Cryptography -- 密码学

Introduction to Cryptography

• Cryptography enables you to store sensitive information or transmit it across insecure networks (like the Internet) so that it cannot be read by anyone except the intended recipient.
• Cryptanalysis is the science of analyzing and breaking secure communication.

Encryption and Decryption

• PlainText or ClearText
  • Data that can be read without any special measures.
• Encryption
  • "Method of disguising plaintext in such a way as tohide its content"
• CipherText
  • "Encrypting plaintext result in unreadable gibberish"

Requirement of computational secure encryption scheme (加密安全的需求)

• Cost of breaking the cipher exceeds the value of the encrypted information
• Time required to break the cipher exceeds the useful lifetime of the information

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Symmetric-Key Encryption

• Single-key, secret-key or conventional encryption
• One key is used both for encryption and decryption.

Pros

• Much faster than asymmetric systems
• Hard to break if using a large key size

　　Cons

• Key distribution 密钥分配
  • It requires a secure mechanism to deliver keys properly
• Scalability 可扩展性
  • Each pair of users needs a unique pair of keys, so the number of keys grow exponentially
• Limited security 有限的安全性
  • It can provide confidentiality, but not authenticity or non-repudiation 它可以提供机密性，但不提供真实性或不可否认性