This morning, the NSA Careers Twitter account posted what looked like a series of nonsense letters:
We looked at this tweet and thought it looked suspciously like a coded message.
It turns out that it was. A couple of our commenters on our earlier post came up with the deciphered message: "Want to know what it takes to work at NSA? Check back each Monday in May as we explore careers essential to protecting our nation."
While some of us were hoping that it would be instructions to secret agents, it's simply a notification of future tweets.
The message was encoded with a simple substitution cipher, one of the most basic ways to encrypt something. In a cipher of this type, the alphabet is scrambled, with each letter in the alphabet assigned to another letter.
For example, T in the encrypted message corresponds to W in the uncoded text, P corresponds to A, F corresponds to N, and C corresponds to T. That makes the first four letters of the encrypted message, "TPFC," turn into the first word of the decrypted message, "Want." Notice that spaces and punctuation don't matter in this code.
This is a very very basic type of encryption, and can be broken fairly easily. The big problem with substitution ciphers is that English letters have a distinct frequency distribution, as explained at Practical Cryptography:
So, to crack the code, the first step is to count up the letter frequencies in the encoded text, and put them into alignment with English-letter frequencies. The most common letters in the coded message will probably be the letters assigned to common letters in normal English, like e, t, or a. Letters that are missing or rare in the coded text will probably be assigned to rare English letters like q, x, and z.