Imagine that you would like to write down your passwords or other critical information into a sheet of paper.
Could an algorithms without using a computer be so good as to avoid people of cracking it? There seems to exists some secret code messages that weren't discovered for centuries. So, theoretically it sound plausible, but how would I proceed?
The simplest, and most secure, form of encryption is the one-time pad. This method is unbreakable, as long as no one discovers the source pad. This method is over a hundred years old, and was used extensively throughout the cold war with paper and pencil.
As long as you can memorize your pad, it's perfectly secure and requires no computers.