"Byte-at-a-time decryption" means creating a scenario where attackers can brute force numbers like 2^8, winning a single byte of "plaintext" (or whatever the equivalent is depending on the primitive you're targeting). If your block size is 16 bytes long, the attacker might have to brute force 2^8 16 times; with a laptop, you might be talking about whole seconds of work.

Block cipher attacks generally never recover crypto keys.

I am being intentionally vague. Not because I want to keep information from you, but because I don't want to create yet another crypto thread that gives developers a false sense of knowing what the risks are when building crypto.

If this is something you're seriously interested in, and you can code in any programming language, email me and I'll give you a syllabus of straightforward things to work on.