In Roviaro v. United States, 353 U.S. 53 (1957), the United States Supreme Court recognized the prosecution's privilege to refuse to disclose the identity of a confidential informant, but imposed various limitations on that privilege to ensure that its application comported with "the fundamental requirements of fairness." (Id. at p. 60.) "Where the disclosure of an informer's identity, or of the contents of his communication, is relevant and helpful to the defense of an accused, or is essential to a fair determination of a cause, the privilege must give way." (Id. at pp. 60-61.) The nature of that determination precluded the establishment of a "fixed rule with respect to disclosure." (Id. at p. 62.) Instead, the Roviaro court favored an approach that balanced the "public interest in protecting the flow of information against the individual's right to prepare his defense. Whether a proper balance renders nondisclosure erroneous must depend on the particular circumstances of each case, taking into consideration the crime charged, the possible defenses, the possible significance of the informer's testimony, and other relevant factors." (Ibid.) The United States Supreme Court recognized the unfairness that can result to a defendant's case with an unlimited informer's privilege. In Roviaro, the confidential informant was the only other party to an illegal drug transaction with the defendant and thus was the only witness who could possibly testify on behalf of the defendant or contradict the testimony of the government witnesses. Id. at 64. The Supreme Court therefore concluded that, under the circumstances, the trial court erred by not compelling disclosure of the informant. Id. at 65. The United States Supreme Court had occasion to define the nature and scope of the informant's privilege. What is usually referred to as the informer's privilege is in reality the Government's privilege to withhold from disclosure the identity of persons who furnish information of violations of law to officers charged with enforcement of that law. . . . The purpose of the privilege is the furtherance and protection of the public interest in effective law enforcement. The privilege recognizes the obligation of citizens to communicate their knowledge of the commission of crimes to law-enforcement officials and, by preserving their anonymity, encourages them to perform that obligation. . . . Roviaro established a test for assessing challenges to the applicability of the informant's privilege. This test involves the balancing of two competing interests: (1) the preservation of the underlying purpose of the privilege; and (2) the fundamental requirements of fairness. . . . The underlying purpose of the privilege is to protect the public interest in the flow of information to law enforcement officials. The fundamental requirements of fairness comprise the defendant's right to a fair trial, including the right to obtain information relevant and helpful to a defense. . . . Whether a proper balance renders nondisclosure erroneous must depend on the particular circumstances of each case, taking into consideration the crime charged, the possible defenses, the possible significance of the informer's testimony, and other relevant factors.