Ask HN: Are OTPs never prime?

https://news.ycombinator.com/item?id=9686311

by shreshthmohan • 11 years ago

2 2 11 years ago

news.ycombinator.com

Indian banks use one-time password for two-factor authentication. Prashant (https://twitter.com/pacificleo/status/600132236730441728) is saying that he has never seen an OTP that is prime. Which makes him ask if this is intentional.

- Is this true? If yes, why?

PS: this is my first submission to HN. Thanks.

shreshthmohan• 11 years ago

I think that this is just probability in play. OTPs are usually 4 or 6 digits. Probability of seeing a prime number is little because of prime number theorem. I might be wrong though.

kylc• 11 years ago

Some digging reveals that a common method for generating OTPs is through repeated hash function applications. A good hash function should produce uniformly distributed outputs.

There are 78,498 primes less than 10^6 (so, primes less than 6 digits), meaning that there is about a 7.9% chance of independently seeing a prime OTP if the OTPs are uniformly distributed. That seems pretty high. The expected value for seeing a prime OTP after multiple trials is just 15.

So I wonder if your friend is just unlucky or if his OTP provider is using another method for OTP generation.

Ask HN: Are OTPs never prime?

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