Question

In my application, i need to encrypt the file before sending it over the network and on the other end, it will get decrypt, This is my code,

-(void)doEncryptTest:(NSString *)pFileName{

    // read the NSData; 
    NSStringEncoding encoding  =NSUTF8StringEncoding;

    NSString *pFileContent = @"xaaaaaaxxaaaaaax";

    NSString *pKey = @"01234567012345670123456701234567";


    NSData *pData = [pFileContent dataUsingEncoding:encoding];

    NSData *pEncryptedData = [pData AES256EncryptWithKey:pKey];


    NSData *decrypted=[pEncryptedData AES256DecryptWithKey:pKey Data:pEncryptedData];

    NSString* pDecryptedDataStr = [[NSString alloc] initWithData:decrypted
                                                      encoding:encoding];

}

This is working fine , only and only if Data size is 16 byte, in real time case, when i sent the file of size 151186 bytes, size of [pEncryptedData] is 15200 , and in fact size of decrypted data is same as original data,
But pDecryptedDataStr is blank, any guess what is going wrong,
please refer below, Encryption and decryption function,

int keySize = kCCKeySizeAES256;
int padding = kCCOptionPKCS7Padding;
char ivKey[16]={0,0,0,0,
             0,0,0,0,
             0,0,0,0,
             0,0,0,0};
//////////////*Encryption*///////////////////
- (NSData *)AES256EncryptWithKey:(NSString *)key{
    // 'key' should be 32 bytes for AES256, will be null-padded otherwise
    char keyPtr[keySize + 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 );
    char ivVector[kCCBlockSizeAES128+1];
    // fetch key data
    [key getCString:ivVector maxLength:sizeof( ivVector ) encoding:NSUTF8StringEncoding];

    bzero( ivVector, sizeof( ivVector ) ); // fill with zeroes (for padding)


    const void *iv=NULL;
    size_t numBytesEncrypted = 0;


    CCCryptorStatus cryptStatus = CCCrypt( kCCEncrypt, kCCAlgorithmAES128, padding,
                                          keyPtr, keySize,
                                          ivKey /* initialization vector (optional) */,
                                          [self bytes], dataLength, /* input */
                                          buffer, bufferSize, /* output */
                                          &numBytesEncrypted );
    if( cryptStatus == kCCSuccess )
    {
        //the returned NSData takes ownership of the buffer and will free it on deallocation
        return [NSData dataWithBytesNoCopy:buffer length:numBytesEncrypted];
    }

    free( buffer ); //free the buffer
    return nil;
}

- (NSData *)AES256DecryptWithKey:(NSString *)key   Data:(NSData*)EncryptedData{
    bool same =[self isEqualToData:EncryptedData];

    // 'key' should be 32 bytes for AES256, will be null-padded otherwise
    char keyPtr[keySize+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 = [EncryptedData 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 );
    const void *iv=NULL;
    char ivVector[kCCBlockSizeAES128+1];
    // fetch key data
    [key getCString:ivVector maxLength:sizeof( ivVector ) encoding:NSUTF8StringEncoding];

    bzero( ivVector, sizeof( ivVector ) ); // fill with zeroes (for padding)

    size_t numBytesDecrypted = 0;
    NSData *output_decrypt = [[NSData alloc] init];
    CCCryptorStatus cryptStatus = CCCrypt( kCCDecrypt, kCCAlgorithmAES128, padding,
                                          keyPtr, keySize,
                                          ivKey /* initialization vector (optional) */,
                                          [EncryptedData bytes], dataLength, /* input */
                                          buffer,bufferSize ,//bufferSize, /* output */
                                          &numBytesDecrypted );
    output_decrypt = [NSMutableData dataWithBytesNoCopy:buffer length:numBytesDecrypted];
    same =[self isEqualToData:output_decrypt];
    if( cryptStatus == kCCSuccess )
    {
        //the returned NSData takes ownership of the buffer and will free it on deallocation

        NSData *pData = [[NSData alloc]initWithBytes:buffer length:numBytesDecrypted];

        return pData;

    }

    free( buffer ); //free the buffer
    return nil;
}

Answer 1

Thanks for looking at this, this was the problem,

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

        **NSData *pData = [[NSData alloc]initWithBytes:buffer length:numBytesDecrypted];**

        return pData;

    }

and solutions was;

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


        return output_decrypt;

    }

Not sure, why it was causing problem, of-course i need to change return type and do some handling in the calling function to release the memory.