Question

我有一个使用AES加密的文件。我使用以下NSData类别：

#import <CommonCrypto/CommonCryptor.h>

@implementation NSData (AES)

- (NSData *)AES256DecryptWithKey:(NSString *)key {

    // 'key' should be 32 bytes for AES256, will be null-padded otherwise
    char keyPtr[kCCKeySizeAES256+1]; // room for terminator (unused)
    bzero(keyPtr, sizeof(keyPtr)); // fill with zeroes (for padding)

    // fetch key data
    [key getCString:keyPtr maxLength:sizeof(keyPtr) encoding:NSUTF8StringEncoding];

    NSUInteger dataLength = [self length];

    //See the doc: For block ciphers, the output size will always be less than or 
    //equal to the input size plus the size of one block.
    //That's why we need to add the size of one block here
    size_t bufferSize = dataLength + kCCBlockSizeAES128;

    void *buffer = malloc(bufferSize);

    size_t numBytesDecrypted = 0;
    CCCryptorStatus cryptStatus = CCCrypt(kCCDecrypt,
                                          kCCAlgorithmAES128,
                                          kCCOptionPKCS7Padding,
                                          keyPtr,
                                          kCCKeySizeAES256,
                                          NULL /* initialization vector (optional) */,
                                          [self bytes], dataLength, /* input */
                                          buffer,       bufferSize, /* output */
                                          &numBytesDecrypted);

    NSLog(@"Bytes decrypted: %d",numBytesDecrypted);

    if (cryptStatus == kCCSuccess) {
        //the returned NSData takes ownership of the buffer and will free it on deallocation
        return [NSData dataWithBytesNoCopy:buffer length:numBytesDecrypted];
    }

    NSLog(@"Decrypt failed with error code %d",cryptStatus);
    free(buffer); //free the buffer;
    return nil;
}

@end

当我使用以下代码从文件系统加载整个文件时，descrypt进程似乎工作正常：

[NSData dataWithContentsOfFile:dataPath];

当前一次调用没有读取文件时出现问题，但是当外部代码将文件分块并仅使用少量数据初始化NSData并尝试解密时，特别是当不同的线程出现问题时使用此代码（或至少是我的想法）：

- (NSData *)readDataOfLength:(NSUInteger)length
{
    HTTPLogTrace2(@"%@[%p]: readDataOfLength:%lu", THIS_FILE, self, (unsigned long)length);

    if (![self openFileIfNeeded])
    {
        // File opening failed,
        // or response has been aborted due to another error.
        return nil;
    }

    // Determine how much data we should read.
    // 
    // It is OK if we ask to read more bytes than exist in the file.
    // It is NOT OK to over-allocate the buffer.

    UInt64 bytesLeftInFile = fileLength - fileOffset;

    NSUInteger bytesToRead = (NSUInteger)MIN(length, bytesLeftInFile);

    // Make sure buffer is big enough for read request.
    // Do not over-allocate.

    if (buffer == NULL || bufferSize < bytesToRead)
    {
        bufferSize = bytesToRead;
        buffer = reallocf(buffer, (size_t)bufferSize);

        if (buffer == NULL)
        {
            HTTPLogError(@"%@[%p]: Unable to allocate buffer", THIS_FILE, self);

            [self abort];
            return nil;
        }
    }

    // Perform the read

    HTTPLogVerbose(@"%@[%p]: Attempting to read %lu bytes from file", THIS_FILE, self, bytesToRead);

    ssize_t result = read(fileFD, buffer, bytesToRead);

    // Check the results

    if (result < 0)
    {
        HTTPLogError(@"%@: Error(%i) reading file(%@)", THIS_FILE, errno, filePath);

        [self abort];
        return nil;
    }
    else if (result == 0)
    {
        HTTPLogError(@"%@: Read EOF on file(%@)", THIS_FILE, filePath);

        [self abort];
        return nil;
    }
    else // (result > 0)
    {
        HTTPLogVerbose(@"%@[%p]: Read %d bytes from file", THIS_FILE, self, result);

        fileOffset += result;

        NSData *data = [NSData dataWithBytes:buffer length:result];
        return [data AES256DecryptWithKey:@"abcdefghijklmnopqrstuvwxyz123456"];
        //return data;
    }
}

在这种情况下，CCCrypt函数失败，错误代码为-4304 AKA“kCCDecodeError - 输入数据未正确解码或解密。”

此外，如果在CCCrypt调用而不是kCCOptionPKCS7Padding，我传递0 - ＆gt;没有填充方法解密第一块数据，但是当线程切换时，它失败了-4300 AKA“kCCParamError - 非法参数值。”

在控制台中显示以下消息：

[Switching to process 13059 thread 0x0]
2011-05-25 18:00:03.631 Drm[1843:6e0b] Bytes decrypted: 131072
2011-05-25 18:00:03.647 Drm[1843:6e0b] Bytes decrypted: 68096
[Switching to process 11779 thread 0x0]
2011-05-25 18:00:04.547 Drm[1843:6e0b] Bytes decrypted: 0
2011-05-25 18:00:04.555 Drm[1843:6e0b] Decrypt failed with error code -4300

有人可以帮忙吗？

Answer 1

AES是一种分组密码。您必须一次解密一个块。 AES块是128位（这与AES256DecryptWithKey中的“256”无关）。因此，您必须确保传递的数据是16个字节的倍数。

我没有试过以这种方式使用CCCrypt()，这不是真正的用途。当您想要进行一次性解密时，CCCrypt()是一项便利功能。如果您想“随时”解密，请使用CCCryptorCreate()，然后多次调用CCCryptorUpdate()，最后CCCryptorFinal()（或者您可以调用CCCryptorFinal()，然后调用{{ 1}}使用相同的密钥解密更多的东西）。最后，请致电CCCryptorReset()以释放您的密码。

编辑我正在考虑这个问题，并意识到即使将输入分解为16字节的块，CCCryptorRelease()也不能以这种方式使用。每个AES加密块都会修改下一个块的IV，因此您不能只在流的中间启动某个人。这就是为什么在整个会话中需要一个持久的CCCrypt()对象。

在多线程环境中使用CCCrypt的NSData

1 个答案: