What is recipient non-repudiation in secure email transport

Question

I’ve read about recipient non-repudiation in the context of secure email, but the concept is a bit unclear. Does it mean that the recipient cannot deny receiving an email? If so, how is this technically enforced, and what protocols or mechanisms enable it? For example, does it rely on delivery confirmations or cryptographic proofs?

Answer 1

Recipient non-repudiation in secure email transport refers to mechanisms that prevent the recipient of an email from denying its receipt. This concept ensures that once an email is delivered, there is verifiable evidence linking the recipient to the received message.

Technical Enforcement Mechanisms

1. Digital Signatures: When an email is digitally signed by the sender, it provides proof of the sender's identity and the message's integrity. However, for recipient non-repudiation, the recipient's actions upon receiving the email need to be captured. This can be achieved if the recipient digitally acknowledges the receipt, creating a verifiable record that they have received and recognized the message.

2. Read Receipts with Cryptographic Proofs: Standard read receipts inform the sender that an email has been opened but lack cryptographic verification. Enhanced mechanisms can involve the recipient's email client generating a digitally signed acknowledgment upon opening the email, providing cryptographic proof of receipt.

3. Secure Email Protocols: Protocols like S/MIME (Secure/Multipurpose Internet Mail Extensions) support digital signing and encryption of emails. While S/MIME primarily ensures message integrity and confidentiality, it can be extended to support non-repudiation by requiring recipients to send digitally signed acknowledgments.

4. Delivery Receipts from Mail Servers: Some email systems can be configured to provide delivery receipts that are digitally signed by the recipient's mail server. This indicates that the email was delivered to the recipient's mailbox, though it doesn't confirm that the recipient has read it.

Challenges and Considerations

• Recipient Cooperation: Achieving recipient non-repudiation often requires the recipient's email client to support and participate in generating cryptographic acknowledgments. Without recipient cooperation, enforcing non-repudiation becomes challenging.

• Privacy Concerns: Automatically sending acknowledgments can raise privacy issues, as recipients may not want to disclose when they have read a message. Therefore, such mechanisms should be implemented with user consent and awareness.

• Technical Limitations: Not all email clients or servers support the necessary features for recipient non-repudiation. Implementing such mechanisms may require additional infrastructure and agreement between communicating parties.