[MUSIC] Welcome back. For this lesson, we´re going to talk about cryptography
as it applies to threat models. The outline of this lesson is, we are
going to basically address the question of how the cryptography
that we learned earlier addresses issues with our threat modeling
that we have been learning so far. With the threats of spoofing, tampering,
and information leakage, connecting these threats with the mitigations that
cryptography offers allows us to have a much better understanding of both threat
modeling and basic applied cryptography. For a larger set of example threats and
possible mitigations, see the very excellent book called Threat Modeling,
Designing for Security by Adam Shostack. Specifically, the appendix
B called threat trees. So after this lesson, you will be
able to explain how cryptography addresses issues with spoofing,
tampering, and information disclosure. You will also be able to connect the
concepts of cryptographic hash functions, message authentication codes, and
encryption to the STRIDE method, and how these cryptography concepts
apply to threat models. But first, let's begin by
reviewing the STRIDE acronym. We're now going to review STRIDE. Remember that STRIDE is
our way of collecting or brainstorming different ways that
threats can occur to our system. S stands for spoofing. T stands for tampering. R stands for repudiation. I stands for information disclosure or
information leakage. D stands for denial of service. E stands for elevation of privilege. So one threat that can happen with systems
is a man-in-the-middle attack when two or more parties in that system
need to authenticate. So this an example of a spoofing threat. This example threat can be
mitigated when we can authenticate the parties that are involved
in the communication process. Here we can use asymmetric key encryption
to perform proper key exchange and authenticate each party. Previously, we had chatted about how
it was recommended that Diffie-Hellman key exchange can be used. And an example of this
is SSH tunneling using diffie-hellman-group14-sha256 key
exchange. And another example is using TLS 1.2
as a means of identifying the server when a client and a web browser need
to talk to a particular server. Moving on to tampering threats, an example
of threat based on tampering threat would be the data flow between two entities
having weak or no message integrity. This example can be mitigated when we can
authenticate messages that we receive before we use them. We can use message authentication
code algorithms in this case. And previously, we talked about how
HMAC using SHA-256 hash functions as building blocks for that as a recommended
way of performing message authentication. Now, in terms of the information
disclosure threats that we can see. We can see threats when
data is in transit. So for example, an example threat would be
unencrypted communication between two or more parties, for example,
HTTP requests and responses. And this when you only use HTTP with session management between
the web browser and the server. Somebody could eavesdrop on that session
and basically obtain session information such that they can further perform
other types of attacks, for example. This example threat can
be mitigated if we can encrypt the data that's being
sent between the two entities. In this case, we can use AES in CBC mode with random
initialization vector or random IV. And another way that information
disclosure threats could happen is during so-called data at rest. An example threat could be
threats to stored data, like data backups that are not encrypted. And again,
we can use encryption in this case. So we would use the recommended
AES algorithm in CBC block cipher mode with random IV. In conclusion, we looked at STRIDE
threats that could be mitigated using the applied cryptography basics
that we learned in the previous lesson. Looking at STRIDE in this point
of view is a good way to connect the applied cryptography
basics that we learned, and also addressing the threats in
your particular threat model. Now, given this information,
this gives you a good base for which to develop your
threat modeling skills.