NSA Code Breaking Puzzle Number 2
Here is the second of the NSA’s code breaking tweets – crack them all and you might have a chance of a job with the super-secret digital spying agency.
After a few attempts it looks a bit harder than the first one – it’s not a substitution cipher like the first code, it’s not a Caesar shift, it’s not a transposition cipher, it’s not an atbash code. The number of letters is 117 – which has factors 1,3,9,13, 39,117 However the frequency distribution is:
This distribution does look similar to the expected frequencies for the alphabet:
so maybe nothing too fancy is happening. However, the fact that there are no letters at all for w,x,y,z is a little odd – you would expect the letters with no frequency to be spaced out if it was any kind of substitution.
Trying something like a Caesar shift and then a transposition cipher doesn’t seem to throw anything up. There’s also a repeated string of letters in the text – isacp which is repeated after 42 letters. This might suggest that this is a Vigenere cipher – with a keyword of length 2,3,6,7 or 21.
We can use the awesome Crypto Corner website to analyse this. However, I’ve not been able to get any further – I’ve tried guessing some obvious keywords (NSA, Careers, agency) etc – without any luck. I have also tried guessing possible first words (The, This, NSA) etc – to try and work out a likely keyword. Again, nothing! Maybe it’s not a Vigenere cipher after all.
So, time to throw in the towel and find the solution online. It turns out that actually it wasn’t as complicated as a Vignere cipher – but it did require using 2 separate actions. First, we need to reverse the text in the message:
and now we then use some software to analyse the string for a substitution cipher.
This gives the solution:
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