Check out The Quantum Thief.
It's set several centuries post singularity.
Crypto ensures privacy. It validates the integrity of your exo-memory. It's how you know your thoughts and memories are really yours.
Update: I was trying not to spoil things for people who have not read the book. I guess I was trying too hard.
WARNING: If you haven't read the book, this contains spoilers.
In The Quantum Thief, the society of the Oubliette makes extensive use of exomemory; non-biological, external, protected storage. It's preferential to biological memory, because it extends beyond biological death, is digitally signed ensuring its authenticity, and is not subject to the smudging or fading of biological memory. Every interaction is subject to gevulot, a cryptographically secure, contractual agreement defining what information is exchanged / retained, and for how long. This all works as long as the crypto is secure.
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However, Jean le Flambeur, the villain, possesses a key which allows the surreptitious editing of the exomemory. So, the populace remember the glorious revolution, not that the Oubliette was a penal colony and they were its prisoners.
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If you control the memories, you control the behavior that arises from the memories.
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There was some point where your rational mind made the realization that your childhood hero did not the measure up to the mythos. If someone had the ability to erase every memory that contradicts the mythos, what evidence would you have to question the Supreme Leader's right to eternal rule?
Conceptually, I find it much scarier thatthan the premise of controlling an individual mind. It's about controlling an entire society.