SMIME

S/MIME stands for Secure/Multipurpose Internet Mail Extensions. It is a standard for public key encryption and signing of MIME data in email, designed to provide confidentiality, data integrity, authentication, and non-repudiation for electronic correspondence.

S/MIME uses public key cryptography and X.509 certificates. Messages can be signed to verify the sender’s identity or encrypted to protect content from eavesdropping. The cryptographic content and its metadata are encoded using the Cryptographic Message Syntax (CMS), a flexible data format defined in standards such as RFC 5652.

Implementation relies on a Public Key Infrastructure. Users obtain certificates from trusted certificate authorities, either from an external CA or an enterprise PKI. Recipients’ public keys are used to encrypt messages; the sender signs messages with their private key, and recipients verify with the sender’s public certificate. S/MIME messages may be carried as multipart/signed or as encrypted content indicated by the CMS structure.

Support: Many major email clients include S/MIME support, including Outlook, Apple Mail, and Mozilla Thunderbird, often requiring user certificates and configuration. Interoperability depends on valid, trusted certificates and proper revocation checking.

Limitations and considerations: S/MIME requires PKI management and user certificates; revocation and expiration can complicate use; key compromise or loss requires revocation. While it provides strong confidentiality and authentication, it does not guarantee against all forms of social engineering and should be used as part of broader security practices.