diff --git a/crates/ruma-client-api/src/r0/membership.rs b/crates/ruma-client-api/src/r0/membership.rs
index 8e2ddd66..b1c94de5 100644
--- a/crates/ruma-client-api/src/r0/membership.rs
+++ b/crates/ruma-client-api/src/r0/membership.rs
@@ -34,7 +34,7 @@ pub struct ThirdPartySigned<'a> {
     pub token: &'a str,
 
     /// A signatures object containing a signature of the entire signed object.
-    pub signatures: BTreeMap<Box<ServerName>, BTreeMap<ServerSigningKeyId, String>>,
+    pub signatures: BTreeMap<Box<ServerName>, BTreeMap<Box<ServerSigningKeyId>, String>>,
 }
 
 impl<'a> ThirdPartySigned<'a> {
@@ -44,7 +44,7 @@ impl<'a> ThirdPartySigned<'a> {
         sender: &'a UserId,
         mxid: &'a UserId,
         token: &'a str,
-        signatures: BTreeMap<Box<ServerName>, BTreeMap<ServerSigningKeyId, String>>,
+        signatures: BTreeMap<Box<ServerName>, BTreeMap<Box<ServerSigningKeyId>, String>>,
     ) -> Self {
         Self { sender, mxid, token, signatures }
     }
diff --git a/crates/ruma-events/src/pdu.rs b/crates/ruma-events/src/pdu.rs
index 7b8d335f..b9bc739b 100644
--- a/crates/ruma-events/src/pdu.rs
+++ b/crates/ruma-events/src/pdu.rs
@@ -89,7 +89,7 @@ pub struct RoomV1Pdu {
     pub hashes: EventHash,
 
     /// Signatures for the PDU.
-    pub signatures: BTreeMap<Box<ServerName>, BTreeMap<ServerSigningKeyId, String>>,
+    pub signatures: BTreeMap<Box<ServerName>, BTreeMap<Box<ServerSigningKeyId>, String>>,
 }
 
 /// A 'persistent data unit' (event) for room versions 3 and beyond.
@@ -146,7 +146,7 @@ pub struct RoomV3Pdu {
     pub hashes: EventHash,
 
     /// Signatures for the PDU.
-    pub signatures: BTreeMap<Box<ServerName>, BTreeMap<ServerSigningKeyId, String>>,
+    pub signatures: BTreeMap<Box<ServerName>, BTreeMap<Box<ServerSigningKeyId>, String>>,
 }
 
 /// Content hashes of a PDU.
diff --git a/crates/ruma-events/src/room/member.rs b/crates/ruma-events/src/room/member.rs
index 059f605f..bbc37290 100644
--- a/crates/ruma-events/src/room/member.rs
+++ b/crates/ruma-events/src/room/member.rs
@@ -178,7 +178,7 @@ pub struct SignedContent {
 
     /// A single signature from the verifying server, in the format specified by the Signing Events
     /// section of the server-server API.
-    pub signatures: BTreeMap<Box<ServerName>, BTreeMap<ServerSigningKeyId, String>>,
+    pub signatures: BTreeMap<Box<ServerName>, BTreeMap<Box<ServerSigningKeyId>, String>>,
 
     /// The token property of the containing `third_party_invite` object.
     pub token: String,
@@ -188,7 +188,7 @@ impl SignedContent {
     /// Creates a new `SignedContent` with the given mxid, signature and token.
     pub fn new(
         mxid: Box<UserId>,
-        signatures: BTreeMap<Box<ServerName>, BTreeMap<ServerSigningKeyId, String>>,
+        signatures: BTreeMap<Box<ServerName>, BTreeMap<Box<ServerSigningKeyId>, String>>,
         token: String,
     ) -> Self {
         Self { mxid, signatures, token }
@@ -495,7 +495,7 @@ mod tests {
                 && mxid == "@alice:example.org"
                 && signatures == btreemap! {
                     server_name!("magic.forest") => btreemap! {
-                        server_signing_key_id!("ed25519:3") => "foobar".to_owned()
+                        server_signing_key_id!("ed25519:3").to_owned() => "foobar".to_owned()
                     }
                 }
                 && token == "abc123"
@@ -580,7 +580,7 @@ mod tests {
                 && mxid == "@alice:example.org"
                 && signatures == btreemap! {
                     server_name!("magic.forest") => btreemap! {
-                        server_signing_key_id!("ed25519:3") => "foobar".to_owned()
+                        server_signing_key_id!("ed25519:3").to_owned() => "foobar".to_owned()
                     }
                 }
                 && token == "abc123"
@@ -654,7 +654,7 @@ mod tests {
                 && mxid == "@alice:example.org"
                 && signatures == btreemap! {
                     server_name!("magic.forest") => btreemap! {
-                        server_signing_key_id!("ed25519:3") => "foobar".to_owned()
+                        server_signing_key_id!("ed25519:3").to_owned() => "foobar".to_owned()
                     }
                 }
                 && token == "abc123"
diff --git a/crates/ruma-events/tests/pdu.rs b/crates/ruma-events/tests/pdu.rs
index 58b460ae..5edd9a86 100644
--- a/crates/ruma-events/tests/pdu.rs
+++ b/crates/ruma-events/tests/pdu.rs
@@ -18,7 +18,7 @@ fn serialize_pdu_as_v1() {
     let mut signatures = BTreeMap::new();
     let mut inner_signature = BTreeMap::new();
     inner_signature.insert(
-        server_signing_key_id!("ed25519:key_version"),
+        server_signing_key_id!("ed25519:key_version").to_owned(),
         "86BytesOfSignatureOfTheRedactedEvent".into(),
     );
     signatures.insert(server_name!("example.com"), inner_signature);
@@ -85,7 +85,7 @@ fn serialize_pdu_as_v3() {
     let mut signatures = BTreeMap::new();
     let mut inner_signature = BTreeMap::new();
     inner_signature.insert(
-        server_signing_key_id!("ed25519:key_version"),
+        server_signing_key_id!("ed25519:key_version").to_owned(),
         "86BytesOfSignatureOfTheRedactedEvent".into(),
     );
     signatures.insert(server_name!("example.com"), inner_signature);
diff --git a/crates/ruma-federation-api/src/discovery.rs b/crates/ruma-federation-api/src/discovery.rs
index b4558ac6..d0e1cdd3 100644
--- a/crates/ruma-federation-api/src/discovery.rs
+++ b/crates/ruma-federation-api/src/discovery.rs
@@ -54,15 +54,15 @@ pub struct ServerSigningKeys {
     pub server_name: Box<ServerName>,
 
     /// Public keys of the homeserver for verifying digital signatures.
-    pub verify_keys: BTreeMap<ServerSigningKeyId, VerifyKey>,
+    pub verify_keys: BTreeMap<Box<ServerSigningKeyId>, VerifyKey>,
 
     /// Public keys that the homeserver used to use and when it stopped using them.
-    pub old_verify_keys: BTreeMap<ServerSigningKeyId, OldVerifyKey>,
+    pub old_verify_keys: BTreeMap<Box<ServerSigningKeyId>, OldVerifyKey>,
 
     /// Digital signatures of this object signed using the verify_keys.
     ///
     /// Map of server name to keys by key ID.
-    pub signatures: BTreeMap<Box<ServerName>, BTreeMap<ServerSigningKeyId, String>>,
+    pub signatures: BTreeMap<Box<ServerName>, BTreeMap<Box<ServerSigningKeyId>, String>>,
 
     /// Timestamp when the keys should be refreshed.
     ///
diff --git a/crates/ruma-federation-api/src/discovery/get_remote_server_keys_batch/v2.rs b/crates/ruma-federation-api/src/discovery/get_remote_server_keys_batch/v2.rs
index d64f221d..70846454 100644
--- a/crates/ruma-federation-api/src/discovery/get_remote_server_keys_batch/v2.rs
+++ b/crates/ruma-federation-api/src/discovery/get_remote_server_keys_batch/v2.rs
@@ -28,7 +28,7 @@ ruma_api! {
         /// notary server must return an empty server_keys array in the response.
         ///
         /// The notary server may return multiple keys regardless of the Key IDs given.
-        pub server_keys: BTreeMap<Box<ServerName>, BTreeMap<ServerSigningKeyId, QueryCriteria>>,
+        pub server_keys: BTreeMap<Box<ServerName>, BTreeMap<Box<ServerSigningKeyId>, QueryCriteria>>,
 
     }
 
@@ -41,7 +41,7 @@ ruma_api! {
 impl Request {
     /// Creates a new `Request` with the given query criteria.
     pub fn new(
-        server_keys: BTreeMap<Box<ServerName>, BTreeMap<ServerSigningKeyId, QueryCriteria>>,
+        server_keys: BTreeMap<Box<ServerName>, BTreeMap<Box<ServerSigningKeyId>, QueryCriteria>>,
     ) -> Self {
         Self { server_keys }
     }
diff --git a/crates/ruma-federation-api/src/thirdparty/bind_callback/v1.rs b/crates/ruma-federation-api/src/thirdparty/bind_callback/v1.rs
index 26201a06..200e7024 100644
--- a/crates/ruma-federation-api/src/thirdparty/bind_callback/v1.rs
+++ b/crates/ruma-federation-api/src/thirdparty/bind_callback/v1.rs
@@ -77,7 +77,7 @@ pub struct ThirdPartyInvite {
     pub sender: Box<UserId>,
 
     /// Signature from the identity server using a long-term private key.
-    pub signed: BTreeMap<Box<ServerName>, BTreeMap<ServerSigningKeyId, String>>,
+    pub signed: BTreeMap<Box<ServerName>, BTreeMap<Box<ServerSigningKeyId>, String>>,
 }
 
 impl ThirdPartyInvite {
@@ -87,7 +87,7 @@ impl ThirdPartyInvite {
         mxid: Box<UserId>,
         room_id: Box<RoomId>,
         sender: Box<UserId>,
-        signed: BTreeMap<Box<ServerName>, BTreeMap<ServerSigningKeyId, String>>,
+        signed: BTreeMap<Box<ServerName>, BTreeMap<Box<ServerSigningKeyId>, String>>,
     ) -> Self {
         Self { medium: Medium::Email, address, mxid, room_id, sender, signed }
     }
diff --git a/crates/ruma-identifiers-macros/src/lib.rs b/crates/ruma-identifiers-macros/src/lib.rs
index a7dacb5c..ea071905 100644
--- a/crates/ruma-identifiers-macros/src/lib.rs
+++ b/crates/ruma-identifiers-macros/src/lib.rs
@@ -91,7 +91,7 @@ pub fn server_signing_key_id(input: TokenStream) -> TokenStream {
     assert!(key_id::validate(&id.value()).is_ok(), "Invalid server_signing_key_id");
 
     let output = quote! {
-        <#dollar_crate::ServerSigningKeyId as ::std::convert::TryFrom<&str>>::try_from(#id).unwrap()
+        <&#dollar_crate::ServerSigningKeyId as ::std::convert::TryFrom<&str>>::try_from(#id).unwrap()
     };
 
     output.into()
diff --git a/crates/ruma-identifiers-validation/src/lib.rs b/crates/ruma-identifiers-validation/src/lib.rs
index 33898e0d..abec1fea 100644
--- a/crates/ruma-identifiers-validation/src/lib.rs
+++ b/crates/ruma-identifiers-validation/src/lib.rs
@@ -16,8 +16,6 @@ pub mod server_name;
 pub mod session_id;
 pub mod user_id;
 
-use std::num::NonZeroU8;
-
 pub use error::Error;
 
 /// All identifiers must be 255 bytes or less.
@@ -36,13 +34,12 @@ fn validate_id(id: &str, valid_sigils: &[char]) -> Result<(), Error> {
     Ok(())
 }
 
-/// Checks an identifier that contains a localpart and hostname for validity,
-/// and returns the index of the colon that separates the two.
-fn parse_id(id: &str, valid_sigils: &[char]) -> Result<NonZeroU8, Error> {
+/// Checks an identifier that contains a localpart and hostname for validity.
+fn parse_id(id: &str, valid_sigils: &[char]) -> Result<usize, Error> {
     validate_id(id, valid_sigils)?;
     let colon_idx = id.find(':').ok_or(Error::MissingDelimiter)?;
     server_name::validate(&id[colon_idx + 1..])?;
-    Ok(NonZeroU8::new(colon_idx as u8).unwrap())
+    Ok(colon_idx)
 }
 
 /// Checks an identifier that contains a localpart and hostname for validity.
diff --git a/crates/ruma-identifiers-validation/src/user_id.rs b/crates/ruma-identifiers-validation/src/user_id.rs
index 51e2f4e1..9880a36d 100644
--- a/crates/ruma-identifiers-validation/src/user_id.rs
+++ b/crates/ruma-identifiers-validation/src/user_id.rs
@@ -1,13 +1,11 @@
-use std::num::NonZeroU8;
-
 use crate::{parse_id, Error};
 
-pub fn validate(s: &str) -> Result<(NonZeroU8, bool), Error> {
+pub fn validate(s: &str) -> Result<(), Error> {
     let colon_idx = parse_id(s, &['@'])?;
-    let localpart = &s[1..colon_idx.get() as usize];
-    let is_historical = localpart_is_fully_conforming(localpart)?;
+    let localpart = &s[1..colon_idx];
+    let _ = localpart_is_fully_conforming(localpart)?;
 
-    Ok((colon_idx, is_historical))
+    Ok(())
 }
 
 /// Check whether the given user id localpart is valid and fully conforming
diff --git a/crates/ruma-identifiers/src/key_id.rs b/crates/ruma-identifiers/src/key_id.rs
index b7d66df0..bb5a3488 100644
--- a/crates/ruma-identifiers/src/key_id.rs
+++ b/crates/ruma-identifiers/src/key_id.rs
@@ -1,25 +1,21 @@
 use std::{
     cmp::Ordering,
-    convert::{TryFrom, TryInto},
+    convert::TryFrom,
     fmt,
     hash::{Hash, Hasher},
     marker::PhantomData,
-    num::NonZeroU8,
     str::FromStr,
 };
 
 use crate::{crypto_algorithms::SigningKeyAlgorithm, DeviceId, KeyName};
 
-/// A key algorithm and key name delimited by a colon
-pub struct KeyId<A, K: ?Sized> {
-    full_id: Box<str>,
-    colon_idx: NonZeroU8,
-    _phantom: PhantomData<(A, K)>,
-}
+/// A key algorithm and key name delimited by a colon.
+#[repr(transparent)]
+pub struct KeyId<A, K: ?Sized>(PhantomData<(A, K)>, str);
 
 impl<A, K: ?Sized> KeyId<A, K> {
     /// Creates a new `KeyId` from an algorithm and key name.
-    pub fn from_parts(algorithm: A, key_name: &K) -> Self
+    pub fn from_parts(algorithm: A, key_name: &K) -> Box<Self>
     where
         A: AsRef<str>,
         K: AsRef<str>,
@@ -32,11 +28,7 @@ impl<A, K: ?Sized> KeyId<A, K> {
         res.push(':');
         res.push_str(key_name);
 
-        let colon_idx =
-            NonZeroU8::new(algorithm.len().try_into().expect("no algorithm name len > 255"))
-                .expect("no empty algorithm name");
-
-        KeyId { full_id: res.into(), colon_idx, _phantom: PhantomData }
+        Self::from_owned(res.into())
     }
 
     /// Returns key algorithm of the key ID.
@@ -44,8 +36,7 @@ impl<A, K: ?Sized> KeyId<A, K> {
     where
         A: FromStr,
     {
-        A::from_str(&self.full_id[..self.colon_idx.get() as usize])
-            .unwrap_or_else(|_| unreachable!())
+        A::from_str(&self.as_str()[..self.colon_idx()]).unwrap_or_else(|_| unreachable!())
     }
 
     /// Returns the key name of the key ID.
@@ -53,33 +44,105 @@ impl<A, K: ?Sized> KeyId<A, K> {
     where
         &'a K: From<&'a str>,
     {
-        self.full_id[self.colon_idx.get() as usize + 1..].into()
+        self.as_str()[self.colon_idx() + 1..].into()
     }
-}
 
-fn try_from<S, A, K: ?Sized>(key_identifier: S) -> Result<KeyId<A, K>, crate::Error>
-where
-    S: AsRef<str> + Into<Box<str>>,
-{
-    let colon_idx = ruma_identifiers_validation::key_id::validate(key_identifier.as_ref())?;
-    Ok(KeyId { full_id: key_identifier.into(), colon_idx, _phantom: PhantomData })
-}
-
-impl<A, K: ?Sized> KeyId<A, K> {
-    /// Creates a string slice from this `KeyId<A, K>`
+    /// Creates a string slice from this `KeyId`.
     pub fn as_str(&self) -> &str {
-        &self.full_id
+        &self.1
     }
 
-    /// Creates a byte slice from this `KeyId<A, K>`
+    /// Creates a byte slice from this `KeyId`.
     pub fn as_bytes(&self) -> &[u8] {
-        self.full_id.as_bytes()
+        self.1.as_bytes()
+    }
+
+    fn from_borrowed(s: &str) -> &Self {
+        unsafe { std::mem::transmute(s) }
+    }
+
+    fn from_owned(s: Box<str>) -> Box<Self> {
+        unsafe { Box::from_raw(Box::into_raw(s) as _) }
+    }
+
+    fn into_owned(self: Box<Self>) -> Box<str> {
+        unsafe { Box::from_raw(Box::into_raw(self) as _) }
+    }
+
+    fn colon_idx(&self) -> usize {
+        self.as_str().find(':').unwrap()
     }
 }
 
-impl<A, K: ?Sized> Clone for KeyId<A, K> {
+/// Algorithm + key name for signing keys.
+pub type SigningKeyId<K> = KeyId<SigningKeyAlgorithm, K>;
+
+/// Algorithm + key name for homeserver signing keys.
+pub type ServerSigningKeyId = SigningKeyId<KeyName>;
+
+/// Algorithm + key name for device keys.
+pub type DeviceSigningKeyId = SigningKeyId<DeviceId>;
+
+impl<A, K: ?Sized> Clone for Box<KeyId<A, K>> {
     fn clone(&self) -> Self {
-        Self { full_id: self.full_id.clone(), colon_idx: self.colon_idx, _phantom: PhantomData }
+        (**self).to_owned()
+    }
+}
+
+impl<A, K: ?Sized> ToOwned for KeyId<A, K> {
+    type Owned = Box<KeyId<A, K>>;
+    fn to_owned(&self) -> Self::Owned {
+        Self::from_owned(self.1.into())
+    }
+}
+
+impl<A, K: ?Sized> From<&KeyId<A, K>> for Box<KeyId<A, K>> {
+    fn from(id: &KeyId<A, K>) -> Self {
+        id.to_owned()
+    }
+}
+
+impl<A, K: ?Sized> AsRef<str> for Box<KeyId<A, K>> {
+    fn as_ref(&self) -> &str {
+        self.as_str()
+    }
+}
+
+impl<A, K: ?Sized> From<&KeyId<A, K>> for std::rc::Rc<KeyId<A, K>> {
+    fn from(s: &KeyId<A, K>) -> std::rc::Rc<KeyId<A, K>> {
+        let rc = std::rc::Rc::<str>::from(s.as_str());
+        unsafe { std::rc::Rc::from_raw(std::rc::Rc::into_raw(rc) as *const KeyId<A, K>) }
+    }
+}
+
+impl<A, K: ?Sized> From<&KeyId<A, K>> for std::sync::Arc<KeyId<A, K>> {
+    fn from(s: &KeyId<A, K>) -> std::sync::Arc<KeyId<A, K>> {
+        let arc = std::sync::Arc::<str>::from(s.as_str());
+        unsafe { std::sync::Arc::from_raw(std::sync::Arc::into_raw(arc) as *const KeyId<A, K>) }
+    }
+}
+
+impl<A, K: ?Sized> PartialEq<KeyId<A, K>> for Box<KeyId<A, K>> {
+    fn eq(&self, other: &KeyId<A, K>) -> bool {
+        self.as_str() == other.as_str()
+    }
+}
+
+impl<A, K: ?Sized> PartialEq<&'_ KeyId<A, K>> for Box<KeyId<A, K>> {
+    fn eq(&self, other: &&KeyId<A, K>) -> bool {
+        self.as_str() == other.as_str()
+    }
+}
+
+impl<A, K: ?Sized> PartialEq<Box<KeyId<A, K>>> for KeyId<A, K> {
+    fn eq(&self, other: &Box<KeyId<A, K>>) -> bool {
+        self.as_str() == other.as_str()
+    }
+}
+
+impl<A, K: ?Sized> PartialEq<Box<KeyId<A, K>>> for &'_ KeyId<A, K> {
+    fn eq(&self, other: &Box<KeyId<A, K>>) -> bool {
+        self.as_str() == other.as_str()
     }
 }
 
@@ -89,54 +152,15 @@ impl<A, K: ?Sized> AsRef<str> for KeyId<A, K> {
     }
 }
 
-impl<A, K: ?Sized> From<KeyId<A, K>> for String {
-    fn from(id: KeyId<A, K>) -> Self {
-        id.full_id.into()
-    }
-}
-
-impl<A, K: ?Sized> FromStr for KeyId<A, K> {
-    type Err = crate::Error;
-
-    fn from_str(s: &str) -> Result<Self, Self::Err> {
-        try_from(s)
-    }
-}
-
-impl<A, K: ?Sized> TryFrom<&str> for KeyId<A, K> {
-    type Error = crate::Error;
-
-    fn try_from(s: &str) -> Result<Self, Self::Error> {
-        try_from(s)
-    }
-}
-
-impl<A, K: ?Sized> TryFrom<String> for KeyId<A, K>
-where
-    A: FromStr,
-    K: From<String>,
-{
-    type Error = crate::Error;
-
-    fn try_from(s: String) -> Result<Self, Self::Error> {
-        try_from(s)
+impl<A, K: ?Sized> fmt::Display for KeyId<A, K> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.as_str().fmt(f)
     }
 }
 
 impl<A, K: ?Sized> fmt::Debug for KeyId<A, K> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        // Using struct debug format for consistency with other ID types.
-        // FIXME: Change all ID types to have just a string debug format?
-        f.debug_struct("KeyId")
-            .field("full_id", &self.full_id)
-            .field("colon_idxs", &self.colon_idx)
-            .finish()
-    }
-}
-
-impl<A, K: ?Sized> fmt::Display for KeyId<A, K> {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(f, "{}", self.as_str())
+        self.as_str().fmt(f)
     }
 }
 
@@ -150,13 +174,13 @@ impl<A, K: ?Sized> Eq for KeyId<A, K> {}
 
 impl<A, K: ?Sized> PartialOrd for KeyId<A, K> {
     fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
-        self.as_str().partial_cmp(other.as_str())
+        PartialOrd::partial_cmp(self.as_str(), other.as_str())
     }
 }
 
 impl<A, K: ?Sized> Ord for KeyId<A, K> {
     fn cmp(&self, other: &Self) -> Ordering {
-        self.as_str().cmp(other.as_str())
+        Ord::cmp(self.as_str(), other.as_str())
     }
 }
 
@@ -165,7 +189,6 @@ impl<A, K: ?Sized> Hash for KeyId<A, K> {
         self.as_str().hash(state);
     }
 }
-
 #[cfg(feature = "serde")]
 impl<A, K: ?Sized> serde::Serialize for KeyId<A, K> {
     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
@@ -176,24 +199,63 @@ impl<A, K: ?Sized> serde::Serialize for KeyId<A, K> {
     }
 }
 
+impl<A, K: ?Sized> From<Box<KeyId<A, K>>> for String {
+    fn from(id: Box<KeyId<A, K>>) -> Self {
+        id.into_owned().into()
+    }
+}
+
 #[cfg(feature = "serde")]
-impl<'de, A, K: ?Sized> serde::Deserialize<'de> for KeyId<A, K> {
+impl<'de, A, K: ?Sized> serde::Deserialize<'de> for Box<KeyId<A, K>> {
     fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
     where
         D: serde::Deserializer<'de>,
     {
-        crate::deserialize_id(deserializer, "Key name with algorithm and key identifier")
+        use serde::de::Error;
+        let s = String::deserialize(deserializer)?;
+        match try_from(s) {
+            Ok(o) => Ok(o),
+            Err(e) => Err(D::Error::custom(e)),
+        }
+    }
+}
+
+fn try_from<S, A, K: ?Sized>(s: S) -> Result<Box<KeyId<A, K>>, crate::Error>
+where
+    S: AsRef<str> + Into<Box<str>>,
+{
+    ruma_identifiers_validation::key_id::validate(s.as_ref())?;
+    Ok(KeyId::from_owned(s.into()))
+}
+
+impl<'a, A, K: ?Sized> TryFrom<&'a str> for &'a KeyId<A, K> {
+    type Error = crate::Error;
+    fn try_from(s: &'a str) -> Result<Self, Self::Error> {
+        (ruma_identifiers_validation::key_id::validate)(s)?;
+        Ok(KeyId::from_borrowed(s))
+    }
+}
+
+impl<A, K: ?Sized> FromStr for Box<KeyId<A, K>> {
+    type Err = crate::Error;
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        try_from(s)
+    }
+}
+
+impl<A, K: ?Sized> TryFrom<&str> for Box<KeyId<A, K>> {
+    type Error = crate::Error;
+    fn try_from(s: &str) -> Result<Self, Self::Error> {
+        try_from(s)
+    }
+}
+
+impl<A, K: ?Sized> TryFrom<String> for Box<KeyId<A, K>> {
+    type Error = crate::Error;
+    fn try_from(s: String) -> Result<Self, Self::Error> {
+        try_from(s)
     }
 }
 
 #[rustfmt::skip]
 partial_eq_string!(KeyId<A, K> [A, K]);
-
-/// Algorithm + key name for signing keys.
-pub type SigningKeyId<K> = KeyId<SigningKeyAlgorithm, K>;
-
-/// Algorithm + key name for homeserver signing keys.
-pub type ServerSigningKeyId = SigningKeyId<KeyName>;
-
-/// Algorithm + key name for device keys.
-pub type DeviceSigningKeyId = SigningKeyId<DeviceId>;
diff --git a/crates/ruma-identifiers/src/signatures.rs b/crates/ruma-identifiers/src/signatures.rs
index 85e31f37..b88f7dc3 100644
--- a/crates/ruma-identifiers/src/signatures.rs
+++ b/crates/ruma-identifiers/src/signatures.rs
@@ -3,7 +3,7 @@ use std::{borrow::Borrow, collections::BTreeMap};
 use crate::{DeviceId, KeyName, ServerName, SigningKeyId, UserId};
 
 /// Map of key identifier to signature values.
-pub type EntitySignatures<K> = BTreeMap<SigningKeyId<K>, String>;
+pub type EntitySignatures<K> = BTreeMap<Box<SigningKeyId<K>>, String>;
 
 /// Map of all signatures, grouped by entity
 ///
@@ -35,7 +35,7 @@ impl<E: Ord, K: ?Sized> Signatures<E, K> {
     pub fn insert(
         &mut self,
         entity: E,
-        key_identifier: SigningKeyId<K>,
+        key_identifier: Box<SigningKeyId<K>>,
         value: String,
     ) -> Option<String> {
         self.0.entry(entity).or_insert_with(Default::default).insert(key_identifier, value)
diff --git a/crates/ruma-identity-service-api/src/keys/get_public_key/v2.rs b/crates/ruma-identity-service-api/src/keys/get_public_key/v2.rs
index 9f8036a8..51f0d05c 100644
--- a/crates/ruma-identity-service-api/src/keys/get_public_key/v2.rs
+++ b/crates/ruma-identity-service-api/src/keys/get_public_key/v2.rs
@@ -16,7 +16,7 @@ ruma_api! {
     request: {
         /// The ID of the key.
         #[ruma_api(path)]
-        pub key_id: ServerSigningKeyId,
+        pub key_id: &'a ServerSigningKeyId,
     }
 
     response: {
@@ -25,9 +25,9 @@ ruma_api! {
     }
 }
 
-impl Request {
+impl<'a> Request<'a> {
     /// Create a `Request` with the given key_id.
-    pub fn new(key_id: ServerSigningKeyId) -> Self {
+    pub fn new(key_id: &'a ServerSigningKeyId) -> Self {
         Self { key_id }
     }
 }
diff --git a/crates/ruma-serde-macros/src/outgoing.rs b/crates/ruma-serde-macros/src/outgoing.rs
index d44fca39..30db0300 100644
--- a/crates/ruma-serde-macros/src/outgoing.rs
+++ b/crates/ruma-serde-macros/src/outgoing.rs
@@ -262,7 +262,9 @@ fn strip_lifetimes(field_type: &mut Type) -> bool {
                     } else if last_seg.ident == "ClientSecret"
                         || last_seg.ident == "DeviceId"
                         || last_seg.ident == "DeviceKeyId"
+                        || last_seg.ident == "DeviceSigningKeyId"
                         || last_seg.ident == "EventId"
+                        || last_seg.ident == "KeyId"
                         || last_seg.ident == "MxcUri"
                         || last_seg.ident == "ServerName"
                         || last_seg.ident == "SessionId"
@@ -271,6 +273,8 @@ fn strip_lifetimes(field_type: &mut Type) -> bool {
                         || last_seg.ident == "RoomId"
                         || last_seg.ident == "RoomIdOrAliasId"
                         || last_seg.ident == "RoomName"
+                        || last_seg.ident == "ServerSigningKeyId"
+                        || last_seg.ident == "SigningKeyId"
                         || last_seg.ident == "UserId"
                     {
                         // The identifiers that need to be boxed `Box<T>` since they are DST's.