Invitation to Computer Science 5 th Edition. Chapter 8 Information Security

Transcription

1 Invitation to Computer Science 5 th Edition Chapter 8 Information Security

2 CIA Triad of Information Security Ensuring that data can be modified only by appropriate mechanisms Ensuring that data is protected from unauthorized access The degree to which authorized users can access information for legitimate purposes Invitation to Computer Science, 6e 2

3 Information Security Rick Analysis Determining the nature and likelihood of the risks to key data Planning for information analysis requires risk analysis Goal is to minimize vulnerability to threats that put a system at the most risk Invitation to Computer Science, 6e 3

4 Cryptography Cryptography The field of study related to encoded information (comes from Greek word for "secret writing") Encryption The process of converting plaintext into ciphertext Decryption The process of converting ciphertext into plaintext Invitation to Computer Science, 6e 4

5 Cryptography Encryption plaintext message ciphertext message Decryption Encrypted(Information) cannot be read Decrypted(Encrypted(Information)) can be Invitation to Computer Science, 6e 5

6 Cryptography Cipher An algorithm used to encrypt and decrypt text Key The set of parameters that guide a cipher Neither is any good without the other Invitation to Computer Science, 6e 6

7 Symmetric-Key Cryptography The general idea behind symmetric-key cryptography. Alice can send a message to Bob over an insecure channel with the assumption that an adversary, Eve, cannot understand the contents of the message by simply eavesdropping on the channel. The original message from Alice to Bob is referred to as plaintext; the message that is sent through the channel is referred to as the ciphertext. Alice uses an encryption algorithm and a shared secret key. Bob uses a decryption algorithm and the same secret key. Invitation to Computer Science, 6e 7

8 Invitation to Computer Science, 6e 8

9 Cryptography Substitution cipher: A cipher that substitutes one character with another Caesar cipher A substitution cipher that shifts characters a certain number of positions in the alphabet Transposition ciphers A cipher that rearranges the order of existing characters in a message in a certain way (e.g., a route cipher) Invitation to Computer Science, 6e 9

10 Caesar cipher A B C D E F G H I J K L M N O P Q R S T U V W X Y Z D E F G H I J K L M N O P Q R S T U V W X Y Z A B C Substitute the letters in the second row for the letters in the top row to encrypt a message Ex: Encrypt(COMPUTER) gives FRPSXWHU Decrypt(Encrypt(COMPUTER)) gives COMPUTER Why is this called the Caesar cipher? What is the key? Invitation to Computer Science, 6e 10

11 Transposition Cipher T O D A Y + I S + M O N D A Y Write the letters in a row of five, using '+' as a blank. Encrypt by starting spiraling inward from the top left moving counter clockwise Ex: Encrypt(TODAY IS MONDAY) gives T+ONDAYMYADOIS+ Decrypt by recreating the grid and reading the letters across the row What is the key? The key are the dimension of the grid and the route used to encrypt the data Invitation to Computer Science, 6e 11

12 Cryptanalysis Cryptanalysis The process of decrypting a message without knowing the cipher or the key used to encrypt it Substitution and transposition ciphers are easy for modern computers to break To protect information more sophisticated schemes are needed Invitation to Computer Science, 6e 12

13 Modern symmetric-key ciphers Modern ciphers normally use a combination of substitution, transposition and some other complex transformations to create a ciphertext from a plaintext. Modern ciphers are bit-oriented (instead of character-oriented). The plaintext, ciphertext and the key are strings of bits. Example DES (Data Encryption Standard) AES Invitation to Computer Science, 6e 13

14 DES DES is a symmetric-key block cipher published by the National Institute of Standards and Technology (NIST) in DES has been the most widely used symmetric-key block cipher since its publication. Invitation to Computer Science, 6e 14

15 DES (continued) Symmetric encryption algorithm Designed for digital data: plaintext is binary string Uses 64-bit binary key (56 bits actually used) Sixteen rounds of same series of manipulations Decryption uses same algorithm, keys in reverse Fast and effective, but requires shared key, 56 bits is too small for modern technology Invitation to Computer Science, 6e 15

16 DES (continued) DES manipulations Split string Duplicating some bits Omit some bits Permute bit order Combine bit strings with XOR (exclusive or) Invitation to Computer Science, 6e 16

17 Invitation to Computer Science, 6e 17

18 AES The Advanced Encryption Standard (AES) is a symmetric-key block cipher published by the US National Institute of Standards and Technology (NIST) in 2001 in response to the shortcoming of DES. Invitation to Computer Science, 6e 18

19 Asymmetric-key Cryptographs Two distinctive keys in asymmetric-key cryptography: a private key and a public key. The key for encryption and the key for decryption are different Person A makes an encryption key public (public key) Anyone can encrypt a message using the public key and send it to A Only A has the decryption key (private key) and can decrypt the message Example RSA Invitation to Computer Science, 6e 19

20 Figure 16.7 The general idea behind asymmetric-key cryptography Invitation to Computer Science, 6e 20

21 RSA RSA key creation Pick 2 large prime numbers: p and q Compute n = p q, and m = (p-1) (q-1) Choose large number e at random, so that e and m are relatively prime (no common factors except 1) Find unique value d, between 0 and m, such that (e d) modulo m = 1 Public key = (n, e), Private key = d Invitation to Computer Science, 6e 21

22 RSA (continued) Example: Choose p = 7, q = 11 n = 7 11 = 77, and m = 6 10 = 60 Choose e = 13 (60 = 2*2*3 * 5, 13=1*13) The corresponding d = 37 (13 37 modulo 60 = 1) Therefore, public key = (77, 13), Private key = 37 Invitation to Computer Science, 6e 22

23 RSA (continued) RSA encryption: Given public key (n, e) Convert message to integer P Calculate C = P e modulo n RSA decryption: Given private key d Calculate C d modulo n Invitation to Computer Science, 6e 23

24 RSA (continued) RSA encryption, example: Given public key (77, 13) Convert message to integer P = 5 Calculate C = 5 13 modulo 77 = 26 RSA decryption: Given private key 37 Calculate modulo 77 = 5 Invitation to Computer Science, 6e 24

25 Public/Private Keys Digital signature Data that is appended to a message, made from the message itself and the sender's private key, to ensure the authenticity of the message Digital certificate A representation of a sender's authenticated public key used to minimize malicious forgeries Invitation to Computer Science, 6e 25

26 Computer Security Authentication credentials Information users provide to identify themselves for computer access User knowledge such as name, password, PIN Smart card with embedded memory chip used for identification Biometrics human characteristics such as fingerprints, retina or voice patterns Invitation to Computer Science, 6e 26

27 Computer Security Malicious Code A computer program that attempts to bypass appropriate authorization and/or perform unauthorized functions Worm: stands alone, targets network resources Trojan horse: disguised as benevolent resource Virus: self-replicating Logic bomb: set up to execute at system event Invitation to Computer Science, 6e 27

28 Computer Security Security Attacks An attack on the computer system itself Password guessing: obvious Phishing: trick users into revealing security information Spoofing: malicious user masquerades as authorized user Back door: unauthorized access to anyone who knows it exists Invitation to Computer Science, 6e 28

29 Computer Security Buffer overflow: defect that could cause a system to crash and leave the user with heightened privileges Denial-of-service: attach that prevents authorized user from accessing the system Man-in-the-middle: network communication is intercepted in an attempt to obtain key data Have you ever experienced one of these? Invitation to Computer Science, 6e 29

30 Invitation to Computer Science, 6e 30

31 Mal-Ware Virus: attaches itself to a program or file and causes damage of hardware, software or files Worm: has the capability to spread itself out to everyone listed in your address book Trojans horse: will appear to be useful software at first glance but will actually do damage once installed or run on your computer Invitation to Computer Science, 6e 31

32 Mal-Ware (con t) Some Famous Computer Virus, Worm, or Trojan Horse Invitation to Computer Science, 6e 32

33 Web Transmission Security E-commerce requires secure transmission of names, passwords, credit card numbers Web protocols: SSL (Secure Sockets Layer) and TLS (Transport Layer Security) Client-server applications Server provides certificate of authentication and server s public key Client sends its DES key, encrypted using RSA Data is sent encrypted by the (now shared) DES key Invitation to Computer Science, 6e 40

34 Invitation to Computer Science, 6e 41

35 Secure Socket Layer (SSL) Invitation to Computer Science, 6e 42

36 Secure Socket Layer (SSL) Invitation to Computer Science, 6e 43

37 Secure Socket Layer (SSL) SSL includes two sub-protocols: the SSL Record Protocol and the SSL Handshake Protocol. Record Protocol - defines the format used to transmit data. Handshake Protocol - using the Record protocol to exchange messages b/t an SSL-enable server and an SSL-enable client. Invitation to Computer Science, 6e 44

38 Secure Socket Layer (SSL) 客戶端認證 (Client Authentication) 伺服端認證 (Server Authentication) 加密連線 (Encrypted Connection) Invitation to Computer Science, 6e 45

39 SSL Record Protocol Operation Application Data abcdefghijklmn Fragment abcdef ghijk lmn Compress Add MAC generate MD5 by using HASH Encrypt Add SSL Record Header Invitation to Computer Science, 6e 46

40 SSL 封包格式 SSL 訊息封裝 ( 一 ) IP Header (a) 變更密文規格協定 (b) 警告協定 (c) 握手協定 (d) 紀錄協定 TCP Header 1 Type = 20 Type = 21 Type =22 Type =23 1 Byte Change SSL Message 1 1 Byte Alter Alert Level Desc. 1 Hand. Type Length 3 >= 0 Bytes Content >= 0 Bytes 0,16, 20 Major Minor HTTP,..., Message Length MAC Version Version (or Compress) 加密編碼 Invitation to Computer Science, 6e 47

41 SSL 訊息封裝 ( 二 ) 握手協定之命令型態 訊息 ( 命令 ) 型態參數 ( 內容 ) hello_request client_hello server_hello certificate server_key_exchange certificate_request server_done certificate_verify client_key_exchange finished 無版本 亂數 會議 ID 加密套件 壓縮方法版本 亂數 會議 ID 加密套件 壓縮方法一連串的 X.509v3 憑證內容鑰匙材料 數位簽章憑證型態 認證中心無數位簽章鑰匙材料 數位簽章雜湊值 Invitation to Computer Science, 6e 48

42 SSL 訊息封裝 ( 三 ) 紀錄層訊息封裝 上層資料 ( 如 HTTP) Application data 分段處理 Fragment Fragment Fragment 壓縮處理 compress compression_method 加入認證標頭 compress MAC client/server_write_mac_secret 加密處理 Encrypt client/server_write_key client/server_write_iv 加入 SSL 紀錄標頭 H Encrypt SSL 協定包裝 TCPInvitation TCP 協定包裝 Header TCP to Computer Message Science, 6e 49 如 https: tcp/443 埠口

43 11-5 SSL 會議連結識別 會議連結 (Session Connection):Session ID 使用於 SSL Record Layer 的參數 Session Identifier Peer Certificate Compression Method Cipher Spec Master Secret Is Re-useable Invitation to Computer Science, 6e 50

44 SSL 握手協定 ( 一 ) 協商步驟 1. 交換 Hello 訊息, 協議演算法, 並檢視是否有 Session ID 可重複使用 2. 交換鑰匙材料, 及製作 前置主秘鑰 (Pre-master Secret) 3. 交換 身份憑證 4. 利用 Pre-master Secret 製作 Master Secret 5. 將安全參數登錄於 Session Connection 6. 保證協議當中未受到駭客攻擊 Invitation to Computer Science, 6e 51

45 客戶端 伺服端 第一階段 (1) client_hello (2) server_hello 雙方利用 Hello 訊息互相傳送相關安全參數 SSL 握手 協定 ( 二 ) 第二階段 (3) certificate * (4) server_key_exchange * (5) certificate_request * 伺服端傳送憑證 交換鑰匙與要求對方憑證 協商階段 第三階段 (6) server_done (7) certificate * (8) client_key_exchange (9) certificate_verify * 客戶端傳送憑證 交換鑰匙與要求確認憑證 第四階段 (10) [change_cipher_spec] (11) finished (12) [change_cipher_spec] (13) finished 客戶端要求變更密文規格及完成協議 伺服端要求變更密文規格及確認完成協議 SSL/TLS 安全連線依照安全機制傳輸應用層資 Record Protocol 料 Invitation to Computer Science, 6e 52 備註 : 有星號 (*) 表示選項訊息

46 網路環境 - 防火牆 Invitation to Computer Science, 6e 53