How to Verify the Authenticity of PGP Signatures

Why Signature Verification Matters

Digital signatures are a crucial aspect of PGP encryption that often doesn't get as much attention as the encryption itself. While encryption ensures that only the intended recipient can read a message, signatures verify who sent it and confirm that it hasn't been altered.

Properly verifying signatures is essential for:

• Confirming the authentic source of a message or file
• Detecting if content has been tampered with
• Preventing impersonation attacks
• Establishing non-repudiation (the sender cannot deny sending the message)

Understanding PGP Signatures

A PGP signature is created using the sender's private key. When you verify a signature, you use the sender's public key to confirm that the signature was indeed created with their corresponding private key. This process also verifies that the message hasn't been modified since it was signed.

Step-by-Step Guide to Verifying Signatures

1. Obtain the Sender's Public Key

Before you can verify a signature, you need the sender's authentic public key. You can obtain it through:

• Direct exchange (the most secure method)
• Public key servers
• The sender's website or social media profiles

2. Verify the Key's Authenticity

It's crucial to ensure you have the sender's authentic public key, not an impostor's. You can verify a key's authenticity by:

• Checking the key fingerprint through a secure channel (phone call, secure messaging)
• Verifying signatures from other trusted parties on the key
• Confirming through multiple independent channels

3. Import the Public Key

Once you've obtained and verified the sender's public key, import it into your PGP software or use our online verification tool.

4. Verify the Signature

To verify a signature using our tool:

1. Go to our Sign & Verify page
2. Select the "Verify" tab
3. Paste the signed message or upload the signed file
4. Paste the sender's public key
5. Click "Verify Signature"

5. Interpret the Verification Results

After verification, you'll see one of these results:

• Valid signature: The message is authentic and hasn't been altered
• Invalid signature: Either the message has been tampered with or it wasn't signed by the owner of the public key you used
• Unknown key: The signature might be valid, but you don't have the correct public key to verify it

Common Verification Scenarios

Verifying Email Messages

Many PGP-enabled email clients automatically verify signatures. Look for indicators like a checkmark or "signature valid" message. For manually signed emails, you may need to copy the signed content into a verification tool.

Verifying Software Downloads

Many software developers sign their releases. To verify:

1. Download both the software and its signature file (.asc or .sig)
2. Obtain the developer's public key from their official website
3. Use PGP software or our verification tool to check the signature against the downloaded file

Verifying Text Documents

For signed text documents:

1. If the signature is detached, you'll need both the original document and the signature file
2. If the signature is inline, the document will contain both the content and the signature block
3. Use the verification tool with the appropriate format

Troubleshooting Verification Issues

Signature Shows as Invalid

If a signature fails verification:

• Check if you're using the correct public key
• Ensure the message hasn't been modified (even adding/removing a single space can invalidate a signature)
• Verify that you've included the entire signed content, including any headers
• For detached signatures, confirm you're using the exact file that was signed

Key Not Found or Expired

If you get a "key not found" or "key expired" error:

• Obtain an updated version of the sender's public key
• Check if the key has been revoked
• Verify that the key hasn't expired

Best Practices for Signature Verification

• Always verify the authenticity of public keys before using them
• Keep your trusted public key collection up to date
• Be suspicious of unexpected signature verification failures
• Use multiple verification methods for highly sensitive communications
• Consider the entire chain of trust when evaluating signatures

By following these guidelines, you can effectively use PGP signatures to verify the authenticity and integrity of your digital communications, adding an essential layer of security beyond encryption alone.