When you have one as long as seven characters, then the most likely scenario Possible but it is unlikely, and it becomes very unlikely when you find repeating runs of text of 4 or more letters. They initially represented different sections of text in the plaintext and, when encrypted by different parts of the key, they just happened to come out as identical chunks of ciphertext. It is possible that these identical sequences have appeared completely by coincidence, i.e. WLSGDXBEUNEESYIHJKOTPANLVMQXV GIFEWNAXHBWXVGLFGHCQVHTUIGGTQHARWXKISPOKTQTMCCLHWHGCPJXCSOXYUXKVSRXBHTWCWDRGXVGZEXGMAISVHWSPZPTXOCLLWZIFGGJDCXJPSLDFSVOZRXYQIUAHACWRACDCRGHXKĮHJGMETJAWPSVXCHZBCXWCLHGLTVGXQTMCJRGXKYGGDMTRCRXWSUIDTKPTCCNMQXKIDNGFCINGXEYWWIENXCPBBYPTWMCPCLJAMXROCEIFKMEIXWXRHSIXVGLLJGLJWHWKIFGGKSTENRWLDXWURXGVNUCECFSWPIFSTWLIįor reference, in this ciphertext, the phrase 'BSWLH' also occurs twice, and so does 'XVGL' and 'IFGG'. MCIOPJQWXBRWZLDXACQBHIQNZLKHRF GIFEWNAWRMQXSTKPIXWPMICRYOTKQGNRWKWSFVOEQFINXDVIMUWHRXLQANVMVXUCWPMQDLXAORYWLCYRLCVCQINIBMGDAQLTFRPOHHEZYOQWIQJXOEWIFHUOIWNVSPIQXBQZFRTQXALR XKIIMFUZZWPAJGWIUMBVNBEAJRRWIAXZKYBCRXMIGALXVCIXTHJAGLTKIFVNXIPYYPLIVEGJOCIDOBSWLHPPGILBTBJRKXKIGGILCSLQEUEFXSTNXHPBQMIXRJORVIXTMJRGFHGCOKPPCXJRGWRSBQTZTLNAIDGKXVGKKSDOCLHOHCKGYFBEUHKRFDGYVUQWTBCEUXRJWVGDPXWBSWLHWWZZERWJAEFXHVONYLWPBJWKMIXCHGMEARNHWSLXWPZEXHLJWHXKIZGTDIWXOXKINIMKYPDSROJHVHRHEGBHPALMSLHVGCXBTBYUSBIGFIVZETZNHPHRJXVEGKQTJWCV Section as long as seven characters that repeats itself: Page), and which was encrypted with a long key of 13 characters (thus making it even less likely that we would get the repeating runs of text in the ciphertext that we are after), there is a In fact, in the example in the box at the top of the screen, which was not contrived in any way (it was simply copied from the previous My example above has very obviously contrived English text, and you might wonder how likely it is that you will get the same section of plaintext encoded by the same section of the key.Īnd the answer is that it happens surprisingly often. Of the key, thus generating the same ciphertext. You hope that the ciphertext is long enough so that, by chance, the same piece of English text gets encrypted by the same section When looking for the key length, this is what you hope for. Occurrence of 'THE', it happens to fall at the same point in the key as the first occurrence and so the ciphertext is the same in both instances, i.e. But, coincidentally, when it comes to the third In the first two occasions, the ciphertext comes out differently because each 'THE' is encrypted by a different part of the key. In the example below, using the same Vigenère cipher from the previous page with a key length of 5, the three letters of 'THE' appear in the contrived phrase in the plaintext three times. Normally, when applying a Vigenère cipher, the same word in English comes out differently in the ciphertext, and this is because it depends on which parts of the key are being used to encrypt

Vigenere cipher decryption tool