Encrypting large files using a public key

问题:

I need to encrypt a 100KB file using a public key. I've been reading some posts claiming that it is not practical to directly encrypt large files using a public key, and that the preferred method is to encrypt the file using a symmetric key and then encrypt this symmetric key using the public key. It seems that a naive solution would be to break the large file to pieces and encrypt each one of them using the same public key. My question is whether and why this solution is wrong?

回答1:

The hybrid approach you mention (generate a random symmetric key, use this to encrypt the data, and encrypt only the key asymmetrically) has a massive performance advantage.

You could "break the large file to pieces and encrypt each one of them using the same public key" as well, there is nothing wrong with that, but it is much slower.

回答2:

If I understand you right, you want to encrypt the file with someone else's public key, to be decrypted by their private key?

The advantage of using symmetric encryption and only using public key cryptography for the (symmetric) key is performance: symmetric cryptography is computationally much less resource-intensive (trade-off: you have to keep the key secret -- and that's what the second, asymmetric step is for).

Breaking up the file adds management overhead (how can you be sure how many chunks there will be? that you have transmitted them all?) and doesn't add any security. On the contrary.

回答3:

Splitting file into smaller pieces and then encrypting them with some asymmetric cipher has nothing to do with the runtime cost of the encryption process. Best practice is encrypting the data with a good symmetric cipher using a relatively strong key and encrypting the secret key you used in symmetric encryption with an asymmetric cipher(using your public key).

回答4:

Asymmetric cryptography is too slow, the most used approach is to encrypt random symmetric key with asymmetric, and encrypt your data with that symmetric key. And, as well, the best way is to use well-known protocol/standard for that purpose (OpenPGP for instance).

回答5:

Aside from the speed-boost of symmetric key encryption, there's another possible benefit: By first encrypting the message with a random securely-generated symmetric key, you can then encrypt the symmetric key for multiple recipients, once in each recipient's own public asymmetric key, without having to re-encrypt the entire message.