
-            j += 1;
+            // We could cast the whole `buffer` to bytes at once,
+            // but IndexMut does not work in const context.
+            let dst_bytes: &mut [u8] =
+                bytemuck::must_cast_slice_mut(core::slice::from_mut(&mut buffer[i]));
+
+            // `copy_from_slice` can be used here when MSRV moves past 1.87
+            let mut j = 0;
+            while j < Limb::BYTES {
+                dst_bytes[j] = src_bytes[j];
+                j += 1;
+            }
+
+            i += 1;
         }
+        Self(buffer)
+    }
 
-        i += 1;
+    const fn new_be(value: &Uint<LIMBS>) -> Self {
+        let mut buffer = [0; LIMBS];
+        let mut i = 0;
+        while i < LIMBS {
+            let src_bytes = &value.limbs[i].0.to_be_bytes();
+
+            // We could cast the whole `buffer` to bytes at once,
+            // but IndexMut does not work in const context.
+            let dst_bytes: &mut [u8] =
+                bytemuck::must_cast_slice_mut(core::slice::from_mut(&mut buffer[LIMBS - 1 - i]));
+
+            // `copy_from_slice` can be used here when MSRV moves past 1.87
+            let mut j = 0;
+            while j < Limb::BYTES {
+                dst_bytes[j] = src_bytes[j];
+                j += 1;
+            }
+
+            i += 1;
+        }
+        Self(buffer)
     }
+}
 
-    ret
+impl<const LIMBS: usize> AsRef<[u8]> for EncodedUint<LIMBS> {
+    fn as_ref(&self) -> &[u8] {
+        bytemuck::must_cast_slice(&self.0)
+    }
 }
 
-/// Encode a [`Uint`] to a little endian byte array of the given size.
-pub(crate) const fn uint_to_le_bytes<const LIMBS: usize, const BYTES: usize>(
-    uint: &Uint<LIMBS>,
-) -> [u8; BYTES] {
-    if BYTES != LIMBS * Limb::BYTES {
-        panic!("BYTES != LIMBS * Limb::BYTES");
+impl<const LIMBS: usize> AsMut<[u8]> for EncodedUint<LIMBS> {
+    fn as_mut(&mut self) -> &mut [u8] {
+        bytemuck::must_cast_slice_mut(&mut self.0)
     }
+}
 
-    let mut ret = [0u8; BYTES];
-    let mut i = 0;
+/// Returned if an object cannot be instantiated from the given byte slice.
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub struct TryFromSliceError;
 
-    while i < LIMBS {
-        let limb_bytes = uint.limbs[i].0.to_le_bytes();
-        let mut j = 0;
+impl fmt::Display for TryFromSliceError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> Result<(), fmt::Error> {
+        write!(f, "TryFromSliceError")
+    }
+}
 
-        while j < Limb::BYTES {
-            ret[i * Limb::BYTES + j] = limb_bytes[j];
-            j += 1;
+impl core::error::Error for TryFromSliceError {}
+
+impl<'a, const LIMBS: usize> TryFrom<&'a [u8]> for EncodedUint<LIMBS> {
+    type Error = TryFromSliceError;
+
+    fn try_from(bytes: &'a [u8]) -> Result<Self, Self::Error> {
+        if bytes.len() != Uint::<LIMBS>::BYTES {
+            return Err(TryFromSliceError);
         }
+        let mut result = Self::default();
+        result.as_mut().copy_from_slice(bytes);
+        Ok(result)
+    }
+}
+
+impl<const LIMBS: usize> Encoding for Uint<LIMBS> {
+    type Repr = EncodedUint<LIMBS>;
+
+    #[inline]
+    fn from_be_bytes(bytes: Self::Repr) -> Self {
+        Self::from_be_slice(bytes.as_ref())
+    }
 
-        i += 1;
+    #[inline]
+    fn from_le_bytes(bytes: Self::Repr) -> Self {
+        Self::from_le_slice(bytes.as_ref())
+    }
+
+    #[inline]
+    fn to_be_bytes(&self) -> Self::Repr {
+        self.to_be_bytes()
     }
 
-    ret
+    #[inline]
+    fn to_le_bytes(&self) -> Self::Repr {
+        self.to_le_bytes()
+    }
 }
 
 /// Decode a single nibble of upper or lower hex
@@ -1057,7 +1138,7 @@ mod tests {
         let n = UintEx::from_be_hex("0011223344556677");
 
         let bytes = n.to_be_bytes();
-        assert_eq!(bytes, hex!("0011223344556677"));
+        assert_eq!(bytes.as_ref(), hex!("0011223344556677"));
 
         #[cfg(feature = "der")]
         assert_eq!(super::der::count_der_be_bytes(&n.limbs), 7);
@@ -1069,7 +1150,7 @@ mod tests {
         let n = UintEx::from_be_hex("00112233445566778899aabbccddeeff");
 
         let bytes = n.to_be_bytes();
-        assert_eq!(bytes, hex!("00112233445566778899aabbccddeeff"));
+        assert_eq!(bytes.as_ref(), hex!("00112233445566778899aabbccddeeff"));
 
         #[cfg(feature = "der")]
         assert_eq!(super::der::count_der_be_bytes(&n.limbs), 15);

src/uint/encoding.rs

@@ -7,42 +7,6 @@ macro_rules! impl_uint_aliases {
             #[doc = $doc]
             #[doc="unsigned big integer."]
             pub type $name = Uint<{ nlimbs!($bits) }>;
-
-            impl $name {
-                /// Serialize as big endian bytes.
-                pub const fn to_be_bytes(&self) -> [u8; $bits / 8] {
-                    encoding::uint_to_be_bytes::<{ nlimbs!($bits) }, { $bits / 8 }>(self)
-                }
-
-                /// Serialize as little endian bytes.
-                pub const fn to_le_bytes(&self) -> [u8; $bits / 8] {
-                    encoding::uint_to_le_bytes::<{ nlimbs!($bits) }, { $bits / 8 }>(self)
-                }
-            }
-
-            impl Encoding for $name {
-                type Repr = [u8; $bits / 8];
-
-                #[inline]
-                fn from_be_bytes(bytes: Self::Repr) -> Self {
-                    Self::from_be_slice(&bytes)
-                }
-
-                #[inline]
-                fn from_le_bytes(bytes: Self::Repr) -> Self {
-                    Self::from_le_slice(&bytes)
-                }
-
-                #[inline]
-                fn to_be_bytes(&self) -> Self::Repr {
-                    encoding::uint_to_be_bytes(self)
-                }
-
-                #[inline]
-                fn to_le_bytes(&self) -> Self::Repr {
-                    encoding::uint_to_le_bytes(self)
-                }
-            }
         )+
      };
 }