Informer Privilege. R v Cook
In R v Cook (Ont CA, 2014) the Court of Appeal took the oppourtunity to make the following salutory comments on the nature of 'informer' privilege, which - if applicable - rendered some evidence inadmissible. The issue relates mostly to criminal law:
Analysis. R v Basi
 In my view, the identity of the author [SS: of an email] is not protected by either informer or public interest privilege. There is therefore no need to determine whether the “innocence at stake” exception is engaged.
 I turn first to the issue of informer privilege. It was submitted that the law is not clear on this point. The leading cases analyze informer privilege on the basis that some promise of confidentiality express or implied is necessary. However, in R. v. Barros, 2011 SCC 51 (CanLII), 2011 SCC 51,  3 S.C.R. 368, Binnie J. seems to have left open the question when he stated, at para. 32: “[i]t might be argued that in a situation of serious potential danger, the informer privilege (or other public interest privilege) might apply even in the absence of the contract-type elements of offer and acceptance.”
 In my view, this matter was settled in R. v. Named Person B, 2013 SCC 9 (CanLII), 2013 SCC 9,  1 S.C.R. 405, at para. 18, when Abella J. confirmed that there must be some conduct on the part of the police from which a promise of confidentiality could be inferred, either expressly or implicitly:
The legal question is whether, objectively, an implicit promise of confidentiality can be inferred from the circumstances. In other words, would the police conduct have led a person in the shoes of the potential informer to believe, on reasonable grounds, that his or her identity would be protected? In this case there was no conduct on the part of the police, express or implied, that could have led the author to believe that his or her identity would be protected. The police merely received an unsolicited anonymous email. The test for informer privilege is not satisfied on the facts of this case. (This is unlike a “crime stoppers” communication which is founded on a promise of anonymity.)
In R v Basi (SCC, 2009) the Supreme Court of Canada re-states the accepted law of informer privilege:
 Before turning to that issue, it will be helpful to consider generally the purpose, scope and operation of the informer privilege and the governing principles set out by Bastarache J. in Named Person v. Vancouver Sun, 2007 SCC 43,  3 S.C.R. 253.
 The privilege arises where a police officer, in the course of an investigation, guarantees protection and confidentiality to a prospective informer in exchange for useful information that would otherwise be difficult or impossible to obtain. In appropriate circumstances, a bargain of this sort has long been accepted as an indispensable tool in the detection, prevention and prosecution of crime.
 The informer privilege has been described as “nearly absolute”. As mentioned earlier, it is safeguarded by a protective veil that will be lifted by judicial order only when the innocence of the accused is demonstrably at stake. Moreover, while a court can adopt discretionary measures to protect the identity of the informer, the privilege itself is “a matter beyond the discretion of a trial judge” (Named Person, at para. 19).
 Whenever informer privilege is claimed, or the court of its own motion considers that the privilege appears to arise, its existence must be determined by the court in camera at a “first stage” hearing. Even the existence of the claim cannot be publicly disclosed. Ordinarily, only the putative informant and the Crown may appear before the judge. In Named Person, however, the Court considered that an amicus curiae may be necessary or appropriate, particularly where the interests of the informant and the Crown are aligned: Named Person, at para. 48.
 In determining whether the privilege exists, the judge must be satisfied, on a balance of probabilities, that the individual concerned is indeed a confidential informant. And if the claim of privilege is established, the judge must give it full effect. As we have seen, Named Person established that trial judges have no discretion to do otherwise.
 Finally, the informer privilege belongs jointly to the Crown and to the informant. Neither can waive it without the consent of the other.