CODEWITHSUNDEEP
iosswiftazureencryptionrsa
Anton Kashpor
Anton Kashpor

Reputation: 1305

RSA public key generation -- Swift

Azure key vault sends e and n parameters which equals RSA modulus(n) and RSA public exponent(e). It's easy to create with 3rd party libraries, but output value wouldn't be valid without modulus and exponent.

I need to create RSA public key of these two parts.Then I have to create RSA OAEP. Has anyone encountered a similar problem? Any idea how to achieve this?

I have tried to use SCZ-BasicEncodingRules-iOS, but it output incorrect code.

Here is code:

let moduleString: String = "mK-g0TLMqtefmosgBSTQi3dWh8h-rn4lQA8sQgNs_Gkf5TvgKWtYGJ4jRGUU-eK2bmyAAomVUojYBBlRYBkTRekm99DlD9T6U9yI3v11pZVl3yQgVXBEkiTZug3Inn_IAOGPQ3Q5OT6fEj1sRRxmMv93CQukQguSKuU4v2tmElgvyhg_eKIQbNx0JRCI4-1Z5GazxNjtwk7tWcA6PAbV0zZe2AaW0TlYVil_U8NckgHBguBoTHqVAbzb-MNa-HRa4QlBmdemcSaiDr5GikoOdmQ1-Lu6koqUkepx16pgqPvVw3o_NuXAZCS37c7bfgtXWTJcAIjiQaEyRcIV2bXsUQ"

    let exponent: String = "AQAB"

    let moduleData: NSData! = (moduleString
        as NSString).dataUsingEncoding(NSUTF8StringEncoding) as NSData!
    let expData: NSData! = (exponent
        as NSString).dataUsingEncoding(NSUTF8StringEncoding) as NSData!
    let arrayaaa: NSArray = [moduleData, expData]
    let key: NSData = arrayaaa.berData()

Upvotes: 4

Views: 1521

Answers (1)

Anton Kashpor
Anton Kashpor

Reputation: 1305

So, I have found a solution. Unfortunately SCZ-BasicEncodingRules-iOS has wrong decoding algorithm for iOS 8 and above. It outputs key with incorrect prefix.

If you faced with the same issue, here is a solution that helped me, but it's Objective-C code. Original source:

   + (NSData *)generateRSAPublicKeyWithModulus:(NSData*)modulus exponent:(NSData*)exponent
{
    const uint8_t DEFAULT_EXPONENT[] = {0x01, 0x00, 0x01,}; //default: 65537
    const uint8_t UNSIGNED_FLAG_FOR_BYTE = 0x81;
    const uint8_t UNSIGNED_FLAG_FOR_BYTE2 = 0x82;
    const uint8_t UNSIGNED_FLAG_FOR_BIGNUM = 0x00;
    const uint8_t SEQUENCE_TAG = 0x30;
    const uint8_t INTEGER_TAG = 0x02;

    uint8_t* modulusBytes = (uint8_t*)[modulus bytes];
    uint8_t* exponentBytes = (uint8_t*)(exponent == nil ? DEFAULT_EXPONENT : [exponent bytes]);

    //(1) calculate lengths
    //- length of modulus
    int lenMod = (int)[modulus length];
    if(modulusBytes[0] >= 0x80)
        lenMod ++;  //place for UNSIGNED_FLAG_FOR_BIGNUM
    int lenModHeader = 2 + (lenMod >= 0x80 ? 1 : 0) + (lenMod >= 0x0100 ? 1 : 0);
    //- length of exponent
    int lenExp = exponent == nil ? sizeof(DEFAULT_EXPONENT) : (int)[exponent length];
    int lenExpHeader = 2;
    //- length of body
    int lenBody = lenModHeader + lenMod + lenExpHeader + lenExp;
    //- length of total
    int lenTotal = 2 + (lenBody >= 0x80 ? 1 : 0) + (lenBody >= 0x0100 ? 1 : 0) + lenBody;

    int index = 0;
    uint8_t* byteBuffer = malloc(sizeof(uint8_t) * lenTotal);
    memset(byteBuffer, 0x00, sizeof(uint8_t) * lenTotal);

    //(2) fill up byte buffer
    //- sequence tag
    byteBuffer[index ++] = SEQUENCE_TAG;
    //- total length
    if(lenBody >= 0x80)
        byteBuffer[index ++] = (lenBody >= 0x0100 ? UNSIGNED_FLAG_FOR_BYTE2 : UNSIGNED_FLAG_FOR_BYTE);
    if(lenBody >= 0x0100)
    {
        byteBuffer[index ++] = (uint8_t)(lenBody / 0x0100);
        byteBuffer[index ++] = lenBody % 0x0100;
    }
    else
        byteBuffer[index ++] = lenBody;
    //- integer tag
    byteBuffer[index ++] = INTEGER_TAG;
    //- modulus length
    if(lenMod >= 0x80)
        byteBuffer[index ++] = (lenMod >= 0x0100 ? UNSIGNED_FLAG_FOR_BYTE2 : UNSIGNED_FLAG_FOR_BYTE);
    if(lenMod >= 0x0100)
    {
        byteBuffer[index ++] = (int)(lenMod / 0x0100);
        byteBuffer[index ++] = lenMod % 0x0100;
    }
    else
        byteBuffer[index ++] = lenMod;
    //- modulus value
    if(modulusBytes[0] >= 0x80)
        byteBuffer[index ++] = UNSIGNED_FLAG_FOR_BIGNUM;
    memcpy(byteBuffer + index, modulusBytes, sizeof(uint8_t) * [modulus length]);
    index += [modulus length];
    //- exponent length
    byteBuffer[index ++] = INTEGER_TAG;
    byteBuffer[index ++] = lenExp;
    //- exponent value
    memcpy(byteBuffer + index, exponentBytes, sizeof(uint8_t) * lenExp);
    index += lenExp;

    if(index != lenTotal)
        NSLog(@"lengths mismatch: index = %d, lenTotal = %d", index, lenTotal);

    NSMutableData* buffer = [NSMutableData dataWithBytes:byteBuffer length:lenTotal];
    free(byteBuffer);

    return buffer;
}

This algorithm matches with standard Java KeyFactory generation class.

Upvotes: 2

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