diff --git a/README.md b/README.md
index cb425c7..020d4e5 100644
--- a/README.md
+++ b/README.md
@@ -43,9 +43,10 @@ Hagrid has it's own URL scheme to fetch keys, verify user IDs and delete keys.
 It's meant to be machine readable, but it's not a REST API. The following URLs
 are handled.
 
-- `GET /by-fingerprint/<fingerprint>` retrieves the key with the given
-  fingerprint.
-- `GET /by-keyid/<key ID>` retrieves the key with the given long key ID.
+- `GET /by-fingerprint/<FINGERPRINT>` retrieves the key with the given
+  fingerprint.  Hexadecimal digits must be uppercase.
+- `GET /by-keyid/<KEY-ID>` retrieves the key with the given long key
+  ID.  Hexadecimal digits must be uppercase.
 - `GET /by-email/<URL-encoded user ID>` retrieves the key with the given user
   ID. Only exact matches are accepted.
 - `GET /vks/verify/<token>` verifies a user ID using a token string send by
diff --git a/examples/import.rs b/examples/import.rs
index 67c2e79..f53d001 100644
--- a/examples/import.rs
+++ b/examples/import.rs
@@ -160,14 +160,14 @@ impl Filesystem {
 
     /// Returns the path to the given KeyID.
     fn path_to_keyid(&self, keyid: &openpgp::KeyID) -> PathBuf {
-        let hex = keyid.to_hex().to_lowercase();
+        let hex = keyid.to_hex();
         self.base_by_keyid.join(&hex[..2]).join(&hex[2..])
     }
 
     /// Returns the path to the given Fingerprint.
     fn path_to_fingerprint(&self, fingerprint: &openpgp::Fingerprint)
                            -> PathBuf {
-        let hex = fingerprint.to_hex().to_lowercase();
+        let hex = fingerprint.to_hex();
         self.base_by_fingerprint.join(&hex[..2]).join(&hex[2..])
     }
 
diff --git a/src/types.rs b/src/types.rs
index f7c68b2..07f29f5 100644
--- a/src/types.rs
+++ b/src/types.rs
@@ -56,7 +56,7 @@ impl TryFrom<sequoia_openpgp::Fingerprint> for Fingerprint {
 
 impl ToString for Fingerprint {
     fn to_string(&self) -> String {
-        hex::encode(&self.0[..])
+        hex::encode_upper(&self.0[..])
     }
 }
 
@@ -133,7 +133,7 @@ impl From<Fingerprint> for KeyID {
 
 impl ToString for KeyID {
     fn to_string(&self) -> String {
-        hex::encode(&self.0[..])
+        hex::encode_upper(&self.0[..])
     }
 }