Email Security with Digital Certificates (Part 1)

If you would like to read the other parts in this article series please go to:

• Email Security with Digital Certificates (Part 2)
• Email Security with Digital Certificates (Part 3)
• Email Security with Digital Certificates (Part 4)
• Email Security with Digital Certificates (Part 5)

Introduction

Email is one of the most widely and commonly used Internet services. The email infrastructure employed on the Internet primarily consists of email servers using Simple Mail Transfer Protocol (SMTP) to accept messages from clients, transport those messages to other servers, and deliver them into a user’s server-based inbox. Clients retrieve email from their inboxes using a variety of protocols such as Post Office Protocol version 3 (POP3), Internet Message Access Protocol (IMAP), Messaging Application Programming Interface (MAPI), and so on.

For email, the basic mechanism in use on the Internet offers the efficient delivery of messages but lacks controls to provide for confidentiality and integrity. In other words, basic email is not secure. However, organizations can, and often do, add security to email in several ways in order to satisfy one or more of the following objectives:

• Provide for nonrepudiation. In general terms, nonrepudiation ensures that the subject of an activity or event cannot deny that the event occurred. In relation to email, nonrepudiation prevents a subject from claiming not to have sent an email;
• Restrict access to emails to their intended recipients (i.e., privacy and confidentiality);
• Maintain the integrity of emails;
• Authenticate and verify the source of emails;
• Verify the delivery of emails;
• Classify sensitive content within or attached to emails.

The first step in deploying email security is to recognize the vulnerabilities specific to email. Although MAPI, POP and IMAP can be encrypted, SMTP usually isn’t. Thus, most emails are transmitted in plain text across the Internet. This makes interception and eavesdropping easy.

The lack of native encryption is, however, only one security issue related to email. Email is a common delivery mechanism for viruses, worms, Trojan horses, documents with destructive macros, and other malicious code. The proliferation of support for various scripting languages, auto-download capabilities, and auto-execute features has transformed hyperlinks within the content of email and attachments into a serious threat to every system.

Spoofing the source address of email is a simple process for even a novice attacker. Email headers can be modified at their source or at any point during transit. And speaking of in-transit modification, there are no native integrity checks to ensure that a message was not altered between its source and destination. To address some of these concerns, organizations rely on SPF, DKIM and DMARC, which have already been discussed in other articles here in MSExchange.org.

In addition, email itself can be used as an attack mechanism. When sufficient numbers of messages are directed to a single user’s inbox or through a specific STMP server, a Denial-of-Service (DoS) attack can result. This attack is often called mail-bombing and is simply a DoS performed by inundating a system with messages. The DoS can be the result of storage capacity consumption or processing capability utilization. Either way, the result is the same: legitimate emails cannot be delivered.

Like email flooding and malicious code attachments, unwanted email can be considered an attack. Sending unwanted, inappropriate, or irrelevant messages is called spamming. Spamming is often little more than a nuisance, but it does waste system resources both locally and over the Internet. It is often difficult to stop spam because the source of the messages is usually spoofed.

In this article series we will focus on encryption as a method of achieving confidentiality, integrity, authentication and nonrepudiation. All these terms, and how encryption helps us meet them, will be explained in detail throughout this article series.

Email Security Solutions

Imposing security on email is possible, but the efforts should be in tune with the value and confidentiality of the emails being exchanged. We can use several protocols, services, and solutions to add security to email without requiring a complete overhaul of the entire Exchange infrastructure. These used to include legacy mechanisms such as MOSS, PEM or PGP. In the Exchange world, nowadays we primarily focus on SPF, DKIM, DMARC and S/MIME. Some of the terms and algorithms mentioned below are explained in more detail later on.

• MIME Object Security Services (MOSS) can provide authentication, confidentiality, integrity and nonrepudiation for email messages. MOSS employs Message Digest 2 (MD2) and MD5 algorithms; Rivest, Shamir, and Adelman (RSA) public key; and Data Encryption Standard (DES) to provide authentication and encryption services;
• Privacy Enhanced Mail (PEM) was an email encryption mechanism that provided authentication, integrity, confidentiality and nonrepudiation. PEM used RSA, DES and X.509 certificates;
• DomainKeys Identified Mail (DKIM) DKIM is a means to assert that valid mail is sent by an organization through verification of domain name identity. More information can be found in the articles DKIM and DMARC in Office 365 and Outbound DKIM Signing in Office 365 here at MSExchange.org;
• Pretty Good Privacy (PGP) secure email system appeared on the computer security scene in 1991. It combines Certificate Authority hierarchy with the “web of trust” concept, that is, we must become trusted by one or more PGP users to begin using the system. We then accept their judgment regarding the validity of additional users and, by extension, trust a multilevel “web” of users descending from our initial trust judgments. PGP is available in two versions: the commercial version uses RSA for key exchange, IDEA for encryption/decryption and MD5 for message digest production; the freeware version (based on the similar OpenPGP standard) uses Diffie-Hellman key exchange, the Carlisle Adams/Stafford Tavares (CAST) 128-bit encryption/decryption algorithm and the SHA-1 hashing function. As of 2010, PGP Corp was acquired by Symantec. Now PGP is available only from Symantec;
• Secure Multipurpose Internet Mail Extensions (S/MIME) is an email security standard that offers authentication and confidentiality to email through public key encryption and digital signatures. Authentication is provided through X.509 digital certificates. Privacy is provided through the use of Public Key Cryptography Standard (PKCS) encryption.

Before delving into S/MIME, we need to first understand how asymmetric cryptography and digital certificates work, as they are the foundation of S/MIME.

Cryptography

Cryptography provides added levels of security to data during processing, storage and communication. Over the years, mathematicians and computer scientists have developed a series of increasingly complex algorithms designed to ensure confidentiality, integrity, authentication and nonrepudiation. While cryptographers spend time developing strong encryption algorithms, hackers and governments alike devote significant resources to undermining them. This is known as an “arms race” in cryptography and resulted in the development of the extremely sophisticated algorithms in use today.

Cryptographic systems are used to meet four fundamental goals: confidentiality, integrity, authentication and nonrepudiation. Achieving each of these goals requires the satisfaction of a number of design requirements, and not all cryptosystems are intended to achieve all four goals.

• Confidentiality ensures that data remains private while at rest, such as when stored on a disk, or in transit, such as during transmission between two or more parties. For many, this is the main purpose of cryptosystems: the preservation of secrecy for stored information or for communications between parties. Two main types of cryptosystems enforce confidentiality: symmetric key cryptosystems use a shared secret key available to all users of the cryptosystem; asymmetric cryptosystems use individual combinations of public and private keys for each user of the system;
• Integrity ensures that data is not altered without authorization. If integrity mechanisms are in place, the recipient of a message can be certain that the message received is identical to the message that was sent. Similarly, integrity checks can ensure that stored data was not altered between the time it was created and the time it was accessed. Integrity controls protect against all forms of alteration: intentional alteration by a third party attempting to insert false information and unintentional alteration by faults in the transmission process. Email integrity is enforced through the use of encrypted message digests, known as digital signatures created upon transmission of an email. The recipient of the email simply verifies that the emails’ digital signature is valid, ensuring that the message was not altered in transit. Integrity can be enforced by both public and secret key cryptosystems;
• Authentication verifies the claimed identity of users and is a major function of cryptosystems. For example, suppose that Nuno wants to establish a communications session with Linda and they are both participants in a shared secret communications system. Linda might use a challenge-response authentication technique to ensure that Nuno is who he claims to be;
• Nonrepudiation provides assurance to the recipient that the message was originated by the sender and not someone masquerading as the sender. It also prevents the sender from claiming that they never sent the message in the first place (known as repudiating the message). Secret key, or symmetric key, cryptosystems do not provide this guarantee of nonrepudiation. If Michael and Nuno participate in a secret key communication system, they can both produce the same encrypted message using their shared secret key. Nonrepudiation is offered only by public key, or asymmetric, cryptosystems.

It is not the purpose of this article series to go into detail about how cryptography works, but rather how it can be used to protect email messages. As such, I will leave out all the extremely complex mathematics involved and, instead, provide a high level overview.

Conclusion

In this first part of this article series, we briefly introduced some email security options such as DKIM and S/MIME and had a look at the four fundamental goals of cryptography: confidentiality, integrity, authentication and nonrepudiation. In the next part we will look at Symmetric vs Asymmetric Key Algorithms and what a Public Key Infrastructure is.

If you would like to read the other parts in this article series please go to:

• Email Security with Digital Certificates (Part 2)
• Email Security with Digital Certificates (Part 3)
• Email Security with Digital Certificates (Part 4)
• Email Security with Digital Certificates (Part 5)