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1、Chapter 2 Cryptography2012Foundation of Information SecuritySteganography1234OverviewOverview of CryptographyClassical CiphersBlock CiphersPublic Key Cryptography5Stream Ciphers61 Overview of Cryptography1.1 History of Cryptography1.2 Cryptosystem1.3 Cryptanalysis1.4 Cryptography Before 1949 Classic
2、al Encryption古典加密Before computer was invented, cryptography was art more than science.There were some cipher algorithms, encryption machines Germany (Enigma), Japan (Purple), Sweden (Hagelin) Rotor Machine (cont d)Each rotor corresponds to a substitution cipher A one-rotor machine produces a polyalp
3、habetic substitution cipher with 26 alphabets After each press, the rotor is rotated 旋转 by one scale Output of each rotor is input to next rotor After a full-round rotation, the adjacent rotor is rotated (like odometer里程表) by one scale - An 3-rotor machine produces 263=17576 alphabetsThree-Rotor Mac
4、hines3 Steganography 隐写术 Hide messages in other messages An alternative to encryption- encryption: make information unreadable- steganography: hide message to conceal its existence Simmons Prisoner Problem (1983) 囚犯问题 Art of covered writing - Character marking - Invisible ink - Pin punctures - First
5、 letter of each word 藏头诗 - Letter position on page - Typewriter correction ribbon - Microdots 微缩胶片 Drawbacks - high overhead to hide relatively few info bits by using redundancy of mediaExampleNews Eight Weather: Tonight increasing snow. Unexpected precipitation Smothers Eastern towns. Be extremely
6、cautious and use snowtires especially heading east. The highway is not knowingly slippery. Highway evacuation is suspected. Police report emergency situations in downtown ending near TuesdayFirst letter of each word yields: Newt is upset because he thinks he is President This example was created by
7、Neil F. Johnson, and was published in Steganography,Technical Report TR_95_11_nfj, 1995. From WWII German spy (Kahn): Apparently neutrals protest is thoroughly discounted and ignored. Isman hard hit. Blockade issue affects pretext for embargo on by products, ejecting suets and vegetable Oils.Second
8、letter of each word yields: Pershing sails from NY June 1.ExerciseWhat is the message embedded in the left figure? LSB algorithm Least Significant Bit 最低有效位 - For digital image, modify the low bits denoting colors to hide information - A pixel contain 24-bit RGB color information, the LSB of which c
9、an be changed without greatly affecting the quality of the image - Advantages: simple, fast - Disadvantages: may be destroyed by some information processing, e.g. signal transforming, size compressionDigital Watermark 数字水印 By adding an undetectable copyright 版权 information (exclusive identity 唯一标识符
10、or serial number) to digital media, digital watermark can be extracted (when needed) to be a proof of unauthorized copy. Characteristic: embed or extract copyright information into or from the host media without affecting the normal use of the host. Requirement: transparent 透明性, robust鲁棒性 Compared w
11、ith covered writingsame technology but for different aimwhether the algorithm and key used is public or privateDigital Watermark (contd)4 Block Ciphers4.1 Block Cipher Principles4.2 Feistel Cipher Structure4.3 DES Cipher Algorithm4.1 Block Cipher PrinciplesnModern ciphersnOne of the most widely used
12、 cryptographic algorithms nProvide secrecy/authentication servicesnMostly based on a Feistel Cipher StructurenUsing idea of a product cipherStream Ciphers else for machine, calculate Cm1=E(M,Km).for attacker, get Ca directly.if Ca=Cm1, then Km=Ka, exit.Triple DES (112/156-bit key) C = EK3(DK2(EK1(P)
13、 P = DK1(EK2(DK3(C) Backwards compatible with existing single DES (K1 = K2 = K3) Two-key TDES (K1 = K3) or three-key TDES Security of TDES - no known practical attacks - brute force search impossible (O(2112), O(2168) Major disadvantage is speed (3x slower)5 Public Key Cryptography5.1 Principles of
14、Public-Key Cryptosystems5.2 RSA AlgorithmDrawback of Conventional Cryptography Hard for key distribution Complex for key administration Difficult for digital signature Secret-Key Cryptography- Secret /single key cryptography uses one key, shared by both sender and receiver - If this key is disclosed
15、, communications are compromised - Because of symmetric, parties are equal, hence cant prevent the sender from denying sending a message, and cant prevent the receiver forging a message & claiming it is sent by sender Public-Key Cryptography- Uses two keys a public key for encryption & a private key
16、 for decryption- The publicity of PU wont disclose PR- Asymmetric since parties are not equal, those who encrypt messages cannot decrypt messages- Based on mathematical functions(number theoretic concepts) rather than on substitution and permutation Public-key cryptography is more secure than symmetric cryptography Public-key cryptography has made symmetric cryptography obsole
