diff --git a/matrix_sdk_crypto/src/machine.rs b/matrix_sdk_crypto/src/machine.rs
index fb9e87f0..c36e1ab1 100644
--- a/matrix_sdk_crypto/src/machine.rs
+++ b/matrix_sdk_crypto/src/machine.rs
@@ -552,6 +552,18 @@ impl OlmMachine {
             } else if user_id == self.user_id() {
                 if let Some(s) = response.user_signing_keys.get(user_id) {
                     let user_signing = UserSigningPubkey::from(s);
+
+                    if master_key.user_id() != user_id
+                        || self_signing.user_id() != user_id
+                        || user_signing.user_id() != user_id
+                    {
+                        warn!(
+                            "User id missmatch in one of the cross signing keys for user {}",
+                            user_id
+                        );
+                        continue;
+                    }
+
                     OwnUserIdentity::new(master_key, self_signing, user_signing)
                         .map(UserIdentities::Own)
                 } else {
@@ -563,7 +575,15 @@ impl OlmMachine {
                     continue;
                 }
             } else {
-                UserIdentity::new(master_key, self_signing).map(UserIdentities::Other)
+                if master_key.user_id() != user_id || self_signing.user_id() != user_id {
+                    warn!(
+                        "User id missmatch in one of the cross signing keys for user {}",
+                        user_id
+                    );
+                    continue;
+                } else {
+                    UserIdentity::new(master_key, self_signing).map(UserIdentities::Other)
+                }
             };
 
             match identity {
@@ -577,7 +597,7 @@ impl OlmMachine {
                 }
                 Err(e) => {
                     warn!(
-                        "Coulnd't update or create new user identity for {}: {:?}",
+                        "Couldn't update or create new user identity for {}: {:?}",
                         user_id, e
                     );
                     continue;
diff --git a/matrix_sdk_crypto/src/user_identity.rs b/matrix_sdk_crypto/src/user_identity.rs
index 2a1206ca..b6c4159a 100644
--- a/matrix_sdk_crypto/src/user_identity.rs
+++ b/matrix_sdk_crypto/src/user_identity.rs
@@ -44,7 +44,6 @@ pub struct UserSigningPubkey(Arc<CrossSigningKey>);
 impl PartialEq for MasterPubkey {
     fn eq(&self, other: &MasterPubkey) -> bool {
         self.0.user_id == other.0.user_id && self.0.keys == other.0.keys
-        // TODO check the usage once `KeyUsage` gets PartialEq.
     }
 }
 
@@ -105,6 +104,11 @@ impl<'a> CrossSigningSubKeys<'a> {
 }
 
 impl MasterPubkey {
+    /// Get the user id of the master key's owner.
+    pub fn user_id(&self) -> &UserId {
+        &self.0.user_id
+    }
+
     /// Get the master key with the given key id.
     ///
     /// # Arguments
@@ -133,6 +137,8 @@ impl MasterPubkey {
             .next()
             .ok_or(SignatureError::MissingSigningKey)?;
 
+        let key_id = DeviceKeyId::try_from(key_id.as_str())?;
+
         // FIXME `KeyUsage is missing PartialEq.
         // if self.0.usage.contains(&KeyUsage::Master) {
         //     return Err(SignatureError::MissingSigningKey);
@@ -145,7 +151,7 @@ impl MasterPubkey {
 
         verify_json(
             &self.0.user_id,
-            &DeviceKeyId::try_from(key_id.as_str())?,
+            &key_id,
             key,
             &mut to_value(subkey.cross_signing_key()).map_err(|_| SignatureError::NotAnObject)?,
         )
@@ -153,6 +159,11 @@ impl MasterPubkey {
 }
 
 impl UserSigningPubkey {
+    /// Get the user id of the user signing key's owner.
+    pub fn user_id(&self) -> &UserId {
+        &self.0.user_id
+    }
+
     /// Check if the given master key is signed by this user signing key.
     ///
     /// # Arguments
@@ -182,6 +193,11 @@ impl UserSigningPubkey {
 }
 
 impl SelfSigningPubkey {
+    /// Get the user id of the self signing key's owner.
+    pub fn user_id(&self) -> &UserId {
+        &self.0.user_id
+    }
+
     /// Check if the given device is signed by this self signing key.
     ///
     /// # Arguments