Having a Glossary meant I could reduce the text on most pages, while expanding background for the definitions, and relating the ideas to other similar, contradictory, or more basic ideas.
The position of the cipher text letter in that row determines the column, and the plain text letter is at the top of that column.
Strength of this cipher is each plaintext letter can have multiple cipher text letters, one for each unique letter of the keyword. The letter frequency information is obscured.
But still not all knowledge of the plaintext structure is lost. Charles Babbage and Friedrich Wilhelm Kasiski demonstrated in the mid and late s respectively that even polyalphabetic ciphers provide is vulnerable to cryptanalysis.
Because the key and the plain text share the same frequency distribution of letters, and statistical method can be applied.
Transposition Ciphers All the techniques we have seen so far include substitution on a cipher text symbol for a plain text symbol. A very different kind of mapping is achieved by performing permutation on the plain text letters.
This technique is called transposition cipher. The simplest of all is rail fence technique. In this technique plaintext is written down as a sequence of diagonals and then read off as a sequence of rows. A more complex way is to write the message in a rectangle.
Write the message row by row and then read the message column by column, but permute the order of the columns. So order of the column will become key to the algorithm.
For this type of columnar transposition, is fairly straightforward and involves lying out the cipher text in a matrix and then playing around the with column position. To make transposition more secure once can perform more than on stage of transposition.
Hence, the foregoing message is reencrypted as below using the same algorithm. So we have 28 letters in the message and the original sequence of letters is 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 And after first order transposition we have 03 10 17 24 04 11 18 25 02 09 16 23 01 08 15 22 05 12 19 26 06 13 20 27 07 14 21 28 So above was somewhat regular structure but after the second order transposition, we have 17 09 05 27 24 16 12 07 10 02 22 20 03 25 15 13 04 23 19 14 11 01 26 21 18 08 06 28 So this is much less structured permutation.
And also much more difficult o cryptanalyze. Contemporary Cryptography As we have seen in the earlier methods breaking viginire cipher is possible and people are starting looking for better unbreakable cipher.
To achieve this we require change to the plaintext even if tis just 3 characters and it must produce significant change in the cipher text. Such that there is no relationship exist between plain and cipher text. It proves to be very secure though its already been broken on paper.
AES is a symmetric cipher which means it uses same key for encryption and decryption.
Simple block cipher As we can see in above graphic the plaintext is broken into blocks. The block size is typically of size bits.
Each block passes through the block algorithm using a key resulting in the final cipher text. And so change in plain text gives corresponding change in the cipher text.
Algorithm produces a block of cipher text and its XORed with the next block of plain text and submitted to the block algorithm using the same key. In case final block of plain text is smaller than a cipher block size, the plain text block is padded with a required number of bits.
Another block cipher mode AES uses a more sophisticated approach including byte substitution, column mixing. Key Management We have discussed a lot about symmetric cipher and the critical component is the use of key.
We have to make sure no unauthorized access occurs to key. And in case we lose a key eventually we lost our data too protected by that key.Open Source and Security.
As a cryptography and computer security expert, I have never understood the current fuss about the open source software movement. Research paper on cryptography and network security full Essay tip #2.
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