PGP Web of Trust: What Key Signatures Prove

PGP key signatures in a web of trust can prove that a specific public key belongs to the person or organization identified on it, but only to the degree that the signing parties actually verified that identity. They cannot prove that the identity itself is truthful, that the key has not been compromised, or that the keyholder is trustworthy. This article explains exactly what a signature does and does not establish, how the web of trust works, and how to interpret signatures you encounter.

Background: Why the Web of Trust Exists

PGP (Pretty Good Privacy) was created in 1991 by Phil Zimmermann as a way to encrypt and sign messages without a central authority. The problem it solved was key distribution: if Alice wants to send Bob an encrypted message, she needs Bob’s public key, but how does she know that the key she has is really Bob’s? A central certificate authority (like those used for HTTPS) could vouch for Bob, but that requires trusting a third party. The web of trust instead lets people vouch for each other directly.

When you sign someone’s key, you are making a statement: “I have verified that this key belongs to the person named on it.” That signature is cryptographically attached to the key and can be checked by anyone else. Over time, a network of these signatures forms a web. If you trust Alice, and Alice has signed Bob’s key, you might decide to trust Bob’s key too, even if you have never met Bob.

What a Signature Can Prove

A valid signature on a PGP key proves three things:

• The key has not been tampered with since the signature was made. Any alteration to the key invalidates the signature.
• The signer possessed the corresponding private key at the time of signing. The signature is made with the signer’s private key, so it proves the signer was in control of their own key.
• The signer intentionally associated their identity with the key. The signature includes the signer’s identity and a timestamp.

In practice, a signature also implies that the signer performed some level of identity verification – but that is a social convention, not a cryptographic guarantee. The OpenPGP specification does not dictate how you must verify identity; it only provides a field for the signer to describe their verification method (e.g., “met in person” or “checked photo ID”).

What a Signature Cannot Prove

Here is where many misconceptions arise. A signature cannot prove:

• The identity itself is real. Someone can create a key with the name “Barack Obama” and get a friend to sign it. The signature only proves that the friend vouches for that key, not that the person is actually Barack Obama.
• The key has not been compromised. If the private key was stolen, the thief can use it to create new signatures that look valid. A signature does not prove the current holder is the original owner.
• The keyholder is trustworthy. A signature says “this key belongs to this person,” not “this person is honest.” Someone can be correctly identified and still be a scammer.
• The key has not been revoked. A key can be revoked after signing. The signature remains valid, but the key may no longer be usable.
• The signer actually verified identity. Some people sign keys without any verification, or with only a cursory email check. The signature proves intent, not diligence.

How to Choose Whom to Trust

When you encounter a PGP key, you must decide how much weight to give its signatures. Here are practical steps:

1. Check the key’s fingerprint – compare it against the fingerprint the owner publishes on their website or social media.
2. Look at the signatures – who signed it? Do you know any of them personally?
3. Check the signer’s verification habits – some people are known for signing keys only after meeting in person and checking ID; others sign anything.
4. Consider the context – if you are using the key for a one-off encrypted email, a single signature from a stranger may be enough. If you are trusting it for a financial transaction, you need a much stronger chain.
5. Use the trust parameters in your keyring – GnuPG lets you assign trust levels to keys you know, and it will compute validity for others based on your settings.

Common Problems and Misconceptions

“I signed their key, so I can trust them.” No – signing is about identity, not character. You can sign a key and still not want to lend that person money.

“The web of trust is broken.” It is not broken, but it is often misused. Many people sign keys without verification, diluting the meaning of a signature. The system works only when participants follow the verification guidelines.

“A revoked key is still safe.” No. Revocation means the owner no longer trusts the key, but old signatures on it remain. Always check for revocation certificates.

“More signatures = more trustworthy.” Not necessarily. Ten signatures from people who never met the keyholder mean less than one signature from someone who carefully checked ID. Quality matters more than quantity.

Comparison with Certificate Authorities

The web of trust is often contrasted with the centralized CA model used for HTTPS. In the CA model, a small number of root authorities vouch for all websites. In the web of trust, there is no hierarchy – anyone can sign anyone. The trade-offs are:

• CA model: easy to verify (your browser does it), but you must trust the CAs. If a CA is compromised, all its certificates are suspect.
• Web of trust: no central point of failure, but verification is manual and inconsistent. You must decide which signatures to trust.

Many systems, including Signal and the Tox chat protocol, use a hybrid approach: they use a central server for key distribution but allow users to manually verify each other’s keys out-of-band.

Safety and Best Practices

To use the web of trust safely:

• Verify fingerprints out-of-band – via a phone call, in person, or a trusted website.
• Only sign keys after real verification – at minimum, confirm the person controls the email address and ideally meet them in person.
• Keep your own private key secure – use a strong passphrase and consider a hardware token.
• Check for revocation – before using a key, see if it has been revoked.
• Understand your keyring’s trust settings – GnuPG’s trust model is powerful but easy to misconfigure.

Frequently Asked Questions

Q: Can I trust a key if it has no signatures?
A: It depends. If you obtained the key directly from the owner (e.g., handed to you on a USB stick), you can verify the fingerprint yourself. If you downloaded it from a keyserver, no signatures means you have no independent confirmation – proceed with caution.

Q: How many signatures are enough?
A: There is no magic number. One signature from someone you trust deeply is worth more than a hundred from strangers. The GnuPG default requires one fully trusted signature or three marginally trusted ones.

Q: What does “marginal” vs “full” trust mean?
A: These are levels you assign to keys you know. If you fully trust Alice, her signature alone can validate another key. If you only marginally trust Bob, you need three of his signatures (or a combination) to validate a key.

Q: Can a signature be faked?
A: No, not cryptographically. A signature is created with the signer’s private key, and forging it would require that key. However, a signer can be tricked into signing a key they think is one person but is actually another – that is a social engineering attack, not a cryptographic one.

Q: What happens if a key is compromised after I signed it?
A: Your signature remains valid, but the key should be revoked. You can also issue a revocation certificate for your signature, but that is rarely done. The important thing is that the keyholder revokes their key, and you check for that revocation before using it.

Q: Is the web of trust still used today?
A: Yes, especially in the Free Software community and among privacy advocates. It is also built into tools like GnuPG, the standard implementation of OpenPGP. While many people now use simpler “trust on first use” models (like SSH), the web of trust remains the strongest decentralized identity system available.

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