.NET Core 3.1 - RSA.ImportRSAPrivateKey raises a CryptographicException

Question

I'm training to asymmetric encryption with RSA. I created a key pair with ssh-keygen, and I managed to encrypt some data with my public key. But when I try to decrypt this data with my private key, this exception is raised :

System.Security.Cryptography.CryptographicException : 'ASN1 corrupted data.'

Here is my PEM file containing my private key :

And here is the code sample using this private key :

static string RSA_Decrypt(byte[] value)
{
    byte[] deciphered;

    using (var reader = File.OpenText(@"C:\Users\???\.ssh\id_rsa"))
    {
        // Extracting the payload
        string privateKeyFile = reader.ReadToEnd();
        var exp = privateKeyFile.Split("

");
        var privateKey = exp[1].Split("
-----")[0];

        // using the payload to decrypt my data
        using(var rsa = RSA.Create())
        {
            rsa.ImportRSAPrivateKey(Convert.FromBase64String(privateKey), out _); //CryptographicException

            deciphered = rsa.Decrypt(value, RSAEncryptionPadding.Pkcs1);
        }
    }

    return Encoding.Default.GetString(deciphered);
}

What am I doing wrong?

user_9014097 · Answer

The posted private key is a PEM encoded PKCS#1 key that is encrypted.

RSA.ImportRSAPrivateKey() can only import unencrypted keys of this format that are furthermore DER encoded. The DER encoding results from the PEM encoding by removing the header and footer and Base64 decoding the rest.

As far as I know, an encrypted PKCS#1 key cannot be imported with .NET Core 3.1 on-board means. But an import is possible with BouncyCastle and its Org.BouncyCastle.OpenSsl.PemReader class, see e.g. here.

Unlike the PKCS#1 format, a DER encoded private key in PKCS#8 format can be imported with .NET Core 3.1 out of the box (RSA.ImportPkcs8PrivateKey()), even if it is encrypted (RSA.ImportEncryptedPkcs8PrivateKey()).
Keys of this format can be generated e.g. with openssl genpkey. With OpenSSL it is also possible to convert between the formats.

user_10755225 · Answer

Thanks Topaco, that's exactly what I needed! I used BouncyCastle to decrypt my private key, and now it's working perfectly! I don't know why RSA class does not have any method to do it...

So here is my code now :

static string RSA_Decrypt(byte[] value)
{
    string deciphered;

    using (var reader = File.OpenText(@"C:\Users\???\.ssh\id_rsa"))
    {
        string privateKeyFile = reader.ReadToEnd();
        var keyReader = new StringReader(privateKeyFile);

        var decryptEngine = new Pkcs1Encoding(new RsaEngine());

        object pemReader = new PemReader(keyReader, new PasswordFinder("azerty")).ReadObject();
        var keyPair = (AsymmetricCipherKeyPair)pemReader;
        decryptEngine.Init(false, keyPair.Private);

        deciphered = Encoding.UTF8.GetString(decryptEngine.ProcessBlock(value, 0, value.Length));
    }

    return deciphered;
}

PS : Yeah, my private key password is "azerty" ^^ I wasn't inspired when I created my key pair ^^

.NET Core 3.1 - RSA.ImportRSAPrivateKey raises a CryptographicException

2 Answers