[mfjordvald]

How would that work? If I write "I just killed a child" here it doesn't mean I actually did that. Similarly. if I used a decryption key "I am guilty of the murder" then it still does not mean I'm guilty of any murder, just that I have a poor sense of humour and used an odd encryption key.

[pantaloons]

That encryption key wouldn't provide proof, but there are many that would. Trivially a sufficiently long key could embed a video of the murder.

[Dylan16807]

And it would be trivially impossible to memorize.