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Encryption is simply the translation of data into a secret code, and it offers the most effective technical weapon against data theft and malicious appropriation. There are of course many human performance aspects to retaining the integrity of a system, some of which may arguably have a greater significance in maintaining security - password policies, authorisation control and the rest - but none of these would mean anything without the necessary hard- and software foundations to build upon. With the routine embedding of Ethernet connections into almost every SCADA and plant automation system, the correct technical choice has never been more important.
By Mark Prowten

Encryption is a formula used to turn data into a secret code. Each algorithm uses a string of bits known as a 'key' to perform the calculations: the larger the key, the greater the number of potential combinations that can be created, thus making it harder to break the code and unscramble the contents. Decryption of an encrypted file requires access to that secret key or password. Until 1996, the US government considered anything stronger than 40-bit encryption a 'munition' and its export, therefore, was illegal. Most government restrictions have now been lifted with 128-bit cryptography emerging as the new digital standard.

Mechanics of cryptography

A system for encrypting and decrypting data is a cryptosystem. These usually involve an algorithm for combining the original data ('plaintext') with one or more 'keys' - numbers or strings of characters known only to the sender and/or recipient. The resulting output is known as 'ciphertext'.

The security of a cryptosystem usually relies on the secrecy of the keys rather than with the supposed secrecy of the algorithm. A strong cryptosystem has a large range of possible keys making it impractical if not impossible to try them all. A strong cryptosystem will produce ciphertext which appears random to all standard statistical tests and can resist all known methods for breaking codes.

Keys

A key allows the encrypted secret code to be decrypted or allows plaintext (data that can be read by anyone) to be encrypted. There are typically two types used with data encryption - secret keys and public keys. There are also two main types of encryption: asymmetric encryption (also known as public-key encryption) and symmetric encryption and many algorithms for encrypting data based on these types. Here are some of the most common varieties presently in use:

• Skipjack - uses an 80-bit key and was designed to run on 'tamperproof' hardware.

• Data Encryption Standard (DES) - uses a 64-bit key to encrypt the data. DES is now considered to be insecure for many applications. This is chiefly due to the 56-bit key size being too small; DES keys have been broken in less than 24 hours.

• Triple-DES - uses three successive DES operations to provide stronger encryption than DES. The algorithm is believed to be practically secure, although it remains theoretically susceptible to some attacks. In recent years, Triple-DES has been superseded by the Advanced Encryption Standard (AES).

• Advanced Encryption Standard (AES) - Also known as Rijndael, it can use 128, 192 or 256 bits to en- and decrypt data in blocks of 128 bits. As of 2004, there have been no successful attacks against AES.

• Secret Key (Symmetric) encryption - uses the same secret key to encrypt and decrypt messages. However this method throws up the implicit problem of transmitting the secret key to its legitimate recipient. Examples of systems that use this technique include:

  • Data Encryption Standard (DES) - an encryption algorithm that operates on 64-bit blocks with a 56-bit key.
  • International Data Encryption Algorithm (IDEA) - an encryption algorithm that operates on 64-bit blocks with a 128-bit key.

The alternative Public Key (Asymmetric) encryption requires that the legitimate recipient has a pair of keys (a public key and a private key). Each person's public key is published while the private key is kept secret by an individual. Messages are encrypted using the intended recipient's openly available public key and can only be decrypted using the private key which is only known to, and held by, the recipient. This method eliminates the need for sender and receiver to share the complete information encoding key over the secure transmission channel; the individual's private key is never transmitted or shared. A further twist allows an individual's [openly available] public key to be 'signed' by a public certification agency - a bit like a passport office - such that a recipient's identity can be assured. Examples of systems that use this type of technique include:

• RSA - used for both encryption and authentication.
• Pretty Good Privacy (PGP) - mainly used to secure email.

The longer the key, the more computing required to crack the code. For example, using the current industry standard 128-bit encryption key, it would be 4.7 x 1021 times more difficult to hack than that generated with a 56-bit encryption key.

Given the current power of computers, a 56-bit key is no longer considered secure whereas a 128-bit key is.

Implementing public-key encryption on a large scale such as on a secure web server requires a digital certificate. This is basically a bit of information that says that the web server is trusted by an independent source known as a certificate authority. The certificate authority acts as a middleman that both computers trust, and confirms that each computer is in fact who it says it is, and then provides the public keys of each computer to the other.

Wireless LAN encryption

Wired Equivalent Privacy (WEP) is a security protocol for wireless local area networks (WLANs) which are defined in the 802.11b standard. WEP is designed to provide the same level of security as that of a wired LAN. However LANs provide more security against unauthorised access by their inherent physical structure. WEP provides security by encrypting the data sent over the radio link such that it is protected in transmission from one end point to another but it has been found that WEP is not as secure as was once believed. WEP is used only at the data link and physical layers of the OSI model and does not offer end-to-end security.

Supported by many newer devices, WiFi Protected Access (WPA) is a WiFi standard that was designed to improve upon the security features of WEP. WPA technology works with existing WiFi products that have been enabled with WEP, but includes two improvements. The first is better data encryption via the temporal key integrity protocol (TKIP), which scrambles keys using a hashing algorithm and adds an integrity checking feature to ensure that keys haven't been tampered with. The second is user authentication through the extensible authentication protocol (EAP).

EAP is built on a secure public-key encryption system, ensuring that only authorised network users have access. EAP is generally missing from WEP - which regulates access to a wireless network based on the computer's hardware-specific MAC Address. Since this information can be easily stolen, there is an inherent security risk in relying on WEP encryption alone.

VPN encryption

IP Security (IPSec) is a set of protocols developed by the Internet Engineering Task Force (IETF) for the secure exchange of data packets at IP layer and is widely used to implement Virtual Private Networks (VPNs). IPSec supports two encryption modes: Transport and Tunnel. Transport mode encrypts only the data portion of each packet, and leaves the header untouched. The Tunnel mode is more secure in that it encrypts both the data and header. On the receiving side, an IPSec-compliant device decrypts each packet. Through the Internet Security Associate and Key Management Protocol/Oakley (ISAKMP/Oakley), both the sending and receiving device share a public key.

The US government has made it compulsory that all its organisations have certified systems in place to protect data transmissions. Starting this year, the Federal Government will begin to retrofit previously installed IT systems with NIST-certified AES equipment, providing opportunities for government VARs. NIST is the National Institute of Standards and Technology, a non-regulatory federal agency. The Federal Information Processing Standards (FIPS) are US Government approved standards and guidelines developed by NIST for federal computer systems. The FIPS program is supervised by the National Security Agency. It is with the oversight of the NSA that the FIP standards are presumed to specify the strongest unclassified encryption technologies available.

There have been numerous FIPS issued since they were first published in 1974, but only several FIPS publications are of significance:

• FIPS46 (Revisions 1, 3, and 3) - the Data Encryption Standard (DES) used by the federal government and private industry to secure sensitive data.

• FIPS140 (Revisions 1 and 2) - the detailed security requirements for encryption software that is required for software using encryption used within the federal government.

• FIPS180 (Revisions 1 and 2) - the Secure Hash Standard for digital signature systems that is used by government and industry to ensure data integrity.

• FIPS186 (Revisions 1 and 2) - the Digital Signature Standard which is an algorithm used for digital signatures within the government as well as other industries.

• FIPS197 - the Advanced Encryption Standard (AES) which superseded the DES as the standard encryption algorithm for government data.

FIPS140 specifies comprehensive implementation and certification requirements for any system which provides encryption. By law, Federal agencies cannot process sensitive information without using an encryption product which has been certified to FIPS140 (when companies claim their products are 'FIPS certified', they typically mean certified to meet the requirements of FIPS140). In effect, anything that is worth encrypting must be encrypted using software certified under FIPS140.

IPS140 certification is particularly valuable for embedded devices which are increasingly network-oriented and frequently lag behind desktop systems in their security offering. With FIPS140 certification, networked embedded products such as medical devices, monitoring systems, alarm and surveillance systems can immediately differentiate themselves and have access to the substantial federal government market, as well as private contractors developing federal systems.

Unlike other FIPS, which describe algorithms for encryption or hashing or digital signatures, FIPS140 specifies how any encryption product must be designed, implemented and tested. Therefore, FIPS140 is a more systematic, broad standard whereas other FIPS are narrowly focused on a particular encryption algorithm. It is a FIPS140 requirement that the cryptographic modules must implement at least one FIPS-certified algorithm. This means that in order to be compliant with FIPS140, a module must first receive FIPS certification for one or more encryption algorithms such as DES (FIPS46-3) or AES (FIPS197). Therefore, it is not possible to receive FIPS140 certification without at least one other FIPS certification, and typically more than one for a useful product.

The importance of AES

The Advanced Encryption Standard supports key sizes of 128, 192 and 256 bits and serves as a replacement for the DES with its key size of 56 bits. DES had been cracked and declared no longer suitable for securing sensitive data. In 1997, NIST started its effort to develop the AES. It brought together researchers from twelve countries who submitted encryption algorithms. Fifteen different formulas were 'attacked' for vulnerabilities and evaluated by the worldwide cryptographic community. Eventually the winning algorithm was selected in October 2000. It incorporates the Rijndael encryption formula that was developed by two Belgian cryptographers, Vincent Rijmen and Joan Daemen, who have agreed that it may be used without royalty fees. The final standard was published in December 2001. In addition to increased security that comes with larger key sizes, AES can encrypt data much faster than Triple-DES, a DES enhancement that essentially encrypts a message or document three times.

AES replaced the DES as the US Government algorithm of choice in 2002 and is described by the standard known as FIPS197. This new standard specifies Rijndael as a FIPS-approved symmetric encryption algorithm that may be used to protect sensitive information. Products that support AES may be validated against the standard to demonstrate that they properly implement the algorithm. A validation certificate issued to the product's vendor which states that the implementation has been tested. In addition, the product is then listed on the NIST website: http://csrc.nist.gov/cryptval/aes/aesval.html

Relating AES to Secure Shell

Rijndael Algorithm is the name for a symmetric block cipher that can encrypt and decrypt information that may be implemented in software, firmware, hardware, or any combination thereof, and is part of AES.

Secure Shell (SSH) is a program that provides strong authentication and secure communications over unsecured channels. It is used as a replacement for Telnet, rlogin, rsh, and rcp, to log into another computer over a network, to execute commands in a remote machine, and to move files from one machine to another.

AES is one of the many encryption algorithms supported by SSH. Once a session key is established SSH uses AES to protect data in transit.

Both SSH and AES are extremely important to overall network security since the combination use maintains strict authentication for protection against intruders as well as symmetric encryption to protect transmission of dangerous packets. AES certification is reliable and can be trusted to handle the highest network security issues.

It seems as though access and security are at opposite ends of the spectrum - the more access you give, the less security you have. Often device servers connect through the Internet, which exposes the serial device data stream to security risks. To keep data secure, it is important to use data encryption as a means of data translation into another format, or alternate language which provides the highest level of security protection.

In the simplest connection scheme where two device servers are set up as a serial tunnel, no encryption application programming is required since both device servers can perform the encryption automatically. However, in the case where a host-based application is interacting with the serial device through its own network connection, modification of the application is required to support data encryption.

Device server security Securing a device server and the attached devices is becoming increasingly important as more sensitive data and devices are being connected to the network. Locking down the server is the first step to protect unauthorised access. This done by:

• Disabling the web browser interface;
• Web browser access for interactive setup can be prevented;
• Shutting down or preventing Telnet access;
• Closing TFTP and FTP ports to prevent network download of new or possibly harmful firmware;
• SNMP can be disabled so preventing the device server from responding to even benign network queries;
• Disabling all unnecessary device server ports so preventing unintended access.

Once a device server is locked down, the only way to reconfigure it is through the serial port. Getting access to the serial port is a physical security issue that needs to be addressed during the actual manufacturing and installation of the device server.

Data encryption of networked devices is seen as the best way to cut the risks associated with misplaced, lost or stolen data. Encryption also helps with the legal liability over information found on misplaced machines, and the growing threat of virus attacks.

To become certified, a device must undergo a series of tests outlined in the Advanced Encryption Standard Validation Suite (AESAVS). The AES algorithm may be implemented in software, hardware, firmware or any combination thereof, and must be used in conjunction with an approved FIPS or a NIST recommended mode of operation. The correct implementation of AES is tested by one of several accredited laboratories. The process of working with the lab can be costly, but it is the only way to perform conformance tests on the device implementation. All non-compliance issues must be corrected prior to certification. When a device receives its certified compliance, NIST will issue a certificate for the implementation, and publish the FIPS certificate number on its website.

NIST formally re-evaluates the standard every five years and continually analyses the AES algorithm for any breakthrough in technology, mathematical weakness or other possible threats that could reduce the security of the standard.

In order to remain certified, an implementation must update its certification with each new product version, which results in an updated certificate reflecting the new version information. Therefore, if the certificate number is valid, but it doesn't specify the version provided by the vendor, the certification is not valid to the product. For example, if a vendor asserts that its 5.0 product is FIPS140-certified with a particular certificate number, then it must provide another certificate number for its 5.1 version or it is not certified.

Additionally, customers need to be aware that if they purchase a NIST-certified product from supplier and they want to load their company's own firmware onto the device, the device may need to be re-certified. This depends on the firmware change. If the customer completely changes the firmware holding the AES encryption, then the product would require recertification. If the modification does not involve changing the AES library within the product in any way, it would retain its NIST certification.

Another requirement of FIPS140 certification is the creation of a security policy document, which describes the module's security policy. The lab will then affirm the module's compliance to NIST, and forward all relevant documentation to the appropriate personnel. The entire certification process can cost up to $75,000 in lab fees alone. If any compliance issues or errors emerge during the process, these costs could go higher.

An easier solution would be to license a FIPS140-certified module from a third-party. As long as the vendor maintains the module's certification, it qualifies as a FIPS certified product. Given the substantial cost and time investment for FIPS140 certification, licensing from a third party is a viable alternative.

Our company for instance offers suitably licensed products incorporating AES to the highest level of publicly available encryption. The technology we provide enables secure communications for IT and edge products such as bar code scanners, thermostats, factory machines, scales, blood analysers and security systems.

To convey an idea of the scope to which encrypted secure comms can be applied, our own product range includes wireless web servers, console servers, device server boxes, and embedded Ethernet modules. Secure COM redirectors will map virtual COM ports to device servers with encryption at both ends of the communication link. This permits transport of sensitive data from remote device servers over the Internet at the highest level of security. Encrypted serial-based communications can multiplex up to 64 secure channels.

Mark Prowten is senior product marketing manager, Lantronix.