Cryptography has long history. Really long one.
Of course, there is no need to say, how important was safety of secret messages between rulers and generals.
In olden times, ciphers used were pretty simple, but they evolved to more complex ones.
Old ciphers had big flaw: they were symmetric, which allowed cracking them.
But what about asymmetric key encryption (for example RSA)? In modern times, it is most important technique of encryption. One could think, that bringing it to Middle Ages would create completely unbreakable cipher.
But mathematical calculations connected to encoding, and decoding RSA messages require a lot of work, thats why we use computers for it. In Middle Ages, there were no computers, and no possibility to create one, even with knowledge from the future.
Of course, it would still be possible - just train some scholars (and accidentally spawn some great mathematical theories, but who cares?), and use sufficiently small keys.
But, would keys small enough, that one (of course only one, because of safety) scholar at kings court, would be able to decrypt message in few hours, still be powerful? Maybe, keys so small will not provide enough protection?
Is using asymmetric key encryption in those terms really safe?
Clarification: all calculations in encryption, and decryption process are done by hand, or using techniques available in late Middle Ages, and shouldn't take more than a day to one man to handle.
RSA is only example of assymetric key encryption - if it turns out, that RSA would be useless, but some other type would be okay, I accept the answer.
By safe, I expect something similar to our time definition: nobody knows a quick method of breaking, and brute force solutions take years at the very least.
Not a duplicate of cryptography in world without computers, as this question is about one, special family of ciphers, while above is about cryptography as whole.