Suppose you knew a message had been coded using a Substitution cipher. Substitution Cipher is defined below?

Question

THIS QUESTION COULD BE BASED ON PERMUTATION OR COMBINATION. Suppose you knew a message had been coded using a Substitution cipher. If it took you one second to try out each key, how many days would it take for you to try all possible keys on this message? Definition of SUBSTITUTION CIPHER from WIKIPEDIA:In cryptography, a substitution cipher is a method of encryption by which units of plaintext are substituted with ciphertext according to a regular system; the "units" may be single letters (the most common), pairs of letters, triplets of letters, mixtures of the above, and so forth. The receiver deciphers the text by performing an inverse substitution.

Answer 1

Without the key or any guidelines, a human would never be able to crack a substitution cipher.

You wouldn't know if the letter A was substituted with a number (like 7), a small group of letters (oGq), or a large group of letters and/or numbers (tU3EWdf98bvsw1dgJJ).

Answer 2

Get a copy of Games or World of Puzzles magazine. Substitution cipher puzzles are in every issue. My wife does these all the time. I assure you it does not take an infinite amount of time. These are never solved by trying permutations and combinations. You use letter frequency and characteristic combinations to solve. Although this is probably the oldest from of cipher, it is useless because it is so easy to solve. One way to make things more difficult is to run all the words together so you cannot use word lengths to help you. Read a book called "The Codebreakers" by David Kahn for more information.

Answer 3

In a simple case where we are only substituting one letter for another, there are 25! or about 1.5 x 10^25 possible cyphers. Trying 1 million per second would still require 491 billion years to try them all. However, there are much more efficient ways of cracking a substitution cypher than trying all possible permutations.

Answer 4

It could take you an infinite amount of time, depending on how many letters are in the cipher. But if it is a good cipher, chances are it would take you more than your lifetime to figure them out...

Answer 5

You didn't specify the message size.

Not having the key is the same as trying to decrypt an intercepted message. I only did a cursory search but I believe NSA or the other agencies that decrypt coded messages aren't going to let the general public in on how long it takes.