Read RSA Public Key from x509 Certificate Bytes in C#

Question

In C#, I'm retrieving an RSA public key from a HTTP request and it gives me the key encoded in base64.

WebClient webClient = new WebClient();
string rsaPublicKeyBase64 = webClient.DownloadString("http://localhost:8000/getkey");
// rsaPublicKeyBase64 = LS0tLS1CRUdJTiBQVUJMSUMgS0VZLS0tLS0KTUlHZk1BMEdDU3FHU0liM0RRRUJBUVVBQTRHTkFEQ0JpUUtCZ1FEdDAwcXQ2Zi9UUXdMQmVsVExRdVlXb05xSQoxbmRkcFpaOGh0WWs4d0NLbmFuRFJpWkJ1NVo5NnBNT01yNi84RS9JUzB0amV4WGdsVjh0WFlKK0NKc1lDUHhoCnBDUkduUW9rYkE2MnpOODVXNEROVUNMQ0cyMXlXcndscFhjSmxLYkY2dFhxdmd3TGRQb2RwZzUwY3RrWkI4R0UKbDBLS3VOV3JHZXRad045V0NRSURBUUFCCi0tLS0tRU5EIFBVQkxJQyBLRVktLS0tLQo=

I then decode the base 64 RSA public key.

byte[] rsaPublicKey = Convert.FromBase64String(rsaPublicKeyBase64);
/*
-----BEGIN PUBLIC KEY-----
MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDt00qt6f/TQwLBelTLQuYWoNqI
1nddpZZ8htYk8wCKnanDRiZBu5Z96pMOMr6/8E/IS0tjexXglV8tXYJ+CJsYCPxh
pCRGnQokbA62zN85W4DNUCLCG21yWrwlpXcJlKbF6tXqvgwLdPodpg50ctkZB8GE
l0KKuNWrGetZwN9WCQIDAQAB
-----END PUBLIC KEY-----
*/

My next step is to convert this byte[] containing my RSA public key certificate into type RSACryptoServiceProvider. I've found answers online, but none seem to work for me.

Here's what I currently have (which does not work).

string rsaPublicKeyFile = Path.Combine(Path.GetTempPath(), Guid.NewGuid().ToString());
X509Certificate2 cert = null;
try {
    File.WriteAllBytes(rsaPublicKeyFile, rsaPublicKey);
    cert = new X509Certificate2(rsaPublicKeyFile);
} finally {
    File.Delete(rsaPublicKeyFile);
}

I receive an unhandled exception error shown in the screenshot below.

System.Security.Cryptography.CryptographicException: 'Cannot find the requested object.

You don't need to create X509Certificate2 object. And you can't create it from just public key. You need to load the key directly to PublicKey object. — Crypt32
@Crypt32 there is no example for that situation. link contains examples: how to work with certificates *.crt & *.pfx files. but question about other type of data. If i am wrong could you provide more exact answer? — Anatoliy

Acidic9 Acidic9 · Accepted Answer · 2017-08-19T07:40:12

Thanks to @Crypt32, I managed to solve it by referencing the PublicKey Class documentation

I wrote a function GetCertificateFromBytes(byte[] cert) which writes to a temporary file in order to read the certificate:

public static X509Certificate2 GetCertificateFromBytes(byte[] cert) {
    string certFile = Path.Combine(Path.GetTempPath(), Guid.NewGuid().ToString());
    try {
        File.WriteAllBytes(certFile, cert);

        X509Store store = new X509Store(StoreLocation.CurrentUser);
        try {
            store.Open(OpenFlags.ReadOnly);
            X509Certificate2Collection certCollection = store.Certificates;
            return certCollection[0];
        } finally {
            store.Close();
        }
    } finally {
        File.Delete(certFile);
    }
}

Then to Encrypt:

X509Certificate2 cert = GetCertificateFromBytes(rsaPublicKey);
RSACryptoServiceProvider publicKeyProvider = (RSACryptoServiceProvider)cert.PublicKey.Key;
byte[] encrypted = publicKeyProvider.Encrypt(data, false);

Read RSA Public Key from x509 Certificate Bytes in C#

1 Answers