The art and science of breaking ciphers, i.e., the extraction of $m$ given $c$, $E$, $D$, and possibly $k_e$. See Wikipedia’s topics in cryptography page. The study of cryptology includes the design of various ciphers, cryptanalysis methods (attacks), key exchange, key authentication, cryptographic hashing, digital signing, and social issues (legal, political, etc.). Public algorithms can be subjected to thousands of experts looking for flaws, so you have some degree of confidence in those that withstand scrutiny.If your secret algorithm is compromised (someone could leave your group), you have to change it, and that is way harder than just changing a key!.Most cryptosystems do not rely on their algorithms being kept secret, because:

The best cryptosystems assume that Eve and Mallory know $E$, $D$, $c$, and, if $k_e eq k_d$, then $k_e$ as well. Nonrepudiation: Alice should not be able to deny sending $m$.Integrity: Bob should be able to verify that $m$ was not tampered with.

Authentication: Bob should be able to verify that it was Alice that sent $m$.

Confidentiality: It should cost Eve more to recover $m$ than $m$ is worth.

Mallory can not only eavesdrop but also spoof one of the participants, add, modify or delete actual messages, hijack the connection, do a denial of service, inject malware, etc. It’s conventional to talk about crypto in a setting with a few characters named Alice (the sender), Bob (the receiver), Eve (the eavesdropper), and Mallroy (a powerful malicious actor).Īlice and Bob can be people, clients and servers, peer computers, data stores, network routers, etc. This has been an important public service announcement. However, you should always check, and ask, whether the library you are using implements an algorithm that has not yet been cracked, and has sufficient key length. Sure, it’s fine to play around at home, but don’t EVER try to implement cryptographic routines for a customer or your own company.

These notes serve as an introduction to crypto and not a complete curriculum to make you an expert who is licensed to implement ciphers, hashes, signatures, or any such thing in any real-world application. To understand what cryptography and cryptanalysis is all about.

Cryptography and network security notes