Reddit mentions of Serious Cryptography: A Practical Introduction to Modern Encryption

Aside from the books others mentioned, I wanted to also suggest one that recently came out: "Serious Cryptography" by Jean-Philippe Aumasson. What I like about this book is that it focuses on teaching crypto via programming practical implementations. So if you like coding and learning by example I highly recommend it.

Link: https://www.amazon.com/dp/1593278268/ref=cm_sw_r_cp_apa_RSWKAbQZMK4FV

Good question. Iterative hashing in the hope of finding a short cycle actually has a name: the "rho method". It's called this because the lowercase greek letter rho (ρ) has the shape of a detected cycle. The idea is that if you find such a cycle, it means you have found a collision in the hash function (the value before you entered the cycle and the last value in the cycle both hash to the first value in the cycle).

A memory-efficient algorithm to search for these cycles is the "tortoise and the hare" algorithm:
https://en.wikipedia.org/wiki/Cycle_detection#Floyd.27s_Tortoise_and_Hare

The algorithm involves keeping two values and double-hashing one (the hare) and single-hashing the other (the tortoise). If the tortoise is ever the same as the hare then you've found a cycle.

The rho method has several applications, but in the context of cryptography there is a very accessible description in this recently published book: https://www.amazon.com/Serious-Cryptography-Practical-Introduction-Encryption/dp/1593278268/

Serious Cryptography is a great introductory book to look at before you dive into Blockchain. Blockchain: A Practical Guide to Developing Business, Law, and Technology Solutions would be my personal recommendation once you're ready to get more specific.

If you have zero background in crypto so far, check out this crash course article. It's a really short read and I think it's an awesome starting point. If you read nothing else before getting into blockchain, at least read through that article once or twice.

I recently read "Serious Cryptography: A Practical Introduction to Modern Cryptography"[1] and I recommend it.

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It explains the basics without going too much into the theoretical bits and proofs and even goes a bit into post-quantum stuff.

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Other than that, I really enjoyed Dan Boneh's Cryptography I course on Coursera [2]. I did not have time to do the assignments when I took it, but even just the theory is very interesting and up to date.

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[1] https://www.amazon.com/Serious-Cryptography-Practical-Introduction-Encryption/dp/1593278268/

[2] https://www.coursera.org/learn/crypto

If you like books and are interested in modern cryptography, "Serious Cryptography" was excellent. A more introductory text with historical context is "The Code Book".

Some CTF’s have good cryptography challenges. picoCTFs has a few good ones

Also this book is pretty good Serious Cryptography: A Practical Introduction to Modern Encryption https://www.amazon.com/dp/1593278268/ref=cm_sw_r_cp_api_i_3cOFDb0DBDJ1N