Network Security. Security Attacks. Normal flow: Interruption: 孫 宏 民 Phone: 國 立 清 華 大 學 資 訊 工 程 系 資 訊 安 全 實 驗 室

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1 Network Security 孫 宏 民 Phone: 國 立 清 華 大 學 資 訊 工 程 系 資 訊 安 全 實 驗 室 Security Attacks Normal flow: sender receiver Interruption: Information source Information destination This is an attack on availability. Examples: cutting of a communication line, denial of service(dos), or destruction of a piece of hardware. sender receiver 1

2 Interception: This is an attack on secrecy. Examples: wiretapping to capture data in network, or illegal copying of files or programs. sender receiver attacker Modification: This is an attack on integrity. Examples: changing values in a data file, or altering a program so that it performs differently. sender receiver attacker 2

3 Fabrication: This is an attack on authenticity. Examples: insertion of fake messages in network, or addition of records to a file. sender receiver attacker Cryptography is the study of creating and using secret writing, or the art and science of keeping messages secure. Encryption (Enciphering): The encryption is the process of disguising a message in such a way as to hide its substance. Notation: C=E(M) or E K (M) or E K1 (M) Decryption (Deciphering): The process of turning ciphertext back into plaintext. Notation: M=D(C) or D K (C) or D K2 (C) 3

4 Cryptographic System Encryption Plaintext Key Ciphertext Decryption Security Services Confidentiality (privacy) The intruder cannot read the encrypted message form the ciphertext. Authentication (who created or sent the data) It should be possible for the receiver of a message to ascertain its origin; an intruder should not be able to masquerade as someone else. 4

5 Integrity (has not been altered) It should be possible for the receiver of a message to verify that it has not been modified in transit; an intruder cannot substitute a false message for a legitimate one. Non-repudiation (the order is final) A sender should not be able to falsely deny later that he sent a message. Cryptographic System Plaintext(M) Encryption Ciphertext(C) Decryption M Cryptanalyst K 1 K 2 Symmetric Cryptosystem: The encryption & decryption keys are the same. (E K (M)= C & D K (C)= M). Asymmetric Cryptosystem: Encryption & decryption keys are different. (E K1 (M)=C & D K2 (C)=M) The encryption key is public, while the decryption key can not be calculated from the public key. 5

6 Symmetric Cryptosystem DES (1977) IDEA (1992) RC5 (1994) AES (2001) Symmetric Cryptosystem Stream ciphers: The operation unit on the plaintext is a single bit (or byte), such as RC4 and A5. Block ciphers: Block ciphers: The operation unit on the plaintext is a group of bits (a block), such as DES, IDEA, and AES. 6

7 Symmetric Cryptosystem Security Service: Confidentiality Authentication Integrity Advantage:High Speed Disadvantages: how to obtain a common secret key between two parities, the number of secret keys is too big, can not achieve nonrepudiation. Average time required for exhaustive key search Key Size (bits) Number of Alternative Keys 2 32 = 4.3 x = 7.2 x = 3.4 x = 3.7 x Time required at 10 6 Decryption/µs 2.15 milliseconds 10 hours 5.4 x years 5.9 x years 7

8 Example of a Cipher M C = = m 1 c 1 m 2 m i, c i {00, 01, 02,..., 25} To encipher: E ( M ) = E ( m ) E ( m ) E c 2 m 3 c 3 ( m K K K 2 K 3 i 1 c = E ( m K 1 i 1 ) E K ( m) = ( m + K1) mod 26 1 )K To decipher: D K c) = ( c + K 2 ( 2 K2 = 26 K 1 Z A B Y A C Y Z B C ) mod 26 8

9 Caesar Cipher Shift each letter in the English alphabet forward by K positions (shift past Z cycle back to A). K is the key to the cipher. Example: k=3 T S I N G H U A U N I V E R S I T Y W V L Q J K X D X Q L Y H U V L W B AES On January 2, 1997, NIST announced a contest to select a new encryption standard to be used for protecting sensitive, nonclassified, U.S. government information. After lots of investigation and discussion in the cryptographic community, NIST chose an algorithm called Rijndael, named sfter two Belgian cryptographers who developed and submitted it. 9

10 Asymmetric Cryptosystem RSA (1978) El-Gamal (1984) McEliece (1978) Knapsack (1978) Public Key Cryptosystems Public Key Cryptosystems: Encryption & decryption keys are different (EK (M) = C & D (C) = M), 1 K 2 such as RSA, ElGamal, and McEliece. The encryption key (public key) can be public while the decryption key (secret key) cannot be calculated from the public key. 10

11 Requirements for Public- Key Cryptography 1. Computationally easy for a party B to generate a pair (public key KUb, private key KR b ). 2. Easy for sender to generate ciphertext. 3. Easy for the receiver to decrypt ciphertect using private key. 4. Computationally infeasible to determine private key (KR b ) knowing public key (KU b ). 5. Computationally infeasible to recover message M, knowing KU b and ciphertext C. 11

12 Asymmetric Cryptosystem Security Service: Confidentiality Integrity Authentication (by Signature) Nonrepudiation (by Signature) Advantage:a pair of keys for each user Disadvantages: Slow speed Public key need to be authenticated by CA RSA (Encryption & Decryption) Public key: n = pq, p and q are large primes (512 bit), e ( gcd (e, (p-1)(q-1)) =1) Private key: d, where ed = 1 mod (p-1)( q-1) Encryption: C=M e mod n Decryption: M= C d mod n p=47, q=71, => n=3337 e=79, => d = 1019 M=688 Encryption: C=M e mod n = mod 3337 = 1570 Decryption: M= C d mod n = mod 3337 =

13 RSA (Encryption & Decryption) Public key: n = pq, p and q are large primes (512 bit), e ( gcd (e, (p-1)(q-1)) =1) Private key: d, where ed = 1 mod (p-1)( q-1) Encryption: C=M e mod n Decryption: M= C d mod n p=47, q=71, => n=3337 e=79, => d = 1019 M=688 Encryption: C=M e mod n = mod 3337 = 1570 Decryption: M= C d mod n = mod 3337 = 688 Ex. Let p = 53 and q = 61, so n = = 3233 and φ(n) = = Choose d = 791 and compute e = 71. To encipher the message RENAISSANCE, we break it into blocks of 4 digits each, where A = 00, B = 01,..., Z = 25, and blank = 26 (in practice, characters would be represented by their 8-bit ASCII codes). Thus we have 13

14 The first block is enciphered as = 3106 mod The entire message is enciphered as M = R E N A I S S A N C E = C = Hash Algorithm A cryptographic hash function is a mathematical transformation that takes a message of arbitrary length and computes from it a fixed-length number. We will call the hash of a message m, h(m). 14

15 One-way hash function Input: X (unlimited length) Output: Y=H(X) (fixed length, e.g., 160 bit) Given X, it is easy to compute Y. Given Y, and H( ), it is computational infeasible to compute X. Given X and Y, it is computational infeasible to find X such that Y=H(X ). There is an additional thing one can do with public key technology, which is to generate a digital signature on a message. signing plaintext Signed message Sender s private key Sender s public key Signed message verification plaintext 15

16 Digital Signature M Hash h(m) S M S Hash Functions : SHA MD5 FFT Snefru N Hash Signature Functions : RSA DSA El-Gamal Elliptive Curve LUC Digital Signature M Message Message Signature Generation Signature(S) Signature Verification Ture or Fal Private Key Public Key Signer A Verifier B 16

17 RSA Digital Signature Public key: n = pq, p and q are large primes (512 bit), e ( gcd (e, (p-1)(q-1)) =1), h is a hash function. Private key: d, where ed = 1 mod (p-1)( q-1) Sign: S= h(m) d mod n Verify: h(m) = S e mod n 憑 證 管 理 中 心 (CA) 的 角 色 與 功 能 發 送 者 戶 政 事 務 所 接 收 者 傳 統 印 章 與 印 鑑 證 明 文 件 印 章 姓 名 文 件 印 鑑 證 明 印 章 印 章 姓 名 文 件 印 章 印 章 比 對 公 開 金 鑰 電 子 簽 章 與 電 子 憑 證 秘 密 金 鑰 CA 認 證 中 心 姓 名 電 子 憑 證 公 開 金 鑰 姓 名 具 有 公 信 力 的 第 三 者 公 開 金 鑰 驗 證 電 子 文 件 電 子 簽 章 電 子 文 件 電 子 簽 章 電 子 文 件 電 子 簽 章 17

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