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From some time I am working with "pen and paper" encrypted text. Keys are not known, algorithm is not known.

I managed to find simple algorithm and key that seems to be nearly perfect. Nearly perfect means that using this algorithm and key I can decrypt part of the encryption into meaningful text (~10 consecutive letters that create words). The composition of the algorithm, the length of the decrypted text and the text itself are definitely not coincidence. It seems that something changes beetween sentence/list of words because the rest of the text cannot be decrypted using simply the same method/key.

What is used in cryptography that could make such outcome?

e.g. If I use simple cipher: Add A letter to decrypted text I would get

DECRYPTEDTEXT AAAAAAAAAAAAA EFDSZQUFEUFYU

but in order to make it harder I could add consecutive letters to consecutive words

DECRYPTEDTEXT AAAAAAAAABBBB EFDSZQUFEVGZV

another example would be encryption using Caesar: DECRYPTEDTEXT CAESARCAESARC ............. but in order to make it more difficult I could add X as space, encrypt and then remove encrypted letter that corresponds to X from encryption string: DECRYPTEDXTEXT CAESARCAESARCA it would then be changed to: DECRYPTEDTEXT <- X removed CAESARCAEARCA <- S removed - corresponds to X

Are there any known and used methods that happen every word or every sentence?

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Keys are not known, algorithm is not known.

Wrong initial assumption. The only secret must be the key by Kerckhoffs's_principle.


but in order to make it harder I could add consecutive letters to consecutive words

and

another example would be encryption using Caesar...

This is not new, this is nearly 600 years old, it is commonly called Vigenère Cipher and it is super weak with nowadays assumption such as Chosen Plaintext Attacks.


Are there any known and used methods that happen every word or every sentence?

Yes, for example Cipher Block Chaining. Note that sentences are not composed of words anymore but seen a bit-string that you are splitting into blocks (sort of words) of the same length.

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  • $\begingroup$ thx for information. I think that I was misunderstood. The text is encrypted and neither the method nor the keys are known. $\endgroup$ – oen Oct 19 '17 at 13:31
  • $\begingroup$ I am working to decrypt this text -> I found method and keys that are "perfect" -> it works for a couple of letters producing meaningful decrypted text (with extremally large possibility of not being accidental) -> it does not work for the whole text -> I suppose that it can happen because something happens in the algorithm after each sentence/set of words -> therefore I am looking for ideas that exist in current pen-and-paper algorithms and change something between words/sets of words. $\endgroup$ – oen Oct 19 '17 at 13:36
  • $\begingroup$ as for the "Cipher Block Chaining" it seems to be not present here since algorithm works perfectly for both words that are of different length. $\endgroup$ – oen Oct 20 '17 at 11:23

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