Block vs Stream Ciphers

Slides:

Advertisements

Similar presentations

Cryptography and Network Security Chapter 6 Block Cipher Modes of Operation Lecture slides by Lawrie Brown for “Cryptography and Network Security”, 5/e,

Advertisements

Block Cipher Modes of Operation and Stream Ciphers

ECE454/CS594 Computer and Network Security

“Advanced Encryption Standard” & “Modes of Operation”

Encipherment Using Modern Symmetric-Key Ciphers. 8.2 Objectives ❏ To show how modern standard ciphers, such as DES or AES, can be used to encipher long.

Modern Symmetric-Key Ciphers

Cryptography and Network Security Chapter 6 Fifth Edition by William Stallings Lecture slides by Lawrie Brown.

Cryptography and Network Security Third Edition by William Stallings Lecture slides by Lawrie Brown.

Cryptography and Network Security Chapter 6. Chapter 6 – Block Cipher Operation Many savages at the present day regard their names as vital parts of themselves,

Cryptography1 CPSC 3730 Cryptography Chapter 6 Triple DES, Block Cipher Modes of Operation.

Chapter 3 – Block Ciphers and the Data Encryption Standard Jen-Chang Liu, 2004 Adopted from lecture slides by Lawrie Brown.

Cryptography and Network Security Chapter 6

Lecture 23 Symmetric Encryption

CSCE 790G: Computer Network Security Chin-Tser Huang University of South Carolina.

Chapter 3 – Block Ciphers and the Data Encryption Standard

Block Cipher Transmission Modes CSCI 5857: Encoding and Encryption.

Modes of Operation. Topics  Overview of Modes of Operation  EBC, CBC, CFB, OFB, CTR  Notes and Remarks on each modes.

symmetric key cryptography

Dr. Lo’ai Tawalbeh 2007 Chapter 3: Block Ciphers and the Data Encryption Standard Dr. Lo’ai Tawalbeh New York Institute of Technology (NYIT) Jordan’s Campus.

Applied Cryptography Example: AES. Advanced Encryption Standard "It seems very simple." "It is very simple. But if you don't know what the key is it's.

Cryptography and Network Security Chapter 6. Multiple Encryption & DES  clear a replacement for DES was needed theoretical attacks that can break it.

Slide 1 Stream Ciphers uBlock ciphers generate ciphertext Ciphertext(Key,Message)=Message  Key Key must be a random bit sequence as long as message uIdea:

Cryptography and Network Security Block Ciphers and DES, and modes of operation M. Sakalli Reviewed, from Stallings.

CSCE 715: Network Systems Security Chin-Tser Huang University of South Carolina.

3DES and Block Cipher Modes of Operation CSE 651: Introduction to Network Security.

Multiple Encryption & DES  clearly a replacement for DES was needed Vulnerable to brute-force key search attacks Vulnerable to brute-force key search.

Chapter 9: Algorithms Types and Modes Dulal C. Kar Based on Schneier.

More About DES Cryptography and Network Security Reference: Sec 3.1 of Stallings Text.

Chapter 2 (B) – Block Ciphers and Data Encryption Standard.

1.1 Chapter 8 Encipherment Using Modern Symmetric-Key Ciphers Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

CSCE 815 Network Security Lecture 4 DES again. Outline History of DES DES revisited.

Modes of Operation INSTRUCTOR: DANIA ALOMAR. Modes of Operation A block cipher can be used in various methods for data encryption and decryption; these.

Data Security and Encryption (CSE348) 1. Lecture # 13 2.

Stream Ciphers and Block Ciphers A stream cipher is one that encrypts a digital data stream one bit or one byte at a time. Examples of classical stream.

Cryptography and Network Security Chapter 6 Fifth Edition by William Stallings Lecture slides by Lawrie Brown.

Chapter 3 – Block Ciphers and the Data Encryption Standard.

Cryptography and Network Security. Chapter 3 – Block Ciphers and the Data Encryption Standard All the afternoon Mungo had been working on Stern's code,

Lecture 23 Symmetric Encryption

Privacy and Integrity: “ Two Essences of Network Security” Presenter Prosanta Gope Advisor Tzonelih Hwang Quantum Information and Network Security Lab,

1 Block Ciphers (DES) Fourth Edition by William Stallings Lecture slides by Lawrie Brown (Changed by Somesh Jha)

Depart. of Computer Science and Engineering

Cipher Transmission and Storage Modes Part 2: Stream Cipher Modes CSCI 5857: Encoding and Encryption.

Block Cipher Modes Last Updated: Aug 25, ECB Mode Electronic Code Book Divide the plaintext into fixed-size blocks Encrypt/Decrypt each block independently.

Modes of Operation block ciphers encrypt fixed size blocks – eg. DES encrypts 64-bit blocks with 56-bit key need some way to en/decrypt arbitrary amounts.

Block Cipher Encrypting a large message Electronic Code Book (ECB) message m1 m2 m3 m4 m5 m6 c1 c2 c3 c4 c5 c6 E E E Secret.

CS480 Cryptography and Information Security

Modes of Operation.

Algorithm Modes ECB, CBC, CFB, OFB.

Computer and Network Security

CSCE 715: Network Systems Security

(Data Encryption Standard)

Chapter 6 – Block Cipher Operation

Cryptography and Network Security Chapter 6

Cryptography and Network Security Chapter 6

Outline Desirable characteristics of ciphers Stream and block ciphers

Feistel Cipher DES.

Block Cipher Modes CS 465 Make a chart for the mode comparisons

Cryptography and Network Security

Cryptography and Network Security Chapter 6

Algorithm Types & Algorithm Modes

Cryptography and Network Security Chapter 6

Cryptography and Network Security Chapter 6

Advanced Encryption Standard (AES)

Cryptography and Network Security Chapter 6

Cryptography and Network Security Chapter 6

Counter Mode, Output Feedback Mode

Elect. Codebook, Cipher Block Chaining

Secret-Key Encryption

Presentation transcript:

Block vs Stream Ciphers Block ciphers process messages in blocks, each of which is then en/decrypted Like a substitution on very big characters 64-bits or more Many current ciphers are block ciphers Stream ciphers process messages a bit or byte at a time when en/decrypting

Modern Block Ciphers One of the most widely used types of cryptographic algorithms Provide secrecy /authentication services Most symmetric block ciphers are based on a Feistel Cipher Structure

Modes of Operation Block ciphers process data in blocks e.g. 64-bits (DES, 3DES) or 128-bits (AES) Longer messages must break up and possibly pad end to blocksize multiple There 5 five modes of operation for this defined in NIST SP 800-38A modes are: ECB, CBC, CFB, OFB, CTR

Modes of Operation Block ciphers encrypt fixed size blocks eg. DES encrypts 64-bit blocks with 56-bit key Need some way to en/decrypt arbitrary amounts of data in practise ANSI X3.106-1983 Modes of Use (now FIPS 81) defines 4 possible modes Subsequently 5 defined for AES & DES Have block and stream modes

Electronic Codebook Book (ECB) Message is broken into independent blocks which are encrypted Each block is a value which is substituted, like a codebook, hence name Each block is encoded independently of the other blocks Ci = DESK1(Pi) Uses: secure transmission of single values

Electronic Codebook Book (ECB)

Advantages and Limitations of ECB Message repetitions may show in ciphertext If aligned with message block Particularly with data such graphics Or with messages that change very little, which become a code-book analysis problem Weakness is due to the encrypted message blocks being independent Main use is sending a few blocks of data

Cipher Block Chaining (CBC) Message is broken into blocks Linked together in encryption operation Each previous cipher blocks is chained with current plaintext block, hence name Use Initial Vector (IV) to start process Ci = DESK1(Pi XOR Ci-1) C-1 = IV Uses: bulk data encryption, authentication

Cipher Block Chaining (CBC)

CBC Cj = Ek [ Cj-1 + Pj ] Dk [ Cj ]=Dk [ Ek( Cj-1 + Pj) ] Dk[ Cj]= ( Cj-1 + Pj) Cj-1 + Dk [Cj ]=Cj-1 + Cj-1 + Pj Cj-1+Dk [Cj] = Pj

Advantages and Limitations of CBC Ciphertext block depends on all blocks before it Any change to a block affects all following ciphertext blocks Need Initialization Vector (IV) Which must be known to sender & receiver If sent in clear, attacker can change bits of first block, and change IV to compensate Hence IV must either be a fixed value Or must be sent encrypted in ECB mode before rest of message

Cipher FeedBack (CFB) Message is treated as a stream of bits Added to the output of the block cipher Result is feed back for next stage (hence name) Standard allows any number of bit (1,8, 64 or 128 etc) to be feed back denoted CFB-1, CFB-8, CFB-64, CFB-128 etc Most efficient to use all bits in block (64 or 128) Ci = Pi XOR DESK1(Ci-1) C-1 = IV Uses: stream data encryption, authentication

Cipher FeedBack (CFB)

Advantages and Limitations of CFB Appropriate when data arrives in bits/bytes Most common stream mode Limitation is need to stall while do block encryption after every n-bits Note that the block cipher is used in encryption mode at both ends Errors propogate for several blocks after the error

Output FeedBack (OFB) Message is treated as a stream of bits Output of cipher is added to message Output is then feed back (hence name) Feedback is independent of message Can be computed in advance Ci = Pi XOR Oi Oi = DESK1(Oi-1) O-1 = IV Uses: stream encryption on noisy channels

Output FeedBack (OFB)

Advantages and Limitations of OFB Bit errors do not propagate More vulnerable to message stream modification. Hence must never reuse the same sequence (key+IV) Sender & receiver must remain in sync Originally specified with m-bit feedback Subsequent research has shown that only full block feedback (ie CFB-64 or CFB-128) should ever be used

Counter (CTR) Similar to OFB but encrypts counter value rather than any feedback value Must have a different counter value for every plaintext block (never reused) Ci = Pi XOR Oi Oi = DESK1(i) Uses: high-speed network encryptions

Counter (CTR)

Advantages and Limitations of CTR Efficiency can do parallel encryptions in h/w or s/w can preprocess in advance of need good for bursty high speed links Random access to encrypted data blocks Provable security (good as other modes) But must ensure never reuse key/counter values, otherwise could break (cf OFB)

Block Cipher Modes of Operation