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PGP: Identifying Message Recipients

G5 Cyber Security

TL;DR

Yes, it’s generally possible to identify all recipients of a PGP encrypted message if the sender doesn’t take specific precautions. The recipient list is often embedded in the message headers.

How Recipients Are Identified

When you send a PGP encrypted email, your email client typically adds information about who you encrypted the message for to the message itself. This isn’t part of the encryption; it’s metadata added before or after the encryption process.

Steps to Understand and Mitigate Recipient Identification

  1. Understanding the ‘To:’ Field: The most obvious way recipients are identified is through the To: field. Even with PGP, this field often remains visible in the unencrypted headers.
  2. Examining Message Headers: Recipient information can be found within the message headers, even if the body is encrypted. Tools like email clients (Thunderbird, Outlook) or command-line tools can display these headers.
    • Using Thunderbird: View > Headers > All
    • Command Line (using openssl): 
      openssl smime -in message.eml -header

      This will show the full headers, including recipient details.

  3. ‘X-PGP-Recipients’ Header: Many PGP implementations add an X-PGP-Recipients header that explicitly lists the email addresses of those who received a copy of the encrypted message.
    • This is often the key indicator.
  4. ‘Resent-From:’ Header: If someone forwards your encrypted email, they may add a Resent-From: header which can also reveal recipients if not handled correctly.
  5. Mitigation Strategies – BCC Equivalent: The best way to prevent recipient identification is to send individual, separately encrypted emails to each person.
    • This mimics the functionality of ‘BCC’ in regular email. Each message has only one intended recipient listed in the headers.
    • Most modern PGP clients allow you to select multiple recipients and encrypt a separate copy for each.
  6. Mitigation Strategies – Removing Headers: Some email clients offer options to strip headers before sending, but this isn’t always reliable and can break compatibility.
    • Be cautious when using header stripping as it may interfere with PGP functionality.
  7. Mitigation Strategies – Using a Mail Transfer Agent (MTA): Advanced users can configure an MTA to remove identifying headers before sending the email.
    • This requires technical expertise and careful configuration.

Important Considerations

Forwarding: If a recipient forwards your encrypted message, they may inadvertently reveal other recipients if the headers aren’t properly handled.

Email Client Behaviour: Different email clients handle PGP and headers differently. Test your setup thoroughly to understand how it behaves.

cyber security Best Practices: Always verify the authenticity of recipient public keys before encrypting sensitive information. Be aware that even with encryption, metadata can still reveal information about communication patterns.

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