Authentication logic is the set of rules a system uses to decide whether a user, device, or session is allowed to sign in. It checks inputs such as passwords, passkeys, tokens, device state, multi-factor prompts, and account status, then combines those checks into a yes-or-no decision. In other words, it is the policy engine behind login.
This logic matters because cryptographic controls are only as strong as the code that verifies them. If the rules are too loose, attackers may bypass checks, abuse enrollment flows, or impersonate privileged users without breaking encryption itself. In real attacks, flaws often appear in edge cases: account recovery, second-factor handling, session binding, or privilege escalation paths. Defenders harden authentication logic by minimizing special cases, validating every step of the trust chain, and testing how systems behave when credentials, devices, or roles change.