diff --git a/src/user_utils.ts b/src/user_utils.ts
new file mode 100644
index 0000000..99957c5
--- /dev/null
+++ b/src/user_utils.ts
@@ -0,0 +1,59 @@
+/** User login format of cohabit `firstname`.`lastname`. */
+export type User = `${string}.${string}`
+
+/**
+ * Transform a {@link UserDomain} into a {@link User}.
+ *
+ * @param userDomain User domain (hostname subdomain) derived from user login.
+ * @returns Corresponding user user login.
+ *
+ * @example
+ * ```ts
+ * import { toUser } from './user_utils.ts'
+ *
+ * const user = toUser('john-doe')
+ * console.assert(user === 'john.doe')
+ * ```
+ */
+export function toUser(userDomain: UserDomain): User {
+	//TODO check is user in cohabit.ressources.users
+	return userDomain.replace('-', '.') as User
+}
+
+/**
+ * User personal portfolio domain rerived from cohabit user login
+ * as `firstname`-`lastname`.
+ */
+export type UserDomain = `${string}-${string}`
+
+/**
+ * Check hostname subdomain against {@link UserDomain}.
+ *
+ * @param userDomain Hostname to assert.
+ * @returns Asserted user domain.
+ *
+ * @example Valid user domain
+ * ```ts
+ * import { toUserDomain } from './user_utils.ts'
+ *
+ * const userDomain = toUserDomain('john-doe')
+ * console.assert(userDomain === 'john-doe')
+ * ```
+ * @example Invalid user domain
+ * ```ts
+ * import { toUserDomain } from './user_utils.ts'
+ *
+ * try {
+ * 	const userDomain = toUserDomain('johndont')
+ * } catch (error) {
+ * 	console.assert(error instanceof Error)
+ * }
+ * ```
+ */
+export function toUserDomain(userDomain: string): UserDomain {
+	//TODO check is user in cohabit.ressources.users
+	if (/\w+-\w+/.test(userDomain)) {
+		return userDomain as UserDomain
+	}
+	throw new TypeError(`invalid user login (${userDomain.replace('-', '.')})`)
+}