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Five years ago, in a contribution to these pages, I suggested that the Supreme Court's oldest precedents and the original intent of the framers of the Constitution precluded the use of evidence produced under a grant of immunity against the producer, even though the material produced included documents that the producer had not been compelled to write. This implied that information concealed with a cryptographic key could not be used in a criminal prosecution against someone from whom the key had been obtained under a grant of immunity.

The issue, however, was doubtful given the tendency of the Court to confine criminal protections and uncontradicted arguments from a dissent in then-recent Supreme Court cases suggesting that the act of producing documents did not confer derivative use immunity as to the contents of the documents. As a result, some thought that the claim of immunity could not be extended to documents produced under a grant of immunity.

Recently Kenneth Starr, the Independent Counsel investigating the "Whitewater" land dealings, sought to use information derived from material produced under a grant of immunity to prosecute Webster Hubbell, the former United States Associate Attorney General and friend of President Clinton. In United States v. Hubbell, decided last Term, the United States Supreme Court reached the issue and held that use immunity under the Fifth Amendment to the United States Constitution required protection of the producer of the information from a prosecution based on the information produced. Chief Justice Rehnquist dissented on the basis of the dissenting judge's decision for the Court of Appeals, which also barred the use of the information in the documents. Justice Thomas, joined by Justice Scalia, concurred with the Court's majority opinion, but suggested that interpreting "witness" in the Fifth Amendment according to its original intent requires a broader protection than that required by the Court.

Starr's action, although it did not seek encrypted information, was consistent with the general practice of the federal government in seeking information on computers. Thus, the Hubbell decision is important not only for ordinary requests of production, but also for requests of production that require the target of the subpoena to produce a cryptographic key.

This article contains three parts. First, it briefly reviews the Hubbell decision, contrasting the decision with alternative approaches that the Court has now implicitly rejected. Second, it discusses the implications of the majority's opinion for information on computers. In particular, the application of the Hubbell decision to encrypted information is likely to lead to several issues not present in applying the decision to tangible documents. This is because encrypted information will have been transmitted often. Multiple transmission creates the possibility of requiring one person to produce evidence against another and the possibility that information will be transmitted in encrypted form between jurisdictions, at least one of which may not recognize the Hubbell decision. Interestingly, the choice of public-key encryption has significant implications in both areas.

Finally, it discusses the implications of Justice Thomas' use of original intent. Although the focus of the Thomas opinion is on the definition of "witness," the use of a historical analysis may lead to a considerably broader protection for private documents. [excerpt]