For example I want to have a hard copy of my backup 2FA codes so that if I lose/destroy/don't have my phone or laptop on me. I, of course, do not want them in plain text. I don't want to encrypt them using a secure cryptographic algorithm that I memorize a key for, as I might not have access to a device that can decrypt them.
Are there any downsides to using mathematical operations such as trig functions, exponents and roots, inversions, truncations, selections of the i-th to j-th digits, addition subtraction division and multiplication, remainders, OR/XOR/AND etc. in a sequence that I memorize?
Would this also be a safe way to store you personal credit card number in case of emergency?