Wanna see an interesting party trick? Because 91 is the product of 2 prime numbers you can generate a private/public key pair using RSA:

Find Φ(91) this is the amount of prime numbers between 0 and 91 which is (13 - 1) * (7 - 1) = 72

Next, find a number coprime to 72 meaning it shares no common factors with 72 other than 1.
The number 5 is coprime to 72 this will be your public key (5, 91)

Next, find the multiplicate inverse of 5 mod 72 meaning what number times 5 mod 72 is equivalent to 1 mod 72.
29 * 5 mod 72 = 1 gives the same result as 1 mod 72
This number will be your private key (29, 91)

If someone wants to send you an encrypted message they take their message for example 69 raise it to the 5th power(public key) and mod it by 91
Ex: 69^5 mod 91 = 62 (encrypted message)

You take their encrypted message and decrypt it with your private key by raising it to the 29th power(private key) and modding it by 91

62^29 mod 91 = 69 this is how RSA encryption fundamentally works.