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diff --git a/browser/components/shell/WindowsUserChoice.h b/browser/components/shell/WindowsUserChoice.h
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+++ b/browser/components/shell/WindowsUserChoice.h
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+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef SHELL_WINDOWSUSERCHOICE_H__
+#define SHELL_WINDOWSUSERCHOICE_H__
+
+#include <windows.h>
+
+#include "mozilla/UniquePtr.h"
+
+/*
+ * Check the UserChoice Hashes for https, http, .html, .htm
+ *
+ * This should be checked before attempting to set a new default browser via
+ * the UserChoice key, to confirm our understanding of the existing hash.
+ * If an incorrect hash is written, Windows will prompt the user to choose a
+ * new default (or, in recent versions, it will just reset the default to Edge).
+ *
+ * Assuming that the existing hash value is correct (since Windows is fairly
+ * diligent about replacing bad keys), if we can recompute it from scratch,
+ * then we should be able to compute a correct hash for our new UserChoice key.
+ *
+ * @return true if we matched all the hashes, false otherwise.
+ */
+bool CheckBrowserUserChoiceHashes();
+
+/*
+ * Result from CheckUserChoiceHash()
+ *
+ * NOTE: Currently the only positive result is OK_V1 , but the enum
+ * could be extended to indicate different versions of the hash.
+ */
+enum class CheckUserChoiceHashResult {
+  OK_V1,         // Matched the current version of the hash (as of Win10 20H2).
+  ERR_MISMATCH,  // The hash did not match.
+  ERR_OTHER,     // Error reading or generating the hash.
+};
+
+/*
+ * Generate a UserChoice Hash, compare it with the one that is stored.
+ *
+ * See comments on CheckBrowserUserChoiceHashes(), which calls this to check
+ * each of the browser associations.
+ *
+ * @param aExt      File extension or protocol association to check
+ * @param aUserSid  String SID of the current user
+ *
+ * @return Result of the check, see CheckUserChoiceHashResult
+ */
+CheckUserChoiceHashResult CheckUserChoiceHash(const wchar_t* aExt,
+                                              const wchar_t* aUserSid);
+
+/*
+ * Get the registry path for the given association, file extension or protocol.
+ *
+ * @return The path, or nullptr on failure.
+ */
+mozilla::UniquePtr<wchar_t[]> GetAssociationKeyPath(const wchar_t* aExt);
+
+/*
+ * Get the current user's SID
+ *
+ * @return String SID for the user of the current process, nullptr on failure.
+ */
+mozilla::UniquePtr<wchar_t[]> GetCurrentUserStringSid();
+
+/*
+ * Generate the UserChoice Hash
+ *
+ * @param aExt          file extension or protocol being registered
+ * @param aUserSid      string SID of the current user
+ * @param aProgId       ProgId to associate with aExt
+ * @param aTimestamp    approximate write time of the UserChoice key (within
+ *                      the same minute)
+ *
+ * @return UserChoice Hash, nullptr on failure.
+ */
+mozilla::UniquePtr<wchar_t[]> GenerateUserChoiceHash(const wchar_t* aExt,
+                                                     const wchar_t* aUserSid,
+                                                     const wchar_t* aProgId,
+                                                     SYSTEMTIME aTimestamp);
+
+/*
+ * Build a ProgID from a base and AUMI
+ *
+ * @param aProgIDBase   A base, such as FirefoxHTML or FirefoxURL
+ * @param aAumi         The AUMI of the installation
+ *
+ * @return Formatted ProgID.
+ */
+mozilla::UniquePtr<wchar_t[]> FormatProgID(const wchar_t* aProgIDBase,
+                                           const wchar_t* aAumi);
+
+/*
+ * Check that the given ProgID exists in HKCR
+ *
+ * @return true if it could be opened for reading, false otherwise.
+ */
+bool CheckProgIDExists(const wchar_t* aProgID);
+
+#endif  // SHELL_WINDOWSUSERCHOICE_H__