Resources

Below are some of the resources I would recommend to anyone wanting to learn more on cryptography, ans specifically for anyone wanting to have a go at crypto challenges in CTFs. Please note these recommendations are highly subjective.

Books

If you are interested to get started in cryptography I would highly recommend the book Serious Cryptography by JP Aumasson. It not only deals with the fundamental concepts, but also illustrates, using examples, what could (and does) go wrong in real-life implementations. Introduction to Modern Cryptography by Jonathan Katz and Yehuda Lindell is an excellent book covering the basics - it is used by a lot of universities as course material for cryptography courses. A third good book is Understanding Cryptography by Christof Paar and Jan Pelzl. Whats is extra nice about this book is that there is an accompanying YouTube channel with lectures by Christof Paar based on the content of the book.

If you are interested in getting a deeper understanding of (the beauty of) the mathematical concepts of a.o. finite fields and elliptic curves (which I fully recommend), the book A Classical Introduction to Modern Number Theory by Kenneth Ireland and Michael Rosen or the book The Arithmetic of Elliptic Curves by Joseph Silverman (who has written much more eleborate and more complex books on elliptic curves) give more mathematical details on these subjects.

Courses

The Coursera Crypto I course by Dan Boneh (Stanford) is an excellent time investment if you want to learn cryptography.

The following platforms allow you to test and train your skills with online challenges:

• CryptoHack provides an elaborate platform with a wide variety of problems, both diverse in difficulty and topics. Highly recommended!
• The Cryptopals Crypto Challenges provide an rich set of challenges dealing with symmetric and asymmetric cryptographic primitives (and their sometimes faulty implementations).
• The site RootMe offers a set of crypto challenges.

Tools

The following tools are very helpful when solving crypto challenges

• Sage is a toolbox which gives you easy Python-based access to all sorts of mathematical implementations that are useful (like finite field arithmetic, linear algebra, lettuce reduction algorithms, elliptic curve arithetic etc.). Simply indispensiblle.
• CyberChef provides a nice GUI for some of the concepts you might encounter. From classical encryption schemes to more relevant cryptographic primitives such as AES, RSA and PBKDF2.

CTFs

Here are some hopefully useful pointers on how to get started with playing CTFs

• CTF time is the website where most CTF competions are announced, and also holds a lot of pointers to the different challenges/tasks hosted at CTFs.
• To play in a CTF competion you might want to be part of a team. Solving challenges can be much more fun when cooperating with team members, sharing thoughts and ideas. If you don't have people to play with, you can always start out by playing with the OpenToAll CTF team. I would prefer playing in a smaller team, but if joining OpenToAll helps you get started, you might want to consider this.
• Reading up on write-ups of crypto challenges from past CTFs is a great way to learn. There are a lot of collection of write-ups out there, for example here or here. As mentioned before, CTF time holds a lot of pointers to write-ups as well (you can search throught them by keyword).

Twitter

Here is a non-complete list of cryptographers I find insightful, you might want to follow then on Twitter:

• @veorg (JP Aumasson)
• @hellman1908 (Aleksei Udovenko) Of the notorious MSLC CTF team, which has a nice collection of write-ups.
• @matthew_d_green (Matthew Green) Also has a blog which has not been as active lately as it used to be.
• @agl__ (Adam Langley)
• @RealWorldCrypto Twitter feed of the most relevant practical crypto conference by far (imho).
• @DucasLeo (Léo Ducas)
• @SmartCryptology (Nigel Smart)
• @kaepora (Nadim Kobeissi) Also has an interesting podcast on cryptography.
• @CryptoOrrDun (Orr Dunkelman) More on the academic side.
• @hsevenps (Samuel Neves) Inactive for a while now.
• @bascule (Tony Arcieri) Mainly on crypto in Rust.