If the question is also applicable to which formulas should be in your cryptographic "toolbox" (and that are beautifully simple) I would add Boltzman's entropy equation (for calculating Entropy which is s = k(logW), but swapped with Claude Shannon's interpretation as it also structurally relates to information theory (and not the decay of gas) and is something that every cryptographer must know how to do, important for combinatorics and security assumption values (passwords, private keys, ciphertext, etc..).
It's also beautifully simple, with a few ways to write it:
${log_2(L^N)}$ = Entropy in bits (where L is the size of the library and N is the length of the string.)
If we equate beauty with its usefulness: then again, I think every cryptographer should be able to - at a minimum - calculate entropy when dealing with any random length of any text character (number or string) in numerous situations related to cryptographic operations in order to calculate the potential message space and determine the potential maximum theoretical Entropy as bits of security (i.e. 128-bit security, 128 bits of entropy).