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Reply To: Stuff to do while you wait (with a message from Harry added)

The Empty Vault Forums Bureau of Security and Signals Intelligence Forum Stuff to do while you wait (with a message from Harry added) Reply To: Stuff to do while you wait (with a message from Harry added)


[As usual, Madness is providing the more advanced cipher crackers with some fun here, so don’t be too put off if you don’t know (yet) what he is taking about! Once you have been here long enough you will be tackling these challenges with ease. Harry

ps, the MD5 lines are what are known as hashes of the original plaintext and the key that madness has used. If you can break the ciphers you can use an online tool to make an MD5 hash of your solution and of your keyword and compare them with madness’s md5s to see if you were right.]

The Mirror Vigenere Cipher

This challenge is so easy, I was debating with myself whether to
even post it. But what the heck.

You know how the Vigenere cipher can be defined by how it acts on
each plaintext character? If we think of the letters A…Z as numbers
0…25, then for a key with period m the ith ciphertext character is

c[i] = (+p[i] + k[i mod m]) mod 26

For the Beaufort it is

c[i] = (-p[i] + k[i mod m]) mod 26

and for the variant Beaufort

c[i] = (+p[i] – k[i mod m]) mod 26

So we have:

sign of p | sign of k | cipher
+ | + | Vigenere
+ | – | variant Beaufort
– | + | Beaufort

Try to imagine that the above table was formatted nicely.
Obviously, one combination is missing, namely – -.
Introducing the Mirror Vigenere Cipher:

c[i] = (-p[i] – k[i mod m]) mod 26

It is so named for reasons that I think are obvious.

Here are two ciphertexts, one long and easy, and one short and less easy.
Your task is to break them and recover the keys. Try to have fun.


MD5 of plaintext: abf514ead6ee8b881a1973b47b4aa1f3
MD5 of keyword: f2af9578745b09c5b22a3575e7a2f5fc


MD5 of plaintext: 05eebdf70355a8019fd7c7c64ad31ab9
MD5 of keyword: b6b818304a439f91fc90b3fe8dd35be8

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