I'm learning about certificate signing requests and signing servers via Nuget package System.Security.Cryptography.Cng. And what better way than to try to re-create one. There seem to be a block I don't get around currently, that is the server signing party, namely in the following code I get System.InvalidOperationException: 'An X509Extension with OID '2.5.29.37' has already been specified.' at request.Create( in the using clause. I see at http://oid-info.com/get/2.5.29.37 it's about extended key usage.
Questions:
- The
MakeLocalhostCertis likely wrong, what should be changed to make it a certificate to sign CSRs? - Is it possible to add/remove extensions/OIDs to the CSR being returned? I believe it is, but somehow this part eludes me currently.
I used the excellent answers of https://stackoverflow.com/users/6535399/bartonjs at https://stackoverflow.com/a/45240640/1332416 and at https://stackoverflow.com/a/44073726/1332416 to get this far. :)
private static void CsrSigningTest()
{
//Both ECDSA and RSA included here, though ECDSA is probably better.
using(ECDsa privateClientEcdsaKey = ECDsa.Create(ECCurve.NamedCurves.nistP256))
//using(RSA privateClientRsaKey = RSA.Create(2048))
{
//A client creates a certificate signing request.
CertificateRequest request = new CertificateRequest(
new X500DistinguishedName("CN=example.com, O=Example Ltd, OU=AllOver, L=Sacremento, ST=\"California\", C=US, E=some@example.com"),
privateClientEcdsaKey,
HashAlgorithmName.SHA256);
/*CertificateRequest request = new CertificateRequest(
new X500DistinguishedName("CN=example.com, O=Example Ltd, OU=AllOver, L=Sacremento, ST=\"California\", C=US, E=some@example.com"),
privateClientRsaKey,
HashAlgorithmName.SHA256,
RSASignaturePadding.Pkcs1);*/
var sanBuilder = new SubjectAlternativeNameBuilder();
sanBuilder.AddDnsName("example.com");
request.CertificateExtensions.Add(sanBuilder.Build());
//Not a CA, a server certificate.
request.CertificateExtensions.Add(new X509BasicConstraintsExtension(false, false, 0, false));
request.CertificateExtensions.Add(new X509EnhancedKeyUsageExtension(new OidCollection { new Oid("1.3.6.1.5.5.7.3.1") }, false));
request.CertificateExtensions.Add(new X509EnhancedKeyUsageExtension(new OidCollection { new Oid("1.3.6.1.5.5.7.3.8") }, false));
byte[] derEncodedCsr = request.CreateSigningRequest();
var csrSb = new StringBuilder();
csrSb.AppendLine("-----BEGIN CERTIFICATE REQUEST-----");
csrSb.AppendLine(Convert.ToBase64String(derEncodedCsr));
csrSb.AppendLine("-----END CERTIFICATE REQUEST-----");
//Thus far OK, this csr seems to be working when using an online checker.
var csr = csrSb.ToString();
//Now, sending this to a server... How does the server function:
//1) Read the CSR to be processed?
//2) How does this CSR get signed?
//In the following, can the signing cert be self-signed could be had from
//https://stackoverflow.com/a/45240640/1332416
byte[] serial = new byte[16];
using(var rng = RandomNumberGenerator.Create())
{
rng.GetBytes(serial);
}
DateTimeOffset notBefore = DateTimeOffset.UtcNow;
DateTimeOffset notAfter = notBefore.AddYears(1);
var issuerCertificate = MakeLocalhostCert();
//For the part 1) there, this doesn't seem to work, likely since CSR isn't a X509 certificate.
//But then again, there doesn't seem to be anything in CertificateRequest to read this.
//In reality in the server the prologue and epilogue strings should be removed and the string read.
//var testRequest = new X509Certificate2(derEncodedCsr);
using(X509Certificate2 responseToCsr = request.Create(issuerCertificate, notBefore, notAfter, serial))
{
//How to add extensions here?
var csrResSb = new StringBuilder();
csrResSb.AppendLine("-----BEGIN CERTIFICATE-----");
csrResSb.AppendLine(Convert.ToBase64String(responseToCsr.GetRawCertData()));
csrResSb.AppendLine("-----END CERTIFICATE-----");
var signedCert = csrResSb.ToString();
}
}
}
private static X509Certificate2 MakeLocalhostCert()
{
using(ECDsa key = ECDsa.Create(ECCurve.NamedCurves.nistP384))
{
var request = new CertificateRequest(
"CN=localhost",
key,
HashAlgorithmName.SHA384);
request.CertificateExtensions.Add(
new X509BasicConstraintsExtension(true, false, 0, true));
const X509KeyUsageFlags endEntityTypicalUsages =
X509KeyUsageFlags.DataEncipherment |
X509KeyUsageFlags.KeyEncipherment |
X509KeyUsageFlags.DigitalSignature |
X509KeyUsageFlags.NonRepudiation |
X509KeyUsageFlags.KeyCertSign;
request.CertificateExtensions.Add(
new X509KeyUsageExtension(endEntityTypicalUsages, true));
var sanBuilder = new SubjectAlternativeNameBuilder();
sanBuilder.AddDnsName("localhost");
sanBuilder.AddIpAddress(IPAddress.Loopback);
sanBuilder.AddIpAddress(IPAddress.IPv6Loopback);
request.CertificateExtensions.Add(sanBuilder.Build());
/*request.CertificateExtensions.Add(
new X509EnhancedKeyUsageExtension(
new OidCollection
{
// server and client authentication
new Oid("1.3.6.1.5.5.7.3.1"),
new Oid("1.3.6.1.5.5.7.3.2")
},
false));*/
DateTimeOffset now = DateTimeOffset.UtcNow.AddMinutes(-1);
return request.CreateSelfSigned(now, now.AddYears(2));
}
}
<edit: Once I apply the fix about OIDs and change the last bits to
using(X509Certificate2 responseToCsr = request.Create(issuerCertificate, notBefore, notAfter, serial))
{
request.CertificateExtensions.Add(new X509SubjectKeyIdentifierExtension(issuerCertificate.PublicKey, false));
var csrResSb = new StringBuilder();
csrResSb.AppendLine("-----BEGIN CERTIFICATE-----");
csrResSb.AppendLine(Convert.ToBase64String(responseToCsr.Export(X509ContentType.Cert), Base64FormattingOptions.InsertLineBreaks));
csrResSb.AppendLine("-----END CERTIFICATE-----");
var signedCert = csrResSb.ToString();
}
I get back a certificate in signedCert that looks like the CSR that's been signed. The missing piece would be to construct the CSR if read from an actual CSR file.
<edit 2: There is an issue in CoreFx GH tracking some of the issues touches here: Security crypto - Roadmap.
<edit 3: Nice to know: How to load a certificate request and create a certificate from it in .NET, How to convert a CSR text file into .NET Core/ Standard CertificateRequest for Signing?, How to convert a CSR text file into .NET Core/ Standard CertificateRequest for Signing? for more about this in .NET5/6.
