Privacy-enhanced Electronic Mail

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Privacy Enhanced Mail (PEM), is a 1993 IETF proposal for securing email using public-key cryptography. Although PEM became an IETF proposed standard it was never widely deployed or used.

One reason for the lack of deployment was that the PEM protocol depended on prior deployment of a hierarchical public key infrastructure (PKI) with a single root. Deployment of such a PKI proved impossible once the operational cost and legal liability of the root and 'policy' CAs became understood.

In addition to being an obstacle to deployment, the single-rooted hierarchy was rejected by some commentators as an unacceptable imposition of central authority. This led Phil Zimmermann to propose the Web of Trust as the PKI infrastructure for the encryption program Pretty Good Privacy (PGP). Efforts to deploy PEM were finally abandoned in response to the need to extend the protocol to support MIME. This led to the development of the protocol MIME Object Security Services (MOSS; never widely implemented, now abandoned) and S/MIME (shares de facto standard status with PGP). This is an IETF standard, a result of a group working for a long time. The basic idea is to have privacy by virtue of hierarchical authentication. A receiver trusts the message of the sender when it is accompanied by a certificate from his trusted authority. These authoritative certificates are distributed from a group called Internet Policy Registration Authority (IPRA) and Policy Certificate Authority (PCA). These trusted authority actually certifies the public key sent by senders.[1]

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RFC 1421
Privacy Enhancement for Internet Electronic Mail: Part I: Message Encryption and Authentication Procedures
RFC 1422
Privacy Enhancement for Internet Electronic Mail: Part II: Certificate-Based Key Management
RFC 1423
Privacy Enhancement for Internet Electronic Mail: Part III: Algorithms, Modes, and Identifiers
RFC 1424
Privacy Enhancement for Internet Electronic Mail: Part IV: Key Certification and Related Services