He seemed to be playing with both substitution codes and numerology.

It was a substitution code based on a prose passage of 682 characters, so that a given character could be replaced by more than one number.

We started with simple substitution codes, then progressed to more complex codes, and hypothesized the impact that computers would shortly have on the field.

"Funny, but this is looking a lot more sophisticated to me than a substitution code."

Furthermore, although Harry thinks it is a substitution code, I think it's a direct visual representation.

That suggested the unintentional code was probably a substitution code, with numbers for letters.

It would take him that long to transcribe the clumsy substitution code using the Bible.

Something easy to remember, such as a substitution code where one letter filled in for another, would be a logical place to start, he determined.

As long as only the author and intended recipients of the message are privy to the substitution code, its hidden meaning remains a mystery.