Cryptography is the practice of establishing a secure connection between two parties in the presence of a third party whom you don't want to be able to read your messages.

Cryptographers design algorithms and protocols, which do exactly this (and many other things). They also use a lot of time looking for security holes in existing protocols to make sure they can still be trusted. This is called cryptanalysis.

If you want a formal introduction to cryptography, you should read An Introduction to Mathematical Cryptography. The creator of the sub also approve of the Udacity course Applied Cryptography - A Science of Secrets.

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Cryptocurrency talk is only allowed if it's to discuss the cryptography subparts of it.

We won't do your homework for you. It is however allowed to help you understand material and or the questions.

we won't solve your ciphers unless you provide us with an algorithm. If anyone sends you a code or a cipher without telling you how they encrypted it, don't bother posting it on this subreddit - your post will get deleted. We redirect you to /r/breakmycode or /r/codes. Thank you for your understanding and for following the rules.

Related Subreddits:

/r/crypto - Tends to have more in depth topics.
/r/math - Modern cryptography is a field of mathematics
/r/compsci - Cryptography is technically a subdisclipline of computer science.
/r/privacy
/r/intelligence
/r/Bitcoin - The Bitcoin protocol is reliant on cryptography.