What is wrong with this authentication code and what attack does it enable?

IF USER_EXISTS(username) THEN
    password_hash=HASH(password)
    IS_VALID=LOOKUP_CREDENTIALS_IN_STORE(username, password_hash)
    IF NOT IS_VALID THEN
        RETURN Error("Invalid Username or Password!")
    ENDIF
ELSE
    RETURN Error("Invalid Username or Password!")
ENDIF

It leaks a timing side-channel: it only hashes when the username exists, so a missing user returns faster — letting an attacker discover valid usernames despite the identical error message.

Even though the error message is the same, the code takes different amounts of time:

• If user EXISTS: Hash calculation + DB lookup (longer)
• If user DOESN'T exist: Immediate return (shorter)

Attack: Attacker can measure response times to determine which usernames exist.

Fix: Always perform the same operations (constant-time):

// Always hash the password, even if user doesn't exist
password_hash = HASH(password)

// Always get a stored hash (use dummy hash if user doesn't exist)
stored_hash = LOOKUP_USER_HASH(username)
IF stored_hash IS NULL THEN
    stored_hash = DUMMY_HASH  // Pre-computed fake hash
ENDIF

// Use constant-time comparison
IS_VALID = CONSTANT_TIME_COMPARE(password_hash, stored_hash)

IF NOT IS_VALID THEN
    RETURN Error("Invalid Username or Password!")
ENDIF

Key principles: Hash the password regardless of user existence, always retrieve/use a hash for comparison, and use constant-time comparison to prevent timing leaks.