How to encrypt a short plain text? I want to encrypt a short plain text (i.e., 30bit), and the requirement is that length of cipher text is the same as the short plain text. 

It seems that stream encryption methods like CFB-8 satisfies my requirement. My question is that 


What is the cracking time of CFB-8 in brute-force-attack? Does the cracking time only depend on the key length? 
Does the length of plain text influence the cracking time? if so, what's the relationship?
Are there any better methods for my requirement?
Do you need to reuse the key for multiple messages without any way to distinguish the role of each message, e.g. by a message sequence number, or file position for a collection of records in a single file?

If you have some short identifier that is never reused between each role, or if you need encrypt only a single message with each key, then you can pick your favorite stream cipher, say ChaCha or AES-CTR, with the message role (sequence number, file position, etc.) as the nonce, and just chop the first 30 bits off the stream.

If you must reuse the key for many messages with no way to distinguish the role of each message, then you need format-preserving encryption.