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
but in order to make it harder I could add consecutive letters to consecutive words
another example would be encryption using Caesar:
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:
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?