1. CIM3681 - Cryptograhic Concepts Overview1 Cryptographic Concepts Overview Chapter 2- Cryptography and Network Security by Stallings for Details

2. CIM3681 - Cryptograhic Concepts Overview 2 Cryptographic Concepts Overview Basic Concepts Glossary Symmetric Cipher Model – p24 Public-Key Cryptosystems – p260 Message Authentication Codes – p.313 Basic Functionality of Cryptographic algorithms Strengths and weaknesses of cryptographic algorithms, and Effect of key length

3. CIM3681 - Cryptograhic Concepts Overview 3 Basic Concepts History Goals of Cryptography

4. CIM3681 - Cryptograhic Concepts Overview 4 History Caesar Cipher 密碼  Shift of letters American Civil War (Code books for substitution ( 替換 ) and transposition ( 換位 )) Ultra vs Enigma (American Vs German)

5. CIM3681 - Cryptograhic Concepts Overview 5 Goals of Cryptography Confidentiality 機密性  Ensures that a message remains private during transmission between 2 or more parties. Integrity 完整性  Ensures that a message is not altered while in transit Authentication 鑑別 鑑定 鑑認  Verifies the claimed identity of system users and is a major function of cryptosystems Non-repudiation 抗抵賴性 不得否認  Provides assurance to the recipient that the message was actually originated by the sender and not someone masquerading as the sender.

6. CIM3681 - Cryptograhic Concepts Overview 6 Glossary Asymmetric 不對稱的 Encryption : A form of cryptosystem in which encryption and decryption are performed using two different keys, one of which is referred to as the public key and one of which is referred to as the private key. Also known as public-key encryption. Authentication: A process used to verify the integrity of transmitted data, especially a message. Message Authentication Code (MAC) – Cryptographic checksum. Symmetric Encryption: A form of cryptosystem in which encryption and decryption are performed using the same key. Digital Signature: An authentication mechanism that enables the creator of a message to attach a code that acts as a signature. The signature guarantees the source and integrity of the messages. Encryption 加密 : The conversion of plaintext or data into unintelligible form by means of a reversible translation, based on a translation table or algorithm. Also called enciphering. Decryption 解密 : The translation of encrypted text or data (called ciphertext) into original text or data (called plaintext). Also called deciphering.

7. CIM3681 - Cryptograhic Concepts Overview 7 Symmetric Cipher Model 5 ingredients: Plaintext, Encryption algorithm, Secret key, Ciphertext, and Decryption algorithm

8. CIM3681 - Cryptograhic Concepts Overview 8 Symmetric Cipher Model Plaintext 明文 - the original intelligible message Encryption Algorithm ( 加密演算法 處理步驟 ) – performs various substitution and transformations on the plaintext Secret Key 秘密鑰匙 – The input to the encryption algorithm Ciphertext 密文 - the coded message Decryption Algorithm ( 解密演算法 處理步驟 ) – reverse run of the encryption algorithm so as to produce the plaintext

9. CIM3681 - Cryptograhic Concepts Overview 9 Public-Key Cryptosystems Six ingredients: plaintext, encryption algorithm, public and private key, ciphertext, decryption algorithm

10. CIM3681 - Cryptograhic Concepts Overview 10 Public-Key Cryptosystems Public and private key ( 公開和私人的鑰匙 ) : if one is used for encryption, the other is used for decryption Public key – this is placed in a public register or other accessible file

11. CIM3681 - Cryptograhic Concepts Overview 11 Message Authentication Codes (MAC) An authentication techniques ( 鑑別技術 ) involves the use of a secret key to generate a small fixed-size block of data, known as a cryptographic checksum ( 校驗和 ) or MAC that is appended ( 附加 ) to the message.

12. CIM3681 - Cryptograhic Concepts Overview 12 Basic Functionality of Cryptographic algorithms Two basic operations to obscure plaintext messages: confusion ( 混亂 ) and diffusion ( 蔓延 ) Confusion occurs when the relationship between the plaintext and the key is so complicated that an attacker can’t merely continue altering the plain text and analyzing the resulting ciphertext ( 密文 ) to determine the key. Diffusion occurs when a change in the plaintext results in multiple changes spread out throughout the ciphertext.

13. CIM3681 - Cryptograhic Concepts Overview 13 Factors affecting strengths and weaknesses of cryptographic algorithms, and Effect of key length The key size  The longer the key size is, the more difficult to crack the key. The nature ( 本質 )of the algorithm  There is no unbreakable ( 不能破解的 )algorithm. It’s a matter of time. Timing attack ( 適時 )  The life cycle for the key is shorter than the cracking time.

14. CIM3681 - Cryptograhic Concepts Overview 14 Strengths and weaknesses of symmetric algorithm Strength 強度, 抵抗力  Operate at a great speed  Apply in hardware implementation ( 實施 ) to acquire at even high speed Weaknesses  Key distribution is a major problem.  Symmetric key cryptography does not implement nonrepudiation.  The algorithm is not scalable 規模可伸縮性.  Keys must be regenerated ( 再生 ) often

15. CIM3681 - Cryptograhic Concepts Overview 15 Strengths and weaknesses of asymmetric algorithm Strength  The addition of new users requires the generation of only one public/private key pair.  Users can be removed far more easily from asymmetric systems.  Key regeneration is required only when a user’s private key is compromised.  Asymmetric key encryption provides nonrepdudiation.  Key distribution ( 分發 ) is a simple process. Weaknesses  Operate at a slow speed.