allow conversion from MessageDigest to Md

This eases porting from legacy const EVP_MD to dynamic EVP_MD,
especially in codebases that need to support boringssl/libressl in
addition to OpenSSL 3

Author: reaperhulk

openssl/src/md.rs

@@ -76,6 +76,12 @@ pub struct Md(Inner);
 unsafe impl Sync for Md {}
 unsafe impl Send for Md {}
 
+impl From<crate::hash::MessageDigest> for Md {
+    fn from(value: crate::hash::MessageDigest) -> Self {
+        unsafe { Md::from_ptr(value.as_ptr() as *mut _) }
+    }
+}
+
 impl Md {
     /// Returns the `Md` corresponding to an [`Nid`].
     #[corresponds(EVP_get_digestbynid)]

openssl/src/md_ctx.rs

@@ -549,4 +549,22 @@ mod test {
         // Validate result of digest of "World"
         assert_eq!(reset_result, world_expected);
     }
+
+    #[test]
+    fn from_message_digest() {
+        let messagedigest = crate::hash::MessageDigest::from_name("sha256").unwrap();
+        let md = Md::from(messagedigest);
+        let mut ctx = MdCtx::new().unwrap();
+        ctx.digest_init(&md).unwrap();
+        ctx.digest_update(b"test").unwrap();
+        let mut digest = [0; 32];
+        ctx.digest_final(digest.as_mut_slice()).unwrap();
+        // It is safe to call EVP_MD_free on const EVP_MDs from MessageDigest, but
+        // let's test it anyway before we use the original const EVP_MD in a hasher
+        drop(md);
+        let mut h = crate::hash::Hasher::new(messagedigest).unwrap();
+        h.update(b"test").unwrap();
+        let output = h.finish().unwrap();
+        assert_eq!(digest, *output);
+    }
 }