primefield: better field element hex parsing (#1473)

Ensures hex strings are always sized appropriately to the modulus, which
also ensures the size is consistent on 32-bit and 64-bit targets.

Notably this impacts P-224 where there was previously some special
casing on `target_pointer_width`.

Author: tarcieri

p224/src/arithmetic.rs

@@ -41,41 +41,17 @@ impl PrimeCurveParams for NistP224 {
     const EQUATION_A: FieldElement = FieldElement::from_u64(3).neg();
 
     /// b = 0xb4050a85 0c04b3ab f5413256 5044b0b7 d7bfd8ba 270b3943 2355ffb4
-    #[cfg(target_pointer_width = "32")]
     const EQUATION_B: FieldElement =
         FieldElement::from_hex_vartime("b4050a850c04b3abf54132565044b0b7d7bfd8ba270b39432355ffb4");
 
-    /// b = 0xb4050a85 0c04b3ab f5413256 5044b0b7 d7bfd8ba 270b3943 2355ffb4
-    #[cfg(target_pointer_width = "64")]
-    const EQUATION_B: FieldElement = FieldElement::from_hex_vartime(
-        "00000000b4050a850c04b3abf54132565044b0b7d7bfd8ba270b39432355ffb4",
-    );
-
     /// Base point of P-224.
     ///
     /// ```text
     /// Gₓ = 0xb70e0cbd 6bb4bf7f 321390b9 4a03c1d3 56c21122 343280d6 115c1d21
     /// Gᵧ = 0xbd376388 b5f723fb 4c22dfe6 cd4375a0 5a074764 44d58199 85007e34
     /// ```
-    #[cfg(target_pointer_width = "32")]
     const GENERATOR: (FieldElement, FieldElement) = (
         FieldElement::from_hex_vartime("b70e0cbd6bb4bf7f321390b94a03c1d356c21122343280d6115c1d21"),
         FieldElement::from_hex_vartime("bd376388b5f723fb4c22dfe6cd4375a05a07476444d5819985007e34"),
     );
-
-    /// Base point of P-224.
-    ///
-    /// ```text
-    /// Gₓ = 0xb70e0cbd 6bb4bf7f 321390b9 4a03c1d3 56c21122 343280d6 115c1d21
-    /// Gᵧ = 0xbd376388 b5f723fb 4c22dfe6 cd4375a0 5a074764 44d58199 85007e34
-    /// ```
-    #[cfg(target_pointer_width = "64")]
-    const GENERATOR: (FieldElement, FieldElement) = (
-        FieldElement::from_hex_vartime(
-            "00000000b70e0cbd6bb4bf7f321390b94a03c1d356c21122343280d6115c1d21",
-        ),
-        FieldElement::from_hex_vartime(
-            "00000000bd376388b5f723fb4c22dfe6cd4375a05a07476444d5819985007e34",
-        ),
-    );
 }

primefield/src/macros.rs

@@ -176,9 +176,35 @@ macro_rules! monty_field_element {
             /// - When input is the wrong length
             /// - If input overflows the modulus
             pub const fn from_hex_vartime(hex: &str) -> Self {
-                Self(
-                    $crate::MontyFieldElement::<$params, { <$params>::LIMBS }>::from_hex_vartime(hex),
-                )
+                use $crate::array::typenum::Unsigned;
+
+                assert!(
+                    hex.len() == <$params as $crate::MontyFieldParams<{ <$params>::LIMBS }>>::ByteSize::USIZE * 2,
+                    "hex is the wrong length"
+                );
+
+                // Build a hex string of the expected size, regardless of the size of `Uint`
+                let mut hex_bytes = [b'0'; { <$uint>::BITS as usize / 4 }];
+
+                let offset = match <$params as $crate::MontyFieldParams<{ <$params>::LIMBS }>>::BYTE_ORDER {
+                    $crate::ByteOrder::BigEndian => hex_bytes.len() - hex.len(),
+                    $crate::ByteOrder::LittleEndian => 0
+                };
+
+                let mut i = 0;
+                while i < hex.len() {
+                    hex_bytes[i + offset] = hex.as_bytes()[i];
+                    i += 1;
+                }
+
+                match core::str::from_utf8(&hex_bytes) {
+                    Ok(padded_hex) => Self(
+                        $crate::MontyFieldElement::<$params, { <$params>::LIMBS }>::from_hex_vartime(padded_hex),
+                    ),
+                    Err(_) => panic!("invalid hex string"),
+                }
+
+
             }
 
             /// Decode [`