AES Encrypt & Decrypt

A: Both are very secure; the choice depends on specific needs. AES-128: Breaking complexity of 2^128 (approximately 3.4×10^38), would take billions of years even using all computers globally. Performance is about 20-40% faster than AES-256. Suitable for most application scenarios. AES-256: Breaking complexity of 2^256 (approximately 1.2×10^77), theoretically more secure, but AES-128 is already sufficient in practice. Used by the U.S. government to protect “top secret” level information. Slightly slower performance, but the difference is minimal. Recommendation: Use AES-128 for general applications, AES-256 for high-security scenarios like government, finance, and healthcare.

A: An IV (Initialization Vector) is a random number used in the encryption process to ensure that the same plaintext produces different ciphertext when encrypted at different times. Why IV is needed: Without an IV, the same plaintext and key always produce the same ciphertext, allowing attackers to identify repeated data blocks, reducing security. IV requirements: 1) Randomness: A different random IV should be used for each encryption. 2) No need for secrecy: The IV can be transmitted publicly, usually prepended to the ciphertext. 3) Length: AES IV length is 128 bits (16 bytes). Note: ECB mode does not use an IV, but has low security and is not recommended.

A: CBC (Cipher Block Chaining): Traditional encryption mode where each plaintext block is XORed with the previous ciphertext block before encryption. Advantages: High security, widely supported. Disadvantages: Cannot parallelize encryption, does not provide authentication (requires additional HMAC). GCM (Galois/Counter Mode): Modern authenticated encryption mode that provides both encryption and authentication. Advantages: Can parallelize encryption, good performance, prevents tampering, TLS 1.3 standard. Disadvantages: Complex implementation, IV reuse causes serious security issues. Recommendation: Use GCM for new projects, CBC + HMAC for legacy projects.

A: Key management is the core of encryption systems; key leakage renders encryption ineffective. Key Generation: Use cryptographically secure random number generators (CSPRNG), do not use simple passwords or predictable values. Key Storage: 1) Use Key Management Services (KMS), such as AWS KMS, Azure Key Vault. 2) Use Hardware Security Modules (HSM). 3) Use key derivation functions (such as PBKDF2, Argon2) to derive keys from passwords. Key Transmission: 1) Use asymmetric encryption (such as RSA) to transmit symmetric keys. 2) Use key exchange protocols (such as Diffie-Hellman). 3) Transmit through secure channels (such as TLS). Best Practices: Regularly rotate keys, use key version management, limit key access permissions.

A: AES and SM4 are both symmetric encryption algorithms, but have different origins and application scenarios. AES: U.S. NIST standard (2001), widely used globally, supports 128/192/256-bit keys, excellent performance. SM4: China State Cryptography Administration standard (2012), only supports 128-bit keys, performance comparable to AES-128. Selection Advice: Use AES for international applications, use SM4 for China's critical sectors (finance, government, telecommunications) to comply with Cryptography Law requirements. If compatibility is needed, both algorithms can be supported simultaneously.

A: Test Vectors: NIST provides official AES test vectors (CAVP) that can be used to verify implementation correctness. Comparison Tools: Use multiple independent implementations (such as OpenSSL, this tool, online tools) to encrypt the same data and compare results for consistency. Decryption Verification: Decrypt immediately after encryption to check if the original data can be recovered. Note: The same plaintext, key, IV, and mode should produce the same ciphertext. If results differ, there may be issues with padding, encoding, or mode settings.