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Information and data security block cipher and the data encryption standard (des)
- 1. Block Cipher andthe data encryption standard (DES) Lecture slides By Rasha
- 2. Content Block CipherPrinciples The Data Encryption Standard DES Details DES Design Issues and Attacks
- 3. The objectives nowlook at modern block ciphers one of the most widely used types of cryptographic algorithms provide secrecy /authentication services focus on DES (Data Encryption Standard) to illustrate block cipher design principles
- 4. Cryptography Classification Theold encryption and decryption techniques before the implementation of computer system are called classical techniques , with implementation computer are called modern techniques. However , cryptography system (classical or modern ) are generally classified along three independent dimensions: 1- type of operations : used for transforming plaintext to ciphertext . all encryption algorithm are based on general principle: a) substitution b) transposition c) bit manipulation
- 5. Cryptography Classification 2- Numberof keys used a) symmetric: if the same key is used by both the sender and receiver for encryption and decryption it might be called single key ,secret key, conventional encryption. b) asymmetric: each sender and receiver using different key. 3- The way in which plaintext processed a) block cipher: the input massage is divided in the blocks of elements and each block is processes at a time ,producing an output block for input. b)stream cipher :the input elements are processed individually , producing an output as one element at a time too.
- 6. Block Ciphers Encryptdata one block at a time „Used in broader range of applications „Typical block size 64 – 128 bits 128 bits „Most algorithms based on a structure referred to as Feistel block cipher
- 7. Block vs StreamCiphers
- 8. Block cipher principles n-bit block cipher takes n bit plaintext and produces n bit ciphertext 2n possible different plaintext blocks Encryption must be reversible (decryption possible) Each plaintext block must produce unique ciphertext block Total transformations is 2n!
- 9. Ideal Block Cipher keyis mapping ; Key length 16 × 4 bits = 64 bits . i.e. concatenate all bits of ciphertext table
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- 11. Ideal Block Cipher n-bit input maps to 2n possible input states Substitution used to produce 2n output states Output states map to n-bit output Ideal block cipher allows maximum number of possible encryption mappings from plaintext block Problems with ideal block cipher: – Small block size: equivalent to classical substitution cipher; cryptanalysis based on statistical characteristics feasible – Large block size: key must be very large; performance/implementation problems Key length : – In general, key length is 2n × n – „Actual block size is at least 64 bit ( „Key length will be 264× 64 ≈ 1021 „bits)
- 12. Feistel Structure forBlock Ciphers Feistel proposed applying two or more simple ciphers in sequence so final result cryptographically stronger than component ciphers n-bit block length; k-bit key length; 2k transformations (rather than 2n !) Feistel cipher alternates: substitutions, transpositions (permutations) Applies concepts of diffusion and confusion Applied in many ciphers today Approach: – Plaintext split into halves – Subkeys (or round keys) generated from key – Round function, F, applied to right half – Apply substitution on left half using XOR – Apply permutation: interchange to halves implements Shannon’s S-P net concept
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- 14. Confusion and Diffusion Diffusion – Statistical nature of plaintext is reduced in ciphertext – E.g. A plaintext letter affects the value of many ciphertext letters – How: repeatedly apply permutation (transposition) to data, and then apply function Confusion – Make relationship between ciphertext and key as complex as possible – Even if attacker can find some statistical characteristics of ciphertext, still hard to find key
- 15. Using the FeistelStructure Exact implementation depends on various design features Block size, e.g. 64, 128 bits: larger values leads to more diffusion Key size, e.g. 128 bits: larger values leads to more confusion, resistance against brute force Number of rounds, e.g. 16 rounds Subkey generation algorithm: should be complex Round function F: should be complex Other factors include fast encryption in software and ease of analysis
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- 17. Data Encryption Standard(DES) Symmetric block cipher – 56-bit key, 64-bit input block, 64-bit output block One of most used encryption systems in world – Developed in 1977 by NBS/NIST – Designed by IBM (Lucifer) with input from NSA – Principles used in other ciphers, e.g. 3DES, IDEA.
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- 20. 3: Expansion permutation(E ) 4 : Permutation Function (P) Permutation Tables for DES
- 21. Single Round ofDES Algorithm 21
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- 23. Definition of DESS-Boxes
- 24. Definition of DESS-Boxes
- 25. DES Key ScheduleCalculation 25
- 26. Table 3.2 DES Example Note: DES subkeysare shown as eight 6-bit values in hex format (Table can be found on page 75 in textbook)
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- 28. Avalanche Effect Aim:small change in key (or plaintext) produces large change in ciphertext Avalanche effect is present in DES (good for security) Following examples show the number of bits that change in output when two different inputs are used, differing by 1 bit – Plaintext 1: 02468aceeca86420 – Plaintext 2: 12468aceeca86420 – Ciphertext difference: 32 bits – Key 1: 0f1571c947d9e859 – Key 2: 1f1571c947d9e859 – Ciphertext difference: 30
- 29. Table 3.3 AvalancheEffect in DES: Change in Plaintext
- 30. Table 3.4 AvalancheEffect in DES: Change in Key
- 31. Table 3.5 Average TimeRequired for Exhaustive Key Search
- 32. Key size Although64 bit initial key, only 56 bits used in encryption (other 8 for parity check) 256 = 7.2 x 1016 – Today: 56 bits considered too short to withstand brute force attack 3DES uses 128-bit keys
- 33. Attacks on DES Timing Attacks – Information gained about key/plaintext by observing how long implementation takes to decrypt – No known useful attacks on DES Differential Cryptanalysis – Observe how pairs of plaintext blocks evolve – Break DES in 247 encryptions (compared to 255); but require 247 chosen plaintexts Linear Cryptanalysis – Find linear approximations of the transformations – Break DES using 243 known plaintexts
- 34. DES Algorithm Design DES was designed in private; questions about the motivation of the design – S-Boxes provide non-linearity: important part of DES, generally considered to be secure – S-Boxes provide increased confusion – Permutation P chosen to increase diffusion
- 35. Summary have considered: –block vs stream ciphers – Feistel cipher design & structure – DES » details » strength