Add methods for conversions between did:key and Ed25519 key

src/identity/did_key.rs

@@ -8,11 +8,44 @@ use super::did::DidParseError;
 
 const DID_KEY_RE: &str = r"did:key:(?P<key>z[a-km-zA-HJ-NP-Z1-9]+)";
 
+// https://github.com/multiformats/multicodec
+// 0xed: Ed25519 public key
+// 0x01: integer encoding
+const MULTICODEC_ED25519_PREFIX: [u8; 2] = [0xed, 0x01];
+
 #[derive(Clone, Debug, PartialEq)]
 pub struct DidKey {
     pub key: Vec<u8>,
 }
 
+#[derive(thiserror::Error, Debug)]
+#[error("multicodec error")]
+pub struct MulticodecError;
+
+impl DidKey {
+    pub fn from_ed25519_key(key: [u8; 32]) -> Self {
+        let prefixed_key = [
+            MULTICODEC_ED25519_PREFIX.to_vec(),
+            key.to_vec(),
+        ].concat();
+        Self { key: prefixed_key }
+    }
+
+    pub fn try_ed25519_key(&self) -> Result<[u8; 32], MulticodecError> {
+        if self.key.len() != 34 {
+            return Err(MulticodecError);
+        };
+        let mut prefix = [0; 2];
+        let mut key = [0; 32];
+        prefix.copy_from_slice(&self.key[0..2]);
+        key.copy_from_slice(&self.key[2..34]);
+        if prefix.as_ref() != MULTICODEC_ED25519_PREFIX {
+            return Err(MulticodecError);
+        };
+        Ok(key)
+    }
+}
+
 #[derive(thiserror::Error, Debug)]
 enum MultibaseError {
     #[error("invalid base string")]
@@ -81,6 +114,8 @@ mod tests {
         let did_str = "did:key:z6MkhaXgBZDvotDkL5257faiztiGiC2QtKLGpbnnEGta2doK";
         let did_key: DidKey = did_str.parse().unwrap();
         assert_eq!(did_key.key.len(), 34); // Ed25519 public key
+        let decoded_key = did_key.try_ed25519_key().unwrap();
+        let did_key = DidKey::from_ed25519_key(decoded_key);
         assert_eq!(did_key.to_string(), did_str);
     }
 }