The new ADSK feature
Werner Koch
wk at gnupg.org
Tue Mar 21 18:22:15 CET 2023
Hi!
Those following my commits may have already noticed the ADSK feature.
To explain for what it is useful I wrote a little piece at
https://gnupg.org/blog/20230321-adsk.html (and pasted below).
The release of GnuPG 2.4.1 is a bit delayed but will definitely be done
in April.
Salam-Shalom,
Werner
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
ADSK: The Additional Decryption Subkey
======================================
This is an introduction to the ADSK feature, we will introduce with
GnuPG 2.4.1.
What and why
~~~~~~~~~~~~
ADSK is short for Additional Decryption SubKey which is a new OpenPGP
feature to make the use of end-to-end encryption more compatible with
standard business procedures.
When deploying end-to-end encryption in larger organizations, a common
concern is how to do
- handle absence management
- access data encrypted to former employees
- comply with the commercial code demanding archival and an option to
access of all business communication.
The standard procedure here is to share the secret key in a group or
to keep a copy of that key at a safe place. Managing this is not
easy, error prone, and may in the worst case turn end-to-end
encryption to a kind of gateway based encryption. Clearly a better
way is required to make large deployments easier.
The new ADSK feature is actually the counterpart to GnuPG's long
existing --encrypt-to feature. It is a flexible method to tell the
recipient to encrypt a reply to several subkeys. If the recipient's
implementation does not support ADSKs encryption still works, but the
encryption to the ADSKs is of course missing.
Another use case for ADSKs is to split keys between several devices
(e.g. desktop and mobile). For example the secret part of the key for
the mobile device could be on a token for additional security but on
the desktop the key is on-disk because the desktop is less likely lost
or stolen. The ADSK of a lost device could then be revoked and a new
one generated for a new device.
Creating an ADSK
~~~~~~~~~~~~~~~~
Adding an ADSK to an existing key is straightforward with GnuPG 2.4.1.
You need to know the fingerprint of the your key and the fingerprint
of the key to be used as ADSK. The latter will in most cases be a
subkey (because the de-facto standard is to use a subkey for
encryption) and thus the subkey's fingerprint is required. By default
gpg does not show the fingerprints of subkeys; but of course there is
an option:
,----
| $ gpg -K --with-subkey-fingerprint B21DEAB4F875FB3DA42F1D1D139563682A020D0A
| sec ed25519 2016-06-22 [SC]
| B21DEAB4F875FB3DA42F1D1D139563682A020D0A
| uid [ultimate] patrice.lumumba at example.net
| ssb cv25519 2016-06-22 [E]
| 8D0221D9B2877A741D69AC4E9185878E4FCD74C0
| ssb# brainpoolP384r1 2021-06-28 [R] [expires: 2027-01-10]
| A1DB793DC23663E7F91475D82B999FA9CE046B1B
| ssb# cv25519 2016-02-14 [R]
| DC9DAC608A8F118FD8D0F332F4EC45F11B457A45
`----
The fingerprints are shown in the easy copy+paste format. If you need
them often it is a good idea to add `with-subkey-fingerprint' to your
`gpg.conf'. The above listing actually shows a key with two ADSKs
(indicated by the "[R]"). The second ADSK was added using this
command:
,----
| gpg --quick-add-adsk B21DEAB4F875FB3DA42F1D1D139563682A020D0A \
| DC9DAC608A8F118FD8D0F332F4EC45F11B457A45
`----
In case you want a bit more interactive use you could have done it
this way:
,----
| $ gpg --edit-key B21DEAB4F875FB3DA42F1D1D139563682A020D0A
| Secret key is available.
|
| sec ed25519/139563682A020D0A
| created: 2016-06-22 expires: never usage: SC
| trust: ultimate validity: ultimate
| ssb cv25519/9185878E4FCD74C0
| created: 2016-06-22 expires: never usage: E
| sub brainpoolP384r1/2B999FA9CE046B1B
| created: 2021-06-28 expires: 2027-01-10 usage: R
| [ultimate] (1). patrice.lumumba at example.net
|
| gpg> addadsk
| Enter the fingerprint of the additional decryption subkey: DC9DAC608A8F118FD8D0F332F4EC45F11B457A45
|
| sec ed25519/139563682A020D0A
| created: 2016-06-22 expires: never usage: SC
| trust: ultimate validity: ultimate
| ssb cv25519/9185878E4FCD74C0
| created: 2016-06-22 expires: never usage: E
| sub brainpoolP384r1/2B999FA9CE046B1B
| created: 2021-06-28 expires: 2027-01-10 usage: R
| sub cv25519/F4EC45F11B457A45
| created: 2016-02-14 expires: never usage: R
| [ultimate] (1). patrice.lumumba at example.net
|
| gpg> save
`----
By using the --edit-key command you may also delete a subkey using
"key N" and "delkey" but this is only useful as long as your modified
key has not yet been published or send to your peers. Thus if you
want to remove an ADSK you need to either expire the ADSK subkey or
revoke it. Let's do the latter:
,----
| $ gpg --edit-key B21DEAB4F875FB3DA42F1D1D139563682A020D0A
| [...]
| gpg> key 3
|
| sec ed25519/139563682A020D0A
| created: 2016-06-22 expires: never usage: SC
| trust: ultimate validity: ultimate
| ssb cv25519/9185878E4FCD74C0
| created: 2016-06-22 expires: never usage: E
| sub brainpoolP384r1/2B999FA9CE046B1B
| created: 2021-06-28 expires: 2027-01-10 usage: R
| sub* cv25519/F4EC45F11B457A45
| created: 2016-02-14 expires: never usage: R
| [ultimate] (1). patrice.lumumba at example.net
|
| gpg> revkey
| Do you really want to revoke this subkey? (y/N) y
| Please select the reason for the revocation:
| 0 = No reason specified
| 1 = Key has been compromised
| 2 = Key is superseded
| 3 = Key is no longer used
| Q = Cancel
| Your decision? 3
| Enter an optional description; end it with an empty line:
| >
| Reason for revocation: Key is no longer used
| (No description given)
| Is this okay? (y/N) y
|
| [...]
| gpg> save
`----
You can check that it has been revoked:
,----
| $ gpg -k --list-options show-unusable-subkeys \
| B21DEAB4F875FB3DA42F1D1D139563682A020D0A
| pub ed25519 2016-06-22 [SC]
| B21DEAB4F875FB3DA42F1D1D139563682A020D0A
| uid [ultimate] patrice.lumumba at example.net
| sub cv25519 2016-06-22 [E]
| 8D0221D9B2877A741D69AC4E9185878E4FCD74C0
| sub brainpoolP384r1 2021-06-28 [R] [expires: 2027-01-10]
| A1DB793DC23663E7F91475D82B999FA9CE046B1B
| sub cv25519 2016-02-14 [R] [revoked: 2023-03-21]
| DC9DAC608A8F118FD8D0F332F4EC45F11B457A45
`----
Now export the key and send it off to all your peers.
Finally let's run an encryption test in verbose mode:
,----
| $ fortune | gpg -vaer patrice.lumumba at example.net
| [...]
| gpg: ECDH/AES256.OCB encrypted for: "2B999FA9CE046B1B patric[...]
| gpg: ECDH/AES256.OCB encrypted for: "9185878E4FCD74C0 patric[...]
| -----BEGIN PGP MESSAGE-----
|
| hJ4DK5mfqc4EaxsSAwMER3V2siJamGy+Z476EJ+ZW8YuhDGtCK5QhuqzTPuXiuWl
| yMNM4MN4tWguoA8IJRY/ZypS7+eHGVesG2vGMfzA1p5e9SoiHis53QZxWTsD3H/2
| n6f9xhev5uzWBDh1uNmOMCR3C2ZrrFULDQib9c3g+0UBGYp1Eu/uJ2GYQ/DIYXwV
| KrofD0/cppryoJB4RFVD1IReA5GFh45PzXTAEgEHQH2+n2658cu7aFU+RXv9AvpO
| xBRTt6lCf5PEecqSgbVRMC5zup2WAkiMbLfGe7Go5BtZnuq06sDUZ/qUhgO6ekpO
| n2Eb08RJ+3zN4MrokP/jLNS6AQkCEG3rE9P8Yuw1nwy49uVnywBLqCTp21wjOZcG
| d6VPBQuWwPcoZeesvdVYt4MwRZsWVMC/TFaMSBHGM5ykrbuoTVWcG20FxP+bvjT0
| DohWrguc0TPvdzyYCId2ipcPwFIymY2miHgUs6IQTmpKnQfprNQ1zIVkmnWmlRWj
| KArSqYFBq2o49zLmRQ3Q2bgSgQ9HsU5v84uvuIw46p4WzOO7NlgWLnXngEiFkaqY
| tkM/4XkOY7W/NMa2
| =UJ0x
| -----END PGP MESSAGE-----
`----
If you compare the shown Key-IDs with the fingerprints from the last
listing, you can see that the message was encrypted to the (regular)
encryption subkey (fingerprint ending in 74C0) and to the Brainpool
ADSK (fingerprint ending in 6B1B).
Looking ahead
~~~~~~~~~~~~~
Because the ADSK feature is required on your peer's implementation, it
will for sure take some years until a sufficient large user base has
updated to an implementation which supports the encryption to an ADSK.
This is actually more important than having the capability to add an
ADSK.
As of 2023-03-21 the current state is
GnuPG 2.4.1
Full support
GnuPG 2.2.42
Encryption support
Thunderbird/RNP
Not yet supported
We track the state of our ADSK implementation at
[https://dev.gnupg.org/T6395] . In particular commit
[https://dev.gnupg.org/rGe4f61df8509e] for GnuPG 2.2 shows that the
effort to implement the encryption part is tractable.
What about the old ARR ?
~~~~~~~~~~~~~~~~~~~~~~~~
The idea of having additional keys is not entirely new. PGP^{(r)}
versions between 5.0 to 6.5.7 implemented an Additional Recipient
Request (ARR or ADK, signature subpacket 10) which specified an
external key as an automatic recipient. Due to an implementation bug
in versions up to 6.5.3 [1], it was possible to insert an ARR into an
arbitrary key and PGP accepted that key and encrypted to it. Although
this bug was fixed right away after its publication, the use of an ARR
was frowned upon and even did not made it into the RFC. GnuPG even
marks such subpackets as "Big Brother's key (ignored)".
The new ADSK is similar in functionality to the ARR but it is
implemented as a key flag on a standard subkey. Thus the public key
is instantly available, avoids key lookups, and gives the user a
better control on whether and how to use an ADSK.
Footnotes
_________
[1] [https://www.kb.cert.org/vuls/id/747124/]
--
The pioneers of a warless world are the youth that
refuse military service. - A. Einstein
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