信息安全(02) Introduction to Cryptography Classical Encryption Techniques(cont 復大软件学院 LiT
LiJT 1 信息安全（02） Introduction to Cryptography -Classical Encryption Techniques (cont.)
et Symmetric Cipher Model Secret key shared by Secret key shared by sender and recipient sender and recipient Transmitted ciphertext Plaintext Plaintext Input Encryption algorithm Decryption algorithm output e,g…DES) (reverse of encryption algorithm) 復大软件学院 LiT
LiJT 2 Symmetric Cipher Model
Discussion 模型合理吗? 什么当保密;什么当公开? 19世纪荷兰人 A Kerckhoffs就提出了一个在密 码学界被公认为基础的假设,也就是著名的 “ Kerckhoffs假设”:秘密必须全寓于密钥。 Other models? 復大软件学院 LiT
LiJT 3 Discussion • 模型合理吗？ • 什么当保密；什么当公开？ – 19世纪荷兰人A.Kerckhoffs就提出了一个在密 码学界被公认为基础的假设，也就是著名的 “Kerckhoffs假设”：秘密必须全寓于密钥。 • Other Models?
Discussion “谁是我们的敌人,谁是我们的朋友,这个 问题是革命的首要问题”——毛选 易用性 秘密仝部寓于密钥≠算法当公开,要看应用 环境(商用,军用,……) ·开放的系统更安全,?? 復大软件学院 LiT
LiJT 4 Discussion • “谁是我们的敌人，谁是我们的朋友，这个 问题是革命的首要问题”——毛选 • 易用性 • 秘密全部寓于密钥≠算法当公开，要看应用 环境(商用，军用，……) • 开放的系统更安全，??
ef Cryptanalytic Attacks ·对于对手而言 最坏情况下,仍有一种攻击方法可用 · Brute Force search,穷举法 5 復大软件学院 LiT
LiJT 5 Cryptanalytic Attacks • 对于对手而言 – 最坏情况下，仍有一种攻击方法可用 • Brute Force Search，穷举法
Brute force search always possible to simply try every key most basic attack, proportional to key size assume either know or recognise plaintext Number of Alternative Time required at I0° Key Size(bits) Keys Time required at I encryption/us encryptions 32 232=43x10P 231 us=35.8 minutes 2. 15 milliseconds 24=72x1010 255 us=1142 year 10.01 hours 128 2128=34x103 As=5.4x10-years 5. 108years (8 2108=37x10 2161s=59×10yca 59 x 10years 26 characters 26!=4x10~6 64x10°ycas (permutation) 2x 106us =6.4x 10 years 6 復大软件学院 LiT
LiJT 6 Brute Force Search • always possible to simply try every key • most basic attack, proportional to key size • assume either know or recognise plaintext
er Monoalphabetic Cipher Security now have a total of 26!=4 X 1026 keys with so many keys, might think is secure but would be wrong!! problem is language characteristIcs 復大软件学院 LiT
LiJT 7 Monoalphabetic Cipher Security • now have a total of 26! = 4 x 1026 keys • with so many keys, might think is secure • • but would be !!!WRONG!!! • problem is language characteristics
o Example Cryptanalysis given ciphertext UZQSOVUOHXMOPVGPOZPEVSGZWSZOPFPESXUDBMETSXAIZ VUEPHZHMDZSHZOWSFPAPPDTSVPQUZWYMXUZUHSX EPYEPOPDZSZUFPOMBZWPFUPZHMDJUDTMOHMQ count relative letter frequencies(see text) guess P&z are e and t guess ZW is th and hence ZWP is the proceeding with trial and error finally get it was disclosed yesterday that several informal but direct contacts have been made with political representatives of the vietcong in moscow 8 復大软件学院 LiT
LiJT 8 Example Cryptanalysis • given ciphertext: UZQSOVUOHXMOPVGPOZPEVSGZWSZOPFPESXUDBMETSXAIZ VUEPHZHMDZSHZOWSFPAPPDTSVPQUZWYMXUZUHSX EPYEPOPDZSZUFPOMBZWPFUPZHMDJUDTMOHMQ • count relative letter frequencies (see text) • guess P & Z are e and t • guess ZW is th and hence ZWP is the • proceeding with trial and error finally get: it was disclosed yesterday that several informal but direct contacts have been made with political representatives of the vietcong in moscow
More Definitions unconditional security no matter how much computer power is available, the cipher cannot be broken since the ciphertext provides insufficient information to uniquely determine the corresponding plaintext computational security given limited computing resources(eg time needed for calculations is greater than age of universe), the cipher cannot be broken Unconditional security would be nice, but the only known such cipher is the one-time pad(later) For all reasonable encryption algorithms, have to assume computational security where it either takes too long, or is too expensive, to bother breaking the cipher 9 復大软件学院 LiT
LiJT 9 More Definitions • unconditional security – no matter how much computer power is available, the cipher cannot be broken since the ciphertext provides insufficient information to uniquely determine the corresponding plaintext • computational security – given limited computing resources (eg. time needed for calculations is greater than age of universe), the cipher cannot be broken • Unconditional security would be nice, but the only known such cipher is the one-time pad (later). – For all reasonable encryption algorithms, have to assume computational security where it either takes too long, or is too expensive, to bother breaking the cipher
t Types of Cryptanalytic Attacks ciphertext only Encryption algorithm Ciphertext to be decoded known plaintext Encryption algorithm Ciphertext to be decoded One or more plaintext-ciphertext pairs formed with the secret key ° chosen plaintext Encryption algorithm Ciphertext to be decoded Plaintext message chosen by cryptanalyst, together with its corresponding ciphertext generated with the secret key 復大软件学院 LiT
LiJT 10 Types of Cryptanalytic Attacks • ciphertext only – Encryption algorithm – Ciphertext to be decoded • known plaintext – Encryption algorithm – Ciphertext to be decoded – One or more plaintext-ciphertext pairs formed with the secret key • chosen plaintext – Encryption algorithm – Ciphertext to be decoded – Plaintext message chosen by cryptanalyst, together with its corresponding ciphertext generated with the secret key
