Question: how to generate a .pem file with private key from an X509Certificate2?

[heng-liu]

Description

May I know if there is any way to generate a .pem file with private key from an X509Certificate2?
I tried the following:
1.Create a self-issued X509Certificate2 certificate(cert) with private key (the key generation algorithm is RSA ), saved as attached file.

byte[] certbytes = cert.Export(X509ContentType.Pfx, "password"); 
File.WriteAllBytes(path, certbytes);

2.Wrote a console to:
1).Read the bytes, create an X509Certificate2 cert, Set a password to protect the cert.
2).Try to get the certificate, public key, private key to create a .pem file.

string path = "c:\\work\\0420\\cert";
            byte[] certbytes = File.ReadAllBytes(path);

            string password = "password";
            X509Certificate2 certWithPrivateKey = new X509Certificate2(certbytes, password, X509KeyStorageFlags.PersistKeySet | X509KeyStorageFlags.Exportable);

            byte[] certificateBytes = certWithPrivateKey.RawData;
            char[] certificatePem = PemEncoding.Write("CERTIFICATE", certificateBytes);

            using (AsymmetricAlgorithm key = certWithPrivateKey.GetRSAPrivateKey())
            {
                byte[] pubKeyBytes = key.ExportSubjectPublicKeyInfo();
                char[] pubKeyPem = PemEncoding.Write("PUBLIC KEY", pubKeyBytes);

                byte[] encryptedPrivKeyBytes = key.ExportEncryptedPkcs8PrivateKey(
                    password,
                    new PbeParameters(
                        PbeEncryptionAlgorithm.Aes256Cbc,
                        HashAlgorithmName.SHA256,
                        iterationCount: 100_000));
                char[] privKeyPem = PemEncoding.Write("PRIVATE KEY", encryptedPrivKeyBytes);
            }

I could see non empty char arrays for above certificatePem, pubKeyPem and privKeyPem. But I'm not sure how to combine them into a .pem file.
I tried to convert them into string and concatenate them, but the .pem file generated in this way failed to be used in CreateFromEncryptedPemFile as CryptographicException as following:
cert = X509Certificate2.CreateFromEncryptedPemFile(options.CertificatePath, options.CertificatePassword)
The exception details is:

I'm wondering if you know how to generate a .pem file with private key (with or without password) from an X509Certificate2 cert? Thanks!

Attached is the cert file from step1.
cert.zip

Configuration

Regression?

Other information

[ghost]

Tagging subscribers to this area: @bartonjs, @vcsjones, @krwq, @GrabYourPitchforks
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Issue Details

---

Description

May I know if there is any way to generate a .pem file with private key from an X509Certificate2?
I tried the following:
1.Create a self-issued X509Certificate2 certificate(cert) with private key (the key generation algorithm is RSA ), saved as attached file.

byte[] certbytes = cert.Export(X509ContentType.Pfx, "password"); 
File.WriteAllBytes(path, certbytes);

2.Wrote a console to:
1).Read the bytes, create an X509Certificate2 cert, Set a password to protect the cert.
2).Try to get the certificate, public key, private key to create a .pem file.

string path = "c:\\work\\0420\\cert";
            byte[] certbytes = File.ReadAllBytes(path);

            string password = "password";
            X509Certificate2 certWithPrivateKey = new X509Certificate2(certbytes, password, X509KeyStorageFlags.PersistKeySet | X509KeyStorageFlags.Exportable);

            byte[] certificateBytes = certWithPrivateKey.RawData;
            char[] certificatePem = PemEncoding.Write("CERTIFICATE", certificateBytes);

            using (AsymmetricAlgorithm key = certWithPrivateKey.GetRSAPrivateKey())
            {
                byte[] pubKeyBytes = key.ExportSubjectPublicKeyInfo();
                char[] pubKeyPem = PemEncoding.Write("PUBLIC KEY", pubKeyBytes);

                byte[] encryptedPrivKeyBytes = key.ExportEncryptedPkcs8PrivateKey(
                    password,
                    new PbeParameters(
                        PbeEncryptionAlgorithm.Aes256Cbc,
                        HashAlgorithmName.SHA256,
                        iterationCount: 100_000));
                char[] privKeyPem = PemEncoding.Write("PRIVATE KEY", encryptedPrivKeyBytes);
            }

I could see non empty char arrays for above certificatePem, pubKeyPem and privKeyPem. But I'm not sure how to combine them into a .pem file.
I tried to convert them into string and concatenate them, but the .pem file generated in this way failed to be used in CreateFromEncryptedPemFile as CryptographicException as following:
cert = X509Certificate2.CreateFromEncryptedPemFile(options.CertificatePath, options.CertificatePassword)
The exception details is:

I'm wondering if you know how to generate a .pem file with private key (with or without password) from an X509Certificate2 cert? Thanks!

Attached is the cert file from step1.
cert.zip

Configuration

Regression?

Other information

Author:	heng-liu
Assignees:	-
Labels:	area-System.Security, untriaged
Milestone:	-

[vcsjones]

You should be able to do something like this:

string certPem = new string(PemEncoding.Write("CERTIFICATE", cert.RawData));
string keyPem;

if (cert.GetRSAPrivateKey() is RSA rsaKey)
{
    keyPem = new string(PemEncoding.Write("PRIVATE KEY", rsaKey.ExportPkcs8PrivateKey()));
}
else if (cert.GetECDsaPrivateKey() is ECDsa ecdsaKey)
{
    keyPem = new string(PemEncoding.Write("PRIVATE KEY", ecdsaKey.ExportPkcs8PrivateKey()));
}
else if (cert.GetDSAPrivateKey() is DSA dsaKey)
{
    keyPem = new string(PemEncoding.Write("PRIVATE KEY", dsaKey.ExportPkcs8PrivateKey()));
}
else
{
    throw new CryptographicException("Unknown certificate algorithm");
}

Console.WriteLine(certPem);
Console.WriteLine(keyPem);

var certAgain = X509Certificate2.CreateFromPem(certPem, keyPem);

That will export the private key in to a PEM-encoded PKCS8 private key.

[vcsjones]

If you want to use ExportEncryptedPkcs8PrivateKey then the PEM label would be ENCRYPTED PRIVATE KEY:

PbeParameters pbe = new PbeParameters(PbeEncryptionAlgorithm.Aes128Cbc, HashAlgorithmName.SHA256, 600_000);
var certPem = new string(PemEncoding.Write("CERTIFICATE", cert.RawData));
string keyPem;

if (cert.GetECDsaPrivateKey() is ECDsa ecdsaKey)
{
    keyPem = new string(PemEncoding.Write("ENCRYPTED PRIVATE KEY", ecdsaKey.ExportEncryptedPkcs8PrivateKey("test", pbe)));
}
else if (cert.GetRSAPrivateKey() is RSA rsaKey)
{
    keyPem = new string(PemEncoding.Write("ENCRYPTED PRIVATE KEY", rsaKey.ExportEncryptedPkcs8PrivateKey("test", pbe)));
}
else if (cert.GetDSAPrivateKey() is DSA dsaKey)
{
    keyPem = new string(PemEncoding.Write("ENCRYPTED PRIVATE KEY", dsaKey.ExportEncryptedPkcs8PrivateKey("test", pbe)));
}
else
{
    throw new CryptographicException("Unknown certificate algorithm");
}

Console.WriteLine(certPem);
Console.WriteLine(keyPem);

var certAgain = X509Certificate2.CreateFromEncryptedPem(certPem, keyPem, "test");

[vcsjones]

And finally, if you want to combine them, one more example:

string aggregatePem = certPem + "\n" + keyPem;

var certAgain = X509Certificate2.CreateFromPem(aggregatePem, aggregatePem);

Or if you write the combined contents to a file:

string aggregatePem = certPem + "\n" + keyPem;
File.WriteAllText("combined.pem", aggregatePem);

var certAgain = X509Certificate2.CreateFromPemFile("combined.pem");

And for encrypted private key PEMs it would be the same but using the Encrypted versions of the appropriate methods.

[heng-liu]

Thanks a lot for your help! @vcsjones
I tried the one with password and it worked :)

[davidfowl]

@vcsjones This feels like recipe we should document somewhere

[vcsjones]

This feels like recipe we should document somewhere

Yeah. Part of me is wondering though if there is a quality-of-life API that's missing. Something like:

public class X509Certificate2 {
    public string ExportCertificatePem();
    public bool TryExportCertificatePem(Span<char> buffer, out int bytesWritten);
}

public class AsymmetricAlgorithm {
    public string ExportPkcs8PrivateKeyPem();
    public bool TryExportPkcs8PrivateKeyPem(Span<char> buffer, out int bytesWritten);
    public string TryExportEncryptedPkcs8PrivateKeyPem();
    public bool TryExportEncryptedPkcs8PrivateKeyPem(Span<char> buffer, out int bytesWritten);
}

Maybe @bartonjs has more thoughts on this.

[bartonjs]

We could add it, but there are some corner cases.

• For the cert, we could easily limit it to X.509 PEM, if you want a PKCS#7 PEM use the collection.
  • But did you want the cert and key together as a multi-PEM? Should we make that easy, too? Or is that easy enough after you grab the key?
  • PFX is a no-go, it has no defined PEM header.
  • Guess we also don't care about SerializedCert and SerializedStore... so we actually maybe don't need to take the "for what format?" parameter.
• For collections I guess PKCS#7 is the only PEM option, so that one doesn't need a format parameter, either.
• For AsymmetricAlgorithm the encrypted versions have some bad copy-pasta 😄.
• We've done everything else, should we also do RSA Public and RSA Private PEM?

[vcsjones]

Or is that easy enough after you grab the key?

My thought was that is easy enough. Concatenating two strings doesn't seem like an especially tall order.

have some bad copy-pasta

And I thought passwordless was all the rage...

Guess we also don't care about SerializedCert and SerializedStore

Personally I have never once used those formats in conjunction with a PEM. Or really ever.

We've done everything else, should we also do RSA Public and RSA Private PEM?

Guess that can't hurt.

---

I will open a new issue with a better formatted proposal (with password parameters!) so I don't hijack this issue any longer.

[bartonjs]

The original question has been answered, and the "maybe there's room for improvement" is an API proposal, so I don't think anything's left from this issue.