tl;dr- Provide your human contact with as many bits of one-time pad cipher as they need. This results in perfect message secrecy; other humans won't ever be able to break it, even with hyper-advanced quantum computers.
Provide one-time pads
Encryption via a one-time pad is perfect and unbreakable, as long as:
- the one-time pad was generated using a process that attackers can't reverse-engineer; and
- each part of the one-time pad is discarded after use, never to be reused.
Humans have plenty of methods for generating high-quality informational entropy, i.e. one-time pads that can't be reverse-engineered. But, as an alien AI, you probably have crazy patterns and methods that we couldn't begin to hope to reverse-engineer, meaning that you're an excellent source of extreme-quality entropy.
Don't use normal encryption methods
The best encryption methods are pretty much unbreakable, we think, as far as modern human knowledge and technology go. But why bother? You can use perfect encryption via one-time pads.
Add junk data if you want
The one big information leak from one-time pad communication is the amount of information and time it was transmitted. This is, if someone sends you a message that's 100-bytes long, then spies can know that you received a 100-byte message. They wouldn't know what it says, but since they know it was sent, that's still technically an information leak.
So, add junk data to fill out messages up to some certain size, let's say 100TB. Then, all a spy knows is that a message was sent that was up to 100TB in size.
This would be impractical for humans, since getting 100TB of high-quality single-use entropy for each message would be really annoying and costly, but since you're an alien AI, it's probably basically free for you.
Schedule regular transmissions
Have your human contact send messages at regularly scheduled times, even if they've got nothing to say and just end up sending pure junk data.
This way, no one knows when you're actually sending messages. All any spy can tell is that they're not sending you more than the message size at the regularly scheduled rate.
Use a hidden transmission channel
Using the above tactics, no spy could ever read your communications or know when you're sending them. At most, they know that you're not sending more than, say, 100TB of messages every communication cycle; which, since that's already pretty much a given, doesn't matter.
But, if you want to go one step further, you could do a directional transmission channel. This would have the advantage of spies not even having the chance to realize that there is some sort of secret communication going on.