Import Public RSA Key From Certificate

Question

Our customer has their public RSA key stored in a certificate.

We need this key hardcoded in our WinRT app, so we can encrypt client-side. However, we\'re having issues

Answer 1

I found this article in the MSDN Forum very helpful. Carlos Lopez postet some code to get the Public Key out of a Base64 encoded Certificate.

http://social.msdn.microsoft.com/Forums/en-US/17e1467a-2de7-47d2-8d8c-130518eaac68/how-to-use-a-x509-certificate-not-a-pfx-to-verify-a-signature

Here the code:

public static CryptographicKey GetCryptographicPublicKeyFromCert(string strCert)
    {
        int length;
        CryptographicKey CryptKey = null;

        byte[] bCert = Convert.FromBase64String(strCert);

        // Assume Cert contains RSA public key 
        // Find matching OID in the certificate and return public key
        byte[] rsaOID = EncodeOID("1.2.840.113549.1.1.1");
        int index = FindX509PubKeyIndex(bCert, rsaOID, out length);

        // Found X509PublicKey in certificate so copy it.
        if (index > -1)
        {
            byte[] X509PublicKey = new byte[length];
            Array.Copy(bCert, index, X509PublicKey, 0, length);

            AsymmetricKeyAlgorithmProvider AlgProvider = AsymmetricKeyAlgorithmProvider.OpenAlgorithm(AsymmetricAlgorithmNames.RsaPkcs1);
            CryptKey = AlgProvider.ImportPublicKey(CryptographicBuffer.CreateFromByteArray(X509PublicKey));
        }

        return CryptKey;
    }

    static int FindX509PubKeyIndex(byte[] Reference, byte[] value, out int length)
    {
        int index = -1;
        bool found;
        length = 0;

        for (int n = 0; n < Reference.Length; n++)
        {
            if ((Reference[n] == value[0]) && (n + value.Length < Reference.Length))
            {
                index = n;
                found = true;

                for (int m = 1; m < value.Length; m++)
                {
                    if (Reference[n + m] != value[m])
                    {
                        found = false;
                        break;
                    }
                }

                if (found) break;
                else index = -1;
            }
        }

        if (index > -1)
        {
            // Find outer Sequence
            while (index > 0 && Reference[index] != 0x30) index--;
            index--;
            while (index > 0 && Reference[index] != 0x30) index--;
        }

        if (index > -1)
        {
            // Find the length of encoded Public Key
            if ((Reference[index + 1] & 0x80) == 0x80)
            {
                int numBytes = Reference[index + 1] & 0x7F;
                for (int m = 0; m < numBytes; m++)
                {
                    length += (Reference[index + 2 + m] << ((numBytes - 1 - m) * 8));
                }

                length += 4;
            }
            else
            {
                length = Reference[index + 1] + 2;
            }
        }

        return index;
    }

    static public byte[] EncodeOID(string szOID)
    {
        int[] OIDNums;
        byte[] pbEncodedTemp = new byte[64];
        byte[] pbEncoded = null;
        int n, index, num, count;

        OIDNums = ParseOID(szOID);

        pbEncodedTemp[0] = 6;

        pbEncodedTemp[2] = Convert.ToByte(OIDNums[0] * 40 + OIDNums[1]);
        count = 1;

        for (n = 2, index = 3; n < OIDNums.Length; n++)
        {
            num = OIDNums[n];

            if (num >= 16384)
            {
                pbEncodedTemp[index++] = Convert.ToByte(num / 16384 | 0x80);
                num = num % 16384;

                count++;
            }

            if (num >= 128)
            {
                pbEncodedTemp[index++] = Convert.ToByte(num / 128 | 0x80);
                num = num % 128;

                count++;
            }


            pbEncodedTemp[index++] = Convert.ToByte(num);
            count++;
        }

        pbEncodedTemp[1] = Convert.ToByte(count);

        pbEncoded = new byte[count + 2];
        Array.Copy(pbEncodedTemp, 0, pbEncoded, 0, count + 2);

        return pbEncoded;
    }

    static public int[] ParseOID(string szOID)
    {
        int nlast, n = 0;
        bool fFinished = false;
        int count = 0;
        int[] dwNums = null;

        do
        {
            nlast = n;
            n = szOID.IndexOf(".", nlast);
            if (n == -1) fFinished = true;
            count++;
            n++;
        } while (fFinished == false);

        dwNums = new int[count];

        count = 0;
        fFinished = false;

        do
        {
            nlast = n;
            n = szOID.IndexOf(".", nlast);
            if (n != -1)
            {
                dwNums[count] = Convert.ToInt32(szOID.Substring(nlast, n - nlast), 10);
            }
            else
            {
                fFinished = true;
                dwNums[count] = Convert.ToInt32(szOID.Substring(nlast, szOID.Length - nlast), 10);
            }

            n++;
            count++;
        } while (fFinished == false);

        return dwNums;
    }