Blog Post Aug 07, 2024

S/MIME encryption: What it is and how does it work?

Malicious emails form the basis for some of today's most devastating cyberattacks. These attacks are dangerous, in part, due to the inherent simplicity and ubiquity of email.

It's easy to see, then, why email often represents a major cybersecurity weak link, even among organizations that otherwise appear to maintain a strong security posture. Thankfully, it is possible to enhance the integrity and confidentiality of email communication—and S/MIME represents the best and most accessible option for achieving this.

1. What is S/MIME Encryption?

2. How does S/MIME work?

3. Implementation of S/MIME

4. Benefits and challenges of S/MIME

5. Sectigo is the leading S/MIME certificate provider

What is S/MIME Encryption?

S/MIME (Secure/Multipurpose Internet Mail Extension) is an important standard that facilitates encryption and digitally signing to secure the contents of an email. This application of PKI (public key infrastructure) technology allows for much-needed authentication, as recipients can verify an email’s origins.

Central to any understanding of S/MIME encryption: basic knowledge surrounding Multipurpose Internet Mail Extensions (MIMEs). This standard expands the scope of email messaging, making it possible to send several types of files. Thanks to MIME, users can easily send a wide range of attachments—including several attachments (of various types) within a single email.

These capabilities are important, but they present distinct security risks. They enable bad actors to send executable scripts or files via email as these can be added as attachments, which unsuspecting recipients are more likely to open.

This is where S/MIME certificates come into play. Use cases abound, with S/MIME commonly seen in many sectors and scenarios, including these:

Government agencies. Given the vital importance of confidential communication, government agencies rely on S/MIME to facilitate strong encryption. This ensures that only intended recipients are able to read emails. This is important for internal communication and can also promote secure communication between agencies. An important consideration: Defense Federal Acquisition Regulation (DFAR) outlines strict cybersecurity policies, and S/MIME can help to ensure that defense-related information is properly secured when sent via email.
Healthcare industry. Crucial for maintaining Health Insurance Portability and Accountability Act (HIPAA) compliance, S/MIME provides enhanced protection in a vulnerable industry. Patients’ Personal Health Information (PHI) may otherwise be at risk if transmitted via email. This is also a must for meeting email retention requirements.
Law firms. Case and client details are routinely discussed via email, so it is imperative that attorneys and other legal professionals abide by strict encryption protocols (available via S/MIME) to keep their clients safe. This is also important for meeting ethical and legal standards related to attorney-client communication.
Corporate email communication. An increasing range of corporations (beyond the scope of the healthcare and legal industries) now look to S/MIME for maximum email security. More attention has been dedicated to email security in light of high-profile spoofing, phishing attacks, and man-in-the-middle (MITM) attacks.

Keep in mind that, while email servers may be encrypted, the same is not necessarily true for emails themselves. This is why S/MIME certificates are so important: they provide an added layer of protection for sensitive communication.

How does S/MIME work?

S/MIME certificates are a type of digital certificate typically requested by a person or organization from a certificate authority (CA). They can then be installed in email clients to sign and encrypt future emails. S/MIME certificates use asymmetric encryption to keep email contents, including attachments, secure. The sender uses the recipient’s public key to encrypt the message and only the recipient’s private key can be used to decrypt it. A digital signature can also be created using the private key to verify the sender’s identity and confirm the email contents have not been altered.

The fundamental components that contribute to this process are outlined below:

Certificates and certificate authorities

Certificate authorities (CAs) play a central role in facilitating S/MIME certificates. CAs are responsible for verifying identities and issuing digital certificates, which, in turn, are used to verify digital signatures and to ensure that email communication is encrypted.

CAs are a linchpin in an overarching PKI solution, capable of producing a trust hierarchy and ensuring that certificates can be traced via a chain of trust.

Public and private keys

S/MIME involves pairs of public and private keys, with the public key used to encrypt messages while the private key makes it possible to decrypt them. The private key must be safeguarded for if it is compromised, the security of the email communication will be as well.

Asymmetric cryptography

A two-key system featuring one public key and one private key, asymmetric cryptography relies on algorithms such as RSA (Rivest-Shamir-Adleman) and ECC (Elliptic Curve Cryptography) to encrypt data, generate digital signatures, and facilitate key exchanges. This can present some key management challenges, but, if properly implemented, asymmetric cryptography can streamline key distribution and also may provide a valuable mechanism to drive effective authentication.

Key escrow and exchange systems

Promoting the secret storage of encryption keys, along with easy access for authorized users, key escrow involves a third party, which is tasked with securely storing private keys so that they cannot be accessed by malicious parties. Access is only permitted under a specific set of circumstances so that they remain secure but also can be recovered by authorized users if the original is lost.

Key exchange allows for the secure sharing of cryptographic keys, which can be distributed among intended email recipients. This is increasingly automated by email clients and is also highly scalable when directory services come into play. It can be difficult to establish initial trust, however, so CAs may be needed to verify that keys belong to the individual or entity who claims to be the sender.

Can also be used for digital signatures

S/MIME allows email clients to leverage cryptographic hash functions, thereby producing message digests that are encrypted with private keys. This supports the creation of a digital signature. The recipient's email client should be able to extract the digital signature and generate a message digest via the sender's cryptographic hash function. Once decrypted with the sender's public key, the message digests can be compared. When they match, the validity of the digital signature is confirmed.

Implementation of S/MIME

S/MIME calls for a structured setup process, which include these crucial steps:

Obtain and install certificates

Digital certificates are central to S/MIME's success, so these must be obtained from trusted CAs and installed in the email client. Automated certificate management is highly recommended as this streamlines an otherwise potentially time-consuming process.

Configure the email client

S/MIME cannot be enabled unless the email client is properly configured. Many email clients have dedicated encryption or security sections, where S/MIME can be enabled and the installed digital certificate can be selected. Keep in mind that processes for configuring S/MIME may vary from one email client to the next.

Benefits and challenges of S/MIME

There is a lot to appreciate about S/MIME, which plays a fundamental role in modern email security. There are also challenges to be considered, however, and these must be fully understood before implementing S/MIME.

Benefits of S/MIME

Critical for safeguarding sensitive information, S/MIME can form the basis for powerful email security. Benefits worth considering include:

Security. Representing a highly secure email encryption protocol, S/MIME leverages asymmetric cryptography and end-to-end encryption to provide enhanced integrity and confidentiality, not to mention, powerful peace of mind.
Privacy. Emails are rarely as private as users assume, but S/MIME can boost privacy via encryption and digital signatures. This limits the potential for unauthorized access to potentially sensitive information.
Integration. Offering seamless integration with many email clients, S/MIME represents a widely adopted standard and is even platform-independent. This leads to strong interoperability and ease of configuration.

Challenges of S/MIME

Despite the many advantages highlighted above, there are some concerns that can prove frustrating or costly if implementation is not carefully planned.

Setup. It takes time to obtain and install S/MIME certificates, and to configure email clients. Despite S/MIME's widespread adoption, interoperability issues can sometimes come into play.
Maintenance. From key management to certificate renewal, many tasks must be continually handled to ensure that S/MIME systems are properly maintained.

These challenges can be resolved by working with a leading S/MIME certificate provider and by leveraging an automated certificate lifecycle management platform to support certificate management.

Sectigo is the leading S/MIME certificate provider

As the leading provider of S/MIME certificates, Sectigo provides solutions that are supported by all major mail applications, such as Microsoft Outlook, Gmail, Exchange, and more. Make the most of our personal email security solutions or work with Sectigo to secure corporate email accounts.

For a comprehensive certificate management platform solution, learn about the Sectigo Certificate Manager.

Sources:

- NIST
- Microsoft
- Veterans Affaires Gov
- Forbes