OpenPGP Email Forwarding Via Diverted Elliptic Curve Diffie-Hellman Key Exchanges

TL;DR

This paper introduces a method for secure email forwarding in OpenPGP using a novel adaptation of proxy-re-encryption, enabling message transformation without exposing decryption keys or plaintexts.

Contribution

It adapts a classic cryptographic technique to the OpenPGP context, providing a practical implementation and security analysis for encrypted email forwarding.

Findings

Successfully implemented email forwarding without key access.

Demonstrated security against common attack vectors.

Provided detailed implementation guidelines.

Abstract

An offline OpenPGP user might want to forward part or all of their email messages to third parties. Given that messages are encrypted, this requires transforming them into ciphertexts decryptable by the intended forwarded parties, while maintaining confidentiality and authentication. It is shown in recent lines of work that this can be achieved by means of proxy-re-encryption schemes, however, while encrypted email forwarding is the most mentioned application of proxy-re-encryption, it has not been implemented in the OpenPGP context, to the best of our knowledge. In this paper, we adapt the seminal technique introduced by Blaze, Bleumer and Strauss in EUROCRYPT'98, allowing a Mail Transfer Agent to transform and forward OpenPGP messages without access to decryption keys or plaintexts. We also provide implementation details and a security analysis.