Understanding Certificate Common Name & Securing Communications with an Email SSL Certificate (S/MIME)

In the realm of online security, verifying identity is fundamental. Whether accessing a website or receiving an email, users need assurance that they are interacting with the legitimate entity. Two key concepts often encountered are the Certificate Common Name (CN) within digital certificates, and the need for an Email SSL Certificate to secure communications. While related to digital identity, they apply in different contexts.

This post will clarify what the Certificate Common Name is, its current relevance, and dive into what’s commonly meant by an “Email SSL Certificate” – primarily focusing on S/MIME certificates for securing email messages.

Key Takeaways

• Certificate Common Name (CN): Historically the primary field in an SSL/TLS certificate identifying the server/host (usually the Fully Qualified Domain Name like www.example.com).
• Subject Alternative Name (SAN): Now the standard field browsers and clients use to match the identity (domain name, IP address, email address). Certificates must typically contain the identifier in the SAN field for validation.^^[CA/Browser Forum Baseline Requirements, Section 7.1.4.2.1, mandate identity information be in SAN.]^^
• “Email SSL Certificate”: This term most accurately refers to S/MIME certificates, used for digitally signing and/or encrypting email messages themselves.
• S/MIME Identifiers: For S/MIME, the crucial identifier is the user’s email address, which is typically placed in the SAN extension (rfc822Name field).
• Server vs. Message Security: Don’t confuse S/MIME (message security) with SSL/TLS certificates used to secure the connection to the mail server (e.g., IMAPS/SMTPS). They are different certificate types.
• Accuracy is Crucial: Whether CN or SAN, providing the correct identifier (domain name for web SSL, email address for S/MIME) during certificate purchase is vital for validation and trust.

Decoding the Certificate Common Name (CN)

The Certificate Common Name field was traditionally the main way an SSL/TLS certificate specified the identity it was validating.

What is the Common Name?

• It’s an attribute within the ‘Subject’ field of an X.509 digital certificate.
• For website SSL/TLS certificates, the CN was historically populated with the Fully Qualified Domain Name (FQDN) of the website being secured (e.g., secure.example.com).

The Shift to Subject Alternative Names (SANs)

While the CN field exists, its role as the primary identifier has been superseded by the Subject Alternative Name (SAN) extension for most modern applications, especially web browsers.

• Why the Shift? SANs offer much more flexibility. A single certificate can list multiple identities (e.g., example.com, www.example.com, mail.example.com) in the SAN fields.
• Browser Validation: Modern browsers primarily check the SAN fields for a matching domain name. If the domain name is present only in the CN but not in the SAN, browsers will typically throw a security warning.^^[RFC 6125 strongly recommends checking SAN first and discourages fallback to CN.]^^
• Requirement: The CA/Browser Forum’s Baseline Requirements mandate that applicable identities (like DNS names) be placed within the SAN extension.

Is the CN Still Relevant?

• It’s often still included in certificates, sometimes mirroring the primary entry in the SAN.
• Some older legacy systems or specific applications might still rely on the CN field.
• However, for broad compatibility and modern security standards, ensuring the correct identifiers are in the SAN fields is essential.

Understanding “Email SSL Certificate” – Focus on S/MIME

The term Email SSL Certificate can be slightly ambiguous, but in the context of user-focused certificates for email, it almost always refers to S/MIME certificates.

Clarifying Terminology: S/MIME vs. Server SSL

• S/MIME Certificates: These secure the email messages themselves. They allow you to digitally sign emails (proving sender identity and message integrity) and encrypt emails (ensuring only the intended recipient can read them). This is likely what someone means when asking for an “Email SSL Certificate” for their personal or business email address.
• SSL/TLS for Mail Servers: These are standard SSL/TLS certificates installed on mail servers (like those running IMAP, POP3, SMTP) to encrypt the connection between the email client (e.g., Outlook, Thunderbird) and the server. This protects login credentials and prevents eavesdropping on the communication channel, but doesn’t secure the message content itself once stored or forwarded.

S/MIME Certificates Explained:

• Purpose: Provides message-level security:
  • Digital Signature: Authenticates the sender, guarantees message integrity (hasn’t been altered), provides non-repudiation.
  • Encryption: Ensures message confidentiality, readable only by the recipient with the corresponding private key.
• Identifier: The crucial identifier in an S/MIME certificate is the email address it validates. This email address is typically included in the certificate’s Subject Alternative Name (SAN) extension, specifically in the rfc822Name field type. While historically the CN might have sometimes contained an email address, standard practice now places it firmly within the SAN.

Benefits of Using S/MIME:

• Trust & Authenticity: Recipients can be sure the email came from you and wasn’t forged.
• Integrity: Assurance that the message content wasn’t modified in transit.
• Confidentiality: Protects sensitive information from prying eyes through encryption.
• Compliance: Helps meet regulatory requirements (like GDPR, HIPAA) for data protection in email communications. ^^[Regulations like HIPAA Security Rule (45 CFR § 164.312(e)(1)) require mechanisms to encrypt electronic protected health information.]^^

The Importance of Correct Identifiers (CN & SAN)

Whether it’s a domain name in a website SSL/TLS certificate or an email address in an S/MIME certificate, accuracy is paramount.

• Validation Failure: If the identifier provided during purchase doesn’t match what the CA verifies, the certificate won’t be issued or won’t work correctly.
• Mismatch Errors: If a certificate is issued but the identifier (especially in SAN) doesn’t match the server name or email address being used, users will encounter security warnings, eroding trust.
• Ordering Process: When purchasing any certificate (SSL/TLS or S/MIME) from providers like SSLRepo, carefully provide the exact domain name(s) or email address you need to secure. For S/MIME, this means entering the specific email address the certificate will be used with.

Wrapping It Up

The Certificate Common Name (CN) is a legacy field in digital certificates, largely superseded by the more flexible and now standard Subject Alternative Name (SAN) field for identity matching. When people refer to an Email SSL Certificate, they usually mean an S/MIME certificate designed to secure individual email messages through digital signatures and encryption. The primary identifier for S/MIME is the user’s email address, located in the SAN field.

Understanding these concepts helps ensure you acquire the right type of certificate with the correct identifiers, building trust and security for your website or email communications. Reputable providers like SSLRepo offer both website SSL/TLS and S/MIME certificates to meet these needs.

Frequently Asked Questions (FAQ)

Q1: What is the Certificate Common Name (CN)?
A: It’s a field in a digital certificate that historically identified the primary name associated with the certificate, usually the domain name for website SSL/TLS. Its importance has diminished in favor of the Subject Alternative Name (SAN).

Q2: What is an Email SSL Certificate?
A: This term typically refers to an S/MIME certificate used to digitally sign and/or encrypt email messages, verifying sender identity and protecting content. It’s different from SSL/TLS certificates used to secure connections to mail servers.

Q3: If my certificate has SANs, do I still need a Common Name?
A: While modern browsers rely on SANs, a CN is often still included for compatibility with older systems. However, the critical identifiers must be in the SAN fields for proper validation by current clients.

Q4: Where is the email address specified in an S/MIME certificate?
A: The validated email address is primarily located in the Subject Alternative Name (SAN) extension of the certificate, usually in a field type called rfc822Name.

Q5: Can I use my website’s SSL certificate for signing emails?
A: No. Website SSL/TLS certificates validate domain names and are used for securing server connections (HTTPS). S/MIME certificates validate email addresses and are used for signing/encrypting email messages. They are different certificate types for different purposes.

Q6: Where can I buy trusted S/MIME (Email SSL) certificates?
A: Reputable certificate resellers like SSLRepo offer S/MIME certificates from trusted Certificate Authorities (CAs), allowing you to secure your email communications effectively.