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Email Architecture

• Involves various agents:

  • Mail Transfer Agent (MTA)

  • Mail Submission Agent (MSA)

  • Message User Agent (MUA)

• Message handling goes through SMTP and ESMTP.

Email Formats

• RFC 5322 format includes To, From, Subject, Data, and Body fields.

Limitations of SMTP/5322

• Cannot send binary/executable files.

• Limited to plain ASCII text and has message size restrictions.

• Poor compatibility with non-standard systems.

MIME Protocol

• Allows sending non-ASCII formatted data.

• Converts data to/from 7-bit ASCII for email transport.

MIME Header Fields

1. MIME-Version: indicates MIME version.

2. Content-Type: defines type/subtype of content.

3. Content-Transfer-Encoding: specifies encoding type.

4. Content-Id: unique message identifier.

5. Content-Description: explanation of non-text content.

MIME Content Types

• Types include:

  • Application

  • Audio

  • Image

  • Message

  • Multipart

  • Text (default)

  • Video

Email Security Threats

• Phishing and spoofed emails.

• Integrity threats from modified invoices.

• Confidentiality threats from data breaches.

• Denial-of-service (DoS) attacks.

Counter Threat Protocols

• Recommended protocols include:

  • STARTTLS: secures paths between mail servers.

  • S/MIME: protects message content.

  • DNSSEC and DANE: secure the DNS.

S/MIME Certificate

• Provides signatures and encryption for email messages.

S/MIME Services

• Digital Signature: RSA/SHA-256 for authenticity.

• Message Encryption: AES-128 with a one-time session key.

• Compression: message may be compressed for storage.

S/MIME Functional Flow

• Sender signs and then encrypts the message.

• Receiver decrypts the message and verifies the signature.

Enhanced Security Services

• Signed receipt for delivery proof.

• Security labels for content sensitivity.

• Secure mailing lists for individual recipient encryption.

• Signing certificates for identity verification.