eec 688/788 secure and dependable computing lecture 3 wenbing zhao department of electrical and...

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

[email protected]@ieee.org

04/18/23EEC688: Secure & Dependable

Computing 1

04/18/2304/18/23 EEC688: Secure & Dependable ComputingEEC688: Secure & Dependable Computing Wenbing ZhaoWenbing Zhao

OutlineOutline

Introduction to cryptography Terminology Basic encryption methods One time pad

Symmetric-key algorithms DES, AES, etc

Cipher modes


Cryptography TerminologyCryptography Terminology Encryption is the process of encoding a message

so that its meaning is not obvious Equivalent terms: encode, encipher

Decryption is the reverse process, transforming an encrypted message back into its normal, original form Equivalent terms: decode, decipher

Plaintext: message to be encrypted Ciphertext: encrypted message


Cryptography TerminologyCryptography Terminology

The cryptosystem involves a set of rules for how to encrypt the plaintext and how to decrypt the ciphertext

Why encryption? It addresses the need for confidentiality of data, also helps

to ensure integrity It forms the basis of protocols that enable us to provide

security while accomplishing system or network tasks


Cryptography TerminologyCryptography Terminology

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


Symmetric EncryptionSymmetric Encryption The encryption and decryption keys are the same,

so P = D(K, E(K,P)) D and E are closely related. They are mirror-image

processes The symmetric systems provide a two-way channel

to their users The symmetry of this situation is a major advantage

of this type of encryption, but it also leads to a problem: key distribution


Symmetric EncryptionSymmetric Encryption DK(EK(P)) = P


Asymmetric EncryptionAsymmetric Encryption

Encryption and decryption keys come in pairs. The decryption key, KD, inverts the encryption of key KE, so that P = D(KD, E(KE,P))

Asymmetric encryption systems excel at key management


CryptologyCryptology Cryptology is the research into and study of

encryption and decryption; it includes both cryptography and cryptanalysis

Cryptography – art of devising ciphers Comes from Greek words for “secret writing”. It refers to the

practice of using encryption to conceal text Cryptanalysis – art of breaking ciphers

Study of encryption and encrypted messages, hoping to find the hidden meanings


CryptanalysisCryptanalysis Attempt to break a single message Attempt to recognize patterns in encrypted messages,

to be able to break subsequent ones Attempt to deduce the key, in order to break

subsequent messages easily Attempt to find weaknesses in the implementation or

environment of use of encryption Attempt to find general weaknesses in an encryption

algorithm


CryptanalysisCryptanalysis

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

short or long


Cryptanalysis Cryptanalysis –– Breaking Encryption Schemes Breaking Encryption Schemes Ciphertext-only: cryptanalyst has a quantity of

ciphertext and no plaintext Known plaintext: cryptanalyst has some matched

ciphertext and plaintext Chosen plaintext: cryptanalyst has the ability to

encrypt pieces of plaintext of his own choosing


Computing 12


Basic Encryption MethodsBasic Encryption Methods

Substitution ciphers: one letter is exchanged for another

Transposition ciphers: order of letters is rearranged


Substitution CiphersSubstitution Ciphers

Idea: each letter or group of letters is replaced by another letter or group of letters

Caesar cipher – circularly shift by 3 letters a -> D, b -> E, … z -> C More generally, shift by k letters, k is the key

Monoalphabetic cipher – map each letter to some other letter A b c d e f … w x y z Q W E R T Y … V B N M <= the key


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


Transposition CiphersTransposition Ciphers Substitution cipher – preserves order of plaintext

symbols but disguises them Transposition cipher – reorders (rearrange) symbols

but does not disguise them. It is also called permutation

With transposition, the cryptography aims for Widely spreading the information from the message or the

key across the ciphertext Transpositions try to break established patterns


Columnar TranspositionColumnar Transposition

Plaintext written in rows, number of columns = key length

Key is used to number the columns Ciphertext read out by columns, starting with

column whose key letter is lowest


Columnar TranspositionColumnar Transposition

A transposition cipher example


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


Computing 19


One-Time PadsOne-Time Pads

Disadvantages The key cannot be memorized, both sender and

receiver must carry a written copy with them Total amount of data can be transmitted is limited

by the amount of key available Sensitive to lost or inserted characters


Computing 20


Symmetric-Key AlgorithmsSymmetric-Key Algorithms

DES – The Data Encryption Standard AES – The Advanced Encryption Standard Other Ciphers Cipher Modes


Computing 21


Data Encryption StandardData Encryption Standard Developed by IBM. US standard for unclassified info (1977)

Same key for encryption as for decryption Encrypts in 64-bit blocks Uses 56-bit key Has 19 stages,

16 parameterized by different functions of the key


Computing 22


Triple DESTriple DES Triple DES – effectively increases the key length. It

uses two keys and three stages In first stage, the plaintext is encrypted using DES in the

usual way with K1

In second stage, DES is run in decryption mode, using K2 as the key

In third stage, another DES encryption is done with K1

Triple DES encryption Triple DES decryption


Computing 23


AES – The Advanced Encryption StandardAES – The Advanced Encryption Standard

AES is a result of a cryptographic contest Organized by NIST in 1997

Rules for AES proposals1. The algorithm must be a symmetric block cipher

2. The full design must be public

3. Key lengths of 128, 192, and 256 bits supported

4. Both software and hardware implementations required

5. The algorithm must be public or licensed on nondiscriminatory terms

Winner: Rijndael (from two Belgian cryptographers: Joan Daemen and Vincent Rijmen)


Computing 24


Other Symmetric-Key Other Symmetric-Key CiphersCiphers


Computing 25


Stream Ciphers Stream Ciphers Stream ciphers: convert one symbol of plaintext immediately into

a symbol of ciphertext The transformation depends only on the symbol, the key, and the

control information of the encryption algorithm


Computing 26


Block CiphersBlock Ciphers Block cipher: encrypts a group of plaintext symbols as one block

It works on blocks of plaintext and produce blocks of ciphertext The columnar transposition is an example of block ciphers


Computing 27


Cipher ModesCipher Modes A block cipher (e.g., AES & DES) is basically a

monoalphabetic substitution cipher using big characters Whenever the same plaintext block goes in the front end,

the same ciphertext block comes out the back end If you encrypt the plaintext abcdefgh 100 times with same

DES key, you get the same ciphertext 100 times An intruder can exploit this property to help subvert the

cipher


Computing 28


Electronic Code Book ModeElectronic Code Book Mode In ECB mode, each plaintext block is encrypted independently with

the block cipher ECB allows easy parallelization to yield higher performance.

However, no processing is possible before a block is seen


Computing 29


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

Example


Computing 30


Cipher Block Chaining ModeCipher Block Chaining Mode To avoid the ECB mode problem: replacing a block

will cause the plaintext decrypted starting at the replaced to become garbage

Exclusive OR the encrypted text with the next block of plaintext before encryption: Need an initialization vector (IV) to boostrap

C0 = E(P0 XOR IV), C1 = E(P1 XOR C0), etc.

Drawback: must wait until full 64-bit (128-bit) block to arrive to decrypt


Computing 31


Cipher Block Chaining ModeCipher Block Chaining Mode Exclusive OR the encrypted text with the next block

of plaintext before encryption: C0 = E(P0 XOR IV), C1 = E(P1 XOR C0), etc.

InitializationVector

Encryption Decryption


Computing 32


Cipher Feedback ModeCipher Feedback Mode Basic operation (Pi and Ci are blocks):

Ci = E(Ci-1) XOR Pi, Pi = E(Ci-1) XOR Ci, C0 = IV Issue: Losing a single bit or byte will ruin all data after that


Computing 33


Cipher Feedback ModeCipher Feedback Mode To enable byte-by-byte encryption

When plaintext byte n (Pn) arrives, DES algorithm operates a 64-bit register to generate a 64-bit ciphertext

Leftmost byte of that ciphertext is extracted and XORed with Pn

That byte is transmitted on the transmission line The shift register is shifted left 8 bits, causing Cn-8 to fall off

the left end, and Cn is inserted in the position just vacated at the right end by C9


Computing 34


Cipher Feedback ModeCipher Feedback Mode



Computing 35


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


Computing 36


Stream Cipher ModeStream Cipher Mode



Computing 37


Stream Cipher ModeStream Cipher Mode It is essential never to use the same (key, IV) pair

twice with a stream cipher because doing so will generate the same keystream each time

Using the same keystream twice exposes the ciphertext to a keystream reuse attack

Stream cipher mode is also called output feedback mode


Computing 38


Keystream Reuse AttackKeystream Reuse Attack Plaintext block, P0, is encrypted with the keystream to get P0

XOR K0 Later, a second plaintext block, Q0, is encrypted with the same

keystream to get Q0 XOR K0

An intruder who captures both ciphertext blocks can simply XOR them together to get P0 XOR Q0, which eliminates the key

The intruder now has the XOR of the two plaintext blocks If one of them is known or can be guessed, the other can also be

found In any event, the XOR of two plaintext streams can be attacked

by using statistical properties of the message


Computing 39


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


Computing 40

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?


Computing 41


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?


Computing 42


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)?


Computing 43


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?


Computing 44

eec 688/788 secure and dependable computing lecture 3 wenbing zhao department of electrical and...

Documents

encryption of key

encryption scheme

type of encryption

p slide

dependable computing

encrypted message slide

key management slide

key distribution slide