We expect the candidate keys to be at least ALPHA percent of PHI_P. So we are using ALPHA to represent a requirement for marking candidate key intervals. The problem is, there are many observed PHI values that meet this criteria. To choose a key length that looks good we want our observed PHI to fall between PHI_R and PHI_P: the closer to PHI_P the better. In this test there is a PHI_R (PHI random) and a PHI_P (PHI plain text) and an observed PHI. A better KAPPA_P value will lead to more accurate key length predictions.ĪLPHA Threshold: The algorithm depends on a PHI test to determine its probable key lengths. One would use this option to get an idea of the possible key lengths and if the KAPPA_P value needs to be adjusted. Only IOC: This means the program will only print the IOC (Index of Coincidence) information about the ciphertext. To get the KAPPA_P for your target plain text use the option for getting KAPPA_P below. One would use this option if the target plain text is something other than standard ASCII. Set KAPPA_P: The KAPPA_P value is described below. You can find the possible key lengths in the data output. This gives the user the option of specifying the length, in the case where the algorithm chooses the wrong one. If it chooses an incorrect value for the key length you will not get the correct solution. The algorithm is perfectly capable of finding this value on its own, but its choice is based on the most probable length. Key Length: This is so you can specify the key length manually. This allows you to take a file encrypted with the XOR cipher and, with only some assumed knowledge about its format, deduce both the plain text and the key. One intelligent guy amazingly figured out the hidden trick and decrypted the avi file, which otherwise can only be played in 3wplayer. Recently, probably the makers of a media player 3wplayer(reported by Norton Antivirus as a spyware threat), encoded DVD rip avi files and put them up on torrent portals. Its primary merit is that it is simple to implement, and that the XOR operation is computationally inexpensive. By itself, using a constant repeating key, a simple XOR cipher can trivially be broken using frequency analysis. The XOR operator is extremely common as a component in more complex ciphers. To decrypt the output, merely reapplying the key will remove the cipher. With this logic, a string of text can be encrypted by applying the bitwise XOR operator to every character using a given key. Where ^ denotes the exclusive disjunction (XOR) operation.
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