SM2 (国密) Encrypt & Decrypt

A: Both SM2 and RSA are asymmetric algorithms: encrypt with the public key, decrypt with the private key. The key differences are: Mathematical basis: RSA relies on integer factorization; SM2 relies on elliptic-curve discrete logarithm. Key size: SM2 uses a 256-bit key that is security-equivalent to RSA-3072, resulting in much shorter keys and smaller ciphertext. Performance: SM2 is faster than RSA at equivalent security levels for both key generation and encryption/decryption operations. Compliance: SM2 is mandated by Chinese regulations; RSA is the international standard. When to choose: Use SM2 for systems that must comply with Chinese cryptography laws; use RSA or standard ECC (P-256) for international interoperability.

A: China's Cryptography Law (2020) and associated regulations require SM algorithms for systems involving national security, critical information infrastructure, and public-facing financial services. Specific scenarios include: Government systems — all e-government platforms and authentication systems. Financial institutions — Chinese banks are required by the People's Bank of China (PBOC) to use SM2/SM3/SM4 in their core systems. Electronic invoices and licenses — mandated by the State Taxation Administration and relevant agencies. Cross-border projects: If your system operates internationally and does not specifically require Chinese regulatory compliance, use standard RSA or ECDSA (P-256) for broader library support and interoperability.

A: The most common cause is a cipher mode mismatch: the ciphertext was encrypted with C1C3C2 but you are attempting to decrypt it with C1C2C3 (or vice versa). Always confirm the mode with the encrypting party. Other causes include: Truncated ciphertext — SM2 ciphertext is at minimum 97 bytes (C1: 64 bytes + C3: 32 bytes + at least 1 byte C2); if the Base64 string is too short, it has been corrupted or truncated. Wrong key — the private key does not correspond to the public key used for encryption. Encoding issue — some systems output ciphertext with a leading "04" byte prefix for the C1 component; ensure both sides agree on whether the prefix is included.

A: SM2's 256-bit key provides security equivalent to approximately RSA-3072, which is well above current recommendations (RSA-2048 is the current minimum). Compared to NIST P-256: both offer similar 128-bit security levels. The SM2 curve uses different domain parameters defined by OSCCA rather than NIST. Cryptographic assessment: No practical attacks against the SM2 curve are known. Independent researchers have reviewed the curve parameters and found no signs of a backdoor, though it has received less public scrutiny than NIST curves. Practical note: SM2 is not widely supported outside China — international libraries (OpenSSL, BouncyCastle) support it, but most cloud providers and browsers do not natively support SM2-based TLS. For international use, standard ECC (P-256) is the better choice.

A: This tool implements SM2 encryption/decryption only. The SM2 standard also defines a digital signature scheme (analogous to ECDSA) and a key exchange protocol, but those are separate functions not covered here. For signature and verification workflows, you will need a dedicated SM2 signature library. In Chinese compliance scenarios, SM2 digital signatures are often required alongside SM3 (the national hash algorithm) and SM4 (the national symmetric cipher) for a complete cryptographic suite.