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# CMPE 471 .

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﻿CMPE 471 BASIC ENCRYPTION AND DECRYPTION

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TERMINOLOGY & BACKGROUND Suppose S (Sender) needs to make an impression on R (Reciever). S depends the message to T , who will convey it to R ; T then turns into the transmission medium . In the event that an untouchable, O , needs the message and tries to get to it, we will call O an interceptor or gatecrasher .

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TERMINOLOGY & BACKGROUND Any time after S transmits through T , the message is uncovered, so O may attempt to get to the message: Block it, by avoiding it to reach to R : accessibility Intercept it, by perusing or listening to the message: mystery Modify it, by grabbing the message and evolving it: uprightness Fabricate a genuine looking message, organizing as though it originated from S : respectability.

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TERMINOLOGY & BACKGROUND Encryption (encode/encipher): Process of encoding a message with the goal that its significance is not all that self-evident. Unscrambling (translate/disentangle): Is the turn around process: changing a scrambled message once more into its typical shape. Cryptosystem: A framework for encryption and decoding Plaintext: The first type of the message Ciphertext: The scrambled type of the message.

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TERMINOLOGY & BACKGROUND Encryption Algorithms: Some encryption calculations utilize a key K , so that the ciphertext message relies on upon both the first plaintext message and the key esteem C = E(K,P) E is an arrangement of encryption calculations, and the key K chooses one particular calculation. Some of the time the encryption and decoding keys are the same; P = D(K, E(K,P)). This is called symmetric encryption since D and E are perfect representation forms. Different circumstances encryption and decoding keys come in sets. At that point a decoding key K alters the encryption of key K so that P = D(K , E(K ,P)) . Encryption calculations of this shape are called hilter kilter , in light of the fact that changing over C back to P is not quite recently switching the means of E . D E D E

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ENCRYPTION ALGORITHMS Original Plaintext Ciphertext Decryption ENCRYPTION

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ENCRYPTION ALGORITHMS Key Original Plaintext Ciphertext Encryption Decryption Symmetric Cryptosystem Encryption Key K Encryption Key K E D Original Plaintext Ciphertext Encryption Decryption Asymmetric Cryptosystem

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ENCRYPTION ALGORITHMS Cryptograpghy: Hidden composition, the act of utilizing encryption to disguise content. Cryptanalyst: Studies encryption and encoded messages, with the objective of finding the shrouded implications of the messages. Cryptology: Is the exploration into and investigation of encryption and decoding; it incorporates both cryptography and cryptanalysis.

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ENCRYPTION ALGORITHMS Substitution: One letter is traded for another Transposition: The request of the letters is modified

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MONOALPHABETIC CIPHERS (SUBSTITUTIONS) The Caesar Cipher: Named after Julius Caeser. Every letter is meant the letter a settled number of letters after it in the letters in order. Caesar used to move 3, so that plaintext letter p was enciphered as ciphertext letter c by the govern c = E (p ) = p +3 Plaintext A B C D E F G H I J K L M N O P Q R S T U V W Y Z Chiphertext d e f g h i j k l m n o p q r s t u v w y z a b c i

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MONOALPHABETIC CIPHERS (SUBSTITUTIONS) Using this encryption encode the underneath message TREATY IMPOSSIBLE Would be encoded as TREATY IMPOSSIBLE wu hd wb l p s r vv le o h

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MONOALPHABETIC CIPHERS (SUBSTITUTIONS) The example p + 3 is anything but difficult to retain and it is a straightforward figure. That undeniable example is likewise the real shortcoming of the Ceasar figure. A protected encryption ought not permit an interceptor to utilize a little piece to anticipate the whole example of the encryption. i

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EXERCISE I Please unravel the accompanying: dh ey vdedk duded wdpluflvlqh jlwwlp vrqud eludc jhcphbh jlwwlp zh rnyod jhoglp eyudgd ghuvlp zdu

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ANSWER ben bu sabah araba tamircisine gittim sonra biraz gezmeye gittim ve okula geldim burada dersim var

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EXERCISE II Please make the cryptanalysis of Caesar chipher.

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ANSWER Suppose you were attempting to break the accompanying ciphertext message: Wklv phvvdjh lv qrw wrr kdug wr euhdn The message has been enciphered with a 27-image letter set Worst of all the clear has been meant itself It demonstrates which are the little words In encryption spaces between words frequently are erased under the supposition that a honest to goodness reciever can breakmostmessagesintowordsfairlyeasily.

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ANSWER English has generally couple of little words, for example, am, is, to, be, he, we, and, are, you, she... One assault is to substitute known short words at fitting spots in the ciphertext and attempt to substituting for coordinating characters different places in the ciphertext. A more grounded piece of information is the rehashed R in the word wrr : see, as well, include, odd, off

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ANSWER The cryptanalysis here is specially appointed Uses finding in view of estimates rather than strong standards. Another approach is to consider which letters usually begin words, which letters ordinarily end words, and which prefixes and postfixes are normal.

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Polyalphabetic Substitution Ciphers The shortcoming of monoalphabetic figures is that their recurrence appropriation mirrors the conveyance of the hidden letters in order. A figure that is all the more cryptographicaly secure would show a fairly level dissemination, which gives no data to cryptanalyst. One approach to level the conveyance is to join appropriations that are high with ones that are low: If T is enciphered as an and b , and if X is additionally enciphered as an and b , the high recurrence of T blends with the low recurrence of X to create a more direct dissemination for an and b .

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Polyalphabetic Substitution Ciphers We can join two appropriations by utilizing two separate encryption letters in order All charaters in odd places of the plaintext message All characters in even positions A B C D E F G H I J K L M N O P Q R S T U V W X Y Z a d g j m p s v y b e h k n q t w z c f i l o r u x A B C D E F G H I J K L M N O P Q R S T U V W X Y Z n s x c h m r w b g l q v a f k p u z e j o t y d i Table for odd positions Table for even positions

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Polyalphabetic Substitution Ciphers The main table uses the change ∏ ı ( λ ) = (3* λ ) mod 26 The second uses the stage ∏ 2 ( λ ) = ((5* λ ) + 13) mod 26 Encryption with these tables would be TREATY IMPOSSIBLE TREAT YIMPO SSIBL E f u m nf dyvtf czysh h

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Polyalphabetic Substitution Ciphers Notice that the twofold S gets to be cz and that the two E s are enciphered as m and h Polyalphabetic encryption straightens the recurrence dispersion of the plaintext extensively.

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EXERCISE 3 Please make the cryptanalysis of polyalphabetic substitutions

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ANSWER With a little assistance from recurrence dispersions and letter designs you can break monoalphabetic substitution by hand With the guide of PC projects and with a satisfactory measure of ciphertext, a great cryptanalyst can break such a figure in 60 minutes. In a few applications the possibility of one day\'s exertion may not bode well and it might be sufficient to secure the message. There are two instruments that can decode messages composed even with an extensive number of letter sets The Kasiski technique for rehashed designs : the strategy depends on the consistency of English. In the event that a message is encoded with n letter sets in cyclic revolution, and if a specific word or letter bunch apperas k times in a plaintext message, it ought to be encoded around k/n times from a similar letters in order. List of Coincidence : to rate how well a specific dispersion coordinates the circulation of letters in English. The list of fortuitous event is a measure of the variety between frequencies in a dispersion.

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Summary of Substitutions are powerful cryptographic gadgets utilized as a part of discretionary interchanges and showed up in the puzzles of Arthur Conan Doyle, Allan Poe, Agatha Cristie... The presentation of substitution figures has likewise presented a few cryptoanalytic devices: Frequency circulation Index of fortuitous event Consideration of exceedingly likely letters and plausible words Repeated example examination and the Kasiski approach Persistence, association, inventiveness, and luckiness

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Transpositions (Permutations) The objective of substitution is disarray, an endeavor to make it hard to decide how a message and key were changed into ciphertext. A transposition is an encryption in which the letters of the message are adjusted. The objective is dispersion, spreading the data from the message or the key out broadly over the ciphertext: stage.

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Transpositions (Permutations) Plaintext message five-segment transposition Ciphertext is framed by navigating the segments

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Transpositions (Permutations) The subsequent ciphertext would then be perused as tssoh oaniw haaso lrsto imghw utpir seeoa mrook istwc nasns The length of this message happened to be a numerous of five, so all sections turned out a similar length If the message length is not a various of the length of a line, the last segments will be a letter short.

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Transpositions (Permutations) Encipherment/Decipherment Complexity Involves no extra work past organizing the letters and understanding them off once more. The calculation is steady in the measure of work per character, and the ideal opportunity for the calculation is corresponding to the length of the message This calculation requires capacity for all characters of the message, so the space required is not consistent but rather depends specifically on the length of the message. Due to the storage room and the defer included, it is not fitting for long messages.

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Transpositions (Permutations) Diagrams: Characteri

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