Last week I wrote to my bank with a general question. Today I received an email inviting me to join their "Secure Email (PGP) system". It made clear I'd have to join to read their response to my question.
You have received an encrypted email:
From: $CST1, Mailbox
Subject: Your pension fund enmquiry Ref CF-123456
This is the first time you have received encrypted email from Lloyds Banking Group so you will need to set up a password (also referred to as a passphrase) to access this email and any future encrypted emails that you receive from us. Lloyds Banking Group takes online security very seriously.
Reluctantly, I followed the link to sign up, inventing an account password. I duly received a passworded PDF by email, with the response to my question.
Out of curiousity, I explored the rest of the site, which appears to run "Symantec Web Email Protection". On the settings page, I saw it gave a choice how the bank would encrypt messages to me; either as passworded PDF documents or with public key cryptography.
I have an OpenPGP Key or digital ID/certificate (X.509, S/MIME) that I want to use to secure messages I exchange with site [sic].
Following to the next page:
If you already have a key or certificate, you may upload it and we will encrypt your messages to it.
You may upload any of the following: a PGP key (.asc), X.509 Certificate (.pem, .crt, .cer), or PKCS#12 file (.p12, .pfx).
Optional: If your key file has an associated passphrase, please enter it.
That last line surprised me. I've used public key cryptography in OpenSSH and GPG. As I understand, private keys can have passphrases, but not public keys. The purpose of a passphrase is to protect your key on your computer; like the private key, it should be kept secret.
Why would a public key have a passphrase? What's the point of a passphrase if you share it with your correspondents?