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How to decrypt an OpenPGP RSA Encrypted text file in C#?

I am hoping that this question can be met with some guidance for someone who is beginning to work with encryption/decryption in C#. There are existing examples on the web regarding this, but I am truthfully struggling to put it all into practice for my given situation.
If given a text file that has been encrypted using OpenPGP with RSA, what is the best method to decrypt this in C#?
This is what I am attempting:
Using Kleopatra OpenPGP, I am generating a key pair using 2048bit RSA. This generates a private and public key.
I am then encrypting/signing a text file with a few word in it as a test.
In C#, I want to decrypt this text file.
Current code:
byte[] encryptedData = File.ReadAllBytes("C:\\PGP Encryption\\test.txt.gpg"); // The encrypted text file generated by Kleopatra.
using (RSACryptoServiceProvider RSA = new RSACryptoServiceProvider())
{
//Import the RSA Key information. This needs
//to include the private key information.
RSA.ImportParameters(RSAKeyInfo);
//Decrypt the passed byte array and specify OAEP padding.
decryptedData = RSA.Decrypt(DataToDecrypt, DoOAEPPadding);
}
return decryptedData;
Unfortunately, the RSA.Decrypt() call throws an exception that reads "The data to be decrypted exceeds the maximum for this modulus of 128 bytes."
I also do not believe that my private key is actually being loaded, as I'm not explicitly stating where the key is. But I don't see how the RSAParameters object is supposed to get populated otherwise.
If anyone can point me in the right direction to decrypt a file in this way, thank you in advance for your time and information.

It's looks like you need this library (see Decrypt section) https://github.com/mattosaurus/PgpCore

OpenSSL.NET How to implement PKCS7?

I am writing an application where I need to encrypt, sign and wrap some content into a PKCS7/CMS structure. I am using OpenSSL.NET for the task (Wrapper for .NET).
I don't have much yet, I have successfully encrypted some data but
RSA toServer = cert.Certificate.PublicKey.GetRSA();
RSA fromClient = cert.Certificate.PublicKey.GetRSA();
byte[] cipherText = toServer.PublicEncrypt(Encoding.UTF8.GetBytes("Hello World!"), RSA.Padding.PKCS1);
byte[] plainText = fromClient.PrivateDecrypt(cipherText, RSA.Padding.PKCS1);
This works, it successfully encrypts and decrypts the data, but now I want to move further and implement PKCS7/CMS so that I have a standard format for my messages. I have found the below object that seem to point in the right direction but I fail to find some documentation on how to use it:
OpenSSL.X509.PKCS7 p7 = new OpenSSL.X509.PKCS7( ? );
Any suggestions or references to documentation are welcome.

AES Decryption - Porting code from C# to Java

I am trying to port the following code from C# into Java. I have made multiple attempts to try and decrypt my encrypted data and I get gibberish every time. The code below uses the org.bouncycastle library and unfortunately there doesn't seem to be a 1-1 mapping between the C# code and the Java code.
I basically know three things:
byte[] file - This contains my encrypted file. Usually a pretty large array of bytes.
byte[] padding - It is 32*bytes* every time and it seems that the first 16 bytes of this are used as the IV.
byte[] aesKey - It is 32*bytes* every time and I do not know how exactly the C# code is using this array.
Original C# Code
private byte[] decryptmessage(byte[] cmessage, byte[] iVector, byte[] m_Key)
{
{
//// randomly generated number acts as inetialization vector
m_IV = new byte[16];
Array.Copy(iVector, 0, m_IV, 0, 16);
// GenerateAESKey();
KeyParameter aesKeyParam = ParameterUtilities.CreateKeyParameter("AES", m_Key);
ParametersWithIV aesIVKeyParam = new ParametersWithIV(aesKeyParam, m_IV);
IBufferedCipher cipher = CipherUtilities.GetCipher("AES/CFB/NoPadding");
cipher.Init(false, aesIVKeyParam);
return cipher.DoFinal(cmessage);
}
}
My attempt in Java
private static byte[] decryptMessage(byte[] file, byte[] iVector, byte[] aesKey) throws Exception {
IvParameterSpec spec = new IvParameterSpec(Arrays.copyOfRange(iVector, 0, 16));
SecretKeySpec key = new SecretKeySpec(Arrays.copyOfRange(aesKey, 0, 16), "AES");
Cipher cipher = Cipher.getInstance("AES/CFB/NoPadding");
cipher.init(Cipher.DECRYPT_MODE, key, spec);
return cipher.doFinal(file);
}
P.S: This is the final step of decryption. Before all this I had to take out some initial set of bytes from my encrypted file and decrypt them using an RSA private key to get this AES key.
If someone has a link / document I can read that properly explains the whole process of using AES to encrypt a file, then using RSA on the key and iv to the begining of the encrypted file, I will be extremely happy. I have just been staring at the C# code, I'd like to see something with pictures.
EDIT: Bytes not bits.
EDIT2: Renamed padding to iVector for consistency and correctness.

In the C# code, you initialize the key with 256 bits (32 bytes) and thus get AES-256. In the Java code, you only use 128 bit (16 bytes) and get AES-128.
So the fix is probably:
SecretKeySpec key = new SecretKeySpec(aesKey, "AES");
You might then find that Java doesn't want to use 256 bit keys (for legal reason). You then have to intall the Java Cryptography Extension (JCE) Unlimited Strength Jurisdiction Policy Files 6.

PFX Self Signed x509 Certificate with password secure

After looking at how to generate self-signed digital signatures from Creating a self-signed certificate in C#, I can call CreateSelfSignCertificatePfx and get PXF data in a byte array back, which can then be used within an X509Certificate2 object to sign and verify. Example...
byte[] pfx = Certificate.CreateSelfSignCertificatePfx("O=Company,CN=Firstname,SN=Lastname", DateTime.Now, DateTime.Now.AddYears(1), "password");
X509Certificate2 cert = new X509Certificate2(pfx, "password");
byte[] publicBytes = cert.RawData;
RSACryptoServiceProvider rsa = (RSACryptoServiceProvider)cert.PrivateKey;
byte[] signedData = rsa.SignData(new System.Text.UTF8Encoding().GetBytes("Test"), new SHA1CryptoServiceProvider());
RSACryptoServiceProvider rsa2 = (RSACryptoServiceProvider)new X509Certificate2(publicBytes).PublicKey.Key;
bool verified = rsa2.VerifyData(new System.Text.UTF8Encoding().GetBytes("Test"), new SHA1CryptoServiceProvider(), signedData);
This works. My concern though is the original bytes, byte[] pfx from above, need to be stored in a DB (to sign stuff). The question becomes, how secure are the bytes in this format? I know you need the password to construct the new X509Certificate2 with a private key, but in a general sense, how secure are the bytes without the password? I have no problems encrypting these bytes as an added layer, but is that necessary?
According to X509Certificate2.X509Certificate2(Byte[], String) Constructor
Calling this constructor with the correct password decrypts the private key and saves it to a key container.
I just want to ensure the private key is safe without the password.

In my eyes the question is not whether you should put the "bytes" in the database, but more, would you put the file with the private key in your file system.
In the way you're doing it, it's essentially the same thing. You're just storing the bytes that make up the cert file.
I may be failing to understand the difference here, but they bytes and the file are essentially the same thing, the only difference being the fact that one has to gain access to the db to get them.

Use a smartcard or token to store your private key.
UPDATE:
The Pvt key can be accessed by anyone who can access the machine.

The private keys in a PFX (PKCS#12) are stored encrypted, which is of course what the password is for. Not all of a PFX is encrypted, the structural pieces stay plaintext to contain metadata about the contents (like what encryption algorithm was used).
Based on inspecting the file, as of Windows 7 the private keys are encrypted using 3-key (168-bit) 3DES. The key is derived via a complex formula involving your password; there's nothing saved in the file which gives any indication as to what your password was, how long it was, et cetera.
The password is usually proven correct by the addition of a MAC on the contents, which uses the same password for its key derivation function. In the possible case of the MAC password and the encryption password being different (which I've personally never seen) the password is verified by the structural information in the encrypted payload.
DES' weakness mainly lay in the small keysize, it's easily brute forcable today. A 3-key 3DES key has 112 more semantic bits than a (1)DES key, making it take 2^112 (~5 x 10^33) times longer to break.
So, at the end of the day, the private key is cryptographically sound. But like anything with a password-based input, if you use a bad password that is easily guessed then it can be cracked by brute force.

Digitally sign in PHP using private key, verify in C#

I am working on a feature that needs me to digitally sign a short string in PHP, and verify the string's signature in C#.
I would really like to use openssl_sign in PHP, because of its simplicity, but all the information I can find on Google indicates that this will not work.
There are some external libraries that claim to do this well, however as this is a hobby project I would rather not purchase such a library.
So what are the alternatives here? Full interoperability between C# and PHP is required. Libraries besides OpenSSL can be used.

I've done something very similar using Bouncy Castle Crypto APIs. It appears PHP openssl_sign uses SHA1 by default. If you are using anything other than the default you'll need to change the algorithm parameter for GetSigner.
string base64pubkey = "<!-- BASE64 representation of your pubkey from open ssl -->";
RsaKeyParameters pubKey = PublicKeyFactory.CreateKey(Convert.FromBase64String(base64pubkey)) as RsaKeyParameters;
byte[] signature = Convert.FromBase64String("<!-- BASE64 representation of your sig -->");
byte[] message = Encoding.ASCII.GetBytes("Something that has been signed");
ISigner sig = SignerUtilities.GetSigner("SHA1WithRSAEncryption");
sig.Init(false, pubKey);
sig.BlockUpdate(message, 0, message.Length);
if (sig.VerifySignature(signature))
{
Console.WriteLine("all good!");
}

You may use to check the digital signature smth like this:
string publicKey = "some key";
// Verifying Step 1: Create the digital signature algorithm object
DSACryptoServiceProvider verifier = new DSACryptoServiceProvider();
// Verifying Step 2: Import the signature and public key.
verifier.FromXmlString(publicKey);
// Verifying Step 3: Store the data to be verified in a byte array
FileStream file = new FileStream(args[0], FileMode.Open, FileAccess.Read);
BinaryReader reader = new BinaryReader(file2);
byte[] data = reader.ReadBytes((int)file2.Length);
// Verifying Step 4: Call the VerifyData method
if (verifier.VerifyData(data, signature))
Console.WriteLine("Signature verified");
else
Console.WriteLine("Signature NOT verified");
reader.Close();
file.Close();

Is there a reason you need something as complex as SSL signing? Can't you just use a simple one-way hash like MD5/SHA-1 on the string? If all you're looking for is verification that the string wasn't tampered with, that should be sufficient.

So looking at this - this guy appears to have asymmetric signing and encrypting working between PHP and C#. Signing should not be a problem, SHA* and MD* are standard, and so it's very very unlikely that is going to not be compatible (although you should be looking at SHA256 as MD* and SHA1 are deprecated due to vulnerabilities)

We're missing some context as to why you need to sign it. You may not need to.
The important question is: what guarantees do you need from your data?
If all you need to do is verify the integrity of the data, a hash will do the job. If you need to verify where it's coming from, you need to sign it. If you need both, hash it, concatenate the payload with the hash, and sign the whole thing.
Regarding cross-platform libraries... you really should need to worry about it. A SHA1 is a SHA1 is a SHA1, no matter which library generated it. Same thing with generating and verifying digital signatures. Use what's easiest in PHP and use what's easiest in C#. If they're both set up correctly you shouldn't need to worry about it.