Decrypting Text

Vignère decryption

Vignère ciphertext can be difficult to decrypt. It is a combination of a Caesar shift combined with a keyword. The length of the keyword determines the number of different encryptions that are applied to the plaintext. For example, if the keyword is 4 chars in length then the plaintext is divided into 4 subtexts and a seperate Caesar shift is applied to each subtext depending on the value of the corresponding letter in the keyword. The cipher is polyalphabetic which means that a character can be enciphered in different ways. A frequency analysis of Vignère often shows a fairly balanced set of frequencies - because the keyword is used to switch between different cipher alphabets.

Charles Babbage worked out a method to decrypt this cipher.

Look for sequences of letters that appear more than once in the ciphertext. These are likely to be the same sequences of plaintext letters that have been enciphered using the same parts of the key. However, a small number of the matching sequences may be purely random.
Find the spacing between the repeated sequences. Try to find the common factor. This may suggest the length of the keyword.
Create a subsets of characters based on the length of the keyword and do a separate frequency analysis on each of them. Compare each to a standard frequency distribution and try to match them up using the distribution of peaks and troughs rather than an exact pattern matching. Find out the character shift which shouldgive the keyword character eg. if shifted 4 places to the left then the letter is probably e.

This program uses a slightly shortened approach. If you don't enter in a keyword then it can break down the text into subtexts so that you can see whether a pattern is present. It will suggest a keyword. It assumes that the most common letter in the subtext is "e" which is not always the case.

Subdivide the ciphertext into a number of subtexts which are then frequency analysed
Assuming that the most common letter is e then it suggests a possible keyword and presents a decryption based on this
You can then go back and try out different keywords and see the result

Here is an example ciphertext which uses a 3 letter key. You can probably work out the keyword from the suggestion - (hint: computer company). Conversely, you can paste into this box any ciphertext of your own that you want to decipher.

Enter your ciphertext below:

BPP OFD ATN WBD LJZ OIA CTQ JJX TJZ OTS IUQ TPZ LPZ AJD QUU AXU BIB TFM AVD MUT IUU IDO MQF GPG ZPR NFD BPP QTO CTE BIQ DBX CFA NUT MNM BFD QBX KPZ KFD VJZ OUT MFZ OMU AIQ VDD GQF QPZ MOS QOQ WON MIM TGA NNK UBE BFD KEM AZA CDM VTQ MJT IWQ UPH MER ZPY APG BIM UQF WOF WBZ IEP ZFE AJZ TPZ LPZ IOP AVS OFE BZA CTT WVX LNQ MUY MUT MSQ IUZ WPZ IXQ MLA VXQ LOQ AEM GQX MBE MDA UFM TPZ MBE QGQ ISF PFY IUQ ZJM TJE AIM TMI MTM GJZ NMM UNM JMQ QUI WVX LCQ UPE BVZ NPD BVZ IUQ QGM ABD MTG TUA NBZ IUF IDW WGZ MSH MTU EFD MUA SOA KLA DFD IMM VUQ ZOA ZQQ ZIM XTP ZPB IMU OIF MEO QHM ZJZ BIQ DJO QOU BZA NUT MTF WDW WGO PFY QDM TTU PBH MTF WSQ LIQ ZFO PFY QTF ZZU ATG KIM XMQ ITM VUP QWQ ZTU WOR WSF PFO CSU WVE UJZ LBZ LXU BIE WNM VZR ITO QOM BJZ OBZ LSM BIQ ZQX MBE QOS IQB TJO IUU WOE QOF PFY WEQ ZOI WSX LJY AVD MZA CXU TMM OSQ MPR KPG ZTQ BIQ XMQ ITG ZFU VTG KIM BSM VTM KUU WOU AOQ KFE ABD QMK UVF CBX IOP QIA XFF PBF IMX COB TFM ABZ BOQ ATM DPU LFP QXU TMN MBN TFF WTM BJE NZK WVD KVD QPE QUK KPZ KFD VJZ OQD WGN ICN IHQ AEQ DJO MBZ LZA CXU TMN MBN TFF WTM BJE NZY GNM AUQ ZTZ MFP NPD KVD ZFZ KZM ABS MOF TFY IOU INE CSQ PFR MMF COM JMQ BPP QTO CTE BFD UTI QUT GPG JVF ITT QTR IJF PGG TTQ ZWM VUU UVE BPR KPG ZTQ MOP MBH WVD BPM ZSU DFM BBZ IHD MFY MOF NBH WVD ICX MUA PJY QUD CTF EFG VEQ ZTF IOP WOQ IOA BIQ ZBZ LUT IUA CSY MFF QOS EJX TJZ LFQ LCQ IQX MBE CSQ NPD CTN WUT XMQ ITQ ZFB TZU VUT MVE CBX NBE PJA VZA CSE MT

 subtexts

 keyword - leave this empty if you want the program to show suggestions