diff --git a/services/identity/src/client_service.rs b/services/identity/src/client_service.rs
index d84981381..1a9010430 100644
--- a/services/identity/src/client_service.rs
+++ b/services/identity/src/client_service.rs
@@ -1,644 +1,671 @@
 pub mod client_proto {
   tonic::include_proto!("identity.client");
 }
 
 use std::str::FromStr;
 
 use crate::{
   client_service::client_proto::{
     DeleteUserRequest, Empty, GenerateNonceResponse, KeyserverKeysRequest,
     KeyserverKeysResponse, OpaqueLoginFinishRequest, OpaqueLoginFinishResponse,
     OpaqueLoginStartRequest, OpaqueLoginStartResponse,
     ReceiverKeysForUserRequest, ReceiverKeysForUserResponse,
     RefreshUserPreKeysRequest, RegistrationFinishRequest,
     RegistrationFinishResponse, RegistrationStartRequest,
     RegistrationStartResponse, SenderKeysForUserRequest,
     SenderKeysForUserResponse, UpdateUserPasswordFinishRequest,
     UpdateUserPasswordStartRequest, UpdateUserPasswordStartResponse,
     UploadOneTimeKeysRequest, VerifyUserAccessTokenRequest,
     VerifyUserAccessTokenResponse, WalletLoginRequest, WalletLoginResponse,
   },
   config::CONFIG,
   database::{DatabaseClient, Error as DBError, KeyPayload},
   id::generate_uuid,
   nonce::generate_nonce_data,
   siwe::parse_and_verify_siwe_message,
   token::{AccessTokenData, AuthType},
 };
 use aws_sdk_dynamodb::Error as DynamoDBError;
 pub use client_proto::identity_client_service_server::{
   IdentityClientService, IdentityClientServiceServer,
 };
 use comm_opaque2::grpc::protocol_error_to_grpc_status;
+use constant_time_eq::constant_time_eq;
 use moka::future::Cache;
 use rand::rngs::OsRng;
 use tonic::Response;
 use tracing::error;
 
 #[derive(Clone)]
 pub enum WorkflowInProgress {
   Registration(UserRegistrationInfo),
   Login(UserLoginInfo),
   Update(UpdateState),
 }
 
 #[derive(Clone)]
 pub struct UserRegistrationInfo {
   pub username: String,
   pub flattened_device_key_upload: FlattenedDeviceKeyUpload,
 }
 
 #[derive(Clone)]
 pub struct UserLoginInfo {
   pub user_id: String,
   pub flattened_device_key_upload: FlattenedDeviceKeyUpload,
   pub opaque_server_login: comm_opaque2::server::Login,
 }
 
 #[derive(Clone)]
 pub struct UpdateState {
   pub user_id: String,
 }
 
 #[derive(Clone)]
 pub struct FlattenedDeviceKeyUpload {
   pub device_id_key: String,
   pub key_payload: String,
   pub key_payload_signature: String,
   pub identity_prekey: String,
   pub identity_prekey_signature: String,
   pub identity_onetime_keys: Vec<String>,
   pub notif_prekey: String,
   pub notif_prekey_signature: String,
   pub notif_onetime_keys: Vec<String>,
 }
 
 #[derive(derive_more::Constructor)]
 pub struct ClientService {
   client: DatabaseClient,
   cache: Cache<String, WorkflowInProgress>,
 }
 
 #[tonic::async_trait]
 impl IdentityClientService for ClientService {
   async fn register_password_user_start(
     &self,
     request: tonic::Request<RegistrationStartRequest>,
   ) -> Result<tonic::Response<RegistrationStartResponse>, tonic::Status> {
     let message = request.into_inner();
     let username_taken = self
       .client
       .username_taken(message.username.clone())
       .await
       .map_err(handle_db_error)?;
 
     if username_taken {
       return Err(tonic::Status::already_exists("username already exists"));
     }
 
     if CONFIG.reserved_usernames.contains(&message.username) {
       return Err(tonic::Status::invalid_argument("username reserved"));
     }
 
     if let client_proto::RegistrationStartRequest {
       opaque_registration_request: register_message,
       username,
       device_key_upload:
         Some(client_proto::DeviceKeyUpload {
           device_key_info:
             Some(client_proto::IdentityKeyInfo {
               payload,
               payload_signature,
               social_proof: _social_proof,
             }),
           identity_upload:
             Some(client_proto::PreKey {
               pre_key: identity_prekey,
               pre_key_signature: identity_prekey_signature,
             }),
           notif_upload:
             Some(client_proto::PreKey {
               pre_key: notif_prekey,
               pre_key_signature: notif_prekey_signature,
             }),
           onetime_identity_prekeys,
           onetime_notif_prekeys,
         }),
     } = message
     {
       let server_registration = comm_opaque2::server::Registration::new();
       let server_message = server_registration
         .start(&CONFIG.server_setup, &register_message, username.as_bytes())
         .map_err(protocol_error_to_grpc_status)?;
       let key_info = KeyPayload::from_str(&payload)
         .map_err(|_| tonic::Status::invalid_argument("malformed payload"))?;
       let registration_state = UserRegistrationInfo {
         username,
         flattened_device_key_upload: FlattenedDeviceKeyUpload {
           device_id_key: key_info.primary_identity_public_keys.curve25519,
           key_payload: payload,
           key_payload_signature: payload_signature,
           identity_prekey,
           identity_prekey_signature,
           identity_onetime_keys: onetime_identity_prekeys,
           notif_prekey,
           notif_prekey_signature,
           notif_onetime_keys: onetime_notif_prekeys,
         },
       };
       let session_id = generate_uuid();
       self
         .cache
         .insert(
           session_id.clone(),
           WorkflowInProgress::Registration(registration_state),
         )
         .await;
 
       let response = RegistrationStartResponse {
         session_id,
         opaque_registration_response: server_message,
       };
       Ok(Response::new(response))
     } else {
       Err(tonic::Status::invalid_argument("unexpected message data"))
     }
   }
 
   async fn register_password_user_finish(
     &self,
     request: tonic::Request<RegistrationFinishRequest>,
   ) -> Result<tonic::Response<RegistrationFinishResponse>, tonic::Status> {
     let message = request.into_inner();
 
     if let Some(WorkflowInProgress::Registration(state)) =
       self.cache.get(&message.session_id)
     {
       self.cache.invalidate(&message.session_id).await;
 
       let server_registration = comm_opaque2::server::Registration::new();
       let password_file = server_registration
         .finish(&message.opaque_registration_upload)
         .map_err(protocol_error_to_grpc_status)?;
 
       let device_id = state.flattened_device_key_upload.device_id_key.clone();
       let user_id = self
         .client
         .add_password_user_to_users_table(state, password_file)
         .await
         .map_err(handle_db_error)?;
 
       // Create access token
       let token = AccessTokenData::new(
         user_id.clone(),
         device_id,
         crate::token::AuthType::Password,
         &mut OsRng,
       );
 
       let access_token = token.access_token.clone();
 
       self
         .client
         .put_access_token_data(token)
         .await
         .map_err(handle_db_error)?;
 
       let response = RegistrationFinishResponse {
         user_id,
         access_token,
       };
       Ok(Response::new(response))
     } else {
       Err(tonic::Status::not_found("session not found"))
     }
   }
 
   async fn update_user_password_start(
     &self,
     request: tonic::Request<UpdateUserPasswordStartRequest>,
   ) -> Result<tonic::Response<UpdateUserPasswordStartResponse>, tonic::Status>
   {
     let message = request.into_inner();
 
     let access_token = self
       .client
       .get_access_token_data(message.user_id.clone(), message.device_id_key)
       .await
       .map_err(handle_db_error)?;
 
     if let Some(token) = access_token {
-      if !token.is_valid() || token.access_token != message.access_token {
+      if !token.is_valid()
+        || !constant_time_eq(
+          token.access_token.as_bytes(),
+          message.access_token.as_bytes(),
+        )
+      {
         return Err(tonic::Status::permission_denied("bad token"));
       }
 
       let server_registration = comm_opaque2::server::Registration::new();
       let server_message = server_registration
         .start(
           &CONFIG.server_setup,
           &message.opaque_registration_request,
           message.user_id.as_bytes(),
         )
         .map_err(protocol_error_to_grpc_status)?;
 
       let update_state = UpdateState {
         user_id: message.user_id,
       };
       let session_id = generate_uuid();
       self
         .cache
         .insert(session_id.clone(), WorkflowInProgress::Update(update_state))
         .await;
 
       let response = UpdateUserPasswordStartResponse {
         session_id,
         opaque_registration_response: server_message,
       };
       Ok(Response::new(response))
     } else {
       Err(tonic::Status::permission_denied("bad token"))
     }
   }
 
   async fn update_user_password_finish(
     &self,
     request: tonic::Request<UpdateUserPasswordFinishRequest>,
   ) -> Result<tonic::Response<Empty>, tonic::Status> {
     let message = request.into_inner();
 
     if let Some(WorkflowInProgress::Update(state)) =
       self.cache.get(&message.session_id)
     {
       self.cache.invalidate(&message.session_id).await;
 
       let server_registration = comm_opaque2::server::Registration::new();
       let password_file = server_registration
         .finish(&message.opaque_registration_upload)
         .map_err(protocol_error_to_grpc_status)?;
 
       self
         .client
         .update_user_password(state.user_id, password_file)
         .await
         .map_err(handle_db_error)?;
 
       let response = Empty {};
       Ok(Response::new(response))
     } else {
       Err(tonic::Status::not_found("session not found"))
     }
   }
 
   async fn login_password_user_start(
     &self,
     request: tonic::Request<OpaqueLoginStartRequest>,
   ) -> Result<tonic::Response<OpaqueLoginStartResponse>, tonic::Status> {
     let message = request.into_inner();
 
     let (user_id, password_file_bytes) = self
       .client
       .get_user_id_and_password_file_from_username(&message.username)
       .await
       .map_err(handle_db_error)?
       .ok_or(tonic::Status::not_found("user not found"))?;
 
     if let client_proto::OpaqueLoginStartRequest {
       opaque_login_request: login_message,
       username,
       device_key_upload:
         Some(client_proto::DeviceKeyUpload {
           device_key_info:
             Some(client_proto::IdentityKeyInfo {
               payload,
               payload_signature,
               social_proof: _social_proof,
             }),
           identity_upload:
             Some(client_proto::PreKey {
               pre_key: identity_prekey,
               pre_key_signature: identity_prekey_signature,
             }),
           notif_upload:
             Some(client_proto::PreKey {
               pre_key: notif_prekey,
               pre_key_signature: notif_prekey_signature,
             }),
           onetime_identity_prekeys,
           onetime_notif_prekeys,
         }),
     } = message
     {
       let mut server_login = comm_opaque2::server::Login::new();
       let server_response = server_login
         .start(
           &CONFIG.server_setup,
           &password_file_bytes,
           &login_message,
           username.as_bytes(),
         )
         .map_err(protocol_error_to_grpc_status)?;
 
       let key_info = KeyPayload::from_str(&payload)
         .map_err(|_| tonic::Status::invalid_argument("malformed payload"))?;
       let login_state = UserLoginInfo {
         user_id,
         opaque_server_login: server_login,
         flattened_device_key_upload: FlattenedDeviceKeyUpload {
           device_id_key: key_info.primary_identity_public_keys.curve25519,
           key_payload: payload,
           key_payload_signature: payload_signature,
           identity_prekey,
           identity_prekey_signature,
           identity_onetime_keys: onetime_identity_prekeys,
           notif_prekey,
           notif_prekey_signature,
           notif_onetime_keys: onetime_notif_prekeys,
         },
       };
       let session_id = generate_uuid();
       self
         .cache
         .insert(session_id.clone(), WorkflowInProgress::Login(login_state))
         .await;
 
       let response = Response::new(OpaqueLoginStartResponse {
         session_id,
         opaque_login_response: server_response,
       });
       Ok(response)
     } else {
       Err(tonic::Status::invalid_argument("unexpected message data"))
     }
   }
 
   async fn login_password_user_finish(
     &self,
     request: tonic::Request<OpaqueLoginFinishRequest>,
   ) -> Result<tonic::Response<OpaqueLoginFinishResponse>, tonic::Status> {
     let message = request.into_inner();
 
     if let Some(WorkflowInProgress::Login(state)) =
       self.cache.get(&message.session_id)
     {
       self.cache.invalidate(&message.session_id).await;
 
       let mut server_login = state.opaque_server_login.clone();
       server_login
         .finish(&message.opaque_login_upload)
         .map_err(protocol_error_to_grpc_status)?;
 
       self
         .client
         .add_password_user_device_to_users_table(
           state.user_id.clone(),
           state.flattened_device_key_upload.clone(),
         )
         .await
         .map_err(handle_db_error)?;
 
       // Create access token
       let token = AccessTokenData::new(
         state.user_id.clone(),
         state.flattened_device_key_upload.device_id_key,
         crate::token::AuthType::Password,
         &mut OsRng,
       );
 
       let access_token = token.access_token.clone();
 
       self
         .client
         .put_access_token_data(token)
         .await
         .map_err(handle_db_error)?;
 
       let response = OpaqueLoginFinishResponse {
         user_id: state.user_id,
         access_token,
       };
       Ok(Response::new(response))
     } else {
       Err(tonic::Status::not_found("session not found"))
     }
   }
 
   async fn login_wallet_user(
     &self,
     request: tonic::Request<WalletLoginRequest>,
   ) -> Result<tonic::Response<WalletLoginResponse>, tonic::Status> {
     let message = request.into_inner();
 
     let wallet_address = parse_and_verify_siwe_message(
       &message.siwe_message,
       &message.siwe_signature,
     )?;
 
     let (flattened_device_key_upload, social_proof) =
       if let client_proto::WalletLoginRequest {
         siwe_message: _,
         siwe_signature: _,
         device_key_upload:
           Some(client_proto::DeviceKeyUpload {
             device_key_info:
               Some(client_proto::IdentityKeyInfo {
                 payload,
                 payload_signature,
                 social_proof: Some(social_proof),
               }),
             identity_upload:
               Some(client_proto::PreKey {
                 pre_key: identity_prekey,
                 pre_key_signature: identity_prekey_signature,
               }),
             notif_upload:
               Some(client_proto::PreKey {
                 pre_key: notif_prekey,
                 pre_key_signature: notif_prekey_signature,
               }),
             onetime_identity_prekeys,
             onetime_notif_prekeys,
           }),
       } = message
       {
         let key_info = KeyPayload::from_str(&payload)
           .map_err(|_| tonic::Status::invalid_argument("malformed payload"))?;
         (
           FlattenedDeviceKeyUpload {
             device_id_key: key_info.primary_identity_public_keys.curve25519,
             key_payload: payload,
             key_payload_signature: payload_signature,
             identity_prekey,
             identity_prekey_signature,
             identity_onetime_keys: onetime_identity_prekeys,
             notif_prekey,
             notif_prekey_signature,
             notif_onetime_keys: onetime_notif_prekeys,
           },
           social_proof,
         )
       } else {
         return Err(tonic::Status::invalid_argument("unexpected message data"));
       };
 
     let user_id = match self
       .client
       .get_user_id_from_user_info(wallet_address.clone(), AuthType::Wallet)
       .await
       .map_err(handle_db_error)?
     {
       Some(id) => {
         // User already exists, so we should update the DDB item
         self
           .client
           .add_wallet_user_device_to_users_table(
             id.clone(),
             flattened_device_key_upload.clone(),
             social_proof,
           )
           .await
           .map_err(handle_db_error)?;
         id
       }
       None => {
         // User doesn't exist yet, so we should add a new user in DDB
         self
           .client
           .add_wallet_user_to_users_table(
             flattened_device_key_upload.clone(),
             wallet_address,
             social_proof,
           )
           .await
           .map_err(handle_db_error)?
       }
     };
 
     // Create access token
     let token = AccessTokenData::new(
       user_id.clone(),
       flattened_device_key_upload.device_id_key,
       crate::token::AuthType::Password,
       &mut OsRng,
     );
 
     let access_token = token.access_token.clone();
 
     self
       .client
       .put_access_token_data(token)
       .await
       .map_err(handle_db_error)?;
 
     let response = WalletLoginResponse {
       user_id,
       access_token,
     };
     Ok(Response::new(response))
   }
 
   async fn delete_user(
     &self,
     request: tonic::Request<DeleteUserRequest>,
   ) -> Result<tonic::Response<Empty>, tonic::Status> {
     let message = request.into_inner();
 
     let access_token = self
       .client
       .get_access_token_data(message.user_id.clone(), message.device_id_key)
       .await
       .map_err(handle_db_error)?;
 
     if let Some(token) = access_token {
-      if !token.is_valid() || token.access_token != message.access_token {
+      if !token.is_valid()
+        || !constant_time_eq(
+          token.access_token.as_bytes(),
+          message.access_token.as_bytes(),
+        )
+      {
         return Err(tonic::Status::permission_denied("bad token"));
       }
 
       self
         .client
         .delete_user(message.user_id)
         .await
         .map_err(handle_db_error)?;
 
       let response = Empty {};
 
       Ok(Response::new(response))
     } else {
       Err(tonic::Status::permission_denied("bad token"))
     }
   }
 
   async fn generate_nonce(
     &self,
     _request: tonic::Request<Empty>,
   ) -> Result<tonic::Response<GenerateNonceResponse>, tonic::Status> {
     let nonce_data = generate_nonce_data(&mut OsRng);
     match self
       .client
       .add_nonce_to_nonces_table(nonce_data.clone())
       .await
     {
       Ok(_) => Ok(Response::new(GenerateNonceResponse {
         nonce: nonce_data.nonce,
       })),
       Err(e) => Err(handle_db_error(e)),
     }
   }
 
   async fn get_receiver_keys_for_user(
     &self,
     _request: tonic::Request<ReceiverKeysForUserRequest>,
   ) -> Result<tonic::Response<ReceiverKeysForUserResponse>, tonic::Status> {
     unimplemented!();
   }
 
   async fn get_sender_keys_for_user(
     &self,
     _request: tonic::Request<SenderKeysForUserRequest>,
   ) -> Result<tonic::Response<SenderKeysForUserResponse>, tonic::Status> {
     unimplemented!();
   }
 
   async fn get_keyserver_keys(
     &self,
     _request: tonic::Request<KeyserverKeysRequest>,
   ) -> Result<tonic::Response<KeyserverKeysResponse>, tonic::Status> {
     unimplemented!();
   }
 
   async fn upload_one_time_keys(
     &self,
     _request: tonic::Request<UploadOneTimeKeysRequest>,
   ) -> Result<tonic::Response<Empty>, tonic::Status> {
     unimplemented!();
   }
 
   async fn refresh_user_pre_keys(
     &self,
     _request: tonic::Request<RefreshUserPreKeysRequest>,
   ) -> Result<tonic::Response<Empty>, tonic::Status> {
     unimplemented!();
   }
 
   async fn verify_user_access_token(
     &self,
-    _request: tonic::Request<VerifyUserAccessTokenRequest>,
+    request: tonic::Request<VerifyUserAccessTokenRequest>,
   ) -> Result<tonic::Response<VerifyUserAccessTokenResponse>, tonic::Status> {
-    unimplemented!();
+    let message = request.into_inner();
+    let token_valid = match self
+      .client
+      .get_access_token_data(message.user_id, message.signing_public_key)
+      .await
+    {
+      Ok(Some(access_token_data)) => {
+        constant_time_eq(
+          access_token_data.access_token.as_bytes(),
+          message.access_token.as_bytes(),
+        ) && access_token_data.is_valid()
+      }
+      Ok(None) => false,
+      Err(e) => return Err(handle_db_error(e)),
+    };
+    let response = Response::new(VerifyUserAccessTokenResponse { token_valid });
+    Ok(response)
   }
 }
 
 pub fn handle_db_error(db_error: DBError) -> tonic::Status {
   match db_error {
     DBError::AwsSdk(DynamoDBError::InternalServerError(_))
     | DBError::AwsSdk(DynamoDBError::ProvisionedThroughputExceededException(
       _,
     ))
     | DBError::AwsSdk(DynamoDBError::RequestLimitExceeded(_)) => {
       tonic::Status::unavailable("please retry")
     }
     e => {
       error!("Encountered an unexpected error: {}", e);
       tonic::Status::failed_precondition("unexpected error")
     }
   }
 }