What is a prime number, and why do banks love them?

A prime number can only be divided by 1 and itself: 2, 3, 5, 7, 11, 13 and so on. Every other whole number can be built by multiplying primes — 12 is 2 × 2 × 3. Primes are the atoms of arithmetic.

There is no end to them. The Greek mathematician Euclid proved 2,300 years ago that the list of primes goes on forever. The largest known prime, found in 2024, has more than 41 million digits.

Here is the useful part. Multiplying two large primes is easy for a computer. Working backwards — taking the product and finding which two primes made it — is so hard that it would take the fastest computers longer than the age of the universe.

That one-way difficulty is the lock on your bank account. Online payments, UPI and WhatsApp messages are protected by maths built on primes.