der: conversions between BitStringRef/OctetStringRef and [u8; N] (#1731)

These are helpful for custom drive when struct fields are byte arrays

Author: dishmaker

der/src/asn1/bit_string.rs

@@ -193,6 +193,24 @@ impl<'a> TryFrom<&&'a [u8]> for BitStringRef<'a> {
     }
 }
 
+impl<'a, const N: usize> TryFrom<&'a [u8; N]> for BitStringRef<'a> {
+    type Error = Error;
+
+    fn try_from(bytes: &'a [u8; N]) -> Result<BitStringRef<'a>> {
+        BitStringRef::from_bytes(bytes)
+    }
+}
+
+impl<'a, const N: usize> TryFrom<BitStringRef<'a>> for [u8; N] {
+    type Error = Error;
+
+    fn try_from(bit_string: BitStringRef<'a>) -> Result<Self> {
+        let bytes: &[u8] = TryFrom::try_from(bit_string)?;
+
+        bytes.try_into().map_err(|_| Tag::BitString.length_error())
+    }
+}
+
 impl<'a> TryFrom<BitStringRef<'a>> for &'a [u8] {
     type Error = Error;
 

der/src/asn1/octet_string.rs

@@ -113,6 +113,25 @@ impl<'a> TryFrom<&&'a [u8]> for OctetStringRef<'a> {
     }
 }
 
+impl<'a, const N: usize> TryFrom<&'a [u8; N]> for OctetStringRef<'a> {
+    type Error = Error;
+
+    fn try_from(byte_slice: &'a [u8; N]) -> Result<Self, Error> {
+        OctetStringRef::new(byte_slice)
+    }
+}
+
+impl<'a, const N: usize> TryFrom<OctetStringRef<'a>> for [u8; N] {
+    type Error = Error;
+
+    fn try_from(octet_string: OctetStringRef<'a>) -> Result<Self, Self::Error> {
+        octet_string
+            .as_bytes()
+            .try_into()
+            .map_err(|_| Tag::OctetString.length_error())
+    }
+}
+
 #[cfg(feature = "alloc")]
 pub use self::allocating::OctetString;
 

der/tests/derive.rs

@@ -528,7 +528,7 @@ mod sequence {
         pub owned_optional_implicit_bytes: Option<Vec<u8>>,
 
         #[asn1(
-            type = "OCTET STRING",
+            type = "BIT STRING",
             context_specific = "6",
             optional = "true",
             tag_mode = "EXPLICIT"
@@ -568,6 +568,85 @@ mod sequence {
         assert_eq!(obj, obj_decoded);
     }
 
+    #[derive(Sequence, Default, Eq, PartialEq, Debug)]
+    #[asn1(tag_mode = "IMPLICIT")]
+    pub struct TypeCheckArraysSequenceFieldAttributeCombinations {
+        #[asn1(type = "OCTET STRING", deref = "true")]
+        pub array_bytes: [u8; 2],
+
+        #[asn1(type = "BIT STRING", deref = "true")]
+        pub array_bits: [u8; 2],
+
+        #[asn1(type = "OCTET STRING", context_specific = "0", deref = "true")]
+        pub array_implicit_bytes: [u8; 2],
+
+        #[asn1(type = "BIT STRING", context_specific = "1", deref = "true")]
+        pub array_implicit_bits: [u8; 2],
+
+        #[asn1(
+            type = "OCTET STRING",
+            context_specific = "2",
+            tag_mode = "EXPLICIT",
+            deref = "true"
+        )]
+        pub array_explicit_bytes: [u8; 2],
+
+        #[asn1(
+            type = "BIT STRING",
+            context_specific = "3",
+            tag_mode = "EXPLICIT",
+            deref = "true"
+        )]
+        pub array_explicit_bits: [u8; 2],
+
+        #[asn1(type = "BIT STRING", context_specific = "4", optional = "true")]
+        pub array_optional_implicit_bits: Option<[u8; 2]>,
+
+        #[asn1(type = "OCTET STRING", context_specific = "5", optional = "true")]
+        pub array_optional_implicit_bytes: Option<[u8; 2]>,
+
+        #[asn1(
+            type = "BIT STRING",
+            context_specific = "6",
+            optional = "true",
+            tag_mode = "EXPLICIT"
+        )]
+        pub array_optional_explicit_bits: Option<[u8; 2]>,
+
+        #[asn1(
+            type = "OCTET STRING",
+            context_specific = "7",
+            optional = "true",
+            tag_mode = "EXPLICIT"
+        )]
+        pub array_optional_explicit_bytes: Option<[u8; 2]>,
+    }
+
+    #[test]
+    fn type_combinations_arrays_instance() {
+        let obj = TypeCheckArraysSequenceFieldAttributeCombinations {
+            array_bytes: [0xAA, 0xBB],
+            array_bits: [0xCC, 0xDD],
+
+            array_implicit_bytes: [0, 1],
+            array_implicit_bits: [2, 3],
+
+            array_explicit_bytes: [4, 5],
+            array_explicit_bits: [6, 7],
+
+            array_optional_implicit_bits: Some([8, 9]),
+            array_optional_implicit_bytes: Some([10, 11]),
+
+            array_optional_explicit_bits: Some([12, 13]),
+            array_optional_explicit_bytes: Some([14, 15]),
+        };
+
+        let der_encoded = obj.to_der().unwrap();
+        let obj_decoded =
+            TypeCheckArraysSequenceFieldAttributeCombinations::from_der(&der_encoded).unwrap();
+        assert_eq!(obj, obj_decoded);
+    }
+
     #[derive(Sequence)]
     #[asn1(error = CustomError)]
     pub struct TypeWithCustomError {