How to decrypt a file in Objective C/IOS that is encrypted in php?

问题

I have googled too much for this error but found nothing useful. I am getting a file that is encrypted in php using the following code:

mcrypt_encrypt(MCRYPT_RIJNDAEL_128, $privateencryptkey, base64_encode(file), MCRYPT_MODE_CBC, $hardvector);

I am unable to decrypt it in IOS. I had tried many libraries like NSDATA+CommonCrypto, NSFileManager-AES, NSDATA-aes but i have not got success in decrypting the file.

Following is the objective-C code used:

- (NSData *)AESDecryptWithPassphrase:(NSString *)pass
{
    NSMutableData *ret = [NSMutableData dataWithCapacity:[self length]];
    unsigned long rk[RKLENGTH(KEYBITS)];
    unsigned char key[KEYLENGTH(KEYBITS)];
    const char *password = [pass UTF8String];
    for (int i = 0; i < sizeof(key); i++)
        key[i] = password != 0 ? *password++ : 0;

    int nrounds = rijndaelSetupDecrypt(rk, key, KEYBITS);
    unsigned char *srcBytes = (unsigned char *)[self bytes];
    int index = 0;
    while (index < [self length])
    {
        unsigned char plaintext[16];
        unsigned char ciphertext[16];
        int j;
        for (j = 0; j < sizeof(ciphertext); j++)
        {
            if (index >= [self length])
                break;

            ciphertext[j] = srcBytes[index++];
        }
        rijndaelDecrypt(rk, nrounds, ciphertext, plaintext);
        [ret appendBytes:plaintext length:sizeof(plaintext)];

    }
    return ret;
}

This code works well for text but unable to decrypt files. When i save the decrypted files then it says the file system error. Those decrypted files cannot be opened on any system, i think the file format is disturbed in the process.

I also tried the following code but no success:

- (NSData *) decryptedDataUsingAlgorithm: (CCAlgorithm) algorithm
                                     key: (id) key      // data or string
                    initializationVector: (id) iv       // data or string
                                 options: (CCOptions) options
                                   error: (CCCryptorStatus *) error
{
    CCCryptorRef cryptor = NULL;
    CCCryptorStatus status = kCCSuccess;

    NSParameterAssert([key isKindOfClass: [NSData class]] || [key isKindOfClass: [NSString class]]);
    NSParameterAssert(iv == nil || [iv isKindOfClass: [NSData class]] || [iv isKindOfClass: [NSString class]]);

    NSMutableData * keyData, * ivData;
    if ( [key isKindOfClass: [NSData class]] )
        keyData = (NSMutableData *) [key mutableCopy];
    else
        keyData = [[key dataUsingEncoding: NSUTF8StringEncoding] mutableCopy];

    if ( [iv isKindOfClass: [NSString class]] )
        ivData = [[iv dataUsingEncoding: NSUTF8StringEncoding] mutableCopy];
    else
        ivData = (NSMutableData *) [iv mutableCopy];    // data or nil

    [keyData autorelease];
    [ivData autorelease];

    // ensure correct lengths for key and iv data, based on algorithms
    FixKeyLengths( algorithm, keyData, ivData );

    status = CCCryptorCreate( kCCDecrypt, algorithm, options,
                              [keyData bytes], [keyData length], [ivData bytes],
                              &cryptor );

    if ( status != kCCSuccess )
    {
        if ( error != NULL )
            *error = status;
        return ( nil );
    }

    NSData * result = [self _runCryptor: cryptor result: &status];
    if ( (result == nil) && (error != NULL) )
        *error = status;

    CCCryptorRelease( cryptor );

    return ( result );
}

2nd function from above code:

- (NSData *) _runCryptor: (CCCryptorRef) cryptor result: (CCCryptorStatus *) status
{
    size_t bufsize = CCCryptorGetOutputLength( cryptor, (size_t)[self length], true );
    void * buf = malloc( bufsize );
    size_t bufused = 0;
    size_t bytesTotal = 0;
    *status = CCCryptorUpdate( cryptor, [self bytes], (size_t)[self length], 
                              buf, bufsize, &bufused );
    if ( *status != kCCSuccess )
    {
        free( buf );
        return ( nil );
    }

    bytesTotal += bufused;

    // From Brent Royal-Gordon (Twitter: architechies):
    //  Need to update buf ptr past used bytes when calling CCCryptorFinal()
    *status = CCCryptorFinal( cryptor, buf + bufused, bufsize - bufused, &bufused );
    if ( *status != kCCSuccess )
    {
        free( buf );
        return ( nil );
    }

    bytesTotal += bufused;

    return ( [NSData dataWithBytesNoCopy: buf length: bytesTotal] );
}

I haven't been able to solve this for a week...

回答1:

One thing to note is the file parameter to mcrypt_encrypt, it appears that the file is being base64 encoded prior to encryption (not that it makes any sense), that would imply you would have to base64 decode after decryption.

The other parameters are straight forward:
MCRYPT_RIJNDAEL_128 is AES, 128 with a 128 bit key
MCRYPT_MODE_CBC is cbc mode, the default for CommonCrypto.
The padding to block size is with null characters, rather nonstandard so the non-padded length may be a problem.

Not that you need yet another AES method, this is the one I use:

#import <CommonCrypto/CommonCryptor.h>

+ (NSData *)doCipher:(NSData *)dataIn
                  iv:(NSData *)iv
                 key:(NSData *)symmetricKey
             context:(CCOperation)encryptOrDecrypt
{
    CCCryptorStatus ccStatus   = kCCSuccess;
    size_t          cryptBytes = 0;    // Number of bytes moved to buffer.
    NSMutableData  *dataOut    = [NSMutableData dataWithLength:dataIn.length + kCCBlockSizeAES128];

    ccStatus = CCCrypt( encryptOrDecrypt,
                       kCCAlgorithmAES128,
                       kCCOptionPKCS7Padding,
                       symmetricKey.bytes, 
                       kCCKeySizeAES128,
                       iv.bytes,
                       dataIn.bytes,
                       dataIn.length,
                       dataOut.mutableBytes,
                       dataOut.length,
                       &cryptBytes);

    if (ccStatus != kCCSuccess) {
        NSLog(@"CCCrypt status: %d", ccStatus);
    }

    dataOut.length = cryptBytes;

    return dataOut;
}
// Also add Security.framework to your project.

Note that it expects NSData input and the padding is specified as standard PKCS.

See CommonCryptor.h

来源：https://stackoverflow.com/questions/10078026/how-to-decrypt-a-file-in-objective-c-ios-that-is-encrypted-in-php