Secure Multipurpose Internet Mail Extensions (S/MIME)

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Multipurpose Internet Mail Extensions (MIME) is an Internet standard that extends the format of an email message to support non-ASCII character sets and multimedia attachments.

Secure Multipurpose Internet Mail Extensions (S/MIME) is an Internet standard for signing and encrypting MIME data.

• provides presentation-layer authentication, message integrity, nonrepudiation, confidentiality, and data security benefits to users
• is a protocol for securing email communications
  • encrypts emails and enables sender authentication to ensure confidentiality and integrity
  • uses public key encryption to secure email content
  • incorporates digital signatures to support sender verification and ensure messages are unmodified
• implementation is often complicated
  • prone to misconfiguration
  • complex certificate management and validation
• requires an X.509 certificate for each email client
• can use RSA, DSA, and ECDSA for digital signatures
• can use AES and 3DES for message encryption
• is an end-to-end security solution
  • defeats/complicates attempts to have enterprise-wide or server-hosted antimalware scanning
    • S/MIME has encrypted such malware or banned content by encrypting the message content and its attachments
• signatures are detached
  • not tied to the content of the message itself
  • so only authenticates sender’s identity, not that the sender sent the message
• in 2019 EFF announced the critical vulnerability EFAIL
  • can allow attackers to hide unknown plaintext within the original message (using various HTML tags)
  • affects many email systems