Where does signing/decrypting take place on a YubiKey 5 for PGP private key?

Question

I have found similar question for PIV (and not PGP)

I assume PIV and OpenPGP would work in similar ways. I also assumes that the GPG private key never leaves the Yubikey.

If these assumptions are correct then:

• at encryption: a symmetric key is generated and encrypted on the sending computer using the GPG public key, then message is encrypted on the sending computer using symmetric key
• at decryption: the symmetric key is decrypted on the yubikey using the GPG private key, then the message is decrypted on the receiving computer using the symmetric key
• when signing: the sending computer computes the fingerprint of the message and then the yubikey encrypt it using the GPG private key

Is it the case? And if not, is the Yubikey a bottleneck when dealing with large messages?

Answer 1

Yes, that's right. It's called a hybrid cryptosystem, and it means that the work for the Yubikey is the same no matter how large the message is.

For encryption, a symmetric key is established for each individual message.

With RSA, this process is called key encapsulation, because a randomly chosen key is encrypted with the recipient's public key.

With elliptic curves, the process is called key exchange. An ephemeral (one-time) key pair is created by the sender, and the one-time public key from this pair is published. The recipient uses EC Diffie-Hellman to discover the symmetric key.

To summarise: Signatures are created by the Yubikey. Encryption only requires the Yubikey to sign the outgoing message. Decryption only requires the Yubikey to recover the encapsulated key or to perform Diffie-Hellman to discover the key.

Note that when signing, you would not use the word 'encrypt' generally to describe the signing process. That is a technical implementation detail that might apply to RSA but not to EC signing.