In cryptography, every layer of the stack has a set of possible vulnerabilities associated with it. In the rarest case, primitives (like ciphers and hash functi

In cryptography, every layer of the stack has a set of possible vulnerabilities associated with it. In the rarest case, primitives (like ciphers and hash functions) are broken; more commonly, the protocol is ill-designed and flawed; but most common of all, the actual implementation itself has security flaws, like side channel attacks. The issues associated with every layer are considered extremely subtle and tricky to both identify and fix. But I would say this is especially true for implementation attacks, which are not really addressed by cryptographic theory. So, no, writing your own protocol implementation is not secure, even if you trust the design of the protocol. You are still vulnerable to the trickiest class of security flaws. However, so long as you clearly label your project as "learning only" or "insecure," no one will think worse of you for having your own protocol implementation. In fact, I'd say re-implementing TLS is one of the few ways to become intimately familiar with its internals.