Difference between PKCS1-padding/RSA encryption ios objc and java

Question

I’m developing a application for ios and Android. I’m relatively new to crypto tasks and for the last 3 Days I keep banging my head against the wall because I’m not able to get RSA encryption running.

Both clients receive a public key from a java server. In android i have (obviously, because it is almost the same code as on server side) no troubles, but the ios part seems to be not compatible at all. I want to encrypt a little piece of data (aes key) with the public key and this is how I do this in Java:

try {
    String publickey  = "MCwwDQYJKoZIhvcNAQEBBQADGwAwGAIRAK+dBpbOKw+1VKMWoFxjU6UCAwEAAQ==";
    byte[] bArr = Crypto.base64Decode(publicKey, false);
    KeyFactory keyFactory = KeyFactory.getInstance("RSA", "BC");
    EncodedKeySpec publicKeySpec = new X509EncodedKeySpec(publicKey);
    PublicKey publicKey = keyFactory.generatePublic(publicKeySpec);

    Cipher cipher = Cipher.getInstance("RSA/NONE/PKCS1Padding", "BC");
    cipher.init(1,publicKey);
    int cipherBlockSize = cipher.getBlockSize();
    ByteArrayOutputStream bArrOut = new ByteArrayOutputStream();
    bArrOut.flush();
    int pos = 0;
    Log.i("ContentBufferLength", contentBuffer.length+"");

    while (true) {
        if (cipherBlockSize > contentBuffer.length - pos) {
            cipherBlockSize = contentBuffer.length - pos;
        }
        Log.i("CipherBlockSize", cipherBlockSize+"");
        byte[] tmp = cipher.doFinal(contentBuffer, pos, cipherBlockSize);
        bArrOut.write(tmp);
        pos += cipherBlockSize;
        if (contentBuffer.length <= pos) {
            break;
        }
    }
    bArrOut.flush();
    encryptedBuffer = bArrOut.toByteArray();
    bArrOut.close();
} catch (Exception ex) {
    throw ex;
}

//  Log.i("Encrypted Buffer Length", encryptedBuffer.length+"");
return encryptedBuffer;

And this is my (not properly working) ios code, borrowed from here:

http://blog.wingsofhermes.org/?p=75 and the apple crypto exercises.

-(NSString* )encryptWithPublicKey:(NSString*)key input:(NSString*) input {
    const size_t BUFFER_SIZE =      16;
    const size_t CIPHER_BUFFER_SIZE = 16;
   //const uint32_t PADDING = kSecPaddingNone;
    const uint32_t PADDING = kSecPaddingPKCS1;

    static const UInt8 publicKeyIdentifier[] = "de.irgendwas.app";

    NSData *publicTag;

    publicTag = [[NSData alloc] initWithBytes:publicKeyIdentifier length:sizeof(publicKeyIdentifier)];

    NSMutableDictionary *publicKey2 = [[NSMutableDictionary alloc] init];
    [publicKey2 setObject:kSecClassKey forKey:kSecClass];
    [publicKey2 setObject:kSecAttrKeyTypeRSA forKey:kSecAttrKeyType];
    [publicKey2 setObject:publicTag forKey:kSecAttrApplicationTag];
    SecItemDelete((CFDictionaryRef)publicKey2);


    NSData *strippedPublicKeyData = [NSData dataFromBase64String:key];

    unsigned char * bytes = (unsigned char *)[strippedPublicKeyData bytes];
    size_t bytesLen = [strippedPublicKeyData length];

    size_t i = 0;
    if (bytes[i++] != 0x30)
        [Exception raise:FAILURE function:__PRETTY_FUNCTION__ line:__LINE__ description:@"Could not set public key."];

    /* Skip size bytes */
    if (bytes[i] > 0x80)
        i += bytes[i] - 0x80 + 1;
    else
        i++;

    if (i >= bytesLen)
        [Exception raise:FAILURE function:__PRETTY_FUNCTION__ line:__LINE__ description:@"Could not set public key."];

    if (bytes[i] != 0x30)
        [Exception raise:FAILURE function:__PRETTY_FUNCTION__ line:__LINE__ description:@"Could not set public key."];

    /* Skip OID */
    i += 15;

    if (i >= bytesLen - 2)
        [Exception raise:FAILURE function:__PRETTY_FUNCTION__ line:__LINE__ description:@"Could not set public key."];

    if (bytes[i++] != 0x03)
        [Exception raise:FAILURE function:__PRETTY_FUNCTION__ line:__LINE__ description:@"Could not set public key."];

    /* Skip length and null */
    if (bytes[i] > 0x80)
        i += bytes[i] - 0x80 + 1;
    else
        i++;

    if (i >= bytesLen)
        [Exception raise:FAILURE function:__PRETTY_FUNCTION__ line:__LINE__ description:@"Could not set public key."];

    if (bytes[i++] != 0x00)
        [Exception raise:FAILURE function:__PRETTY_FUNCTION__ line:__LINE__ description:@"Could not set public key."];

    if (i >= bytesLen)
        [Exception raise:FAILURE function:__PRETTY_FUNCTION__ line:__LINE__ description:@"Could not set public key."];

    strippedPublicKeyData = [NSData dataWithBytes:&bytes[i] length:bytesLen - i];

    DLog(@"X.509 Formatted Public Key bytes:\n%@",[strippedPublicKeyData description]);

    if (strippedPublicKeyData == nil)
        [Exception raise:FAILURE function:__PRETTY_FUNCTION__ line:__LINE__ description:@"Could not set public key."];



    CFTypeRef persistKey = nil;
    [publicKey2 setObject:strippedPublicKeyData forKey:kSecValueData];
    [publicKey2 setObject: (kSecAttrKeyClassPublic) forKey:kSecAttrKeyClass];
    [publicKey2 setObject:[NSNumber numberWithBool:YES] forKey:kSecReturnPersistentRef];

    OSStatus secStatus = SecItemAdd((CFDictionaryRef)publicKey2, &persistKey);

    if (persistKey != nil) CFRelease(persistKey);

    if ((secStatus != noErr) && (secStatus != errSecDuplicateItem))
        [Exception raise:FAILURE function:__PRETTY_FUNCTION__ line:__LINE__ description:@"Could not set public key."];

    SecKeyRef keyRef = nil;
    [publicKey2 removeObjectForKey:kSecValueData];
    [publicKey2 removeObjectForKey:kSecReturnPersistentRef];
    [publicKey2 setObject:[NSNumber numberWithBool:YES] forKey:kSecReturnRef];
    [publicKey2 setObject: kSecAttrKeyTypeRSA forKey:kSecAttrKeyType];

    SecItemCopyMatching((CFDictionaryRef)publicKey2,(CFTypeRef *)&keyRef);
    if (!keyRef)
    [Exception raise:FAILURE function:__PRETTY_FUNCTION__ line:__LINE__ description:@"Could not set public key."];  

    uint8_t *plainBuffer;
    uint8_t *cipherBuffer;
    uint8_t *decryptedBuffer;


    const char inputString[] = "1234";
    int len = strlen(inputString);
    // TODO: this is a hack since i know inputString length will be less than BUFFER_SIZE
    if (len > BUFFER_SIZE) len = BUFFER_SIZE-1;
    plainBuffer = (uint8_t *)calloc(BUFFER_SIZE, sizeof(uint8_t));
    cipherBuffer = (uint8_t *)calloc(CIPHER_BUFFER_SIZE, sizeof(uint8_t));
    decryptedBuffer = (uint8_t *)calloc(BUFFER_SIZE, sizeof(uint8_t));

    strncpy( (char *)plainBuffer, inputString, len);

    size_t plainBufferSize = strlen((char *)plainBuffer);
    size_t cipherBufferSize = CIPHER_BUFFER_SIZE;

    NSLog(@"SecKeyGetBlockSize() public = %lu", SecKeyGetBlockSize(keyRef));
    //  Error handling
    // Encrypt using the public.
    OSStatus status = noErr;

    status = SecKeyEncrypt(keyRef,
                           PADDING,
                           plainBuffer,
                           plainBufferSize,
                           &cipherBuffer[0],
                           &cipherBufferSize
                           );
    NSLog(@"encryption result code: %ld (size: %lu)", status, cipherBufferSize);

    return [[[NSString stringWithFormat:@"%s",cipherBuffer] dataUsingEncoding:NSUTF8StringEncoding] base64EncodedString];
}

For testing purposes and simplicity for the moment I am trying to encrypt only a input with a length of 4 bytes. This should be small enough to fit one block. The public key import and the encrypting process seems to work, however I always receive a much longer output compared to the android method.

The only difference I encountered so far is the fact, that SecKeyGetBlockSize returns 16 and in java cipher.blocksize returns 5. I think the other 11 bytes are reserved for the pkcs1 padding, but how can force the same behaviour in ios/objc?

Answer 1

Try up with splitting cipher text into multiple parts so that each contains 16 char long and separately decode them. I too faced the same problem but that was in PHP for a long time and above trick worked for me.

This may be help you to get-out of the problem.

Answer 2

Decoding the Base64 key gives:

MCwwDQYJKoZIhvcNAQEBBQADGwAwGAIRAK+dBpbOKw+1VKMWoFxjU6UCAwEAAQ==
-> 302c300d06092a864886f70d0101010500031b003018021100af9d0696ce2b0fb554a316a05c6353a50203010001

Interpreting this as DER-encoded ASN.1, we find:

30(2c) //SEQUENCE
  30(0d)  //SEQUENCE
    06(09): 2a 86 48 86 f7 0d 01 01 01  //OID 1.2.840.113548.1.1.1 (RSA Encryption)
    05(00): //NULL                           
    03(1b): [00] 30 18 02 11 00 af 9d 06 96 ce 2b 0f b5 54 a3 16 a0 5c 63 53 a5 02 03 01 00 01 //BITSTRING

Where the BITSTRING also seems to contain DER-encoded ASN.1:

30(18) //SEQUENCE
  02(11): 00 af 9d 06 96 ce 2b 0f b5 54 a3 16 a0 5c 63 53 a5 02 03 01 00 01 //INTEGER

 = 0xaf9d0696ce2b0fb554a316a05c6353a50203010001

Walking through the IOS code, you can see that it is parsing the DER-encoded ASN.1. It correctly identifies the first two SEQUENCE tags, and skips over the OID field without even verifying that it is an OID. Then the problem occurs: the IOS code expects the next tag to be BITSTRING(0x03)---but in our data, we have an additional NULL(0x05) field to denote that the public exponent is implicit. The IOS code raises an exception upon encountering the 0x05 tag. If the NULL weren't there, we see that the IOS code would have successfully extracted the contents of the BITSTRING.

So: either the NULL is an optional field, and the IOS code isn't permitting it, or the IOS code is expecting a different ASN.1 structure. For example, it appears that the BITSTRING is also a DER-encoded ASN.1 INTEGER (presumably the RSA modulus). Yet the IOS code makes no attempt to parse it. It may be that the IOS SecKeyEncrypt routine expects this format for the modulus, or it may be that the caller is supposed to extract the raw bytes of the modulus.

So there's a little bit of experimentation still needed. But the following additional conditional is definately necessary if this code is to parse the supplied data object:

/* Skip OID */
i += 15;

if (i >= bytesLen - 2)
    [Exception raise:FAILURE function:__PRETTY_FUNCTION__ line:__LINE__ description:@"Could not set public key."];

if (bytes[i] == 0x05)    /* This should handle the spurious ASN.1 NULL field */
    i += 2;

if (bytes[i++] != 0x03)