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explain use of card URL field to obtain public key from keyserver

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Signed-off-by: Phill Kelley <34226495+Paraphraser@users.noreply.github.com>
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Phill Kelley 2023-06-29 11:35:30 +10:00
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@ -1751,6 +1751,75 @@ If you need to set up a second host when you are travelling and don't have ready
$ gpg --card-status
```
Another approach is to add the URL of your public key to your YubiKey:
1. Define your KEYID. For example:
``` console
$ KEYID=0xFF3E7D88647EBCDB
```
2. Construct the URL (based on [Shaw 2003](https://datatracker.ietf.org/doc/html/draft-shaw-openpgp-hkp-00)):
```
$ [[ ! "$KEYID" =~ ^"0x" ]] && KEYID="0x${KEYID}"
$ URL="hkps://keyserver.ubuntu.com:443/pks/lookup?op=get&search=${KEYID}"
$ echo $URL
hkps://keyserver.ubuntu.com:443/pks/lookup?op=get&search=0xFF3E7D88647EBCDB
```
3. Insert your YubiKey into a USB port.
4. Add the URL to your YubiKey (will prompt for your YubiKey's admin PIN):
```
$ gpg --edit-card
gpg/card> admin
gpg/card> url
URL to retrieve public key: hkps://keyserver.ubuntu.com:443/pks/lookup?op=get&search=0xFF3E7D88647EBCDB
gpg/card> quit
```
Note:
* You do not have to use a *keyserver* URL. You can export your public key as an armored ASCII file and upload it to any place on the web where it can be downloaded using HTTP/HTTPS.
Once the URL of your public key is present on your YubiKey, setting up a new host becomes:
1. Insert your YubiKey into a USB port.
2. Use the `fetch` sub-command to retrieve your public key using the URL stored on the card:
```
$ gpg --edit-card
gpg/card> fetch
gpg: requesting key from 'hkps://keyserver.ubuntu.com:443/pks/lookup?op=get&search=0xFF3E7D88647EBCDB'
gpg: /home/pi/.gnupg/trustdb.gpg: trustdb created
gpg: key FF3E7D88647EBCDB: public key "Dr Duh <doc@duh.to>" imported
gpg: Total number processed: 1
gpg: imported: 1
gpg/card> quit
```
This step also imports the private key stubs from the YubiKey.
3. Define your KEYID (which appears in the output in the previous step):
``` console
$ export KEYID=0xFF3E7D88647EBCDB
```
4. Set ultimate trust:
``` console
$ gpg --edit-key $KEYID
gpg> trust
Your decision? 5
Do you really want to set this key to ultimate trust? (y/N) y
gpg> quit
```
# Cleanup
Before finishing the setup, ensure you have done the following: