Im about to implement GPG for signing documents in my company. Here is the Setup I came up with:
- A personal key for each employee on a smartcard
- A Company signing key that signs all the personal keys for validity.
During implementation, I’m struggling with the following points:
Distinguish between “good signature” and “valid signature”
In my understanding, a “good signature” is a cryptogaphically sound signature, a “valid signature” is one where I can trust the signers key actually belongs to the real person, is not expired, etc. See validity
I have the case where I have a signature where the validity of a key is “unknown” and I have no trust in the key. I have imported the public cert, but I have not signed the key with my private or the company key. Here is the ouput of the --edit-key output:
gpg: using pgp trust model
pub rsa3072/F6475CA4D40800F1
created: 2025-03-26 expires: never usage: SC
trust: unknown validity: unknown
sub rsa3072/6F884B933496DEC5
created: 2025-03-26 expires: never usage: E
[ unknown] (1). Employee A <mployee_a@company.com>
When I validate a signature made with this key, I get the expected (good, but not valid):
> $ gpg --verify "System_Integration_Test_Bench_00014.pdf.asc"
gpg: assuming signed data in 'System_Integration_Test_Bench_00014.pdf'
gpg: Signature made Mo 19 Mai 2025 11:15:51 CEST
gpg: using RSA key 41BDF6C5822010ABC6652ED3F6475CA4D40800F1
gpg: using pgp trust model
gpg: Good signature from "Employee A <emplyee_a@company.com>" [unknown]
gpg: WARNING: This key is not certified with a trusted signature!
gpg: There is no indication that the signature belongs to the owner.
Primary key fingerprint: 41BD F6C5 8220 10AB C665 2ED3 F647 5CA4 D408 00F1
gpg: binary signature, digest algorithm SHA512, key algorithm rsa3072
However, if i get the machine-readable version, I get the info that the signature is valid (because of the [GNUPG:] VALIDSIG line):
> $ gpg --status-fd 1 --verify "System_Integration_Test_Bench_00014.pdf.asc"
gpg: assuming signed data in 'System_Integration_Test_Bench_00014.pdf'
[GNUPG:] NEWSIG
gpg: Signature made Mo 19 Mai 2025 11:15:51 CEST
gpg: using RSA key 41BDF6C5822010ABC6652ED3F6475CA4D40800F1
[GNUPG:] KEY_CONSIDERED 41BDF6C5822010ABC6652ED3F6475CA4D40800F1 0
[GNUPG:] SIG_ID 0OytdFIocjafx9/5fsyKdWNrMkg 2025-05-19 1747646151
gpg: using pgp trust model
[GNUPG:] GOODSIG F6475CA4D40800F1 Employee A <emplyee_a@company.com>
gpg: Good signature from "Employee A <emplyee_a@company.com>" [unknown]
[GNUPG:] VALIDSIG 41BDF6C5822010ABC6652ED3F6475CA4D40800F1 2025-05-19 1747646151 0 4 0 1 10 00 41BDF6C5822010ABC6652ED3F6475CA4D40800F1
[GNUPG:] KEY_CONSIDERED 41BDF6C5822010ABC6652ED3F6475CA4D40800F1 0
[GNUPG:] TRUST_UNDEFINED 0 pgp
gpg: WARNING: This key is not certified with a trusted signature!
gpg: There is no indication that the signature belongs to the owner.
Primary key fingerprint: 41BD F6C5 8220 10AB C665 2ED3 F647 5CA4 D408 00F1
gpg: binary signature, digest algorithm SHA512, key algorithm rsa3072
[GNUPG:] VERIFICATION_COMPLIANCE_MODE 23
To my understanding, this signature should not be valid. Please help me understand what makes this valid or where I get the concept of validity wrong.
Setting up a easy-to-use web of trust
Some of my employees are not tech-savvy, I want the setup an usage for them as easy as possible. I would like to
- Get everybody a ready-to use key on a smartcard
- Have the possibility of a backup
- Make it easy for them to have all the keys of other employees in a valid way
For this, I thought of the following:
- Have a offline (tails) environment to create the keys for a new employee
- Sign the key with the company key (to show other employees that this is valid)
- Sign the company key (to show the employees computer that the company key is valid)
- I was told that it is good practice to not have the Cert key on the smartcard, but offline. This creates the need to sign the company key at this stage, because afterwards the Cert key is not available anymore.
- Export the private keys to the smartcard and offer the public keys (with exported signatures) over a static html site that offers the export of the public key
However, Im facing the following problem that I cannot get the web of trust to work. Even though I signed the company key with my cert key (in a exportable way), exported it and import it on my working - environment, I do not get the key to be shown as valid. To verify that the certification of the keys made it with the export and import, here is my output of gpg --list-signatures on my working maschine:
pub rsa4096 2022-06-21 [C]
AF3295423E1B6B50C42006C59F8921B4BAEA00F2
uid [ultimate] Me myself and I <me@company.com>
sig 3 9F8921B4BAEA00F2 2022-06-21 Me myself and I <me@company.com>
sig N E373B7E181E3C1B5 2025-05-16 company offline certification key <cert@company.com>
sub rsa4096 2022-06-21 [S]
sig 9F8921B4BAEA00F2 2022-06-21 Me myself and I <me@company.com>
sub rsa4096 2022-06-21 [E]
sig 9F8921B4BAEA00F2 2022-06-21 Me myself and I <me@company.com>
sub rsa4096 2022-06-21 [A]
sig 9F8921B4BAEA00F2 2022-06-21 Me myself and I <me@company.com>
pub rsa4096 2025-05-16 [C]
4A0DA3776273007C32CCBAC1E373B7E181E3C1B5
uid [ unknown] company offline certification key <cert@company.com>
sig 3 E373B7E181E3C1B5 2025-05-16 company offline certification key <cert@company.com>
sig N 9F8921B4BAEA00F2 2025-05-16 Me myself and I <me@company.com>
sub rsa4096 2025-05-16 [S]
sig E373B7E181E3C1B5 2025-05-16 company offline certification key <cert@company.com>
sub rsa4096 2025-05-16 [E]
sig E373B7E181E3C1B5 2025-05-16 company offline certification key <cert@company.com>
sub rsa4096 2025-05-16 [A]
sig E373B7E181E3C1B5 2025-05-16 company offline certification key <cert@company.com>
Please not that the “company offline certification key” still has validity [ unknown] even though it is signed by me and my key is ultimately trusted. I have the feeling that my own imported signature from another company does not count, it is also not listed in the check command in the --edit-key command. Can I make this work? If not, having an offline Cert key does not seem feasible for me. Feel free for other suggestion/comments about this setup.
Thank you a lot for your help!
Dominik
Edit: I just found out that there is a --trusted-key option. I assume that this is an option that you have to add each call. Even tough this would be possible, I would prefer a solution that is cryptography-based / without needing an argument in every call.