In Apple/Gov dispute on the San Bernardino iPhone case, Gov brought up the Burr case from 1807, arguing that a 3rd party could be compelled to decipher the contents, provided there was no self-incrimination (Apple argued Burr did not apply): http://www.macworld.com/article/3046095/legal/burrs-cipher-s...
Does anyone know if there has been an attempt to equate decryption to deciphering in US courts?