How to attack a classical cipher using known partial plaintext? I have a ciphertext generated by a classical cipher. I do not know what was cipher used to generate it. I do however have the beginning of the plaintext. 


What are the cryptanalysis approaches for classical ciphers? 
How do you suggest trying to recover the plaintext in this instance?
How can I leverage the known plaintext to attack the cipher?


I have this ciphertext:


  NUVRRIEZWBRVRFFAPUUKFVRYNQBCINHZKNETUVDVUMRTEANPAFIELBPOANIXLCMEZQETKDKVWBMNCTRVUMRFNTFDGWRUMCSCTHSGKDULRFCNABNCGSNMMGRFNTLOWQRBETZRMUBQYURFRFASANGPEQRFFAFCZACAZCTUBTHKUNEZHPPWSGPNABHZKZQNREANLCHUCZDLMLPUNTMNETDAUINQADUNSUMGETETHFBNMNNONTFBBPOQEMZPETEQUIDTHKNABKDZWBDBRZUSRASUMNIRARETETHFBZKRHPNQFAKRICMGETMULCTCULSLGBIVWBDBIEMUFCUVNUGPINFBQTMPNUMSRNNANTNANCMNNTFUMUKRILRFFAPUUKFVRYNTCMETNUVFWDABBRFNRIEFRFNTHGPUHPNKCILEGBUTWUSCPQSLMNIBWQINFBQTFAVRLCKRHKLZNFAZLSNTBDBEASTD


It begins with the partial plaintext: 


  it turns out that
I do not have a solution, but I pursued the cipher long enough to establish it wasn't one of the easy classical ciphers. This approach should get you started.

The first thing you want to do is convert the text into numbers as many classic ciphers are mathematically-based (or at least easy represented mathematically). Using $A=0$, $B=1$, $\ldots$, the ciphertext is:

{13,20,21,17,17,8,4,25,22,1,17,21,17,5,5,0,15,20,20,10,5,21,17,24,13,16,1,2,8,13,7,
 25,10,13,4,19,20,21,3,21,20,12,17,19,4,0,13,15,0,5,8,4,11,1,15,14,0,13,8,23,11,2,
 12,4,25,16,4,19,10,3,10,21,22,1,12,13,2,19,17,21,20,12,17,5,13,19,5,3,6,22,17,20,
 12,2,18,2,19,7,18,6,10,3,20,11,17,5,2,13,0,1,13,2,6,18,13,12,12,6,17,5,13,19,11,14,
 22,16,17,1,4,19,25,17,12,20,1,16,24,20,17,5,17,5,0,18,0,13,6,15,4,16,17,5,5,0,5,2,
 25,0,2,0,25,2,19,20,1,19,7,10,20,13,4,25,7,15,15,22,18,6,15,13,0,1,7,25,10,25,16,
 13,17,4,0,13,11,2,7,20,2,25,3,11,12,11,15,20,13,19,12,13,4,19,3,0,20,8,13,16,0,3,
 20,13,18,20,12,6,4,19,4,19,7,5,1,13,12,13,13,14,13,19,5,1,1,15,14,16,4,12,25,15,4,
 19,4,16,20,8,3,19,7,10,13,0,1,10,3,25,22,1,3,1,17,25,20,18,17,0,18,20,12,13,8,17,
 0,17,4,19,4,19,7,5,1,25,10,17,7,15,13,16,5,0,10,17,8,2,12,6,4,19,12,20,11,2,19,2,
 20,11,18,11,6,1,8,21,22,1,3,1,8,4,12,20,5,2,20,21,13,20,6,15,8,13,5,1,16,19,12,15,
 13,20,12,18,17,13,13,0,13,19,13,0,13,2,12,13,13,19,5,20,12,20,10,17,8,11,17,5,5,0,
 15,20,20,10,5,21,17,24,13,19,2,12,4,19,13,20,21,5,22,3,0,1,1,17,5,13,17,8,4,5,17,5,
 13,19,7,6,15,20,7,15,13,10,2,8,11,4,6,1,20,19,22,20,18,2,15,16,18,11,12,13,8,1,22,
 16,8,13,5,1,16,19,5,0,21,17,11,2,10,17,7,10,11,25,13,5,0,25,11,18,13,19,1,3,1,4,0,
 18,19,3}


The plaintext (assuming spaces are eliminated, you can rerun the process with spaces included as an additional character) is:

{8,19,19,20,17,13,18,14,20,19,19,7,0,19}


If $p_i$ be the $i^\mathrm{th}$ plaintext character (an integer between 0 and 25) and likewise $c_i$. Many classic ciphers map plaintext characters to ciphertext characters one-to-one. In other words, the value of $c_i$ depends only on $p_i$ and some key character $k_i$. First consider an encryption scheme like: $c_i=p_i + k_i \mod 26$. Since we know $p_i$ for the first few values, we can compute what $k_i$ would be:

{5,1,2,23,0,21,12,11,2,8,24,14,17,12}


Putting this into a chart, where the first row is the plaintext, the second row is the key, and the third row is the ciphertext, we can start an analysis.

$\begin{array}{llllllllllllll}
 8 &amp; 19 &amp; 19 &amp; 20 &amp; 17 &amp; 13 &amp; 18 &amp; 14 &amp; 20 &amp; 19 &amp; 19 &amp; 7 &amp; 0 &amp; 19 \\
 5 &amp; 1 &amp; 2 &amp; 23 &amp; 0 &amp; 21 &amp; 12 &amp; 11 &amp; 2 &amp; 8 &amp; 24 &amp; 14 &amp; 17 &amp; 12 \\
 13 &amp; 20 &amp; 21 &amp; 17 &amp; 17 &amp; 8 &amp; 4 &amp; 25 &amp; 22 &amp; 1 &amp; 17 &amp; 21 &amp; 17 &amp; 5
\end{array}$

The key is not the same value, ruling out a Ceaser cipher. The key as characters is not English (FBCXAVMLCIYORM), it does not appear to repeat, and applying the same key to the next block of ciphertext decrypts to garbage (AZNXUPTKPQPCKQ). For these reasons, it is unlikely a Vigenere cipher. The same $p_i$ is not always mapped to the same $c_i$ value (for example, 19 is mapped to 20, 21, 1, 17, 5), so it is not a simple substitution cipher. Finally, if you sort the plaintext values and ciphertext values, they are not similar sets of integers so it is unlikely a permutation cipher.

I also did a frequency analysis of the ciphertext:



It tells us two things: it resembles English text and there is no j. If you are familiar with the classical ciphers, there is one in particular that conflates i and j: the Playfair cipher.

This should be enough to get you started.