Part Two: Building Smart Client Applications

Security Primer

Before discussing the security issues surrounding enterprise applications, we are going to take a look at some security concepts and technologies. We will begin by looking at the major components involved in creating a secure environment, followed by the leading security threats that you need to be aware of. The last part of this primer covers the leading security technologies and other security measures that you will want to consider in your solutions.

Creating a Secure Environment

For end-to-end security you have to consider the entire environment, including enterprise access, middle-tier components, and client applications. End-to-end security means that the transmission of data is secure along the entire path from the sender to the receiver—usually the client application to the enterprise server. Contrary to popular belief, this endeavor involves more than just data encryption. In this section we examine five objectives involved in creating a secure mobile environment. Understanding these objectives and the impact they have on mobile application development is crucial for creating secure applications.

Authentication

Authentication is the process of proving that people and organizations are who or what they claim to be. For wireless networks, this is often done at two layers: the network layer and the application layer. The network requires the user to be authenticated before that person is granted access. This can be done implicitly, based on the device or modem being used, or explicitly, using a variety of mechanisms. At the application layer, authentication is important at two levels: the client and the enterprise server. To gain access to enterprise data, the client has to prove to the server that it is what it says it is. At the same time, before a client allows an outside server to connect to it—for example, to push some content—the server has to authenticate itself to the client application. The simplest, and probably least secure, method of authentication is a username/password combination. More advanced methods include digital certificates or digital signatures.

Data Integrity

Data integrity is assurance that the data in question has not been altered or corrupted in any way during the transmission from the sender to the receiver. This can be accomplished by using data encryption in combination with a cryptographic checksum or Message Authentication Code (MAC). This information is encoded into the message itself by applying an algorithm to the message. When recipients receive the message, they compute the MAC and compare it with the MAC encoded in the message to see if the codes are the same. If they are, recipients can be confident that the message has not been tampered with. If the codes are different, recipients can discard the data as inaccurate.

Confidentiality

Confidentiality is one of the most important aspects of security, and certainly the most talked about. Confidentiality is about maintaining data privacy, making sure it cannot be viewed by unwanted parties. Most often, when people are worried about the security of a system, they are concerned that sensitive information, such as a credit card number or health records, can be viewed by parties with malicious intent. The most common way of preventing this intrusion is by encrypting the data. This process involves encoding the content of a message into a form that is unreadable by anyone other than the intended recipient. More information on encryption is provided later in this chapter in the Security Technologies section.

Authorization

Authorization is the process of determining the user's level of access—whether a user has the right to perform certain actions. Authorization is often closely tied to authentication. Once a user is authenticated, the system can determine what that party is permitted to do. Access control lists (ACLs) are often used to help determine this. For example, all users may have read-only access to a set of data, while the administrator, or another trusted source, may also have write access to the data.

Nonrepudiation

Nonrepudiation is about making parties accountable for transactions in which they have participated. It involves identifying the parties in such a way that they cannot at a later time deny their involvement in the transaction. In essence, it means that both the sender and the recipient of a message can prove to a third party that the sender did indeed send the message and the recipient received the identical message. To accomplish this, each transaction has to be signed with a digital signature that can be verified and time-stamped by a trusted third party.

Security Threats

Building a secure solution is difficult without awareness of the potential risks, so now that we have looked at the requirements for a secure environment, we will look at four common security threats: spoofing, sniffing, tampering, and theft. Whenever data is being transferred, whether over a wireless or wired network, you need to take precautions against these risks.

Note 

To simplify terminology, any access to data or systems through a security hole will be considered unauthorized access.

Spoofing

Spoofing is the attempt by a party to gain unauthorized access to an application or system by pretending to be someone he or she is not. If the spoofer gains access, he or she can then create fake responses to messages in an attempt to gain further knowledge and access to other parts of the system. Spoofing is a major problem for Internet security, hence, wireless Internet security because a spoofer can make application users believe that they are communicating with a trusted source, such as their bank, when in reality they are communicating with an attacker machine. Unknowingly, users often provide additional information that is useful to the attacker to gain access to other parts and other users of the system.

The process of sniffing, described next, is often used in conjunction with spoofing to get enough information to access the system in the first place. For this reason, implementing both authentication and encryption is required to combat spoofing.

Sniffing

Sniffing is a technique used to monitor data flow on a network. While sniffing can be used for proper purposes, it is more commonly associated with the unauthorized copying of network data. In this sense, sniffing is essentially electronic eavesdropping. By "listening" to network data, unauthorized parties are able to obtain sensitive information that will allow them to do further damage to the application users, the enterprise systems, or both.

Sniffing is dangerous because it is both simple to do and difficult to detect. Moreover, sniffing tools are easy to obtain and configure. In fact, Ethernet sniffing tools come with the Microsoft Windows NT and 2000 installs; fortunately, these tools are simple to detect. To combat the more sophisticated sniffing tools, data encryption is the best defense. If an unauthorized user is able to access encrypted data, he or she will lack a way to decrypt it, essentially making it useless. That said, you must ensure that the encryption protocol you are using is nearly impossible to break. Many wireless LAN users have discovered the hard way that Wired Equivalent Privacy (WEP) encryption is often not enough to protect their data. [For more information on this issue, refer back to the Wireless Local Area Networks (WLANs) section in Chapter 3, "Wireless Networks."]

Tampering

Data tampering, also called an integrity threat, involves the malicious modification of data from its original form. Very often this involves intercepting a data transmission, although it also can happen to data stored on a server or client device. The modified data is then passed off as the original. Employing data encryption, authentication, and authorization are ways to combat data tampering.

Theft

Device theft is a problem inherent in mobile computing. Not only do you lose the device itself but also any confidential data that may reside on this device. This can be a serious threat for smart client applications, as they contain persistent data, often confidential in nature. For this reason, you should follow these rules when it comes to securing mobile devices:

1. Lock down devices with a username/password combination to prevent easy access.

2. Require authentication to access any applications residing on the device.

3. Do not store passwords on the device.

4. Encrypt any persistent data storage facilities.

5. Enforce corporate security policies for mobile users.

Authentication and encryption, along with a security policy, are required to help prevent malicious data access from a lost or stolen device. Fortunately, this is not as serious a problem for wireless Internet applications, as they rarely store data outside of the browser's cache.

Security Technologies

Given the security threats just outlined, companies need to understand the technologies that are available to help them minimize security risks. Though the requirements for each company will be different, all companies will benefit by implementing a well-thought-out security plan. This section provides information on the main concepts and technologies that are required to implement end-to-end security for your m-business applications.

Cryptography

The basic objective of cryptography is to allow two parties to communicate over an insecure channel without a third party being able to understand what is being transmitted. This capability is one of the core requirements of a secure environment, as it deals with all aspects of secure data transfer, including authentication, digital signatures, and encryption. On the face of it, cryptography is a simple concept, but it is actually quite complex, especially for large-scale mobile implementations.

Algorithms and Protocols

Cryptography works on many levels. At the lowest level are cryptographic algorithms. These algorithms describe the steps required to perform a particular computation, typically based around the transformation of data from one format to another. Building on these algorithms, is a protocol. The protocol describes the complete process of executing a cryptographic activity, including explicit information on how to handle any contingency that might arise. Making this distinction is important, because an excellent cryptographic algorithm does not necessarily translate into a strong protocol. The protocol is responsible for more than just the encoding of data; data transmission and key exchange are also properties of a protocol.

Finally, on top of the protocol are the applications. Once again, a strong protocol does not guarantee strong security, as the application itself may lead to further problems. Thus, in order to create a secure solution, a strong protocol is required, as well as a good, robust application implementation.

Data Encryption

The core of any cryptographic system is encryption, the process of taking a regular set of data, called plaintext, and converting it into an unreadable form, called ciphertext. Encryption allows you to maintain the privacy of sensitive data, even when accessed by unauthorized users. The only way the data can be read is by transforming it back to its original form using a process called decryption. The method of encryption and decryption is called an algorithm or cipher. Figure 6.1 demonstrates the concept of encryption. As the message is transported over an insecure public channel, it is encrypted, preventing anyone eavesdropping on the line from being able to understand the data being sent.

Figure 6.1: Sending a message using encryption.

Modern algorithms use keys to control the encryption and decryption of data. Once a message has been encrypted, it can only be decrypted by users who have the corresponding key. Key-based algorithms come in two classes: symmetric and asymmetric.

Symmetric algorithms are efficient: They use a single key to encrypt and decrypt all messages. The sender uses the key to encrypt the message and then sends the message to the intended recipient. Once the message is received, the recipient uses the same key to decrypt the message. This type of algorithm works well when there is a safe way to transmit the key between users, such as by meeting before the transmission takes place. Unfortunately a substantial problem arises when exchanging data between loosely related parties, such as an e-commerce Web site and a customer. Exchanging the key is a problem that symmetric encryption itself is incapable of solving; and without a secure method of exchange keys, this method is only useful between private parties.

Symmetric encryption is also referred to as secret-key encryption. The most popular form of this method is the Data Encryption Standard (DES), which was developed in the 1970s. Since then more secure forms of symmetric encryption have been developed; leaders among them include the Advanced Encryption Standard (AES), which is based on the Rijndael algorithm; Triple DES; International Data Encryption Algorithm (IDEA); Blowfish; and the Rivest family of algorithms, RC2, RC4, RC5, and RC6.

Asymmetric encryption addresses the main problem that has plagued symmetric key systems: the use of a single key. For years, cryptologists worked to find a solution to the key distribution problem with little success. Just as many mathematicians started to think that a solution was not possible, Whitfield Diffie and Martin Hellman proved them wrong. In 1975, Diffie and Hellman developed a solution using two separate but related keys: one to encrypt the data and another to decrypt it. The key used to encrypt the data is called the public key. This key can be widely distributed over insecure lines, for general public use. The key used to decrypt the corresponding data is called the private key. This key is never transmitted, as it is only required by the party that needs to decrypt the data. These keys are related in an obscure way using extremely large prime numbers and one-way functions. This technique makes it computationally infeasible to calculate the private key based on the public key. The larger the key is, the more difficult it becomes to break the system. Sixty-four-bit key systems, such as DES, are capable of being attacked by brute force—that is, trying every single key combination until the attacker finds the correct one. The more common 128-bit systems, such as ECC, so far have proven invulnerable to brute-force attacks.

Here is an example of how asymmetric, or public key, cryptography works. Suppose Alice wants to send a secure message to Bob. Alice can use Bob's public key to encrypt the message, since it is publicly available. She then sends the message to Bob. When he receives the message, he uses his private key, to which only he has access, to decrypt the message. Now Alice is able to send a secure message to Bob without having to do a key exchange. If information is to be exchanged in both directions using asymmetric encryption, each party must have his or her own public key and private key combination.

Encrypting a message with the private key and decrypting it with the public key is possible as well, but this has a different objective. It can be used on nonsensitive data simply to prove that the party who encrypted it actually has access to the private key.

The first, and best-known, asymmetric key algorithm was released in 1977 by Ron Rivest, Adi Shamir, and Leonard Adelman; who are commonly known as RSA. Other popular algorithms include Elliptic Curve Cryptography (ECC) and Diffie-Hellman (DH). RSA is being challenged in the mobile space by ECC, which is much less expensive in terms of processing power and key size, which are essential attributes in mobile computing.

Asymmetric ciphers are not, however, a perfect solution. Choosing a private key is not trivial, as a poor choice can lead to an easily broken scheme. Also, asymmetric ciphers provide a solution to the key distribution problem by using a public key and a private key, but they also are much more complicated, and therefore computationally slower than symmetric ciphers. For large sets of data, this can be problematic. In these cases, a combination of symmetric and asymmetric systems is an ideal solution. This allows you to take advantage of the higher performance of symmetric algorithms, by sending the secret key over insecure channels using public key systems. Once all parties have the secret key, the remainder of the data for that session can be encrypted and decrypted using symmetric algorithms. This is the basis for public key cryptography as used by many of today's leading protocols.

Digital Certificates

Digital certificates provide a way to guarantee that a public key belongs to the party it represents. For this to be successful, the certificate itself also has to be verified to ensure that it represents the claimed entity (a person or organization). This is accomplished using a trusted third party called a certificate authority (CA). The most common certificate authorities include VeriSign, Entrust, and Certicom. Users can purchase a digital certificate from the CA and use this certificate for authentication and to circulate their private key. Once a recipient has their private key and they are confident the recipient is who he or she claims to be (which is largely based on the trust of the CA), they can then send encrypted messages to this person, knowing they can decrypt them using their private key. Digital certificates typically contain the following:

• The name of the holder, as well as other information that uniquely identifies the holder. Additional information can include the URL of a Web server using the certificate or an email address.

• The holder's public key.

• The name of the CA that issued the certificate.

• The lifetime that the certificate if valid for (usually a start and end date).

• A digital signature from the CA to make it easy to detect if the transmission has been tampered with.

Certified users also have the option of self-signing a digital certificate, thereby becoming a CA themselves. This additional party can be considered trustworthy if he or she was signed by another trusted key. In this way, you can continue to navigate to the root CA, thereby determining who provided the initial certificate. If the root CA is not recognized or trusted, then each certificate in the chain is considered invalid.

The leading format for digital certificates is X.509, an industry standard for authentication. These certificates are prominent in Internet applications. In the wireless space, another form of digital certificate, called a WAP Server WTLS Certificate, is available. These certificates, commonly shortened to WTLS certificates, are a simpler version of X.509, created because X.509 certificates were too big for wireless applications. The WTLS certificates are primarily used in WAP applications where the microbrowser wants to authenticate the identity of a WAP server and encrypt information using the Wireless Transport Layer Security (WTLS). (There is more information on WTLS later in this chapter.)

Digital Signatures

Digital signatures are used to verify that a message really came from the claimed sender. It is based upon the notion that only the creator of the signature has the private key and that it can be verified using a corresponding public key. The digital signature is created by computing the message digest of a document, then concatenating it with information about the signer, a timestamp, and any other required information. A message digest is a function that takes arbitrary-sized input data (the message) and generates a fixed-size output, called a digest. This set of information is then encrypted using the private key of the sender using a suitable asymmetric algorithm. The resulting encrypted block of information is the digital signature.

The message digest that was calculated is a bit value that represents the current state of a document. If the document changes in any way, the message digest will also change. By incorporating the message digest into the digital signature, it is possible for the recipient of the document to easily detect if the document has been altered since the digital signature was created.

Public Key Infrastructure

Public key infrastructure (PKI) is the term used to describe a complete organization of systems and rules defining a single security system. The Internet Engineering Task Force (IETF) X.509 Working Group (www.ietf.org/internet-drafts/draft-ietf-pkix-roadmap-09.txt) defines PKI as "the set of hardware, software, people, and procedures needed to create, manage, store, distribute, and revoke certificates based on public key cryptography."

The components of PKI include the following:

• Certificate authorities responsible for issuing and revoking certificates

• Registration authorities responsible for binding between public keys and the identities of their holders

• Certificate holders who have been issued certificates that they can use to sign digital documents

• Repositories that store certificates as well as certificate revocation lists

• Security policy that defines an organization's top-level direction on security

Leading Protocols

The following are some leading protocols that are used for secure data transmission.

Secure Sockets Layer (SSL)

SSL is the dominant security protocol being used on the Internet today. It was developed by Netscape to provide secure and private Internet sessions, typically on top of HTTP, although it can also be used over FTP and other relevant protocols. SSL uses a combination of symmetric and asymmetric algorithms to maximize performance.

There are four phases in an SSL session:

1. Handshake and cipher negotiation. Both the client and server agree on the algorithms or ciphers to use.

2. Authentication. The server, and optionally the client, is authenticated using digital certificates.

3. Key exchange. The client creates a secret key and sends it to the server using its public key for encryption. The server decrypts the message using its private key; and for the remainder of that session, the client and server can communicate using the secret key.

4. Application data exchange. Once the secure symmetric session has been established, encrypted data can be communicated between the client and the server.

SSL can be used by many of the more powerful wireless clients, including laptops and Pocket PC devices. You can tell you are using SSL when the URL starts with https://, rather than just http://.

Encrypting the data within the protocol is more efficient than encrypting the data yourself and sending it over HTTP. Within the SSL protocol, the data will be encrypted on a packet level and can be decrypted on a packet level after it has arrived and has passed any sort of integrity test. If you encrypt your data as one large cipher text, then you will not be able to decrypt the data until all the packets have arrived.

Transport Layer Security (TLS)

Transport Layer Security (TLS) is the next generation of SSL. It consists of two layers. The lower layer is the TLS Record protocol, which is layered on top of a reliable transport protocol such as TCP. The two main features of the Record protocol are private and reliable connections. The higher level is the TLS Handshake protocol. This layer provides connection security that authenticates using asymmetric cryptography, negotiates a secret key, and provides a reliable negotiation. Like SSL, TLS is independent and can use a range of ciphers. The overall goals of TLS include cryptographic security, interoperability, and extensibility.

Wireless Transport Layer Security (WTLS)

WTLS is the security layer defined in the WAP specification. It operates above the Transport Protocol Layer, making it suitable for a variety of underlying wireless protocols. It is similar to TLS, but is optimized for low-bandwidth networks with high latency. It also adds new features such as datagram support, optimized handshakes, and key refreshing. It also supports the use of WTLS certificates for server-side authentication, in contrast to SSL/TLS, which typically use X.509 certificates. Overall, WTLS has similar goals to both SSL and TLS, in that it aims to provide privacy, data integrity, and authentication between two communicating parties. (A more in-depth look at WTLS and its authentication models is provided in the WAP Security section later in this chapter.)

IP Security (IPSec)

IPSec is different from the other protocols in that it does not operate on the application layer. Whereas SSL, TLS, and WTLS are aimed at providing secure communications over an inherently insecure network, IPSec is aimed at making the Internet itself secure. It does this by providing authentication, integrity, and privacy services at the IP datagram layer. While it is mainly targeted at laptop clients in the mobile space, IPSec-based virtual private network (VPN) products are starting to emerge for PDAs as well (see the upcoming section for more on VPNs). IPSec will become a more prominent solution when mobile devices start to support IP6, which includes IPSec as part of the standard. It is important to know that IPSec supports TCP/IP, not WAP.

Other Security Measures

The following subsections describe other security measures that you should consider when implementing your mobile solutions. You may find that some of these measures are already in place in your organization, as they are commonly used techniques for increasing overall system security.

Firewalls

Firewalls are the most common form of security implemented within organizations. They establish a network perimeter between what is public and what is private. A firewall is a set of software programs, usually located on a separate gateway server that restricts access to the private network resources from users on other networks. As soon as an enterprise installs Internet access to its site, a firewall is required to protect its own resources and, sometimes, to control outside resources to which its own users have access.

At a lower level, a firewall will examine each packet of network data to determine whether it should be forwarded to its destination. Where outside access is permitted, as in the case of a Web server, the firewall will allow outside traffic through the firewall on a specified port, for communication with a specified application. In this case, it would permit access from all outside users. At times, however, access from outside the firewall will be restricted to only known users, usually based on their IP addresses. This is used when a finite number of known users require access to a system.

For mobile devices that have always-on connections, a personal firewall may also be useful. This works in the same way as the firewall just described above, except it prevents unauthorized access to laptops and high-end PDAs. A personal firewall will prevent not only suspicious connections but also inappropriate content. Many devices contain hidden instructions that are used by maintenance professionals to repair a device. Lately, viruses and other inappropriate content have taken advantage of these hidden instructions to cause harm. A personal firewall can help prevent this from happening, although it should not be used a replacement for appropriate antivirus software.

Virtual Private Networks (VPNs)

A VPN allows a company to turn a public network (most commonly the Internet) into a private network. This technology allows remote workers to communicate with the corporate network in a secure fashion. Before VPN technology became available, dedicated leased lines were required to achieve the same result. Actually, VPNs present an additional benefit over leased lines, by providing secure access from many locations—essentially anywhere an Internet connection is available.

VPN technology is currently being used to help overcome the wireless LAN security problems by providing a direct link through a WLAN past the corporate firewall. The drawbacks of this configuration are cost and the inability to roam between WLAN access points. Mobile VPNs for devices on public networks are still in the early phases of adoption. As larger amounts of data are accessed remotely, mobile VPN usage is expected to increase.

Two-Factor Authentication

For some purposes, usually dealing with financial transactions, strong authentication is required. This involves using a two-factor approach, where users have to apply two factors to authenticate themselves. One factor is usually something the user knows, such as a PIN number; the other is something the user has, such a token card to generate a one-time password. This combination makes it much more difficult for unauthorized users to gain to access the system.

Smart client applications inherently provide a form of two-factor authentication: First you must have the device to access the application; second, you must authenticate yourself to gain access to the application, as well as to any back-end system to which it connects. This is not the traditional sense of the term, but it does provide additional security over thin client applications where any device with a microbrowser has the capability to access the content.

Biometrics

Even with the increased security of two-factor authentication, unauthorized users can compromise the system, for example, by obtaining the PIN code and token card, thereby gaining access to the enterprise system.

To avoid this situation, PIN codes can be replaced with a stronger form of authentication: biometric authentication. Biometrics provides a wide range of techniques for authenticating an individual based on his or her unique physical characteristics. Such techniques include fingerprint identification, face recognition, voice recognition, or iris and retina scanning. Using biometric techniques, you can ensure that the identification token is indeed unique. While this use of biometrics does improve security, this type of authentication does have some drawbacks. Many of these systems are somewhat intrusive and therefore not widely accepted by users. Also, the reliability of these technologies varies and so can lead to what's called "false refusal." That said, biometric systems are growing in popularity due to increased security concerns among all users.

Security Policy

The final, and often most important, security measure is the adoption of a corporate security policy. Such a policy will outline all aspects of a corporation's security measures, including both technology and the use and disclosure of confidential information within the enterprise. Even if a corporation has implemented a very strong technical security solution, the overall system will still be insecure if its users do not follow corporate security guidelines. Remember, intruders will always attack the weakest link in a system. Unfortunately, this link is often the users themselves.

Sometimes, very simple measures will dramatically increase overall security. For example, many PDA users do not lock the operating system when it is not in use. If the device is lost, nothing prevents another user from accessing the applications and corresponding data on the device. The same security measures that are in place for desktop users must be extended to remote workers.