It seems to be standard in that field, but to be honest, I hate cryptographic notation. AFAICT, about half of the numbers in that pseudo-code count a number of bytes, and the other half a number of bits, and without already knowing the algorithms, it's almost impossible to tell which is which. E.g., N[:12] seems to be twelve bytes, while 0¹²⁸ is 16 bytes. X is actually the character 'X', i.e. the bit string 01011000. While L is actually a variable, not the bit string 01001100. And so on. It's clear that mathematicians don't like unambiguous notation nearly as much as CS people do.