Add ability to retrieve private keys as bytes

src/bdk.udl

@@ -283,6 +283,8 @@ interface DescriptorSecretKey {
 
   DescriptorPublicKey as_public();
 
+  sequence<u8> secret_bytes();
+
   string as_string();
 };
 

src/lib.rs

@@ -907,6 +907,21 @@ impl DescriptorSecretKey {
         })
     }
 
+    /// Get the private key as bytes.
+    fn secret_bytes(&self) -> Vec<u8> {
+        let descriptor_secret_key = self.descriptor_secret_key_mutex.lock().unwrap();
+        let secret_bytes: Vec<u8> = match descriptor_secret_key.deref() {
+            BdkDescriptorSecretKey::XPrv(descriptor_x_key) => {
+                descriptor_x_key.xkey.private_key.secret_bytes().to_vec()
+            }
+            BdkDescriptorSecretKey::SinglePriv(_) => {
+                unreachable!()
+            }
+        };
+
+        secret_bytes
+    }
+
     fn as_string(&self) -> String {
         self.descriptor_secret_key_mutex.lock().unwrap().to_string()
     }
@@ -1131,4 +1146,14 @@ mod test {
         let derived_dpk = &derive_dpk(&master_dpk, "m/84h/1h/0h");
         assert!(derived_dpk.is_err());
     }
+
+    #[test]
+    fn test_retrieve_master_secret_key() {
+        let master_dpk = get_descriptor_secret_key();
+        let master_private_key = master_dpk.secret_bytes().to_hex();
+        assert_eq!(
+            master_private_key,
+            "e93315d6ce401eb4db803a56232f0ed3e69b053774e6047df54f1bd00e5ea936"
+        )
+    }
 }