Written by 1.1 Evolution of cryptography Over the past four decades or so, cryptography has evolved from an exotic field known to a select few into a fundamental skill for the design and operation of modern IT systems. Today, nearly every modern product, from the bank card in your pocket to the server farm running your favorite […]
1.4.1 Complexity versus security – features The following thought experiment illustrates why complexity arising from the number of features or options is a major security risk. Imagine an IT system, say a small web server, whose configuration consists of 30 binary parameters (that is, each parameter has only two possible values, such as on or […]
2.4 Integrity Integrity is the ability to detect data manipulation by unauthorized entities. By data manipulation, we mean unauthorized acts such as the insertion, deletion, or substitution of individual data chunks or entire messages. It is not required that manipulations as such are rendered impossible; given the multitude of possible communication channels, this would be […]
1.5 Example attacks The combination of these two trends – increase in complexity and increase in connectivity – results in an attack surface explosion. The following examples shall serve to illustrate this point. 1.5.1 The Mirai botnet In late 2016, the internet was hit by a series of massive Distributed Denial-of-Service (DDoS) attacks originating from […]
3.1 Secret keys and Kerckhoffs’s principle Let’s assume a plaintext m is mapped onto a ciphertext c. Earlier, we formalized this situation in the equation c = fK(m). You may have wondered why there is a parameter K. In cryptography, we distinguish between the encryption algorithm f and the key K. We can think of […]
3.2.1 One key for each task In addition to the three basic requirements, it is a good practice in cryptography to have a unique key for each unique task. As an example, if Alice and Bob first authenticate each other (entity authentication) and then use message authentication to ensure their communication is not being manipulated […]
3.4 Key length The key space 𝒦 is large but finite. So, in principle, it is possible to search through it completely until the correct key K has been found. Such an attack is called a brute-force attack. Whether a brute-force attack is possible within a reasonable time frame, that is, within the time span […]
3.6 Key establishment To communicate securely, Alice and Bob need to share the secret key in advance. According to [117], Def. 1.63, ”key establishment is any process whereby a shared secret key becomes available to two or more parties for subsequent cryptographic use”. In principle, Alice and Bob might meet in person in a cafe […]
3.7 Randomness and entropy In cryptography, the security of most protocols and mechanisms depends on the generation of random sequences of bits or numbers. These sequences must have a sufficient length and be random in a very specific sense: we do not want an attacker to be able to guess part of or the whole […]
3.7.2 Entropy in cryptography So, why is entropy so fundamental to cryptography? If the source used to generate secrets (or unique values used in cryptographic protocols) has a poor entropy, the number of values that can be possibly drawn from that source will be limited, and some values will be (much) more likely than others. […]