SIDH a quantum resistant algorithm for DH exchange
All algorithms that actually performs DH exchange are susceptible to be solved in polynomial time by a quantum computer: say based on discrete logarithm problem or based on elliptic curve point multiplication. Beside these, the other big algorithm that can perform public key encryption is RSA, which has its strength in the impossibility of factoring big numbers, but it also can be solved in polynomial time by a quantum computer. In fact, factoring is shown as an example of how a quantum computer works by means of the Shor’s algorithm.
Side-channel attack to modular inversion
Modular inversion is a common mathematical operation that is given within cryptographic algorithms based on finite groups generated by a prime number. Mainly, these algorithms are related to public key cryptographic, specially, to Elliptic Curve Cryptography (ECC). The way to compute a modular inverse is always hard; at least, it is roughly 100 times harder than the opposite operation, the modular product.
The maths of Secret Santa
Secret Santa is a way to share gifts among work mates, family, etc. It is organized in such a way that every person is commited to gift secretly to another, and the latter does not know who is the gifter. In Spanish this game is called amigo invisible (invisible friend).
RC4 as pencil & paper cipher
RC4 is a well-known stream cipher, extremely simple —I’d say minimalist— and strong enough to be still used, spite of some documented weaknesses which, mostly, fall on the key schedule.