关于OpenSSL ENGINE中RSA_METHOD公私钥加解密函数作用的咨询

I totally get why this is confusing—OpenSSL’s RSA_METHOD struct uses callback names that can feel counterintuitive at first, especially when you’re mapping them to real-world RSA use cases. Let’s break down each of these four callbacks, their intended use, and which keys they rely on:

rsa_pub_enc #

• Key used: Only the RSA public key (e and n from the RSA struct)
• Purpose: This is the standard "public key encryption" callback. It takes plaintext and encrypts it using the recipient’s public key, so only the holder of the corresponding private key can decrypt it.
• Maps to: OpenSSL’s top-level RSA_public_encrypt() API.

rsa_pub_dec #

• Key used: Only the RSA public key (e and n)
• Purpose: This is not for decrypting arbitrary ciphertext encrypted with a public key—instead, it’s used for verifying digital signatures. When someone signs data with their private key, they’re essentially "encrypting" a hash of the data with their private key. To verify, you use this callback to "decrypt" the signature with the signer’s public key, then compare the result to the hash of the original data.
• Maps to: OpenSSL’s top-level RSA_public_decrypt() API, which is often called internally by RSA_verify().

rsa_priv_enc #

• Key used: Only the RSA private key (d, plus optional p, q, etc., for optimized operations)
• Purpose: This is for generating digital signatures, not encrypting plaintext for confidentiality. It takes a hash of the data you want to sign and performs the RSA private key operation on it to produce a signature that only the corresponding public key can verify.
• Maps to: OpenSSL’s top-level RSA_private_encrypt() API, which is often called internally by RSA_sign().

rsa_priv_dec #

• Key used: Only the RSA private key (d, p, q, etc.)
• Purpose: This is the standard "private key decryption" callback. It takes ciphertext that was encrypted with the corresponding public key and decrypts it back to plaintext.
• Maps to: OpenSSL’s top-level RSA_private_decrypt() API.

A Quick Math Context #

RSA operations are symmetric in terms of modular exponentiation: (m^e)^d ≡ m mod n and (m^d)^e ≡ m mod n. That’s why you can "encrypt" with a private key and "decrypt" with a public key, but we don’t use that for confidentiality (since anyone with the public key could decrypt it). Instead, that asymmetric property is leveraged for digital signatures.

Tips for Your ENGINE Implementation #

• Ensure rsa_pub_enc and rsa_pub_dec never attempt to access private key material (like d, p, q) from the RSA struct—they should only use public key parameters.
• rsa_priv_enc and rsa_priv_dec will need access to your hardware-stored private key, so make sure your implementation handles this securely (no leaking private key data to the host system).
• If your hardware only supports signature/verify operations (not general-purpose encryption/decryption), you can implement the rsa_sign and rsa_verify callbacks instead (set the RSA_FLAG_SIGN_VER flag in the flags field of RSA_METHOD) to bypass the generic rsa_priv_enc/rsa_pub_dec path.

内容的提问来源于stack exchange，提问作者Anisyanka