cms: make MessageDigest a newtype around OctetString (#1967)

Author: baloo

cms/Cargo.toml

@@ -57,7 +57,7 @@ builder = [
     "dep:ansi-x963-kdf",
     "dep:cbc",
     "dep:cipher",
-    "dep:digest",
+    "digest",
     "elliptic-curve/ecdh",
     "elliptic-curve/pkcs8",
     "dep:rsa",

cms/src/attr.rs

@@ -1,5 +1,10 @@
 //! Attribute-related types
-use der::asn1::OctetString;
+use alloc::{boxed::Box, vec};
+use der::{
+    DecodeValue, EncodeValue, FixedTag, Length, Tag,
+    asn1::{OctetString, OctetStringRef},
+    referenced::OwnedToRef,
+};
 
 use x509_cert::time::Time;
 
@@ -12,7 +17,89 @@ use crate::signed_data::SignerInfo;
 /// ```
 ///
 /// [RFC 5652 Section 11.2]: https://www.rfc-editor.org/rfc/rfc5652#section-11.2
-pub type MessageDigest = OctetString;
+#[derive(Clone, Debug, Eq, Ord, PartialEq, PartialOrd)]
+pub struct MessageDigest(pub OctetString);
+
+impl MessageDigest {
+    /// Borrow the inner byte slice.
+    #[inline]
+    pub fn as_bytes(&self) -> &[u8] {
+        self.0.as_bytes()
+    }
+
+    /// Take ownership of the octet string.
+    #[inline]
+    pub fn into_bytes(self) -> Box<[u8]> {
+        self.0.into_bytes()
+    }
+
+    /// Get the length of the inner byte slice.
+    #[inline]
+    pub fn len(&self) -> Length {
+        self.0.len()
+    }
+
+    /// Create a [`MessageDigest`] from a [`digest::Digest`]
+    #[cfg(feature = "digest")]
+    pub fn from_digest<D>(digest: D) -> der::Result<Self>
+    where
+        D: digest::Digest,
+    {
+        Ok(MessageDigest(OctetString::new(digest.finalize().to_vec())?))
+    }
+
+    /// Return an [`OctetStringRef`] pointing to the underlying data
+    #[inline]
+    pub fn as_octet_string_ref<'a>(&'a self) -> OctetStringRef<'a> {
+        self.0.owned_to_ref()
+    }
+}
+
+impl AsRef<[u8]> for MessageDigest {
+    #[inline]
+    fn as_ref(&self) -> &[u8] {
+        self.0.as_ref()
+    }
+}
+
+impl AsRef<OctetString> for MessageDigest {
+    #[inline]
+    fn as_ref(&self) -> &OctetString {
+        &self.0
+    }
+}
+
+impl<'a> DecodeValue<'a> for MessageDigest {
+    type Error = <OctetString as DecodeValue<'a>>::Error;
+
+    #[inline]
+    fn decode_value<R: der::Reader<'a>>(reader: &mut R, header: der::Header) -> der::Result<Self> {
+        OctetString::decode_value(reader, header).map(Self)
+    }
+}
+
+impl EncodeValue for MessageDigest {
+    #[inline]
+    fn value_len(&self) -> der::Result<Length> {
+        self.0.value_len()
+    }
+
+    #[inline]
+    fn encode_value(&self, writer: &mut impl der::Writer) -> der::Result<()> {
+        self.0.encode_value(writer)
+    }
+}
+
+impl FixedTag for MessageDigest {
+    const TAG: Tag = OctetString::TAG;
+}
+
+impl From<MessageDigest> for vec::Vec<u8> {
+    #[inline]
+    fn from(value: MessageDigest) -> vec::Vec<u8> {
+        value.0.into_bytes().into()
+    }
+}
 
 /// The `SigningTime` attribute is defined in [RFC 5652 Section 11.3].
 ///