- 1 It is about trust
- 2 Hash results
- 3 Hash-based Message Authentication Codes
- 4 Using private/public key cryptography
- 5 Trust chain
- 6 An implementation: the Trusted Computing Group functionality
It is about trust
Integrity is about trusting components within your environment, and in our case the workstations, servers and machines you work on. You definitely want to be certain that the workstation you type your credentials on to log on to the infrastructure is not compromised in any way. This "trust" in your environment is a combination of various factors: physical security, system security patching process, secure configuration, access controls and more.
Integrity plays a role in this security field: it tries to ensure that the systems have not been tampered with by malicious people or organizations. And this tamperproof-ness extends to a wide range of components that need to be validated. You probably want to be certain that the binaries that are ran (and libraries that are loaded) are those you built yourself (in case of Gentoo) or were provided to you by someone (or something) you trust. And that the Linux kernel you booted (and the modules that are loaded) are those you made, and not someone else.
Most people trust themselves and look at integrity as if it needs to prove that things are still as you've built them. But to support this claim, the systems you use to ensure integrity need to be trusted too: you want to make sure that whatever system is in place to offer you the final yes/no on the integrity only uses trusted information (did it really validate the binary) and services (is it not running on a compromised system). To support these claims, many ideas, technologies, processes and algorithms have passed the review.
In this document, we will talk about a few of those, and how they play in the Gentoo Hardened Integrity subprojects' vision and roadmap.
Algorithmically validating a file's content
Hashes are a primary method for validating if a file (or other resource) has not been changed since it was first inspected. A hash is the result of a mathematical calculation on the content of a file (most often a number or ordered set of numbers), and exhibits the following properties:
- The resulting number is represented in a small (often fixed-size) length. This is necessary to allow fast verification if two hash values are the same or not, but also to allow storing the value in a secure location (which is, more than often, much more restricted in space).
- The hash function always returns the same hash (output) when the file it inspects has not been changed (input). Otherwise it'll be impossible to ensure that the file content hasn't changed.
- The hash function is fast to run (the calculation of a hash result does not take up too much time or even resources). Without this property, it would take too long to generate and even validate hash results, leading to users being malcontent (and more likely to disable the validation altogether).
- The hash result cannot be used to reconstruct the file. Although this is often seen as a result of the first property (small length), it is important because hash results are often also seen as a "public validation" of data that is otherwise private in nature. In other words, many processes rely on the inability of users (or hackers) to reverse-engineer information based on its hash result. A good example are passwords and password databases, which should store hashes of the passwords, not the passwords themselves.
- Given a hash result, it is near impossible to find another file with the same hash result (or to create such a file yourself). Since the hash result is limited in space, there are many inputs that will map onto the same hash result. The power of a good hash function is that it is not feasible to find them (or calculate them) except by brute force. When such a match is found, it is called a collision .
Compared with checksums, hashes try to be more cryptographically secure (and as such more effort is made in the last property to make sure collisions are very hard to obtain). Some even try to generate hash results in a way that the duration to calculate hashes cannot be used to obtain information from the data (such as if it contains more 0s than 1s, etc.)
Hashes in integrity validation
Integrity validation services are often based on hash generation and validation. Tools such as tripwire or AIDE generate hashes of files and directories on your systems and then ask you to store them safely. When you want the integrity of your systems checked, you provide this information to the program (most likely in a read-only manner since you don't want this list to be modified while validating) which then recalculates the hashes of the files and compares them with the given list. Any changes in files are detected and can be reported to you (or the administrator).
A popular hash functions is SHA-1 (which you can generate and validate using the sha1sum command) which gained momentum after MD5 (using md5sum) was found to be less secure (nowadays collisions in MD5 are easy to generate). SHA-2 also exists (but is less popular than SHA-1) and can be played with using the commands sha224sum, sha256sum, sha384sum and sha512sum.
Hashes are a means, not a solution
Hashes, in the field of integrity validation, are a means to compare data and integrity in a relatively fast way. However, by itself hashes cannot be used to provide integrity assurance towards the administrator. Take the use of sha1sum by itself for instance.
You are not guaranteed that the sha1sum application behaves correctly (and as such has or hasn't been tampered with). You can't use sha1sum against itself since malicious modifications of the command can easily just return (print out) the expected SHA-1 sum rather than the real one. A way to thwart this is to provide the binary together with the hash values on read-only media.
But then you're still not certain that it is that application that is executed: a modified system might have you think it is executing that application, but instead is using a different application. To provide this level of trust, you need to get insurance from a higher-positioned, trusted service that the right application is being ran. Running with a trusted kernel helps here (but might not provide 100% closure on it) but you most likely need assistance from the hardware (we will talk about the Trusted Platform Module later).
Likewise, you are not guaranteed that it is still your file with hash results that is being used to verify the integrity of a file. Another file (with modified content) may be bind-mounted on top of it. To support integrity validation with a trusted information source, some solutions use HMAC digests instead of plain hashes.
Finally, checksums should not only be taken on file level, but also its attributes (which are often used to provide access controls or even toggle particular security measures on/off on a file, such as is the case with PaX markings), directories (holding information about directory updates such as file adds or removals) and privileges. These are things that a program like sha1sum doesn't offer (but tools like AIDE do).
Hash-based Message Authentication Codes
Trusting the hash result
In order to trust a hash result, some solutions use HMAC digests instead. An HMAC digest combines a regular hash function (and its properties) with a a secret cryptographic key. As such, the function generates the hash of the content of a file together with the secret cryptographic key. This not only provides integrity validation of the file, but also a signature telling the verification tool that the hash was made by a trusted application (one that knows the cryptographic key) in the past and has not been tampered with.
By using HMAC digests, malicious users will find it more difficult to modify code and then present a "fake" hash results file since the user cannot reproduce the secret cryptographic key that needs to be added to generate this new hash result. When you see terms like HMAC-SHA1 it means that a SHA-1 hash result is used together with a cryptographic key.
Managing the keys
Using keys to "protect" the hash results introduces another level of complexity: how do you properly, securely store the keys and access them only when needed? You cannot just embed the key in the hash list (since a tampered system might read it out when you are verifying the system, generate its own results file and have you check against that instead). Likewise you can't just embed the key in the application itself, because a tampered system might just read out the application binary to find the key (and once compromised, you might need to rebuild the application completely with a new key).
You might be tempted to just provide the key as a command-line argument, but then again you are not certain that a malicious user is idling on your system, waiting to capture this valuable information from the output of ps , etc.
Again rises the need to trust a higher-level component. When you trust the kernel, you might be able to use the kernel key ring for this.
Using private/public key cryptography
Validating integrity using public keys
One way to work around the vulnerability of having the malicious user getting hold of the secret key is to not rely on the key for the authentication of the hash result in the first place when verifying the integrity of the system. This can be accomplised if you, instead of using just an HMAC, you also encrypt HMAC digest with a private key.
During validation of the hashes, you decrypt the HMAC with the public key (not the private key) and use this to generate the HMAC digests again to validate.
In this approach, an attacker cannot forge a fake HMAC since forgery requires access to the private key, and the private key is never used on the system to validate signatures. And as long as no collisions occur, he also cannot reuse the encrypted HMAC values (which you could consider to be a replay attack).
Ensuring the key integrity
Of course, this still requires that the public key is not modifyable by a tampered system: a fake list of hash results can be made using a different private key, and the moment the tool wants to decrypt the encrypted values, the tampered system replaces the public key with its own public key, and the system is again vulnerable.
Handing over trust
As you've noticed from the methods and services above, you always need to have something you trust and that you can build on. If you trust nothing, you can't validate anything since nothing can be trusted to return a valid response. And to trust something means you also want to have confidence that that system itself uses trusted resources.
For many users, the hardware level is something they trust. After all, as long as no burglar has come in the house and tampered with the hardware itself, it is reasonable to expect that the hardware is still the same. In effect, the users trust that the physical protection of their house is sufficient for them.
For companies, the physical protection of the working environment is not sufficient for ultimate trust. They want to make sure that the hardware is not tampered with (or different hardware is suddenly used), specifically when that company uses laptops instead of (less portable) workstations.
The more you don't trust, the more things you need to take care of in order to be confident that the system is not tampered with. In the Gentoo Hardened Integrity subproject we will use the following "order" of resources:
- System root-owned files and root-running processes. In most cases and most households, properly configured and protected systems will trust root-owned files and processes. Any request for integrity validation of the system is usually applied against user-provided files (no-one tampered with the user account or specific user files) and not against the system itself.
- Operating system kernel (in our case the Linux kernel). Although some precautions need to be taken, a properly configured and protected kernel can provide a higher trust level. Integrity validation on kernel level can offer a higher trust in the systems' integrity, although you must be aware that most kernels still reside on the system itself.
- Live environments . A bootable (preferably) read-only medium can be used to boot up a validation environment that scans and verifies the integrity of the system-under-investigation. In this case, even tampered kernel boot images can be detected, and by taking proper precautions when running the validation (such as ensuring no network access is enabled from the boot up until the final compliance check has occurred) you can make yourself confident of the state of the entire system.
- Hypervisor level . Hypervisors are by many organizations seen as trusted resources (the isolation of a virtual environment is hard to break out of). Integrity validation on the hypervisor level can therefor provide confidence, especially when "chaining trusts": the hypervisor first validates the kernel to boot, and then boots this (now trusted) kernel which loads up the rest of the system.
- Hardware level . Whereas hypervisors are still "just software", you can lift up trust up to the hardware level and use the hardware-offered integrity features to provide you with confidence that the system you are about to boot has not been tampered with.
In the Gentoo Hardened Integrity subproject, we aim to eventually support all these levels (and perhaps more) to provide you as a user the tools and methods you need to validate the integrity of your system, up to the point that you trust. The less you trust, the more complex a trust chain might become to validate (and manage), but we will not limit our research and support to a single technology (or chain of technologies).
Chaining trust is an important aspect to keep things from becoming too complex and unmanageable. It also allows users to just "drop in" at the level of trust they feel is sufficient, rather than requiring technologies for higher levels.
- A hardware component that you trust (like a Trusted Platform Module or a specific BIOS-supported functionality) verifies the integrity of the boot regions on your disk. When ok, it passes control over to the bootloader.
- The bootloader now validates the integrity of its configuration and of the files (kernel and initramfs) it is told to boot up. If it checks out, it boots the kernel and hands over control to this kernel.
- The kernel, together with the initial ram file system, verifies the integrity of the system components (and for instance SELinux policy) before the initial ram system changes to the real system and boots up the (verified) init system.
- The (root-running) init system validates the integrity of the services it wants to start before handing over control of the system to the user.
An even longer chain can be seen with hypervisors:
- Hardware validates boot loader
- Boot loader validates hypervisor kernel and system
- Hypervisor validates kernel(s) of the images (or the entire images)
- Hypervisor-managed virtual environment starts the image
Integrity on serviced platforms
Sometimes you cannot trust higher positioned components, but still want to be assured that your service is not tampered with. An example would be when you are hosting a system in a remote, non-accessible data center or when you manage an image hosted by a virtualized hosting provider (I don't want to say "cloud" here, but it fits).
In these cases, you want a level of assurance that your own image has not been tampered with while being offline (you can imagine manipulating the guest image, injecting trojans or other backdoors, and then booting the image) or even while running the system. Instead of trusting the higher components, you try to deal with a level of distrust that you want to manage.
Providing you with some confidence at this level too is our goal within the Gentoo Hardened Integrity subproject.
From measurement to protection
When dealing with integrity (and trust chains), the idea behind the top-down trust chain is that higher level components first measure the integrity of the next component, validate (and take appropriate action) and then hand over control to this component. This is what we call protection or integrity enforcement of resources.
If the system cannot validate the integrity, or the system is too volatile to enforce this integrity from a higher level, it is necessary to provide a trusted method for other services to validate the integrity. In this case, the system attests the state of the underlying component(s) towards a third party service, which appraises this state against a known "good" value.
In the case of our HMAC-based checks, there is no enforcement of integrity of the files, but the tool itself attests the state of the resources by generating new HMAC digests and validating (appraising) it against the list of HMAC digests it took before.
An implementation: the Trusted Computing Group functionality
Trusted Platform Module
Years ago, a non-profit organization called the Trusted Computing Group was formed to work on and promote open standards for hardware-enabled trusted computing and security technologies, including hardware blocks and software interfaces across multiple platforms.
One of its deliverables is the Trusted Platform Module, abbreviated to TPM, to help achieve these goals. But what are these goals exactly (especially in light of our integrity project)?
- Support hardware-assisted record (measuring) of what software is (or was) running on the system since it booted in a way that modifications to this record (or the presentation of a different, fake record) can be easily detected
- Support the secure reporting to a third party of this state (measurement) so that the third party can attest that the system is indeed in a sane state
The idea of providing a hardware-assisted method is to prevent software-based attacks or malpractices that would circumvent security measures. By running some basic (but important) functions in a protected, tamper-resistant hardware module (the TPM) even rooted devices cannot work around some of the measures taken to "trust" a system.
The TPM chip itself does not influence the execution of a system. It is, in fact, a simple request/reply service and needs to be called by software functions. However, it provides a few services that make it a good candidate to set up a trusted platform (next to its hardware-based protection measures to prevent tampering of the TPM hardware itself):
- Asymmetric crypto engine, supporting the generation of asymmetric keys (RSA with a keylength of 2048 bits) and standard operations with those keys
- A random noise generator
- A SHA-1 hashing engine
- Protected (and encrypted) memory for user data and key storage
- Specific registers (called PCRs) to which a system can "add" data to
Platform Configuration Registers, Reporting and Storage
PCR registers are made available to support securely recording the state of (specific parts of) the system. Unlike processor registers that software can reset as needed, PCR registers can only be "extended": the previous value in the register is taken together with the new provided value, hashed and stored again. This has the advantage that a value stores both the knowledge of the data presented to it as well as its order (providing values AAA and BBB gives a different end result than providing values BBB and AAA), and that the PCR can be extended an unlimited number of times.
A system that wants to securely "record" each command executed can take the hash of each command (before it executes it), send that to the PCR, record the event and then execute the command. The system (kernel or program) is responsible for recording the values sent to the PCR, but at the end, the value inside the PCR has to be the same as the one calculated from the record. If it differs, then the list is incorrect and the "secure" state of the system cannot be proven.
To support secure reporting of this value to a "third party" (be it a local software agent or a remote service) the TPM supports secure reporting of the PCR values: an RSA signature is made on the PCR value as well as on a random number (often called the "nonce") given by the third party (proving there is no man-in-the-middle or replay attack). Because the private key of this signature is securely stored on the TPM this signature cannot be forged.
The TPM chip has (at least) 24 PCR registers available. These registers will contain the extended values for
- BIOS, ROM and memory block data (PCR 0-4)
- OS loaders (PCR 5-7)
- Operating System-provided data (PCR 8-15)
- Debugging data (PCR 16)
- Localities and Trusted Operating System data (PCR 17-22)
- Application-specific data (PCR 23)
The idea of using PCRs is to first measure the data a component is about to execute (or transfer control to), then extend the appropriate PCR, then log this event in a measurement log and finally transfer control to the measured component. This provides a trust "chain".
Trusting the TPM
In order to trust the TPM, the TCG basis its model on asymmetric keys. Each TPM chip has a 2048-bit private RSA key securely stored in the chip. This key, called the Endorsement Key, is typically generated by the TPM manufacturer during the creation of the TPM chip, and is backed by an Endorsement Key certificate issued by the TPM manufacturer. This EK certificate guarantees that the EK is in fact an Endorsement Key for a given TPM (similar to how an SSL certificate is "signed" by a Root CA). The private key cannot leave the TPM chip.
A second key, called the Storage Root Key, is generated by the TPM chip when someone takes "ownership" of the TPM. Although the key cannot leave the TPM chip, it can be removed (when someone else takes ownership). This key is used to encrypt data and other keys (user Storage Keys and Signature Keys).
The other keys (storage and signature keys) can leave the TPM chip, but always in an encrypted state that only the TPM can decrypt. That way, the system can generate specific user storage keys securely and extract them, storing them on non-protected storage and reload them when needed in a secure manner).