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NETWORK SECURITY

NETWORK SECURITY. Outline. Conventional Encryption Principles Conventional Encryption Algorithms Cipher Block Modes of Operation Location of Encryption Devices Key Di s tribution. Conventional Encryption Principles. An encryption scheme has five ingredients:

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NETWORK SECURITY

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  1. NETWORK SECURITY

  2. Outline • Conventional Encryption Principles • Conventional Encryption Algorithms • Cipher Block Modes of Operation • Location of Encryption Devices • Key Distribution

  3. Conventional Encryption Principles • An encryption scheme has five ingredients: • Plaintext: The original message or data • Encryption algorithm: Performs various substitutions and transformations on the plaintext • Secret Key: Input to the encryption algorithm • Ciphertext: Scrambled message produced as output • Decryption algorithm: Encryption algorithm run in reverse • Security depends on the secrecy of the key, not the secrecy of the algorithm

  4. Conventional Encryption Principles

  5. Cryptography • Classified along three independent dimensions: • The type of operations used for transforming plaintext to ciphertext • substitution / transposition 代换/置换 • The number of keys used • symmetric (single key) • asymmetric (two-keys, or public-key encryption) • The way in which the plaintext is processed • block / stream

  6. Cryptanalysis • Process of attempting to discover the plaintext or key • An encryption scheme is computationally secure if the ciphertext meets one of these criteria • cost of breaking the cipher exceeds the value of the information • time requires to break the cipher exceeds the useful lifetime of the information

  7. Attacks On Encrypted Msgs

  8. Average time required for exhaustive key search

  9. Feistel Cipher Structure • Virtually all conventional block encryption algorithms, including DES have a structure first described by Horst Feistel of IBM in 1973 • The realisation of a Fesitel Network depends on the choice of the following parameters and design features .

  10. 第i轮加密变换： Li＝Ri－1 Ri＝Li－1 F(Ri－1,Ki)

  11. Feistel Cipher Structure • Block size: larger block sizes mean greater security • Key Size: larger key size means greater security • Number of rounds: multiple rounds offer increasing security • Subkey generation algorithm: greater complexity will lead to greater difficulty of cryptanalysis. • Fast software encryption/decryption: the speed of execution of the algorithm becomes a concern

  12. Conventional Encryption Algorithms • Data Encryption Standard (DES) • The most widely used encryption scheme • DES is a block cipher • The plaintext is processed in 64-bit blocks • The key is 56-bits in length

  13. 数据加密标准DES • C = DES (K, M) • Block size = 64 bits • Key size = 56 bits • Number of rounds = 16 • IP - Initial Permutation • IP-1 - The inverse of IP • f - A nonlinear function • Ki - Round i subkey (48 bits) • Each Feistel block can be described as • Li = Ri-1 • Ri = Li-1 f (Ri-1, Ki) 32-bit Swap

  14. 明文64位输入 初始密钥64位 移位变换16次 初始置换IP Feistel Cipher 产生16个48位子密钥 初始逆置换IP－1 密文64位输出 数据加密标准DES DES 简化示意图

  15. 数据加密标准DES-The Initial Permutation How to read this table? The 58th bit of input x will be the 1st bit of outputIP(x), the 50th bit of x is the second bit ofIP(x), etc. • even bits to LH half, odd bits to RH half • Do not affect security

  16. Li-1 Ri-1 f  Ki+1 Li Ri 数据加密标准DES －Feistel Block • In Round i, • Separate message block into two 32-bit halves, Li and Ri • Introduce a “complex” nonlinear function f • f has two inputs: Ri and a 48-bit round key, Ki • “adding”Li and the output of f Li+1 = Ri Ri+1 = Li f(Ri, Ki+1)

  17. 数据加密标准DES －Nonlinear Function f(Ri-1, Ki) : (32 bits, 48 bits)  32 bits • expand R to 48 bits R’ using permutation and repetition • R’ XOR with K • subdivide the result into 8 blocks of 6 bits each • each 6-bit block goes through a S-box to produce 4 bits • permute the 32-bit string

  18. 数据加密标准DES －Nonlinear Function f(Ri-1, Ki) : (32 bits, 48 bits)  32 bits

  19. 数据加密标准DES －Nonlinear Function f(Ri-1, Ki) : (32 bits, 48 bits)  32 bits Ri－1（32bit）

  20. 数据加密标准DES － Expansion

  21. 数据加密标准DES － S-Box • There are eight S-boxes. • Each takes 6 bits to 4 bits. • Take the 6-bit input b1, b2, b3, b4, b5, b6 • Interpret b1 b6 as a row number, between 0 and 3. • Interpret b2 b3 b4 b5 as a column number, 0 through 15. S-box 1:Bit Bits 2, 3, 4, 5: 1 6 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 0 0 14 4 13 1 2 15 11 8 3 10 6 12 5 9 0 7 0 1 0 15 7 4 14 2 13 1 10 6 12 11 9 5 3 8 1 0 4 1 14 8 13 6 2 11 15 12 9 7 3 10 5 0 1 1 15 12 8 2 4 9 1 7 5 11 3 14 10 0 6 13

  22. 数据加密标准DES － S-Box example: S2(010010)= 0111 S5(111101)= 0101

  23. 数据加密标准DES － S-Box

  24. 数据加密标准DES － Permutation How to read this table? The 16th bit of input x will be the 1st bit of outputP(x), the 7th bit of x is the second bit ofP(x), etc. 置换函数P 16 7 20 21 29 12 28 17 1 15 23 26 5 18 31 10 2 8 24 14 32 27 3 9 19 13 30 6 22 11 4 25

  25. 56-bit keyK Permutation PC1 C0 D0 Cyclic L Shift Cyclic L Shift C1 D1 56 Permutation PC2 48 K1 数据加密标准DES －Key Schedule 8-byte key has 8 parity check bits： 8，16，24，32，40，48，56，64 Each has 28 bits Number of shifts are dependent on i for each Ki

  26. 数据加密标准DES －Key Schedule PC1 PC2 9,18,22,25 35,38,43,54

  27. 数据加密标准DES －example • 设明文M=(0123456789ABCDEF)16=K=(133457799BBCDFF1)16=

  28. 数据加密标准DES －example • 第1轮：[R1,L1]=(EF4A6544F0AAF0AA)16第2轮：[R2,L2]=(CC017709EF4A6544)16第3轮：[R3,L3]=(A25C0BF4CC017709)16第4轮：[R4,L4]=(77220045A25C0BF4)16第5轮：[R5,L5]=(8A4FA63777220045)16……第16轮：[R16,L16]=(0A4CD99543423234)16 C=(85E813540F0AB405) 16

  29. Triple DEA • Use three keys and three executions of the DES algorithm (encrypt-decrypt-encrypt) • C = ciphertext • P = Plaintext • EK[X] = encryption of X using key K • DK[Y] = decryption of Y using key K • Effective key length of 168 bits C = EK3[DK2[EK1[P]]]

  30. Triple DEA

  31. Other Symmetric Block Ciphers • Advanced Encryption Standard • NIST（国家标准技术协会） call for proposals in 1997 • Nov, 2001 – Rijndael • Symmetric block cipher (128 bits) and key lengths 128, 192, 256

  32. Other Symmetric Block Ciphers 4Transformations: • Substitute Bytes • Shift Rows • Mix Columns • Add Round Keyflash演示

  33. Other Symmetric Block Ciphers • International Data Encryption Algorithm (IDEA) • 128-bit key • Used in PGP • Blowfish • Easy to implement • High execution speed • Run in less than 5K of memory

  34. Other Symmetric Block Ciphers RC5 • Suitable for hardware and software 适合软硬件实现 • Fast, simple 快速、简单 • Adaptable to processors of different word lengths适合不同字长的处理器 • Variable number of rounds 可变循环次数 • Variable-length key 可变密钥长度 • Low memory requirement 低存储器要求 • High security 高安全性 • Data-dependent rotations 依赖于数据的循环

  35. Cipher Block Modes of Operation Electronic Codebook Book 电码本模式(ECB) • message is broken into independent blocks which are encrypted将信息划分成独立的分组进行加密 • each block is encoded independently of the other blocks每一分组单独进行加密 Ci = DESK1 (Pi) • uses: secure transmission of single values

  36. Cipher Block Modes of Operation(ECB)

  37. Cipher Block Modes of Operation • Cipher Block Chaining Mode (CBC) • The input to the encryption algorithm is the XOR of the current plaintext block and the preceding ciphertext block. • Repeating pattern of 64-bits are not exposed

  38. Cipher Block Modes of Operation CFB Cipher FeedBack密码反馈模式 (CFB) • message is treated as a stream of bits • result is feed back for next stage • standard allows any number of bit (1,8 or 64 or whatever) to be feed back • denoted CFB-1, CFB-8, CFB-64 etc

  39. Cipher FeedBack (CFB)

  40. Cipher FeedBack (CFB)

  41. Location of Encryption Device • Link encryption: • A lot of encryption devices • High level of security • Decrypt each packet at every switch • End-to-end encryption • The source encrypt and the receiver decrypts • Payload encrypted • Header in the clear • High Security: Both link and end-to-end encryption are needed (see Figure 2.9)

  42. Key Distribution • A key could be selected by A and physically delivered to B. • A third party could select the key and physically deliver it to A and B. • If A and B have previously used a key, one party could transmit the new key to the other, encrypted using the old key. • If A and B each have an encrypted connection to a third party C, C could deliver a key on the encrypted links to A and B.

  43. Key Distribution • Session key: • Data encrypted with a one-time session key. At the conclusion（结束） of the session the key is destroyed • Permanent key: • Used between entities for the purpose of distributing session keys

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