A firewall enables you to control which local resources are available on the Internet and which IP addresses can access those resources. But firewalls filter only on the information in IP and other headers. They can’t identify specific, authorized users who may be using IP addresses that the firewall doesn’t know about ahead of time. A solution is to provide authorized users with a password and to require users to enter the password before accessing a resource. For additional security and to identify who is accessing the resources, you can require a user name in addition to a password. Each user name and password combination can be unique to a user, so different users can have different access. The accepted passwords and user names may be hard-coded into the firmware, with authorized users informed of the values to use. Or you may want to allow users to obtain access to a resource by filling out a form that requires selecting a user name and password. The form can request additional information as well. Two words you’ll encounter relating to password protection are authentication and authorization. A user who wants to access a protected resource must provide authentication, or proof that the user has permission to access the resource. On receiving a valid user name and password, the server grants authorization, or permission, to access the resource.

Basic Authentication and Digest Authentication

The examples at the beginning of this networking tutorial showed how to use Basic Authentication to require a user name and password before accessing a resource. A more secure option than Basic Authentication is Digest Authentication. To access a resource protected with Digest Authentication, the user must provide a message digest, which is a 32-character ASCII hex string created from information provided by both the client requesting the resource and the server that is hosting the resource. The information that goes into creating the message digest includes a nonce value that the server returns in response to a request for a protected resource, a user name, a password, a realm, and the request. The default method for obtaining the message-digest string is the MD5 algorithm described in RFC 1321: The MD5 Message- Digest Algorithm. The nonce value provided by the server typically incorporates a time stamp and an Etag value that identifies the resource being requested. The time stamp enables the server to allow access for a specified time before requiring re-authentication. A server can use the Etag value to prevent replay attacks, where an unauthorized user requests an updated version of a resource previously returned to an authorized user. Rabbit Semiconductor’s Dynamic C includes functions that support Digest Authentication on Web servers hosted by Rabbit modules. Some older Web browsers don’t support Digest Authentication.

Encrypt Private Data

The fourth rule for securing network resources is to encrypt data that must remain private. Basic and Digest Authentication encrypt passwords. It’s also possible to encrypt any data exchanged between two computers. Encrypting and decrypting large amounts of data can take up a lot of CPU cycles and time. On small embedded systems, the challenge is to obtain the needed security without overwhelming the system’s resources. Options such as AES encryption and stand-alone firewalls that support Virtual Private Network protocols are two possible solutions for embedded systems.

AES (Rijndael) Encryption

In 1997, the U.S. National Institute of Standards and Technology (NIST) began a search for a new encryption standard that was royalty-free, easy to implement even on small embedded systems, and able to withstand attack. In 2001, Federal Information Processing Standard (FIPS) 197 designated the Rijndael algorithm the winner of the search. The algorithm was named the government’s Advanced Encryption Standard (AES) to use for sensitive but unclassified information. Entities other than the U.S. Government are welcome to use the algorithm as well, of course. Rabbit Semiconductor’s Dynamic C offers a library module with support for the Rijndael Advanced Encryption Standard (AES) cipher.

Virtual Private Networks

Another option for securing network data is a virtual private network (VPN). The computers at each end of the VPN can use authentication and encryption to ensure that the data is secure from spying and to block all other traffic from entering the VPN. The program code required to implement a VPN can be too complex and time-consuming to develop for a small embedded system. However, just about any system can communicate over a VPN by VPNs use IP Security (IPsec) protocols for encryption and authentication. A variety of RFC documents cover the protocols. A good place to start is with RFC 2411: IP Security Document Roadmap. To establish a VPN, a computer at each end of the network must have software that knows how to use the required protocols to establish a connection to the other end. Windows XP includes an IPSec Security Manager that enables PCs to communicate over a VPN. For embedded systems, the easiest way to support VPN is to connect the system to a firewall device that supports VPNs.

Firewalls that support VPNs typically include a variety of configuration options. At the local network, you can enable a single IP address, an entire subnet, or a user-specified range of addresses within a subnet to access the VPN. You can specify that the local network will accept VPN communications from a specified IP address or domain name, or from any requesting host. To use encryption, both ends of the VPN must agree on the type of encryption to use and they must share a key that enables each end to encrypt and decrypt network traffic. Encryption options include AES and the older methods 3DES and DES. Authentication options include MD5 and the more secure 160-bit Secure Hash Algorithm (SHA). When both ends have been configured, the devices can communicate and attempt to establish the VPN. When the VPN has been established, the two devices can use encryption to transfer data securely.

Secure Sockets Layer Encryption

Many Web browsers support the Secure Sockets Layer (SSL) protocol for encrypting data such as the credit-card numbers customers send to on-line retailers. SSL uses public-key cryptography, which uses separate keys for encrypting and decrypting. The computer requesting the encrypted data generates a public key for encrypting and a private key for decrypting. The sender of the data uses the public key in encrypting the data. Decrypting requires the private key, which only the receiving computer has access to.

SSL encryption is very secure but requires more resources than many small embedded systems can provide. Netburner is one company that offers SSL support for its products, which use Motorola’s 32-bit ColdFire processors.