Add optional der feature (#61)

Adds support for decoding/encoding `UInt` as an ASN.1 DER `INTEGER`.

.github/workflows/crypto-bigint.yml

@@ -34,11 +34,12 @@ jobs:
           override: true
       - run: cargo build --target ${{ matrix.target }} --release --no-default-features
       - run: cargo build --target ${{ matrix.target }} --release --no-default-features --features alloc
+      - run: cargo build --target ${{ matrix.target }} --release --no-default-features --features der
       - run: cargo build --target ${{ matrix.target }} --release --no-default-features --features generic-array
       - run: cargo build --target ${{ matrix.target }} --release --no-default-features --features rand_core
       - run: cargo build --target ${{ matrix.target }} --release --no-default-features --features rlp
       - run: cargo build --target ${{ matrix.target }} --release --no-default-features --features zeroize
-      - run: cargo build --target ${{ matrix.target }} --release --no-default-features --features alloc,generic-array,rand_core,rlp,zeroize
+      - run: cargo build --target ${{ matrix.target }} --release --no-default-features --features alloc,der,generic-array,rand_core,rlp,zeroize
 
   test:
     runs-on: ubuntu-latest

Cargo.toml

@@ -20,6 +20,7 @@ rust-version = "1.57"
 subtle = { version = "2.4", default-features = false }
 
 # optional dependencies
+der = { version = "=0.6.0-pre", optional = true, default-features = false }
 generic-array = { version = "0.14", optional = true }
 rand_core = { version = "0.6", optional = true }
 rlp = { version = "0.5", optional = true, default-features = false }
@@ -41,3 +42,6 @@ rand = ["rand_core/std"]
 [package.metadata.docs.rs]
 all-features = true
 rustdoc-args = ["--cfg", "docsrs"]
+
+[patch.crates-io]
+der = { git = "https://github.com/RustCrypto/formats.git" }

src/uint/encoding.rs

@@ -2,6 +2,9 @@
 
 mod decoder;
 
+#[cfg(all(feature = "der", feature = "generic-array"))]
+mod der;
+
 #[cfg(feature = "rlp")]
 mod rlp;
 

src/uint/encoding/der.rs

@@ -0,0 +1,63 @@
+//! Support for decoding/encoding [`UInt`] as an ASN.1 DER `INTEGER`.
+
+use crate::{generic_array::GenericArray, ArrayEncoding, UInt};
+use ::der::{
+    asn1::{Any, UIntBytes},
+    EncodeValue, Encoder, FixedTag, Length, Tag,
+};
+
+#[cfg(feature = "der")]
+#[cfg_attr(docsrs, doc(cfg(feature = "der")))]
+impl<'a, const LIMBS: usize> TryFrom<Any<'a>> for UInt<LIMBS>
+where
+    UInt<LIMBS>: ArrayEncoding,
+{
+    type Error = der::Error;
+
+    fn try_from(any: Any<'a>) -> der::Result<UInt<LIMBS>> {
+        UIntBytes::try_from(any)?.try_into()
+    }
+}
+
+#[cfg(feature = "der")]
+#[cfg_attr(docsrs, doc(cfg(feature = "der")))]
+impl<'a, const LIMBS: usize> TryFrom<UIntBytes<'a>> for UInt<LIMBS>
+where
+    UInt<LIMBS>: ArrayEncoding,
+{
+    type Error = der::Error;
+
+    fn try_from(bytes: UIntBytes<'a>) -> der::Result<UInt<LIMBS>> {
+        let mut array = GenericArray::default();
+        let offset = array.len().saturating_sub(bytes.len().try_into()?);
+        array[offset..].copy_from_slice(bytes.as_bytes());
+        Ok(UInt::from_be_byte_array(array))
+    }
+}
+
+#[cfg(feature = "der")]
+#[cfg_attr(docsrs, doc(cfg(feature = "der")))]
+impl<const LIMBS: usize> EncodeValue for UInt<LIMBS>
+where
+    UInt<LIMBS>: ArrayEncoding,
+{
+    fn value_len(&self) -> der::Result<Length> {
+        // TODO(tarcieri): more efficient length calculation
+        let array = self.to_be_byte_array();
+        UIntBytes::new(&array)?.value_len()
+    }
+
+    fn encode_value(&self, encoder: &mut Encoder<'_>) -> der::Result<()> {
+        let array = self.to_be_byte_array();
+        UIntBytes::new(&array)?.encode_value(encoder)
+    }
+}
+
+#[cfg(feature = "der")]
+#[cfg_attr(docsrs, doc(cfg(feature = "der")))]
+impl<const LIMBS: usize> FixedTag for UInt<LIMBS>
+where
+    UInt<LIMBS>: ArrayEncoding,
+{
+    const TAG: Tag = Tag::Integer;
+}