RNDecryptor addData方法中的混淆

Question

- (IBAction)decryptWithSemaphore:(id)sender {
dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);

__block int total = 0;
int blockSize = 32 * 1024;

NSArray *docPaths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *input = [[docPaths objectAtIndex:0] stringByAppendingPathComponent:@"zhuge.rncryptor"];
NSString *output = [[docPaths objectAtIndex:0] stringByAppendingPathComponent:@"zhuge.decrypted.pdf"];

NSInputStream *cryptedStream = [NSInputStream inputStreamWithFileAtPath:input];
__block NSOutputStream *decryptedStream = [NSOutputStream outputStreamToFileAtPath:output append:NO];
__block NSError *decryptionError = nil;

[cryptedStream open];
[decryptedStream open];

RNDecryptor *decryptor = [[RNDecryptor alloc] initWithPassword:@"12345678901234567890123456789012" handler:^(RNCryptor *cryptor, NSData *data) {
    @autoreleasepool {
        NSLog(@"Decryptor recevied %d bytes", data.length);
        [decryptedStream write:data.bytes maxLength:data.length];
        dispatch_semaphore_signal(semaphore);

        data = nil;
        if (cryptor.isFinished) {
            [decryptedStream close];
            decryptionError = cryptor.error;
            // call my delegate that I'm finished with decrypting
        }
    }
}];

while (cryptedStream.hasBytesAvailable) {
    @autoreleasepool {
        uint8_t buf[blockSize];
        NSUInteger bytesRead = [cryptedStream read:buf maxLength:blockSize];
        if (bytesRead > 0) {
            NSData *data = [NSData dataWithBytes:buf length:bytesRead];

            total = total + bytesRead;
            [decryptor addData:data];
            NSLog(@"New bytes to decryptor: %d Total: %d", bytesRead, total);

            dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
        }
    }
}

[cryptedStream close];
[decryptor finish];

dispatch_release(semaphore);

而RNDecryptor的addData方法如下：

- (void)addData:(NSData *)theData
{
  if (self.isFinished) {
    return;
  }

  [self.inData appendData:theData];
  if (!self.engine) {
    [self consumeHeaderFromData:self.inData];
  }
  if (self.engine) {
    NSUInteger HMACLength = self.HMACLength;
    if (self.inData.length > HMACLength) {
      NSData *data = [self.inData _RNConsumeToIndex:self.inData.length - HMACLength];
      [self decryptData:data];
    }
  }
}

我不明白这一行实际上是要做什么，它对加密流的每一个块都调用：

  NSData *data = [self.inData _RNConsumeToIndex:self.inData.length - HMACLength];

假设我有一个1000字节的块，而HMACLength是32。

提前道谢。

Answer 1

问题是流通常不知道剩下多少数据。在您的情况下，似乎验证标记（HMAC值）被发送密文的一方放在了密文的末尾。

现在的问题是，您应该只更新数据，而不是身份验证标记。由于您不知道还有多少数据可用，可能是您已经在读入末尾的身份验证标记。显然，如果在计算中包含HMAC本身的输出，HMAC计算将失败。

所以基本上您读取流并更新HMAC状态直到结束。然后执行最后一次HMAC更新,直到密文结束。从流的末尾提取给定的身份验证标记，然后比较计算值和给定值。如果它们是相同的，那么将检查密文（以及明文）的完整性和身份验证--当然，假定密钥从未泄露给攻击者。