WinRT中 导入X509 RSA公钥时问题

最近开发WinRT应用时，需要使用服务端提供的公钥加密，可总出现：“ASN1 bad tag value met.”，错误提示。寻寻觅觅多时未找到解决方案。最后终于在一个国处的论坛找到一种解决办法。

链接地址为：https://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

出错位置在以下语句，即加载公钥的地方：

IBuffer keyBuffer = CryptographicBuffer.DecodeFromBase64String(publicKey);AsymmetricKeyAlgorithmProvider asym = AsymmetricKeyAlgorithmProvider.OpenAlgorithm(AsymmetricAlgorithmNames.RsaPkcs1);CryptographicKey key = asym.ImportPublicKey(keyBuffer, CryptographicPublicKeyBlobType.Pkcs1RsaPublicKey);  // 出错

经过反复查阅论坛，得出一个结论，就是不要尝试直接使用public key，一定要进行解析转换才行，要不然，就只能要求服务端改变成适合于WinRT的形式再重新提供public key。

解析转换原文提供了源代码，使用其提供的函数，则上述代码替换如下即可：

</pre><pre name="code" class="csharp">CryptographicKey key = GetCryptographicPublicKeyFromCert(publicKey);

GetCryptographicPublicKeyFromCert 函数的提供源码如下：

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;    }