session-ios/SignalServiceKit/src/Messages/OWSOutgoingNullMessage.m

//
//  Copyright (c) 2018 Open Whisper Systems. All rights reserved.
//

#import "OWSOutgoingNullMessage.h"
#import "Cryptography.h"
#import "NSDate+OWS.h"
#import "OWSSignalServiceProtos.pb.h"
#import "OWSVerificationStateSyncMessage.h"
#import "TSContactThread.h"

NS_ASSUME_NONNULL_BEGIN

@interface OWSOutgoingNullMessage ()

@property (nonatomic, readonly) OWSVerificationStateSyncMessage *verificationStateSyncMessage;

@end

#pragma mark -

@implementation OWSOutgoingNullMessage

- (instancetype)initWithContactThread:(TSContactThread *)contactThread
         verificationStateSyncMessage:(OWSVerificationStateSyncMessage *)verificationStateSyncMessage
{
    self = [super initOutgoingMessageWithTimestamp:[NSDate ows_millisecondTimeStamp]
                                          inThread:contactThread
                                       messageBody:nil
                                     attachmentIds:[NSMutableArray new]
                                  expiresInSeconds:0
                                   expireStartedAt:0
                                    isVoiceMessage:NO
                                  groupMetaMessage:TSGroupMessageUnspecified
                                     quotedMessage:nil];
    if (!self) {
        return self;
    }
    
    _verificationStateSyncMessage = verificationStateSyncMessage;
    
    return self;
}

#pragma mark - override TSOutgoingMessage

- (NSData *)buildPlainTextData:(SignalRecipient *)recipient
{
    OWSSignalServiceProtosContentBuilder *contentBuilder = [OWSSignalServiceProtosContentBuilder new];
    OWSSignalServiceProtosNullMessageBuilder *nullMessageBuilder = [OWSSignalServiceProtosNullMessageBuilder new];

    NSUInteger contentLength = self.verificationStateSyncMessage.unpaddedVerifiedLength;

    OWSAssert(self.verificationStateSyncMessage.paddingBytesLength > 0);

    // We add the same amount of padding in the VerificationStateSync message and it's coresponding NullMessage so that
    // the sync message is indistinguishable from an outgoing Sent transcript corresponding to the NullMessage. We pad
    // the NullMessage so as to obscure it's content. The sync message (like all sync messages) will be *additionally*
    // padded by the superclass while being sent. The end result is we send a NullMessage of a non-distinct size, and a
    // verification sync which is ~1-512 bytes larger then that.
    contentLength += self.verificationStateSyncMessage.paddingBytesLength;

    OWSAssert(contentLength > 0);

    nullMessageBuilder.padding = [Cryptography generateRandomBytes:contentLength];
    
    contentBuilder.nullMessage = [nullMessageBuilder build];

    return [contentBuilder build].data;
}

- (BOOL)shouldSyncTranscript
{
    return NO;
}

- (BOOL)shouldBeSaved
{
    return NO;
}

@end

NS_ASSUME_NONNULL_END