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Security

Revisiting Security Fundamentals Part 2: Integrity

Steve Goeringer
Distinguished Technologist, Security

Oct 24, 2019

Let’s revisit the fundamentals of security during this year’s security awareness month – part 2: Integrity.

As I discussed in Part 1 of this series, cybersecurity is complex. Security engineers rely on the application of fundamental principles to keep their jobs manageable. The first blog focused on confidentiality. This second part will address integrity. The third and final part of the serious will review availability. Application of these three principles in concert is essential to ensuring excellent user experiences on broadband.

Nearly everyone who uses broadband has some awareness of confidentiality, though most may think of it exclusively as enabled by encryption. That’s not a mystery – our browsers even tell us when a session is “secure” (meaning the session they have initiated with a given server is at least using HTTPS which is encrypted). Integrity is a bit more obscure and less known. It’s also less widely implemented and then not always well.

Defining Integrity

In their special publication, “An Introduction to Information Security,” NIST defines integrity as “a property whereby data has not been altered in an unauthorized manner since it was created, transmitted, or stored.” This definition is a good starting place, but it can be extended in today’s cybersecurity context. Integrity needs to be applied not only to data, but also to the hardware and software systems that store and process that data and the networks that connect those systems. Ultimately, integrity is about proving that things are as they should be and that they have not been changed, intentionally or inadvertently or accidentally (like magic), in unexpected or unauthorized ways.

How is this done? Well, that answer depends on what you are applying integrity controls to. (Again, this blog post isn’t intended to be a tutorial in-depth on the details but a simple update and overview.) The simplest and most well-known approach to ensuring integrity is to use a signature. Most people are familiar with this as they sign a document or write a check.  And most people know that the bank, or whomever else we’re signing a document for, knows that signatures are not perfect so you often have to present an ID (passport, driver’s license, whatever) to prove that you are the party to which your signature attests on that document or check.

We can also implement similar steps in information systems, although, the process is a bit different. We produce a signature of data by using math; in fact, integrity is a field of cryptography that complements encryption. A signature is comprised of two parts, or steps. First, data is run through a mathematical function called hashing. Hashing is simply a one-way process which reduces a large piece of data to a few bits (128-256 bits is typical) in a way that is computationally difficult to reverse and unlikely to be copied using different data. This is often referred to as a digest and the digest is unlikely to be duplicated using a different source data (we call this collisions). This alone can be useful and is used in many ways in information systems. But it doesn’t attest the source of the data or the authenticity of the data. It just shows whether it has been changed. If we encrypt the digest, perhaps using asymmetric cryptography supported by a public key infrastructure, we produce a signature. That signature can now be validated through a cryptographic challenge and response. This is largely equivalent to being asked to show your ID when you sign a check.

One thing to be mindful of is that encryption doesn’t ensure integrity. Although an adversary who intercepts an encrypted message may not be able to read that message, they may be able to alter the encrypted text and send it on its way to the intended recipient. That alteration may be decrypted as valid. In practice this is hard because without knowledge of the original message, any changes are likely to just be gibberish. However, there are attacks in which the structure of the original message is known. Some ciphers do include integrity assurances, as well, but not all of them. So, implementors need to consider what is best for a given solution.

Approaches to Integrity

Integrity is applied to data at motion and at rest, and to systems, software, and even supply chains somewhat differently. Here’s a brief summary of the tools and approaches for each area:

  • Information in motion: How is the signature scheme above applied to transmitting data? The most common means uses a process very similar to what is described above. Hash-based Message Authentication Codes are protocols that create a digest of a packet and then encrypt the digest with a secret key. A public key is used to prove that the digest was produced by an authorized source. One old but still relevant description of HMAC is RFC 2104 available from IETF here.
  • Information at rest: In many ways, assuring integrity of files on a storage server or a workstation is more challenging than integrity of transmitted information. Usually, storage is shared by many organizations or departments. What secret keys should be used to produce a signature of those files? Sometimes, things are simplified. Access controls can be used to ensure only authorized parties can access a file. When access controls are effective, perhaps only hashing of the data file is sufficient to prove integrity. Again, some encryption schemes can include integrity protection. The key problem noted above still remains a challenge there. Most storage solutions, both proprietary and open source, provide a wide range of integrity protection options and it can be challenging for the security engineer to architect the best solution for a given application.
  • Software: Software is, of course, a special type of information. And so, the ideas of how to apply integrity protections to information at rest can apply to protecting software. However, how software is used in modern systems with live builds adds additional requirements. Namely, this means that before a given system uses software, that software should be validated as being from an authorized source and that it has not been altered since being provided by that source. The same notion of producing a digest and then encrypting the digest to form a signature applies, but that signature needs to be validated before the software is loaded and used. In practice, this is done very well in some ecosystems and either done poorly or not at all in other systems. In cable systems, we use a process referred to as Secure Software Download to ensure the integrity of firmware downloaded to cable modems. (See section 14 of Data-Over-Cable Service Interface Specifications 3.1)
  • Systems: Systems are comprised of hardware and software elements, yet the overall operation of the hardware tends to be governed by configurations and settings stored in files and software. If the files and software are changed, the operation of the system will be affected. Consequently, the integrity of the system should be tracked and periodically evaluated. Integrity of the system can be tracked through attestation – basically producing a digest of the entire system and then storing that in protected hardware and reporting it to secure attestation servers. Any changes to the system can be checked to ensure they were authorized. The processes for doing this are well documented by the Trusted Computing Group. Another process well codified by the Trusted Computing Group is Trusted Boot. Trusted boot uses secure modules included in hardware to perform a verified launch of an OS or virtual environment using attestation.
  • Supply chain: A recent focus area for integrity controls has been supply chains. How do you know where your hardware or software is coming from? Is the system you ordered the system you received? Supply chain providence can be attested using a wide range of tools, and application of distributed ledger or blockchain technologies is a prevalent approach.

Threats to Integrity

What are the threats to integrity? One example that has impacted network operators and their users repeatedly is changing of the DNS settings on gateways. If an adversary can change the DNS server on a gateway to a server they control (incorporation of strong access controls minimizes this risk), then they can selectively redirect DNS queries to spoofed hosts that look like authentic parties (e.g., banks, credit card companies, charity sites) and get customer’s credentials. The adversary can then use those credentials to access the legitimate site and do whatever that allows (e.g., empty bank accounts). This can also be done by altering the DNS query in motion at a compromised router or other server through which the query traverses (HMAC or encryption with integrity protection would prevent this.) Software attacks can occur even at the source but also at intermediate points in the supply chain. Tampered software is a prevalent way malware is introduced to end-points and can be very hard to detect because the software appears legitimate. (Consider the Wired article, “Supply Chain Hackers Snuck Malware Into Videogames.”)

The Future of Integrity

Cable technology has already addressed many integrity threats. As mentioned above, DOCSIS technology already includes support for Secure Software Download to provide integrity verification of firmware. This addresses both software supply chain providence and tampering of firmware. Many of our management protocols are also supported by HMAC. Future controls will include trusted boot and hardened protection of our encryption keys (also used for signatures). We are designing solutions for virtualized service delivery and integrity controls are pervasively included across the container and virtual machine architectures being developed.

Addition of integrity controls to our tools to ensure confidentiality provides defense in depth. It is a fundamental security component and should include in any security by design strategy. You can read more about security by clicking below.


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