While the precise math is pretty complex, encryption is essentially multiplying two large prime numbers together, reversing that with the key is the equivalent of factoring that result back into the two primes.
Multiplying two numbers, even two 100 digit numbers, is an easy step by step process that directly produces the resulting 200 digit number. Getting those two numbers back boils down to a guessing a bunch of 100 digit numbers until you find the right one, and can take years on a supercomputer.
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