EEC 688/788 EEC 688/788 Secure and Dependable Secure and Dependable Computing Computing Lecture 3 Lecture 3 Wenbing Zhao Wenbing Zhao Department of Electrical and Computer Department of Electrical and Computer Engineering Engineering Cleveland State University Cleveland State University [email protected][email protected]06/16/22 EEC688: Secure & Dependable Computing 1
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EEC 688/788 Secure and Dependable Computing Lecture 3 Wenbing Zhao Department of Electrical and Computer Engineering Cleveland State University [email protected].
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EEC 688/788EEC 688/788Secure and Dependable Secure and Dependable ComputingComputing
Lecture 3Lecture 3
Wenbing ZhaoWenbing ZhaoDepartment of Electrical and Computer EngineeringDepartment of Electrical and Computer Engineering
Cleveland State UniversityCleveland State University
The encryption and decryption rules are called encryption and decryption algorithms
Encryption/decryptions algorithms often use a device called a key, denoted by K, so that the resulting ciphertext depends on the original plaintext message, the algorithm, and the key value
An encryption scheme that does not require the use of a key is called a keyless cipher
Traffic analysis: attempt to infer some meaning without even breaking the encryption, e.g., Noticing an unusual frequency of communication Determining something by whether the communication was
Cryptanalysis of Substitution Cryptanalysis of Substitution Ciphers Ciphers Brute force cryptanalysis would have to try 26! permutations of
a particular ciphertext message Smarter way: use frequencies of letters, pairs of letter etc., or by
guessing a probable word or phrase. Most frequently occurred Letters: e, t, o, a, n, … Digrams: th, in, er, re, an, … Trigrams: the, ing, and, ion, ent Words: the, of, and, to, a, in, that, …
When messages are long enough, the frequency distribution analysis quickly betrays many of the letters of the plaintext
One-Time PadsOne-Time Pads One-time pad: construct an unbreakable cipher
Choose a random bit string as the key Convert the plaintext into a bit string Compute the XOR of these two strings, bit by bit The resulting ciphertext cannot be broken, because in a
sufficiently large sample of ciphertext, each letter will occur equally often, as will every digram, every trigram, and so on
=> There is simply no information in the message because all possible plaintexts of the given length are equally likely
Electronic Code Book Mode - Electronic Code Book Mode - ProblemsProblems In ECB, plaintext patterns are not concealed
Each identical block of plaintext gives an identical block of ciphertext. The plaintext can be easily manipulated by removing, repeating, or interchanging blocks
Stream Cipher ModeStream Cipher Mode To be insensitive to transmission error, an arbitrarily
large sequence of output blocks, called the keystream, is treated like a one-time pad and XORed with the plaintext to get the ciphertext It works by encrypting an IV, using a key to get an output
block The output block is then encrypted, using the key to get a
second output block This block is then encrypted to get a third block, and so on
The keystream is independent of the data, so (1) It can be computed in advance (2) It is completely insensitive to transmission errors
Counter ModeCounter Mode To allow random access to encrypted data
The IV plus a constant is encrypted, and the resulting ciphertext XORed with the plaintext
By stepping the IV by 1 for each new block, it is easy to decrypt a block anywhere in the file without first having to decrypt all of its predecessors
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04/18/2304/18/23 EEC693: Secure and Dependable ComputingEEC693: Secure and Dependable Computing Wenbing ZhaoWenbing Zhao
ExerciseExercise
Q1. Assuming that the DES block cipher is used in the Electronic Code Book mode. If one bit in a block of ciphertext is inverted during transmission, how many bits will likely be damaged after decryption at the receiver?
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04/18/2304/18/23 EEC693: Secure and Dependable ComputingEEC693: Secure and Dependable Computing Wenbing ZhaoWenbing Zhao
ExerciseExercise
Q2. Assuming that the DES block cipher is used in the Cipher Block Chaining mode. If one bit of ciphertext is inverted during transmission, how many bits will likely be damaged after decryption at the receiver?
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04/18/2304/18/23 EEC693: Secure and Dependable ComputingEEC693: Secure and Dependable Computing Wenbing ZhaoWenbing Zhao
ExerciseExercise
Q3. Assuming that the DES block cipher is used in the Cipher Feedback mode. If one bit of ciphertext is inverted during transmission, how many bits will likely be damaged after decryption at the receiver (for both variations)?
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04/18/2304/18/23 EEC693: Secure and Dependable ComputingEEC693: Secure and Dependable Computing Wenbing ZhaoWenbing Zhao
ExerciseExercise
Q4. Assuming that the DES block cipher is used in the Stream Cipher mode (it is also called output feedback mode). If one bit of ciphertext is inverted during transmission, how many bits will likely be damaged after decryption at the receiver?