keymgr: Use Box<dyn EncodableKey> instead of Box<dyn Any>.

Prompted by https://gitlab.torproject.org/tpo/core/arti/-/merge_requests/1337#note_2917701

This will make it harder to accidentally return the wrong value from
`Keystore::get` (the returned value is now at least guaranteed to
implement `EncodableKey`).

Closes #937

Cargo.lock

@@ -4550,6 +4550,7 @@ name = "tor-keymgr"
 version = "0.1.0"
 dependencies = [
  "derive_more",
+ "downcast-rs",
  "dyn-clone",
  "fs-mistrust",
  "itertools 0.10.5",

crates/tor-keymgr/Cargo.toml

@@ -25,6 +25,7 @@ experimental = ["keymgr"]
 
 [dependencies]
 derive_more = "0.99.3"
+downcast-rs = "1.2.0"
 dyn-clone = "1.0.11"
 fs-mistrust = { path = "../fs-mistrust", version = "0.7.1", features = ["serde", "walkdir"] }
 itertools = "0.10.1"

crates/tor-keymgr/src/key_type/ssh.rs

@@ -216,7 +216,10 @@ mod tests {
     fn wrong_key_type() {
         let key_type = KeyType::Ed25519Keypair;
         let key = UnparsedOpenSshKey::new(OPENSSH_DSA.into(), PathBuf::from("/test/path"));
-        let err = key_type.parse_ssh_format_erased(key).unwrap_err();
+        let err = key_type
+            .parse_ssh_format_erased(key)
+            .map(|_| "<type erased key>")
+            .unwrap_err();
 
         assert_eq!(
             err.to_string(),
@@ -232,7 +235,10 @@ mod tests {
     fn invalid_ed25519_key() {
         let key_type = KeyType::Ed25519Keypair;
         let key = UnparsedOpenSshKey::new(OPENSSH_ED25519_BAD.into(), PathBuf::from("/test/path"));
-        let err = key_type.parse_ssh_format_erased(key).unwrap_err();
+        let err = key_type
+            .parse_ssh_format_erased(key)
+            .map(|_| "<type erased key>")
+            .unwrap_err();
 
         assert_eq!(
             err.to_string(),

crates/tor-keymgr/src/keystore.rs

@@ -8,10 +8,10 @@ use tor_llcrypto::pk::{curve25519, ed25519};
 use crate::key_type::KeyType;
 use crate::{KeySpecifier, Result};
 
-use std::any::Any;
+use downcast_rs::{impl_downcast, Downcast};
 
 /// A type-erased key returned by a [`Keystore`].
-pub type ErasedKey = Box<dyn Any>;
+pub type ErasedKey = Box<dyn EncodableKey>;
 
 /// A generic key store.
 //
@@ -63,13 +63,15 @@ pub trait Keystore: Send + Sync + 'static {
 
 /// A key that can be serialized to, and deserialized from, a format used by a
 /// [`Keystore`](crate::Keystore).
-pub trait EncodableKey {
+pub trait EncodableKey: Downcast {
     /// The type of the key.
     fn key_type() -> KeyType
     where
         Self: Sized;
 }
 
+impl_downcast!(EncodableKey);
+
 impl EncodableKey for curve25519::StaticSecret {
     fn key_type() -> KeyType
     where

crates/tor-keymgr/src/keystore/arti.rs

@@ -299,12 +299,13 @@ mod tests {
         let erased_kp = KeyType::Ed25519Keypair
             .parse_ssh_format_erased(key)
             .unwrap();
+
+        let Ok(key) = erased_kp.downcast::<ed25519::Keypair>() else {
+            panic!("failed to downcast key to ed25519::Keypair")
+        };
+
         key_store
-            .insert(
-                &*erased_kp.downcast::<ed25519::Keypair>().unwrap(),
-                &TestSpecifier,
-                KeyType::Ed25519Keypair,
-            )
+            .insert(&*key, &TestSpecifier, KeyType::Ed25519Keypair)
             .unwrap();
 
         // Found!