Encryption, the Fifth Amendment, and Aaron Burr:
Following my posts last week on encryption and the Fifth Amendment, a few readers asked about how courts have dealt with such issues before. As far as I know, there is only one other judicial decision specifically addressing the Fifth Amendment implications of decrypting ciphertext. Remarkably, it arose 200 years ago, in the treason trial of former Vice-President Aaron Burr.

  The prosecution had charged Burr with treason for leading a failed rebellion against the United States in the western territories. The evidence included an encrypted communication Burr had sent to his alleged co-conspirators. To decrypt the communication, the prosecutor subpoenaed Burr's private secretary, Willie, (who knew the key to Burr's cryptography) and forced him to testify as to the communication's plaintext meaning. Willie objected on Fifth Amendment grounds.

  In United States v. Burr, 25 F. Cas. 38 (C.C. Va. 1807), Chief Justice Marshall ruled that Willie could be compelled to answer because revealing his knowledge of the cipher would not incriminate him. Here was Marshall's analysis:
To know and conceal the treason of another is misprision of treason, and is punishable by law. No witness, therefore, is compellable by law to disclose a fact which would form a necessary and essential part of this crime. If the letter in question contain evidence of treason, which is a fact not dependent on the testimony of the witness before the court, and, therefore, may be proved without the aid of his testimony; and if the witness were acquainted with that treason when the letter was written, he may probably be guilty of misprision of treason, and, therefore, the court ought not to compel him to answer any question, the answer to which might disclose his former knowledge of the contents of that letter.

But if the letter should relate to misdemeanor and not to the treason, the court is not apprized that a knowledge and concealment of the misdemeanor would expose the witness to any prosecution whatever. On this account the court was, at first, disposed to inquire whether the letter could be deciphered, in order to determine from its contents how far the witness could be examined respecting it. The court was inclined to this course from considering the question as one which might require a disclosure of the knowledge which the witness might have had of the contents of this letter when it was put in cipher, or when it was copied by himself; if, indeed, such were the fact. But, on hearing the question more particularly and precisely stated, and finding that it refers only to the present knowledge of the cipher, it appears to the court that the question may be answered without implicating the witness, because his present knowledge would not, it is believed, in a criminal prosecution, justify the inference that his knowledge was acquired previous to this trial, or afford the means of proving that fact.

The court is, therefore, of opinion that the witness may answer the question now propounded.