/// <summary>
/// Unlocks (i.e. decrypts) the archive with the given <see cref="UserKey"/>.
/// </summary>
/// <param name="userKey">The <see cref="UserKey"/> to use to unlock the archive.</param>
public void Unlock(UserKey userKey)
{
ArgCheck.NotNull(userKey, nameof(userKey));
// Setting the ArchiveKey property puts the archive into the "unlocked" state.
// Wait to set the property until after everything is properly unlocked.
var archiveKey = this.DecryptArchiveKey(userKey);
this.FileIndex = this.ArchiveMetadata.EncryptedFileIndex is null || this.ArchiveMetadata.EncryptedFileIndex.IsEmpty()
? new FileIndex()
: FileIndex.DecryptFrom(this.ArchiveMetadata.EncryptedFileIndex, archiveKey, this.ArchiveMetadata.SecuritySettings);
this.ArchiveKey = archiveKey;
}