反向使用RSA解密许可证代码：使用私钥加密，使用公钥解密

Question

I want to encrypt some values pertaining to a licence code with a secret private key, and then when it's entered in the user's app install it will be decrypted with the public key (stored with the app) to view the encoded data and ensure it was only created by me.

The trouble is it seems that you encrypt with the public key and decrypt with the private key, which is the reverse of what I want.

It's also worth mentioning that the library I'm using called SwiftyRSA only supports encrypting with the public key, and doesn't like it when I use the private key instead. I believe this is because it's being saved to the keychain with kSecAttrKeyClassPublic , because that's what it's expecting, and that causes things to fail.

I have read that the keys are technically interchangeable, but it seems I can't get it to work in my instance. Is this because they public key has a smaller exponent? Is there a way to get the public key to be as "long" as the private key using ssh-keygen , and therefore be able to swap them around? If not, how could I proceed?

Answer 1

The keys aren't always interchangeable (eg RSA private keys with CRT parameters) and it is pretty likely that the encryption procedure doesn't protect the key against side channel attacks. You should not use private keys to encrypt, period.

You could use signatures with message recovery if you're really careful.

Otherwise - if you've enough space - you could of course always sign-then-encrypt your license. For this to work (without additional AES encryption) your encryption key pair would have to be quite a bit larger than your signing key though.