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Polyalphabetic systems in .NET Build USS Code 128 in .NET Polyalphabetic systems

Polyalphabetic systems generate, create uss code 128 none for .net projects iReport ALTHOUGH I AM A Code 128 Code Set C for .NET N OLD MAN NIGHT IS GENERALLY MY TIME FOR WALKING IN THE SUMMER I OFTEN LEAVE HOME EARLY IN THE MORNING AND ROAM ABOUT FIELDS AND LANES ALL DAY. (which, with pu nctuation removed, is the opening of 1 of Dickens s The Old Curiosity Shop). Whilst such polyalphabetic Julius Caesar systems are harder to solve than the simple version they still have the inherent weakness that once we have identi ed the cipher letter for space , or some other high frequency letter, in a shifted alphabet the 25 other letters of that alphabet follow immediately. Then, if we are unsure of the cipher letter for space in one or more alphabets, knowledge of some of the letters in incomplete words may be suf cient to enable us to complete them and so obtain the full solution, as we did in the example above.

This weakness disappears if, instead of using alphabets in normal order shifted by different numbers of places according to a key, we use a set of alphabets all in different orders and independent of each other. This modi cation however raises two other problems:. (1) how do we o barcode standards 128 for .NET btain such alphabets (2) how can the shuf ed alphabets be made available to the intended recipient(s) without revealing them to unauthorised interceptors . These questions are of fundamental importance for if the shuf ed alphabets are obtained by some simple method, as in Julius Caesar for example, the cryptanalyst will soon spot the method and decryption will be made that much easier. On the other hand the intended recipient must know which alphabets are being used or, alternatively, how to obtain them. There are a number of ways of resolving both of these problems, some of which we shall come to later, but two of the possible solutions to (2) we describe below.

First, however, we de ne two terms that are relevant to cryptographic systems generally.. Indicators When the origin ator of a message has a choice of some parameters relating to its encipherment which he needs to make available to the recipient(s) he will probably provide this information, possibly in an enciphered form, in an indicator which may precede or follow the message or be hidden within it.. chapter 3 Thus in the exa mple above the key, 7-5-11, would need to be provided somewhere, either in information sent in advance, or enciphered in some pre-arranged cipher and perhaps hidden in the message at some speci ed place. In this case the key itself, 7-5-11, can serve as the indicator, but it is unlikely to be sent in that form..

Depths When two or mor e messages are enciphered by the same system with identical parameters (components, keys, parts, settings etc.) they are said to be in depth. So, if two Vigen re messages are sent using the same keyword they are in depth; but if they are sent using different keywords, even if the keywords are of the same length, they are not in depth.

If, however, two Vigen re messages have keywords which are of the same length and which have some identical letters in corresponding positions the cipher messages will be in partial depth. This would not necessarily be true of other systems of encipherment where the slightest change in the indicator puts the messages out of depth. Whether a cryptanalyst can take advantage of nding two or more messages in depth depends upon the system which has been used for enciphering them.

In some cases, such as simple substitution or Vigen re systems, he should certainly be able to do so but in others, such as the two-letter cipher systems described in 5, depths are of much less use. Broadly speaking, if the encipherment system is done on a letter-by-letter basis then depths may be identi able and useful to the cryptanalyst but if the encipherment involves two or more letters at a time the depths, if recognisable at all, may not be of much use..

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