What is Time-Based One-Time Password?
An algorithmic extension of HMAC-based One-Time Passwords that computes temporary, single-use authentication codes by hashing a secret seed with the current 30-second Unix time slice.
Synchronized Time-Window Derivation
TOTP operates by dividing the current Unix timestamp by a standardized timestep window (defaulting to 30 seconds): $T = \lfloor(CurrentTime - T_0) / X\rfloor$.
Because time flows identically everywhere, both the client phone and the authenticating server arrive at the same counter integer simultaneously without transmitting counter state over the network.
Cryptographic Dynamic Truncation
The counter integer is hashed with the shared secret key using HMAC-SHA1 or HMAC-SHA256. The lower 4 bits of the resulting hash determine an offset byte, from which a 31-bit integer is extracted.
Taking that integer modulo $10^6$ produces the familiar 6-digit verification code. Because the code expires every 30 seconds, intercepted codes cannot be reused by attackers.
Key Advantages & Impact
- Operates completely offline with zero cellular or Wi-Fi connectivity required on the mobile authenticator
- Neutralizes credential-stuffing and password-spraying attacks by requiring a physical second factor
- Open, standardized protocol supported across millions of websites and open-source applications
Common Misconceptions vs Reality
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Frequently Asked Questions
What is the standard timestep used by TOTP apps?
The RFC 6238 standard and major authenticator apps (Google, Microsoft, Apple) default to a 30-second timestep window.