RSA/ECB/PKCS1Padding C# 解密错误字符串到 PrivateKey

Question

I'm getting an error in Private Key conversion, I can't decrypt. Error: System.InvalidCastException: Could not cast object of type 'Org.BouncyCastle.Crypto.AsymmetricCipherKeyPair' to type 'Org.BouncyCastle.Crypto.AsymmetricKeyParameter'.

When I convert it to AsymmetricCipherKeyPair type, the type does not match in the bottom line. I am waiting for your help.

    static void Main()
        {
            var plainData = "plain_text";
            RSA publicKeyEncryptor = getRSAPublic(@"MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAlYB5JrwA9fMxZxTRhG0NnKRwJizMZGJNq/xFfFxaEmKp3O6vZgsZMlFTi2kSC++yR/KriGKuGgbIYrgomn7BueoooAw5KLVO9CKKtNyQgg28vdOBbnQqljA+KID0PouAD8MqpDk9opi41zeEQPOSkAUsq5sHMptG7h9cgj0mNr2c4ffNolHAhPsrZVtGYtswhtznDkG463VOKLAmDLDeY9bASUsQXFOY+Em93GHFjStgZSTIEBof6HbUqIQf2rGjuPYCQsB/94BFma58epGz12zUPwKFMuxg89wbLOCjyAkocgS9zDnwKr7DVv08GmCUVVqI6ySzbWpKhiqWQvz4hwIDAQAB");
            var plainBytes = Encoding.ASCII.GetBytes(plainData);
            string encryptedPayload = System.Convert.ToBase64String(publicKeyEncryptor.Encrypt(plainBytes, RSAEncryptionPadding.Pkcs1));
         
            RSA privateKeyDecyrpt = getRSAPrivate();
            var y = privateKeyDecyrpt.Decrypt(Encoding.ASCII.GetBytes(encryptedPayload), RSAEncryptionPadding.Pkcs1);
            Console.WriteLine(encryptedPayload);
        }
        public static RSA getRSAPublic(string publicKey)
        {
            string publicKeyPem = $"-----BEGIN PUBLIC KEY-----\r\n{ publicKey }\r\n-----END PUBLIC KEY-----\r\n";
            var pemReader = new PemReader(new StringReader(publicKeyPem));
            AsymmetricKeyParameter keyPairRaw = (AsymmetricKeyParameter)pemReader.ReadObject();
            RSAParameters rsaParams = DotNetUtilities.ToRSAParameters((RsaKeyParameters)keyPairRaw);
            RSA rsaObj = System.Security.Cryptography.RSA.Create();
            rsaObj.ImportParameters(rsaParams);
            return rsaObj;
        }

        public static RSA getRSAPrivate()
        {
            string privateKeyPem = @"-----BEGIN RSA PRIVATE KEY-----
MIIEogIBAAKCAQEAlYB5JrwA9fMxZxTRhG0NnKRwJizMZGJNq/xFfFxaEmKp3O6v
ZgsZMlFTi2kSC++yR/KriGKuGgbIYrgomn7BueoooAw5KLVO9CKKtNyQgg28vdOB
bnQqljA+KID0PouAD8MqpDk9opi41zeEQPOSkAUsq5sHMptG7h9cgj0mNr2c4ffN
olHAhPsrZVtGYtswhtznDkG463VOKLAmDLDeY9bASUsQXFOY+Em93GHFjStgZSTI
EBof6HbUqIQf2rGjuPYCQsB/94BFma58epGz12zUPwKFMuxg89wbLOCjyAkocgS9
zDnwKr7DVv08GmCUVVqI6ySzbWpKhiqWQvz4hwIDAQABAoIBAEJFR+8Gqbpcykpy
bQmxubX1Io2ZkCTzepDBbB/bZEYAHGIGIBQw2UN3z3vd4JUP9Mx14tm7PIfm987i
6YTKqZ97D/UaVgAYlt4brbbMivZLlp3i8t3+ep5G1lboCtzqw6K5Fd7kTNEVt+IX
BvYvwok68flD6GXjdQa7Oiu1ZYofxXOBg84RDWu6E6edDj/XaaX12WOYaQ4p6zDn
jyYNMiYYkKoUI1a884cN50WAAIGs0n3TxgJGXZa0n/Y3CpvuphAoHPQQs7ZyJPlP
k29gaFXdotzv1TDKk0I+eTIlonMl1EyW0wDPvfMuV7EYNNfmtmqrT3OWkOpUU8fO
9eDKhukCgYEA5nlVlTKAx5LsKI3KGRQF3jiWAaQAUKoON68Sx47fSJSv317QyZqM
qbQTLz5vSytbTyHhgIEc2dtdRbl7MYkvvzaizjPDEHEJSwlTmnzt81qGYtjNGC+j
tz6Bx5MGDE8Ax3ls60Lm0uH9TMQyJZWTepZGght5WXP5NpmzJ6zG7D0CgYEApg9S
zZyMN1rNtcz6Pqaqll33hchIQd+vh2LLKkHAoQP93oWtipxyT08yr4tQ5ke3nwjj
onIt6KR0U2aSIv9OUwXw2cSwghCIqTKmtont88WXYh9zRtnZ+U0An9r3x+fr7e8y
wpk7lFPP840pzQQjyEQ2s3woMlssqOiS6SyGMBMCgYB04KVBEypxixWN/1G05A2R
wxp3XIb4YTTykisw3khnU1fZLAkvo9ufl/1+oOfps+QLPkBQXamW5YLogAZ0eYCo
NHndnixW4yv2TJWEK8Sz+31ZFV702/vnSqCf5/RSO6JGhlJxAC10VjyROJHBs5fl
u92nz2z7qy9/u/Q5s4nxdQKBgAx6qFFVS2A5ja302nVs1vL32ssN8wgoRCubbAMf
79bp0uEvEIyTFzAIlpmEka7MgusLovepNvP9r9Q4qBDDOOKaVrA2zMDpdyun58ld
8ijYl3jDPkl7w5qtg7d/oBFAx4UY7aqcE1MhPUZjPFnwzrOVFLtGQEsQePm0iJ3H
P8pLAoGAUCC6KxGF20y0uBnSLS2q3iqAqZNO57ZLnJRAscsViNEhqn4G5OZbcANa
RsyFluCwjuKFg9yW9s6ZKzjPXsAEWTyXpayICSG4gJBRfdzUol5Sjo9JweqMhiVh
5G9uBYx3+kEpYNvJDcYK89HQ3fAcfeZ1o9sXYtwccOulSQ5euAE=
-----END RSA PRIVATE KEY-----";
            var pemReader = new PemReader(new StringReader(privateKeyPem));
            AsymmetricKeyParameter keyPairRaw = (AsymmetricKeyParameter)pemReader.ReadObject();
            RSAParameters rsaParams = DotNetUtilities.ToRSAParameters((RsaKeyParameters)keyPairRaw);
            RSA rsaObj = System.Security.Cryptography.RSA.Create();
            rsaObj.ImportParameters(rsaParams);
            return rsaObj;
        }

Answer 1

The code essentially contains casting-related bugs that are most easily identified during debugging by determining the object types:

I'm getting an error in Private Key conversion, I can't decrypt. Error: System.InvalidCastException: Could not cast object of type 'Org.BouncyCastle.Crypto.AsymmetricCipherKeyPair' to type 'Org.BouncyCastle.Crypto.AsymmetricKeyParameter'.

You can't import the private key 1:1 like the public key, because both have different formats. The PemReader returns a different object type in the case of the private key, namely AsymmetricCipherKeyPair , which you cannot cast into an AsymmetricKeyParameter . This is what the error message says. So it must be:

AsymmetricCipherKeyPair keyPairRaw = (AsymmetricCipherKeyPair)pemReader.ReadObject();

When I convert it to AsymmetricCipherKeyPair type, the type does not match in the bottom line.

You need to modify this line as well. Here you have to pass a keyPairRaw.Private that must be cast to RsaPrivateCrtKeyParameters :

RSAParameters rsaParams = DotNetUtilities.ToRSAParameters((RsaPrivateCrtKeyParameters)keyPairRaw.Private);

With these changes the import of the private key works.

---

Another bug is in the decryption. The ciphertext is Base64 encoded during encryption, therefore it must be Base64 decoded during decryption and not ASCII encoded, ie correct is:

var decrypted = privateKeyDecyrpt.Decrypt(Convert.FromBase64String(encryptedPayload), RSAEncryptionPadding.Pkcs1);

---

Keep in mind that the options for importing keys in .NET are highly dependent on the version. There are versions where you can import PEM keys out-of-the-box (eg as of .NET 5), so that BouncyCastle is not needed.