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?
答案 0 :(得分:3)
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。