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i like to know if there is any way to decrypt a plain-text that is encrypted 55 rounds using Caesar ciphers with different key for each round. and using brute force attack without knowing the length of the keys and character used

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    Sure, you brute-force the keys and character lengths. Is this a homework question? This is an awfully specific and arbitrary scenario. – schroeder May 28 at 8:04
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An encrypted plain-text is a cipher-text. A Plain-text - like its name says - is an unencrypted text.

The Caesar cipher is a simple encryption in which every plain-text letter of a given string is shifted by a given key.

Like schroeder said, your question looks awfully specific and it seems to be a homework question.

Anyway: If you rotate a given string 55 rounds with a different key (e.g. 1-26 for the latin character set), at the end there is only one single key to decrypt the whole cipher-text to a meaningful plain-text which doesn't looks like garbage. That is simply because the function is nothing less or more than:

encrypt(encrypt(encrypt(encrypt(string,key1), key2), key3), keyx)

The algorithm uses the output of the first round as input for the second round and so on. If one knows the cipher-text and the plain-text he doesn't need to reproduce all X rounds. He simply is able to figure out one key from cipher-text to plain-text. If one only knows the cipher-text, a analysis of the quantity of every letter in the alphabet in known words would help to guess this one key. A simple explanation of this you can find here:

Cracking a Caesar-Cipher

I hope, that this "answer" helps you to put a little bit effort in solving your specific 55-Round encryption problem.

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