C# .NET “公鑰證書” （.cer .pem）轉換為 RSACryptoServiceProvider 對象。導出“公鑰”

“公鑰證書” .cer 文件是直接可以用X509Certificate2 對象來讀取的，但 .cer 文件 不便於存儲。

“公鑰證書” .pem 文件內容如下：

-----BEGIN CERTIFICATE----- MIICnzCCAggCCQDbr9OvJHgzmDANBgkqhkiG9w0BAQUFADCBkzELMAkGA1UEBhMC RUUxETAPBgNVBAgMCEhhcmp1bWFhMRAwDgYDVQQHDAdUYWxsaW5uMREwDwYDVQQK DAhFZXRhc29mdDERMA8GA1UECwwIYmFua2xpbmsxFjAUBgNVBAMMDXBhbmdhbGlu ay5uZXQxITAfBgkqhkiG9w0BCQEWEmVldGFzb2Z0QG9ubGluZS5lZTAeFw0xNDEy MjQxNjI1MjdaFw0zNDEyMTkxNjI1MjdaMIGTMQswCQYDVQQGEwJFRTERMA8GA1UE CAwISGFyanVtYWExEDAOBgNVBAcMB1RhbGxpbm4xETAPBgNVBAoMCEVldGFzb2Z0 MREwDwYDVQQLDAhiYW5rbGluazEWMBQGA1UEAwwNcGFuZ2FsaW5rLm5ldDEhMB8G CSqGSIb3DQEJARYSZWV0YXNvZnRAb25saW5lLmVlMIGfMA0GCSqGSIb3DQEBAQUA A4GNADCBiQKBgQDCi9usnv8rJLBeBgHhbM/80zPUOlkuH0uYrdzj/zxQSZS0QEoH yVvosEWqO34+17CzQ0sc34kzK5qhLws7AKetjLH7cpgmVH5YnQxKgZ2a5I5mpj02 v9rjwDSKX2ZDXCeRUdnZa26beMcX/i1C9jxk9tpE2NWud910yHPRpnAQFwIDAQAB MA0GCSqGSIb3DQEBBQUAA4GBAFmlXa8NkR2OP0bC0EapmCaoObUG2WOi6Fhl2dBB PInJq6tzv+YtMbaIAPANo1EzBeBcQTRvxgGIrqE/JJkjDhOvwxjdNcxp7Mt+7hkk PI55KkKAfOeE0ss0EUcCdnyCGAXdhJfUCxJydg0PaVpE70FXUcLZcZXfT968mDOC NpaS -----END CERTIFICATE-----

如果導出公鑰，或轉為RSACryptoServiceProvider 對象呢？參照了：https://stackoverflow.com/questions/27642180/how-to-create-public-key-from-certificate
代碼如下：

string pemPublicCert = Path.Combine(Application.StartupPath, "CCQTGBtest.pem");
            string base64X509Cert = "";//"公鑰證書"字符串

            /*
             -----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
             
             */

            using (FileStream fs = new FileStream(pemPublicCert, FileMode.Open, FileAccess.Read))
            {
                using (StreamReader sr = new StreamReader(fs))
                {
                    base64X509Cert = sr.ReadToEnd().Trim();
                }
            }

            //提取純字符串
            base64X509Cert = base64X509Cert.Replace("-----BEGIN CERTIFICATE-----", "").Replace("-----END CERTIFICATE-----", "").Replace("\r", "").Replace("\n", "");
            //https://stackoverflow.com/questions/27642180/how-to-create-public-key-from-certificate
            //轉換為“公鑰”對象
            var derCert = Convert.FromBase64String(base64X509Cert);
            var x509 = new X509Certificate2(derCert);
            RSACryptoServiceProvider rsa = (RSACryptoServiceProvider)x509.PublicKey.Key;

            MessageBox.Show(rsa.ToXmlString(false));

            //Console.WriteLine(rsa.ToXmlString(false));

拿到RSACryptoServiceProvider  對象 （公鑰）后，就可以驗證簽名了，或加密。

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