S/MIME and PGP are protocols used to authenticate and encrypt messages through the Internet. Both use public key encryption to sign and encrypt email. In simple terms, the main differences are:
Public key credibility: In the S/MIME standard, users must apply for an X.509 v3 digital certificate from a trusted certificate authority. Authoritative CA institutions verify the user’s true identity and sign the public key. Ensure that the user’s public key is trusted, and the recipient verifies the authenticity of the sender’s identity through the certificate public key.
PGP does not provide a policy to enforce trust creation. The sender creates and signs his own key pair or signs the public key for other communication users to increase the credibility of his key. There is no trusted authority to verify it. Identity information, each user must decide whether to trust each other.
Scope of encryption protection: PGP was born to solve the security problem of plain text messages, while S/MIME not only protects text messages, but also aims to protect various attachments / data files.
Centralized management: From a management perspective, S/MIME is considered better than PGP because it has powerful features that support centralized key management through an X.509 certificate server.
Compatibility and ease of use: S/MIME has broader industry support. The S/MIME protocol is already built into most email client software, such as Outlook, Thunderbird, and iMail, which support S/MIME encryption. From an end-user perspective, S/MIME is also easier to use than PGP, because PGP requires downloading additional plugins to run, and the S/MIME protocol allows most vendors to send and receive encrypted email without using other plugins.
Therefore, in general, the applicability of the S/MIME standard is more extensive, which can more fully protect the security and credibility of e-mail.
Trusted Guide from NIST
In the “Trusted Mail” standard issued by the National Institute of Standards and Technology, also recommend S/MIME for Federal Use:
Security Recommendation 5-5: For Federal use, OpenPGP is not preferred for message confidentiality. The use of S/MIME with a certificate signed by a known CA is preferred.
Security Recommendation 4-11: Use S/MIME signatures for assuring message authenticity and integrity.
Security Consideration 7-2: Enterprises should establish a cryptographic key management system (CKMS) for keys associated with protecting email sessions with end users. For federal agencies, this means compliance with all relevant policy and best practice for the protection of key material [SP800-57pt1].
Supported Documents and more details about PGP and S/MIME can be found here: https://www.nist.gov/publications/trustworthy-email-0
MeSince is a free email encryption client in S/MIME standard, making S/MIME certificates deployed easier in individuals’ or enterprises’ email communication systems. We also support enterprises to deploy key management system on their premises. We deliver bespoke email encryption solutions for individual and enterprise users based on their demands.