A cryptographic system is a collection of software and hardware that can encrypt or decrypt information. A typical cryptographic system is the combination of a desktop computer, a web browser, a remote web server, and the computer on which the web server is running. A cryptographic protocol, by contrast, describes how information moves throughout the cryptographic system. In our examples, the web browser and the remote web server communicate using the Secure Sockets Layer (SSL) cryptographic protocol.
More than a dozen cryptographic protocols have been developed for Internet security and commerce. These systems fall into two categories. The first category of cryptographic programs and protocols is used for encryption of offline messages—mostly email. The second category of cryptographic protocols is used for confidentiality, authentication, integrity, and nonrepudiation for online communications.
Offline encryption systems are designed to take a message, encrypt it, and either store the ciphertext or transmit it to another user on the Internet. Some popular programs that are used for email encryption are shown in Table 4-1 and described in the sections that follow.
Table 4-1. Cryptographic protocols for offline communications
What does it do?
Programs and systems
Encryption and digital signatures for email and electronic documents
PGP (Network Associates)
Hushmail (Hush Communications)
GNU Privacy Guard (GNU)
Encryption and digital signatures for email
Netscape Communicator (Netscape Communications)
Outlook Express (Microsoft)
PGP (Pretty Good Privacy) is a complete working system for the cryptographic protection of electronic mail and files. OpenPGP is a set of standards (RFC 2440) that describe the formats for encrypted messages, keys, and digital signatures. PGP offers confidentiality, integrity, and nonrepudiation.
PGP was the first widespread public key encryption program. The original version was written between 1990 and 1992 by Phil Zimmermann and released on the Internet in June 1991. Later versions were the result of efforts by Zimmermann and programmers around the world.
PGP is available in two ways: as a command-line program, which can be run on many different operating systems, and as an integrated application, which is limited to running on the Windows and Macintosh platforms. The integrated application comes with plug-in modules that allow it to integrate with popular email packages such as Microsoft Outlook, Outlook Express, Eudora, and Netscape Communicator. With these plug-ins, the standard email packages can automatically send and receive PGP-encrypted messages.
Current versions of PGP allow users to create two kinds of private keys: encryption keys, which are used for actually encrypting email messages, and signing keys, which are used for digitally signing messages. Older versions of PGP supported only a single key that was used for both encryption and signing.
Each PGP key consists of two parts: a person’s name and the actual mathematical key that is used to perform cryptographic operations, as shown in Figure 4-1.
Figure 4-1. PGP keys consist of the actual public key that is used to encrypt or decrypt information, one or more email addresses, and one or more digital signatures attached to each email address.
Because PGP keys have names, one logical question to ask is this: how do you know that a given PGP key really belongs to the person whose name is on that key? This can be a very difficult question to answer.
The simplest way to be sure of a key’s authenticity is to get a copy of the key from its owner. Unlike other email encryption systems, every PGP user can certify any key that he wishes: if you have a key, it is up to you to decide if you believe that the key actually belongs to the person who is named on the key’s certificate. When you get a person’s key and add it to your PGP key ring , you can tell your copy of PGP whether or not to trust the key.
In practice, it is not always possible to have face-to-face meetings with people before you get their keys. As an alternative to face-to-face meetings, PGP has a provision for key signing. That is, one PGP key can be used to sign a second key. Essentially, this means that the first person is using his key to attest to the validity of the second person. I may not be able to meet directly with Sam, but if I trust Deborah, and I get a copy of Sam’s key with Deborah’s signature on the key attesting to the fact that Sam’s key is valid, I may trust the key. This process is called certification. Certification is discussed in detail in Chapter 7. Chapter 6 contains additional discussion of PGP and provides examples of its use.
PGP’s name-based system also assumes that everybody who signed and who uses the key thinks that the name on the key refers to the same person. This may be true for some names and email addresses, but it is certainly not true for others. There might be many John Smiths; several people might use the email address firstname.lastname@example.org over the course of a few years. On this issue, PGP is silent.
When you send an email with an attachment over the Internet, the attachment is encoded with a protocol called the Multipurpose Internet Mail Extensions, or MIME. The MIME standard codifies the technique by which binary files, such as images or Microsoft Word documents, can be encoded in a format that can be sent by email.
Secure/MIME (S/MIME) extends the MIME standard to allow for encrypted email. On the surface, S/MIME offers similar functionality to PGP; both allow email messages to be encrypted and digitally signed. But S/MIME is different from PGP in an important way: to send somebody a message that is encrypted with PGP you need a copy of that person’s key. With S/MIME, on the other hand, to send somebody an encrypted message you need a copy of that person’s S/MIME certificate. In general, people cannot create their own S/MIME certificates. Instead, these certificates are issued by third parties called certification authorities. This extra layer of complexity has somewhat limited the widespread adoption of S/MIME as a system for encrypted email.
Online cryptographic protocols generally require real-time interplay between a client and a server to work properly. The most popular online protocol is SSL, which is used to protect information as it is sent between a web browser and a web server. Some popular systems that fall into this category are summarized in Table 4-2 and described in the following sections.
Table 4-2. Cryptographic protocols for online communications
What does it do?
Programs and systems
DNSSEC (Secure DNS)
Provides secure hostname to IP address resolution
BIND, Version 9 (Internet Software Consortium)
IPsec and IPv6
Secures IP traffic
Provides secure authentication and cryptographic key exchange for higher-level protocols
Windows 2000 (Microsoft)[a]
PCT (Private Communications Technology)
Provides privacy for web transactions
Internet Explorer (Microsoft)
Internet Information Server (Microsoft)
SET (Secure Electronic Transactions)
Provides privacy and nonrepudiation for credit card transactions; prevents merchants from getting credit card numbers
Capital One Wallet
SSH (Secure Shell)
Provides secure remote access (telnet) protocol and additional provisions for encrypting other protocols such as email and X Windows
SSH Version 1.x, 2.x
SecureCRT (Vandyke Communications)
SSL (Secure Sockets Layer)
Encrypts stream communications; mostly used for downloading web pages and email
Apache Web Server
Internet Information Server (Microsoft)
Commerce Server (Netscape)
Most web browsers
[a] The Microsoft version of Kerberos contains proprietary extensions not compatible with open standards.
The Secure Sockets Layer (SSL) is a general-purpose web cryptographic protocol for securing bidirectional communication channels. SSL is commonly used with TCP/IP. SSL is the encryption system that is used by web browsers such as Netscape Navigator and Microsoft’s Internet Explorer, but it can be used with any TCP/IP service.
SSL connections are usually initiated with a web browser
using a special URL prefix. For example, the prefix
is used to indicate an SSL-encrypted HTTP connection,
snews: is used to
indicate an SSL-encrypted NNTP connection.
SSL offers confidentiality through the use of:
User-specified encryption algorithms
Integrity, through the use of user-specified cryptographic hash functions
Authentication, through the use of X.509 v3 public key certificates
Nonrepudiation, through the use of cryptographically signed messages
The Private Communications Technology (PCT) is a transport layer security protocol similar to SSL that was developed by Microsoft because of shortcomings in SSL 2.0. The SSL 2.0 problems were also addressed in SSL 3.0 and, as a result, use of PCT is decreasing. Nevertheless, Microsoft intends to continue supporting PCT because it is being used by several large Microsoft customers on their corporate intranets.
The Secure Electronic Transaction (SET) protocol is an online payment protocol designed to facilitate the use of credit cards on the Internet.
The fundamental motivation behind SET is to speed transactions while reducing fraud. To speed transactions, the protocol automates the “buy” process by having the consumer’s computer automatically provide the consumer’s credit card number and other payment information, rather than forcing the consumer to type this information into a form in a web browser. To reduce fraud, SET was designed so that the merchant would never have access to the consumer’s actual credit card number. Instead, the merchant would receive an encrypted credit card number that could only be decrypted by the merchant’s bank.
There are three parts to the SET system: an “electronic wallet” that resides on the user’s computer; a server that runs at the merchant’s web site; and the SET Payment Server that runs at the merchant’s bank. All of these parts need to be operational before any transactions can be processed. Largely because of this complexity, SET has not been successful in the marketplace to date.
A more detailed explanation of SET appears in Chapter 25.
The Domain Name Service Security (DNSSEC) standard is a system designed to bring security to the Internet’s Domain Name System (DNS). DNSSEC creates a parallel public key infrastructure built upon the DNS system. Each DNS domain is assigned a public key. A domain’s public key can be obtained in a trusted manner from the parent domain or it can be preloaded into a DNS server using the server’s “boot” file.
DNSSEC allows for secure updating of information stored in DNS servers, making it ideal for remote administration. The DNSSEC standard is built into the current version of bind , the DNS server that is distributed by the Internet Software Consortium.
IPsec is a cryptographic protocol designed by the Internet Engineering Task Force to provide end-to-end confidentiality for packets traveling over the Internet. IPsec works with IPv4, the standard version of IP used on today’s Internet. IPv6, the “next generation” IP, includes IPsec.
IPsec does not provide for integrity, authentication, or nonrepudiation, but leaves these features to other protocols. Currently, the main use of IPsec seems to be as a multivendor protocol for creating virtual private networks (VPNs) over the Internet. But IPsec has the capacity to provide authentication, integrity, and optionally, data confidentiality for all communication that takes place over the Internet, provided that vendors widely implement the protocol and that governments allow its use.
Kerberos is a network security system developed at MIT and used throughout the United States. Unlike the other systems mentioned in this chapter, Kerberos does not use public key technology. Instead, Kerberos is based on symmetric ciphers and secrets that are shared between the Kerberos server and each individual user. Each user has his own password, and the Kerberos server uses this password to encrypt messages sent to that user so that they cannot be read by anyone else.
Support for Kerberos must be added to each program that is to be protected. Currently, “Kerberized” versions of Telnet, FTP, POP, SSH, and Sun RPC are in general use. Several systems that use Kerberos to provide authentication and confidentiality for HTTP have been developed but not widely deployed.
Kerberos is a difficult system to configure and administer. To operate a Kerberos system, each site must have a Kerberos server that is physically secure. The Kerberos server maintains a copy of every user’s password. In the event that the Kerberos server is compromised, every user’s password must be changed.
Despite the fact that Microsoft built support for Kerberos into its Windows 2000 operating system, to date Kerberos has not been widely deployed beyond a few academic environments. That Microsoft built proprietary, non-standard extensions into their version of Kerberos has probably worked against its interoperability and more general adoption.
The Secure Shell (SSH) provides for cryptographically protected virtual terminal (telnet) and file transfer (rcp) operations. Originally developed as free software for Unix, a wide variety of both commercial and noncommercial programs that implement the SSH protocol are now available for Unix, Windows, Mac OS, and other platforms. These implementations also allow for the creation of cryptographically secured "tunnels” for other protocols.
 RFC 2045, RFC 2046, RFC 2047, RFC 2048, and RFC 2049.
 Kerberos didn’t adopt public key technology for two reasons. The first was that when Kerberos was developed in 1985, computers were much slower. The developers thought that public key encryptions and decryptions would be too slow to use Kerberos to authenticate logins and requests for email. The second reason was because of the Stanford and MIT patents. Kerberos’ developers wanted to be able to distribute the code freely over the Internet. They were worried that they would have trouble if the system required the licensing of patents. (Phil Zimmermann struggled with this same issue six years later when he wrote PGP, but he resolved it differently.)