Encryption algorithms don't allow you to limit the output values: they generate a stream of eight bit byte values which can contain any value in the range 0 to 255 inclusive. Any attempt to restrict the values will result in data which cannot be decrypted.
If you need a "human readable" form that can be sent over a Text only channel, then consider translate the encrypted bytes to either hex (inefficient) or Base64 - (more efficient, and designed for it): Convert.ToBase64String Method
] and Convert.FromBase64String
Do not be tempted to assume that converting your original data to Base64 will provide any security - it doesn't, it is purely a translation system, there is no encryption involved at all.