I'm trying to embed a date inside an encryption key so that I can warn a user if his encryption key is older than a certain number days / months / years. Due to legacy systems (that I don't have control over) using this encryption system, they can't store a date that the key was created and for security reasons, I can't store keys next to dates in the encryption system, so the most feasible way to store it is inside the keys themselves. Visibility is not much of an issue, as long as it's not easy to change it.
The concept of Steganography is exactly what I'm looking for, but all the algorithms for it usually hides stuff in noise / data that can be tampered with, I only have exact data, no noise.
I've started writing my own compression algorithm to reduce the bits in the key, I'll just need to mathematically prove that I will always have those number of bits available after compression, if not, I risk increasing the length of the key.