How to Apply Cryptography

Cryptography is present in several technologies. Applying cryptography is a big topic that covers several independent technologies. For the exam, be familiar with the high-level concepts around applying cryptography and its related technologies more so than understanding the details of implementing or supporting them. The subtopics below are the key topics outlined for this section. If you are new to cryptography or have limited exposure to it, consider additional sources to dive deeper.


  • Cryptographic lifecycle (e.g., cryptographic limitations, algorithm/protocol governance). When we think about the lifecycle of technologies, we often think about the hardware and software support, performance and reliability. When it comes to cryptography, things are a bit different: The lifecycle is focused squarely around security. As computing power goes up, the strength of cryptographic algorithms goes down. It is only a matter of time before there is enough computing power to brute-force through existing algorithms with common key sizes. You must think through the effective life of a certificate or certificate template, and of cryptographic systems. Beyond brute force, you have other issues to think through, such as the discovery of a bug or an issue with an algorithm or system. NIST defines the following terms that are commonly used to describe algorithms and key lengths: approved (a specific algorithm is specified as a NIST recommendation or FIPS recommendation), acceptable (algorithm + key length is safe today), deprecated (algorithm and key length is OK to use, but brings some risk), restricted (use of the algorithm and/or key length is deprecated and should be avoided), legacy (the algorithm and/or key length is outdated and should be avoided when possible), and disallowed (algorithm and/or key length is no longer allowed for the indicated use).


  • Cryptographic methods. This subtopic covers the following three types of encryption. Be sure you know the differences.


    • Symmetric. Symmetric encryption uses the same key for encryption and decryption. Symmetric encryption is faster than asymmetric encryption because you can use smaller keys for the same level of protection. The downside is that users or systems must find a way to securely share the key and then hope that the key is used only for the specified communication.


    • Asymmetric. Asymmetric encryption uses different keys for encryption and decryption. Since one is a public key that is available to anybody, this method is sometimes referred to as “public key encryption.” Besides the public key, there is a private key that should remain private and protected. Asymmetric encryption doesn’t have any issues with distributing public keys. While asymmetric encryption is slower, it is best suited for sharing between two or more parties. RSA is one common asymmetric encryption standard.


    • Elliptic curves. Elliptic Curve Cryptography (ECC) is a newer implementation of asymmetric encryption. The primary benefit is that you can use smaller keys, which enhances performance.


  • Public key infrastructure (PKI). A PKI is a foundational technology for applying cryptography. A PKI issues certificates to computing devices and users, enabling them to apply cryptography (for example, send encrypted email messages, encrypt web sites, or use IPsec to encrypt data communications). There are multiple vendors providing PKI services. You can run a PKI privately and solely for your own organization, you can acquire certificates from a trusted third-party provider, or you can do both, which is very common. A PKI is made up of certification authorities (CAs) (servers that provide one or more PKI functions, such as providing policies or issuing certificates), certificates (issued to other certification authorities or to devices and users), policies and procedures (such as how the PKI is secured), and templates (a predefined configuration for specific uses, such as a web server template). There are other components and concepts you should know for the exam:


    • A PKI can have multiple tiers. Having a single tier means you have one or more servers that perform all the functions of a PKI. When you have two tiers, you often have an offline root CA (a server that issues certificates to the issuing CAs but remains offline most of the time) in one tier, and issuing CAs (the servers that issue certificates to computing devices and users) in the other tier. The servers in the second tier are often referred to as intermediate CAs or subordinate CAs. Adding a third tier means you can have CAs that are only responsible for issuing policies (and they represent the second tier in a three-tier hierarchy). In such a scenario, the policy CAs should also remain offline and brought online only as needed. In general, the more tiers, the more security (but proper configuration is critical). The more tiers you have, the more complex and costly the PKI is to build and maintain.


    • A PKI should have a certificate policy and a certificate practice statement (CSP). A certificate policy documents how your company handles items like requestor identities, the uses of certificates and storage of private keys. A CSP documents the security configuration of your PKI and is usually available to the public.


    • Besides issuing certificates, a PKI has other duties. For example, your PKI needs to be able to provide certificate revocation information to clients. If an administrator revokes a certificate that has been issued, clients must be able to get that information from your PKI. Another example is the storage of private keys and information about issued certificates. You can store these in a database or a directory.


  • Key management practices. Remember, key management can be difficult with symmetric encryption but is much simpler with asymmetric encryption. There are several tasks related to key management:


    • Key creation and distribution. Key creation is self-explanatory. Key distribution is the process of sending a key to a user or system. It must be secure and it must be stored in a secure way on the computing device; often, it is stored in a secured store, such as the Windows certificate store.


    • Key protection and custody. Keys must be protected. You can use a method called split custody which enables two or more people to share access to a key — for example, with two people, each person can hold half the password to the key.


    • Key rotation. If you use the same keys forever, you are at risk of having the keys lost or stolen or having your information decrypted. To mitigate these risks, you should retire old keys and implement new ones.


    • Key destruction. A key can be put in a state of suspension (temporary hold), revocation (revoked with no reinstatement possible), expiration (expired until renewed), or destruction (such as at the end of a lifecycle or after a compromise).


    • Key escrow and key backup recovery. What happens if you encrypt data on your laptop but then lose your private key (for example, through profile corruption)? Normally, you lose the data. But key escrow enables storage of a key for later recovery. This is useful if a private key is lost or a court case requires escrow pending the outcome of a trial. You also need to have a method to back up and recover keys. Many PKIs offer a backup or recovery method, and you should take advantage of that if requirements call for it.


  • Digital signatures. Digital signatures are the primary method for providing non-repudiation. By digitally signing a document or email, you are providing proof that you are the sender. Digital signatures are often combined with data encryption to provide confidentiality.


  • Non-repudiation. For this section, non-repudiation refers to methods to ensure that the origin of data is can be deduced with certainty. The most common method for asserting the source of data is to use digital signatures, which rely on certificates. If User1 sends a signed email to User2, User2 can be sure that the email came from User1. It isn’t foolproof though. For example, if User1 shares his credentials to his computer with User3, then User3 can send an email to User2 purporting to be User1, and User2 wouldn’t have a way to deduce that. It is common to combine non-repudiation with confidentiality (data encryption).


  • Integrity. A hash function implements encryption with a specified algorithm but without a key. It is a one-way function. Unlikely encryption, where you can decrypt what’s been encrypted, hashing isn’t meant to be decrypted in the same way. For example, if you hash the word “hello”, you might end up with “4cd21dba5fb0a60e26e83f2ac1b9e29f1b161e4c1fa7425e73048362938b4814”. When apps are available for download, the install files are often hashed. The hash is provided as part of the download. If the file changes, the hash changes. That way, you can figure out if you have the original install file or a bad or modified file. Hashes are also used for storing passwords, with email and for other purposes. Hashes are susceptible to brute force. If you try to hash every possible word and phrase, eventually you will get the hash value that matches whatever hash you are trying to break. Salting provides extra protection for hashing by adding an extra, usually random, value to the source. Then, the hashing process hashes the original value of the source plus the salt value. For example, if your original source value is “Hello” and your salt value is “12-25-17-07:02:32”, then “hello12-25-17-07:02:32” gets hashed. Salting greatly increased the strength of hashing.


  • Methods of cryptanalytic attacks. There are several methods to attack cryptography. Each has strengths and weaknesses. The primary methods are:


    • Brute force. In a brute-force attack, every possible combination is attempted. Eventually, with enough time, the attack will be successful. For example, imagine a game where you have to guess the number between 1 and 1,000 that I chose. A brute-force attack would try all numbers between 1 and 1,000 until it found my number. This is a very simplified version of a brute-force attack, but the key point is that a brute-force attack will eventually be successful, provided it is using the correct key space. For example, if an attempt is made to brute force a password, the key space must include all the characters in the password; if the key space includes only letters but the password includes a number, the attack will fail.


    • Ciphertext only. In a ciphertext-only attack, you obtain samples of ciphertext (but not any plaintext). If you have enough ciphertext samples, the idea is that you can decrypt the target ciphertext based on the ciphertext samples. Today, such attacks are very difficult.


      • Known plaintext. In a known plaintext attack, you have an existing plaintext file and the matching ciphertext. The goal is to derive the key. If you derive the key, you can use it to decrypt other ciphertext created by the same key.


  • Digital rights management. When people think of digital rights management (DRM), they think of protections placed on movies and games. But for the CISSP exam, it is really about protection of data, such as spreadsheets and email messages. Organizations often refer to data protection as enterprise digital rights management (E-DRM) or information rights management (IRM). Several vendors offer solutions to protect data in individual files. The solutions all provide a common set of foundational features:
    • Restrict viewing of a document to a defined set of people
    • Restrict editing of a document to a defined set of people
    • Expire a document (rendering it unreadable after a specified date)
    • Restrict printing of a document to a defined set of people
    • Provide portable document protection such that the protection remains with the document no matter where it is stored, how it is stored, or which computing device or user opens it You can use DRM, E-DRM or IRM to protect data for your organization. Many of the solutions also enable you to securely share data with external organizations. Sometimes, this sharing is enabled through federation. Other times, the use of a public cloud provider enables cross-organization sharing. DRM, E-DRM and IRM provide companies with a way to provide confidentiality to sensitive documents. Additionally, some of the solutions enable you to track when and where documents were viewed. Last, some solutions enable you to update the protection of a document (such as removing a previously authorized viewer) even after a document has been sent and shared with external parties.

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