How to use crypto/rsa EncryptOAEP with private key?

[James Foster]

I am trying to use RSA encryption in Go.  What I am trying to do is encrypt a buffer with a private key (on the server side) and decrypt it with a public key (on the client).

I stumbled upon the EncryptOAEP method in crypto/rsa/rsa.go but it only takes a PublicKey as a parameter, not a private key.

How can I get EncryptOAEP to encrypt using a PrivateKey? Has anyone figured this out?

[Péter Szilágyi]

Hi,

  There are a few conceptual problems with what you're trying to do. Usually people sign and decrypt with a secret key, and verify and encrypt with a public key. Although mathematically possible to do the reverse, think about the roles of the two keys:

• secret key - meant to be secret, owned by a single entity
• public key - as the name suggests, *not* secret, known to anyone

Hence if you encrypt with a private key, there's really no protection, since everybody can know the public key. Of course you *may* try and keep the public key private too, but that is just asking for trouble.

So in short, use only the public RSA for encryption. Usually encrypted protocols have a handshake where each side uses their RSA keys to agree upon a single symmetric key (i.e. key exchange protocol) and use that key afterwards. There are various exchange protocols that have different requirements and support a different set of guarantees (DH, STS, etc).

  Take a bit of time to read up on encryption in general: symmetric ones (AES specifically), asymmetric ones (RSA specifically) and a few extras like ElGamal. If you're not in too much of a hurry, I would suggest going through this Stanford online crypto course. It started today if I'm not mistaking and is a brilliant one!

Cheers,

  Peter

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[James Foster]

Hi Peter. Thanks for your reply.

Digging further, I found out that what I am trying to do is called RSA signing:

- encryption: encrypt with public key, decrypt with private
- signing:  encrypt the content digest (hash) with private key, verify with public

Seems I need to look at SignPKCS1v15 and VerifyPKCS1v15 functions. Is there anything else I should look at?

P.S. Thanks for the Stanford course reference!

[agl]

Encrypting with a private key is signing.

See http://golang.org/pkg/crypto/rsa/#SignPKCS1v15.

The ensures that the message came from the holder of the private key, but doesn't keep the message confidential.

Cheers

AGL