system-config/doc/source/signing.rst

title

Signing System

Signing System

Our standard signing automation model leverages an OpenPGP signing subkey, encrypted as a Zuul secret, to create detached signatures for release artifacts (tarballs, wheels, et cetera) and to sign and push Git tags as part of managed release automation. For OpenStack's releases, the master key corresponding to this subkey is replaced near the start of each new development cycle and set to expire soon after the cycle is scheduled to conclude (with enough overlap to allow for graceful replacement).

At a Glance

Secrets

• gpg_key

Roles

• add-gpgkey
• sign-artifacts

Key Management Overview

The signing system is implemented as a set of Zuul jobs; these utilize the signing subkey via an encrypted Zuul secret imported into a job's ~/.gnupg/secring.gpg at runtime. It's used by jobs to create detached signatures of release artifacts and to sign Git tags in release management automation.

Storage

While the signing subkey is installed unencrypted on some job nodes, so that it can be used unattended by job automation, for OpenStack the corresponding master key is kept symmetrically encrypted in the root home directory of the Infra systems management bastion. At the time of key creation a revocation certificate is also generated, for which sysadmins are encouraged to retrieve and keep local copies in case control over or access to the original master key is lost.

Rotation

For OpenStack, the master key is rotated at the start of each development cycle (usually shortly after cycle-trailing deliverables are released), signed by the previous key before being put into service, and has an expiration date set for at least a month after the end of the targeted development cycle (or best guess, often longer for safety). New key fingerprints are also submitted to the openstack/releases repository, for publication on the releases.openstack.org Web site.

Revocation

Under normal circumstances, keys should be allowed to expire gracefully. If the key is compromised but still accessible, a revocation certificate can be generated and published to the key network at that time. If access to the private key is lost completely, the revocation certificate generated at key creation time should be used as a last resort.

Key Management Process

Configuration

This is the content of the /root/signing.gnupg/gpg.conf file on our management bastion host:

# A basic gpg.conf using secure keyserver transport and some more
# verbose display options. This configuration assumes you have
# installed both the gnupg and gnupg-curl packages. Set your umask
# to 077, create a /root/signing.gnupg directory and place this
# configuration file in it.

# Receive, send and search for keys in the SKS keyservers pool using
# HKPS (OpenPGP HTTP Keyserver Protocol via TLS/SSL).
keyserver hkps://keys.openpgp.org

# Display key IDs in a more accurate 16-digit hexidecimal format
# and add 0x at the beginning for clarity.
keyid-format 0xlong

# Display the calculated validity of user IDs when listing keys or
# showing signatures.
list-options show-uid-validity
verify-options show-uid-validity

Generation

Make sure we start with a restrictive umask so that files and directories we write from this point forward are only accessible by the root user:

root@bridge:~# umask 077

Now create a master key for the coming development cycle, taking mostly the GnuPG recommended default values. Set a validity period sufficient to last through the release process at the conclusion of the cycle. Use a sufficiently long, randomly-generated passphrase string (it's fine to reuse the one stored in our passwords list for earlier keys unless we know it to have been compromised):

root@bridge:~# gpg --homedir signing.gnupg --full-generate-key --expert
gpg (GnuPG) 2.2.4; Copyright (C) 2017 Free Software Foundation, Inc.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Please select what kind of key you want:
   (1) RSA and RSA (default)
   (2) DSA and Elgamal
   (3) DSA (sign only)
   (4) RSA (sign only)
   (7) DSA (set your own capabilities)
   (8) RSA (set your own capabilities)
   (9) ECC and ECC
   (10) ECC (sign only)
   (11) ECC (set your own capabilities)
   (13) Existing key
Your selection? 9
Please select which elliptic curve you want:
   (1) Curve 25519
   (3) NIST P-256
   (4) NIST P-384
   (5) NIST P-521
   (6) Brainpool P-256
   (7) Brainpool P-384
   (8) Brainpool P-512
   (9) secp256k1
Your selection? 1
Please specify how long the key should be valid.
         0 = key does not expire
      <n>  = key expires in n days
      <n>w = key expires in n weeks
      <n>m = key expires in n months
      <n>y = key expires in n years
Key is valid for? (0) 9m
Key expires at Thu 02 Feb 2017 08:41:39 PM UTC
Is this correct? (y/N) y

You need a user ID to identify your key; the software constructs the user ID
from the Real Name, Comment and Email Address in this form:
    "Heinrich Heine (Der Dichter) <heinrichh@duesseldorf.de>"

Real name: OpenStack Infra
Email address: infra-root@openstack.org
Comment: Some Cycle
You selected this USER-ID:
    "OpenStack Infra (Some Cycle) <infra-root@openstack.org>"

Change (N)ame, (C)omment, (E)mail or (O)kay/(Q)uit? o
You need a Passphrase to protect your secret key.

Enter passphrase: ********************************
Repeat passphrase: ********************************

We need to generate a lot of random bytes. It is a good idea to perform
some other action (type on the keyboard, move the mouse, utilize the
disks) during the prime generation; this gives the random number
generator a better chance to gain enough entropy.
.+++++
......+++++
We need to generate a lot of random bytes. It is a good idea to perform
some other action (type on the keyboard, move the mouse, utilize the
disks) during the prime generation; this gives the random number
generator a better chance to gain enough entropy.
.+++++
+++++
gpg: key 0x120D3C23C6D5584D marked as ultimately trusted
gpg: revocation certificate stored as '/root/signing.gnupg/openpgp-revocs.d/7222E5A05730B7670F93035A120D3C23C6D5584D.rev'
public and secret key created and signed.

pub   ed25519/0x120D3C23C6D5584D 2016-07-07 [expires: 2017-02-02]
      7222E5A05730B7670F93035A120D3C23C6D5584D
uid                              OpenStack Infra (Some Cycle) <infra-root@openstack.org>
sub   cv25519/0x1F215B56867C5D9A 2016-07-07 [E] [expires: 2017-02-02]

Save the revocation certificate for the master key, for use in the case extreme case that this master key itself becomes inaccessible, for example because the decryption passphrase is lost (under any other circumstances, a revocation certificate with a more detailed description can be generated using the master key on an as-needed basis). Replace some in the output filename with the lower-cased cycle name:

root@bridge:~# mv \
> signing.gnupg/openpgp-revocs.d/7222E5A05730B7670F93035A120D3C23C6D5584D.rev
> signing.gnupg/some.revoke.asc

Use the interactive key editor to add a subkey constrained to signing purposes only. It does not need an expiration since it will be valid only for as long as its associated master key is valid:

root@bridge:~# gpg --homedir signing.gnupg --expert --edit-key 0x120D3C23C6D5584D
gpg (GnuPG) 2.2.4; Copyright (C) 2017 Free Software Foundation, Inc.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Secret key is available.

sec  ed25519/0x120D3C23C6D5584D
     created: 2016-07-07  expires: 2017-02-02  usage: SC
     trust: ultimate      validity: ultimate
ssb  cv25519/0x1F215B56867C5D9A
     created: 2016-07-07  expires: 2017-02-02  usage: E
[ultimate] (1). OpenStack Infra (Some Cycle) <infra-root@openstack.org>

gpg> addkey
Please select what kind of key you want:
   (3) DSA (sign only)
   (4) RSA (sign only)
   (5) Elgamal (encrypt only)
   (6) RSA (encrypt only)
   (7) DSA (set your own capabilities)
   (8) RSA (set your own capabilities)
  (10) ECC (sign only)
  (11) ECC (set your own capabilities)
  (12) ECC (encrypt only)
  (13) Existing key
Your selection? 10
Please select which elliptic curve you want:
   (1) Curve 25519
   (3) NIST P-256
   (4) NIST P-384
   (5) NIST P-521
   (6) Brainpool P-256
   (7) Brainpool P-384
   (8) Brainpool P-512
   (9) secp256k1
Your selection? 1
Please specify how long the key should be valid.
         0 = key does not expire
      <n>  = key expires in n days
      <n>w = key expires in n weeks
      <n>m = key expires in n months
      <n>y = key expires in n years
Key is valid for? (0)
Key does not expire at all
Is this correct? (y/N) y
Really create? (y/N) y
Key is protected.

You need a passphrase to unlock the secret key for
user: "OpenStack Infra (Some Cycle) <infra-root@openstack.org>"
ID 0x120D3C23C6D5584D, created 2016-07-07

Enter passphrase: ********************************

We need to generate a lot of random bytes. It is a good idea to perform
some other action (type on the keyboard, move the mouse, utilize the
disks) during the prime generation; this gives the random number
generator a better chance to gain enough entropy.
+++++
........+++++

sec  ed25519/0x120D3C23C6D5584D
     created: 2016-07-07  expires: 2017-02-02  usage: SC
     trust: ultimate      validity: ultimate
ssb  cv25519/0x1F215B56867C5D9A
     created: 2016-07-07  expires: 2017-02-02  usage: E
ssb  ed25519/0xC0224DB5F541FB68
     created: 2016-07-07  expires: never       usage: S
[ultimate] (1). OpenStack Infra (Some Cycle) <infra-root@openstack.org>

gpg> save

Next, sign the new master key with the key from the previous cycle (specified with the --default-key option). This proves that the new key was created by a party with access to its predecessor, so provides some added assurance of its validity:

root@bridge:~# gpg --homedir signing.gnupg --default-key 0x70CA2E45DF30B1B8 --sign-key 0x120D3C23C6D5584D

This key is due to expire on 2017-02-02.
Are you sure that you want to sign this key with your
key "OpenStack Infra (Previous Cycle) <infra-root@openstack.org>" (0x70CA2E45DF30B1B8)

Really sign? (y/N) y

You need a passphrase to unlock the secret key for
user: "OpenStack Infra (Previous Cycle) <infra-root@openstack.org>"
ID 0x70CA2E45DF30B1B8, created 2016-11-03

Enter passphrase: ********************************

Now send the master key to the keyserver network. The subkeys are all submitted along with it, so do not need to be specified separately:

root@bridge:~# gpg --homedir signing.gnupg --send-keys 0x120D3C23C6D5584D
gpg: sending key 0x120D3C23C6D5584D to hkps://keys.openpgp.org

Check the infra-root inbox (the address associated with the key) for a notification about the key upload to keys.openpgp.org and follow the URL within it. Once there, click the button to send a verification message. Now check the inbox again and follow the URL provided in the new message which should arrive. Once that's done, the key will be searchable on the keyserver.

The rest of this process shouldn't happen until we're ready for the signing system to transition to our new key. In a typical, non-emergency rotation this should not happen until release activities for the previous cycle have concluded so that we don't inadvertently sign their artifacts with the new key.

Create a new GnuPG keychain by exporting a copy of just the signing subkey to a file and then importing that (and only that) in a new GnuPG directory:

root@bridge:~# umask 077
root@bridge:~# mkdir temporary.gnupg
root@bridge:~# gpg --homedir signing.gnupg \
> --output temporary.gnupg/secret-subkeys \
> --export-secret-subkeys 0xC0224DB5F541FB68\!
root@bridge:~# gpg --homedir temporary.gnupg \
> --import temporary.gnupg/secret-subkeys
gpg: keyring `temporary.gnupg/secring.gpg' created
gpg: keyring `temporary.gnupg/pubring.gpg' created
gpg: key C6D5584D: secret key imported
gpg: temporary.gnupg/trustdb.gpg: trustdb created
gpg: key C6D5584D: public key "OpenStack Infra (Some Cycle) <infra-root@openstack.org>" imported
gpg: Total number processed: 1
gpg:               imported: 1
gpg:       secret keys read: 1
gpg:   secret keys imported: 1

Check that the exported version does not contain a usable primary secret key by listing all secret keys and looking for a sec# in front of it instead of just sec:

root@bridge:~# gpg --homedir temporary.gnupg --list-secret-keys

/root/temporary.gnupg/pubring.kbx
---------------------------------
sec#  ed25519 2016-07-07 [SC] [expires: 2017-02-02]
      120D3C23C6D5584D
uid           [unknown] OpenStack Infra (Some Cycle) <infra-root@openstack.org>
ssb   ed25519 2016-07-07 [S]

So that our CI jobs will be able to make use of this subkey without interactively supplying a passphrase, the old passphrase (exported from the master key) must be reset to an empty string in the new temporary copy. Here we override the default pinentry mode to loopback as a workaround for other pinentry frontends refusing to accept an empty passphrase (unfortunately the prompting and feedback from the loopback pinentry leaves something to be desired). Note that the first password prompt will be for the original key's password, but the second password prompt should be left blank thus removing that password. This is again done using an interactive key editor session:

root@bridge:~# gpg --homedir temporary.gnupg --pinentry-mode loopback \
> --edit-key 0xC0224DB5F541FB68
gpg (GnuPG) 2.2.4; Copyright (C) 2017 Free Software Foundation, Inc.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Secret subkeys are available.

pub  ed25519/0x120D3C23C6D5584D  created: 2016-07-07  expires: 2017-02-02  usage: SC
                 trust: unknown       validity: unknown
sub  ed25519/0xC0224DB5F541FB68  created: 2016-07-07  expires: never       usage: S
[ unknown] (1). OpenStack Infra (Some Cycle) <infra-root@openstack.org>

gpg> passwd
gpg: key 120D3C23C6D5584D/120D3C23C6D5584D: error changing passphrase: No secret key
Enter passphrase: ********************************
Enter passphrase:

gpg> save
Key not changed so no update needed.

Test the subkey can be used without a passphrase:

root@bridge:~# echo foo | gpg --homedir temporary.gnupg --sign --armor
-----BEGIN PGP MESSAGE-----

SR43vh1iK66BbmlsONWxII74fIPEDHDeCqVnkzxdhleDf7DOd9HhYmI8WNOKtTIU
7hcy6cYqHBjEgVr5oViNiveiwGsKlOUhh8x1eYDIxEEoGQEHDJDKq9YOMMjRdsO8
fOw0TD/1r8Lmi8QLkCfGvFdrSY6EoCHqCMx3+JmGUD+iFGp2rCOucw==
=LxND
-----END PGP MESSAGE-----

This leaves us with a temporary keyring containing only an unencrypted copy of the signing subkey. Export this keyring so that we can add it as a secret to Zuul for use by release jobs.

root@bridge:~# gpg --homedir temporary.gnupg \
> --output temporary.gnupg/for-zuul --armor \
> --export-secret-subkeys 0xC0224DB5F541FB68\!
root@bridge:~# wget https://opendev.org/zuul/zuul/raw/branch/master/tools/encrypt_secret.py
root@bridge:~# python3 encrypt_secret.py --tenant openstack \
> --infile temporary.gnupg/for-zuul --outfile temporary.gnupg/zuul.yaml \
> https://zuul.opendev.org openstack/project-config
writing RSA key
Public key length: 4096 bits (512 bytes)
Max plaintext length per chunk: 470 bytes
Input plaintext length: 1490 bytes
Number of chunks: 4

Copy temporary.gnupg/zuul.yaml to your workstation and make a commit to zuul.d/secrets.yaml file in the openstack/project-config repo to update the gpg_key secret with its contents. Be sure to replace <name> and <fieldname> as appropriate.

Safely clean up, doing your best to securely remove the temporary copy of the unencrypted signing subkey and any associated files:

root@bridge:~# find temporary.gnupg/ -type f -exec shred {} \;
root@bridge:~# rm -rf temporary.gnupg encrypt_secret.py

To document this transition, export a minimal text version of the public master key:

root@bridge:~# ( gpg --fingerprint --list-sigs \
> 0x120d3c23c6d5584d6fc2464664dbb05acc5e7c28 ; gpg --armor \
> --export 0x120d3c23c6d5584d6fc2464664dbb05acc5e7c28 ) > \
> 0x120d3c23c6d5584d6fc2464664dbb05acc5e7c28.txt

Add the file to a change for the openstack/releases repo placing it in the doc/source/static directory, and then link it similarly to other exported public keys are linked in the Cryptographic Signatures <https://releases.openstack.org/#cryptographic-signatures> section of doc/source/index.rst (noting the appropriate end date for use of the prior key as the start date for the new one).

Attestation

Since the collapse of the SKS keyserver network, no popular keyservers publish third-party key signatures any longer (the pollution thereof was a big part of SKS's downfall). As such, until something like a caff-style signature approval mechanism is integrated into the OpenPGP keyserver infrastructure or some alternative comes along, there's little point in generating our own individual key signatures we can't easily distribute to users. For now, we simply make sure the signature made by the previous key is included in the export we publish from our own sites so that users can have some path to confirm new keys.

Still, please retrieve a copy of the /root/signing.gnupg/some.revoke.asc fallback revocation certificate (some to be replaced with the lower-cased release name) from the management bastion and keep it stashed somewhere secure, for emergency use in the (hopefully very unlikely) event that our OpenPGP master private key is completely lost to us (for example, if we lose the file containing its decryption passphrase and all backups thereof).