diff --git a/src/authd.cr b/src/authd.cr
index 9e4ea3f..b7c544e 100644
--- a/src/authd.cr
+++ b/src/authd.cr
@@ -10,8 +10,8 @@ require "ipc"
 
 require "baguette-crystal-base"
 
-# In any message, a user can be referred by its Int32 uid or its login.
-alias UserID = Int32 | String
+# In any message, a user can be referred by its UInt32 uid or its login.
+alias UserID = UInt32 | String
 
 # Allows get configuration from a provided file.
 # See Baguette::Configuration::Base.get
diff --git a/src/authd/client.cr b/src/authd/client.cr
index 8530f66..15a3745 100644
--- a/src/authd/client.cr
+++ b/src/authd/client.cr
@@ -40,7 +40,7 @@ module AuthD
 			parse_message [Response::User], read
 		end
 
-		def get_user?(uid_or_login : Int32 | String)
+		def get_user?(uid_or_login : UserID)
 			send_now Request::GetUser.new uid_or_login
 			parse_message [Response::User], read
 		end
@@ -84,12 +84,12 @@ module AuthD
 			parse_message [Response::UserValidated], read
 		end
 
-		def ask_password_recovery(uid_or_login : String | Int32)
+		def ask_password_recovery(uid_or_login : UserID)
 			send_now Request::AskPasswordRecovery.new uid_or_login
 			parse_message [Response::PasswordRecoverySent], read
 		end
 
-		def change_password(uid_or_login : String | Int32, new_pass : String, renew_key : String)
+		def change_password(uid_or_login : UserID, new_pass : String, renew_key : String)
 			send_now Request::PasswordRecovery.new uid_or_login, renew_key, new_pass
 			parse_message [Response::PasswordRecovered], read
 		end
@@ -137,12 +137,12 @@ module AuthD
 			parse_message [Response::MatchingUsers], read
 		end
 
-		def edit_profile_content(user : Int32 | String, new_values)
+		def edit_profile_content(user : UserID, new_values)
 			send_now Request::EditProfileEntries.new user, new_values
 			parse_message [Response::User], read
 		end
 
-		def delete(user : Int32 | String)
+		def delete(user : UserID)
 			send_now Request::Delete.new user
 			parse_message [Response::UserDeleted], read
 		end
diff --git a/src/authd/token.cr b/src/authd/token.cr
index 5c8d844..b2b5dd2 100644
--- a/src/authd/token.cr
+++ b/src/authd/token.cr
@@ -4,7 +4,7 @@ class AuthD::Token
 	include JSON::Serializable
 
 	property login : String
-	property uid   : Int32
+	property uid   : UInt32
 
 	def initialize(@login, @uid)
 	end
@@ -23,7 +23,7 @@ class AuthD::Token
 	def self.from_s(key, str)
 		payload, meta = JWT.decode str, key, JWT::Algorithm::HS256
 
-		self.new payload["login"].as_s, payload["uid"].as_i
+		self.new payload["login"].as_s, payload["uid"].as_i64.to_u32
 	end
 end
 
diff --git a/src/authd/user.cr b/src/authd/user.cr
index 962e2d6..dd1b058 100644
--- a/src/authd/user.cr
+++ b/src/authd/user.cr
@@ -32,7 +32,7 @@ class AuthD::User
 
 	# Public.
 	property login         : String
-	property uid           : Int32
+	property uid           : UInt32
 	property admin         : Bool = false
 	property profile       : Hash(String, JSON::Any)?
 
@@ -60,7 +60,7 @@ class AuthD::User
 		include JSON::Serializable
 
 		property login   : String
-		property uid     : Int32
+		property uid     : UInt32
 		property admin   : Bool = false
 		property profile : Hash(String, JSON::Any)?
 
diff --git a/src/client.cr b/src/client.cr
index 58abf87..db90810 100644
--- a/src/client.cr
+++ b/src/client.cr
@@ -172,7 +172,7 @@ class Actions
 
 	def user_deletion
 		args = Context.args.not_nil!
-		userid = args[0].to_i
+		userid = args[0].to_u32
 
 		res = authd.delete userid
 
@@ -203,7 +203,7 @@ class Actions
 	def permission_check
 		args = Context.args.not_nil!
 		user, application, resource = args[0..2]
-		res = @authd.check_permission user.to_i, application, resource
+		res = @authd.check_permission user.to_u32, application, resource
 		case res
 		when Response::PermissionCheck
 			s = res.service
@@ -218,7 +218,7 @@ class Actions
 		args = Context.args.not_nil!
 		user, application, resource, permission = args[0..3]
 		perm = AuthD::User::PermissionLevel.parse(permission)
-		res = @authd.set_permission user.to_i, application, resource, perm
+		res = @authd.set_permission user.to_u32, application, resource, perm
 		case res
 		when Response::PermissionSet
 			s = res.service
diff --git a/src/responses/contact.cr b/src/responses/contact.cr
index cc6409e..c4cc413 100644
--- a/src/responses/contact.cr
+++ b/src/responses/contact.cr
@@ -1,6 +1,6 @@
 class AuthD::Response
 	IPC::JSON.message Contacts, 12 do
-		property user : Int32
+		property user : UInt32
 		property email : String? = nil
 		def initialize(@user, @email)
 		end
diff --git a/src/responses/login.cr b/src/responses/login.cr
index c73c6af..493ddaf 100644
--- a/src/responses/login.cr
+++ b/src/responses/login.cr
@@ -1,6 +1,6 @@
 class AuthD::Response
 	IPC::JSON.message Login, 1 do
-		property uid    : Int32
+		property uid    : UInt32
 		property token  : String
 		def initialize(@token, @uid)
 		end
diff --git a/src/responses/permissions.cr b/src/responses/permissions.cr
index cff5ce8..720ef99 100644
--- a/src/responses/permissions.cr
+++ b/src/responses/permissions.cr
@@ -1,6 +1,6 @@
 class AuthD::Response
 	IPC::JSON.message PermissionCheck, 7 do
-		property user       : Int32
+		property user       : UInt32
 		property service    : String
 		property resource   : String
 		property permission : ::AuthD::User::PermissionLevel
@@ -10,7 +10,7 @@ class AuthD::Response
 	AuthD.responses << PermissionCheck
 
 	IPC::JSON.message PermissionSet, 8 do
-		property user       : Int32
+		property user       : UInt32
 		property service    : String
 		property resource   : String
 		property permission : ::AuthD::User::PermissionLevel
diff --git a/src/responses/users.cr b/src/responses/users.cr
index 021bcc2..0097cd0 100644
--- a/src/responses/users.cr
+++ b/src/responses/users.cr
@@ -14,7 +14,7 @@ class AuthD::Response
 	AuthD.responses << UserAdded
 
 	IPC::JSON.message UserEdited, 4 do
-		property uid    : Int32
+		property uid    : UInt32
 		def initialize(@uid)
 		end
 	end
@@ -42,7 +42,7 @@ class AuthD::Response
 	AuthD.responses << MatchingUsers
 
 	IPC::JSON.message UserDeleted, 12 do
-		property uid    : Int32
+		property uid    : UInt32
 		def initialize(@uid)
 		end
 	end
diff --git a/src/service.cr b/src/service.cr
index 20cc6a7..1bd9da7 100644
--- a/src/service.cr
+++ b/src/service.cr
@@ -63,13 +63,12 @@ class AuthD::Service < IPC
 	#
 	# WARNING: to record this new UID, new_uid_commit must be called.
 	# WARNING: new_uid isn't thread safe.
-	def new_uid
-		begin
-			uid = File.read(@last_uid_file).to_i
+	def new_uid : UInt32
+		uid : UInt32 = begin
+			File.read(@last_uid_file).to_u32
 		rescue
-			uid = 999
+			999.to_u32
 		end
-
 		uid += 1
 	end
 
@@ -92,7 +91,7 @@ class AuthD::Service < IPC
 	end
 
 	def user?(uid_or_login : UserID)
-		if uid_or_login.is_a? Int32
+		if uid_or_login.is_a? UInt32
 			@users_per_uid.get? uid_or_login.to_s
 		else
 			@users_per_login.get? uid_or_login