The problem I see with this is that decryption is not really possible or useful without encryption. And in many if not most occasions the encryption method is as likely the cause of any issues (and thus questions) as the decryption.
Furthermore, I do think that there are already enough tags that have to do with cryptography. In the end we should not so much worry about the exact meaning of the word. Instead we should be worried about the visibility of the questions (and thus answers). Having to check yet another tag is not going to help me for sure.
Finally, a question can only have 5 tags. As there are many questions that would merit both the encryption as well as the decryption tag, I'm afraid that users will run out of tag space.
For these reasons I've also nominated cipher (439 followers) to be a synonym of encryption (over 11K followers, about 25 times as many), even though both are not identical in a linguistic sense. There's just too many questions "flying under the radar" as I like to call it. Once in a few weeks I clean up those kind of tags, but I'd rather not and some users have to wait for weeks to get an answer.
The difference between cryptography and encryption on the other hand is entirely practical. Most questions that have to do only with encryption are correctly tagged with encryption and most questions that have to do with anything else are tagged cryptography. That's useful to me and both have enough followers to merit a split.