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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?

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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.

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  • $\begingroup$ Very clear and thanks for the link. $\endgroup$
    – AlexVal
    Mar 26, 2023 at 2:51
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    $\begingroup$ Strictly speaking, the Yubikey does not sign the outgoing message, which it can't receive for performance reasons. It receives a hash of that message (what the question calls fingerprint), computed externally. Then there is the question of if the Yubikey yields a signature of the outgoing message, or a signature of the hash of the outgoing message. For ECDSA, if the signature is that of the outgoing message (which I do not know for sure), then the Yubikey can't actually sign per ECDSA, but with a hashless variant thereof. EdDSA has an Ed25519ph variant to solve that issue. $\endgroup$
    – fgrieu
    Mar 26, 2023 at 9:10

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