Privacy Enhanced Mail (PEM), is an early 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 as 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 PGP.

Attempts to deploy PEM were finally abandoned in response to the need to extend the protocol to support MIME leading to the development of MOSS (never widely implemented; now abandoned) and S/MIME (shares de facto standard status with PGP).

See also

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

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