Do you know a good encryption algorithm that is not so difficult to implement in C ++ (but not too easy)?
Check out the awesome comics on AES . It explains the cryptographic background of the algorithm, and you can practice by writing a highly efficient implementation of one in different languages.
I think RSA is good and also pretty easy to implement. You might want to study it. Another that I would suggest that is also simple is DES .
If you're just looking for something to encode your learning experience, I always liked the Advanced Encryption Standard . The actual standard document in NIST contains all the details needed to write and test your code.
I like RC5 for its simplicity. It is much easier to implement than, for example, AES , and although it is not so strong, it is still a good cypher.