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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

AES Encryption Not Working Properly in UWP

We have an C++ application where it encrypts password using AES algorithm through openssl API's (EVP_EncryptInit_ex, EVP_EncryptUpdate, EVP_EncryptFinal_ex).
Now I try to develop UWP application through which I want to do same authentication as the above legacy app does.
But when I'm using API's from "Windows.Security.Cryptography" my authentication fails as the encrypted data output is different from the legacy app, so decryption fails and there by authentication.
Legacy app uses one Key and initialization vector and same are used in my UWP app also. Legacy app uses cipher mode CBC, I had used "AesCbcPKCS7" in my UWP but the encrypted output is different.
I also tried just "AesCbc" and padded data as per PKCS7 padding manually still the encrypted output is different and my authentication is failing.
Kindly help with the above issue.
Some of Legacy App Code API Flow Sample:
EVP_CIPHER_CTX_init(&ctx);
EVP_EncryptInit_ex(&ctx, EVP_aes_128_cbc(), NULL, key, iv);
EVP_EncryptUpdate(&ctx, output, &nOutputSize, input, strlen((const char *)input)+1);
EVP_EncryptFinal_ex(&ctx, output + nOutputSize, &nTmplen);
EVP_CIPHER_CTX_cleanup(&ctx);
UWP Code sample below:
IBuffer iBuf = CryptographicBuffer.ConvertStringToBinary(strPwd, BinaryStringEncoding.Utf8);
IBuffer iPubKey = CryptographicBuffer.CreateFromByteArray(PUB_KEY);
SymmetricKeyAlgorithmProvider objAlg = SymmetricKeyAlgorithmProvider.OpenAlgorithm(SymmetricAlgorithmNames.AesCbcPkcs7);
CryptographicKey cryptKey = objAlg.CreateSymmetricKey(iPubKey);
IBuffer iv = CryptographicBuffer.CreateFromByteArray(INIT_VECTOR);
IBuffer encryptPwd = CryptographicEngine.Encrypt(cryptKey, iBuf, iv);
string strEncPwd = CryptographicBuffer.EncodeToBase64String(encryptPwd);

Cannot figure out why PHP to C# .NET will not symmetrically encrypt/decrypt

I've been working on this for a while and I don't know much about either PHP or C#. We are building an application that is using AES 128 CBC mode encryption to store things in the database. One part is PHP and JS, the other is C# .NET WPF.
The person who wrote the PHP used the Mcrypt library to crypt/decrypt. I'm using the Chilkat library to encrypt/decrypt. Chilkat had a default C# example that is supposed to mimic the PHP Mcrypt.
Currently I can symmetrically encrypt/decrypt things on .Net, and .Net can decrypt anything from PHP. However the PHP side cannot decrypt anything that I encrypt to the database from the .Net side.
I've narrowed at least part of down to encoding issues, but I'm not sure how to fix it. The decryption scheme on PHP side usually decrypts to ASCII, but for things that I send it decrypts to UTF-8. I've tried to decrypt it then encode from UTF-8 to ASCII to no avail.
I'll show you the in/outputs and the functions. The IV is being set to 16 ASCII 0s to help my debugging along even though it shouldn't really matter.
input from .Net to mcrypt_encrypt func: string "1220"
output: 3tRIG7qUxUsU7WoXDybRRcdQRobOfeFGtQ438V7XRD8=
Parameter input into database = 'same as above'
input of PHP side decrypt func = 3tRIG7qUxUsU7WoXDybRRcdQRobOfeFGtQ438V7XRD8= 'same as above'
output of the mcrypt_decrypt function = ��J���{$�Z'?�u 'iconv says utf-8 encoding'
Ask for anything else and I'll get it if it would help. I'm sure this is some stupid easy problem I can't see.
PHP side - if it matters the PHP charset is set to UTF-8
function encrypt($input)
{
$this->iv_size = mcrypt_get_iv_size(MCRYPT_RIJNDAEL_128, MCRYPT_MODE_CBC);
/*No longer using random iv :(
$this->iv = mcrypt_create_iv($this->iv_size, MCRYPT_RAND);*/
//use 16 zeros
$this->iv = '0000000000000000';
$encrypted = $this->iv .mcrypt_encrypt(MCRYPT_RIJNDAEL_128, KEY, $input, MODE, $this->iv);
//Finally encode this as base 64, see http://php.net/manual/en/function.base64-encode.php
$encrypted = base64_encode($encrypted);
return $encrypted;
}
function decrypt($input)
{
/*Get our message back!!!
First decode the base 64 string. Note, de/encoding bas 64 != encryption*/
$ciphertext_dec = base64_decode($input);
//Get the iv back out for decryption
$iv_size = mcrypt_get_iv_size(MCRYPT_RIJNDAEL_128, MCRYPT_MODE_CBC);
//$iv_dec = substr($ciphertext_dec, 0, $iv_size);*/
$iv_dec = '0000000000000000';
//Now get the text of encrypted message (all but the iv in front)
$ciphertext_dec = substr($ciphertext_dec, $iv_size);
//Now decrypt the message
$plaintext_dec = mcrypt_decrypt(MCRYPT_RIJNDAEL_128, KEY, $ciphertext_dec, MCRYPT_MODE_CBC, $iv_dec);
//Test
//test
//$plaintext_dec = iconv("UTF-8", "ASCII", $plaintext_dec);
//echo mb_detect_encoding($plaintext_dec, "auto");
//echo $plaintext_dec;
/*However, we might now have blank space # end of output b/c
remember we de/encrypt via block, so a 10 char long message
could be padded to 16 char long with blank spaces. Get rid of those.*/
$plaintext_dec = trim($plaintext_dec);
//return so we can compare to, i.e., original input
return $plaintext_dec;
}
.NET C#
public string mcrypt_encrypt(string plainText)
{
plainText = Encoding.ASCII.GetString(crypt.IV) + plainText;
byte[] myText = Encoding.ASCII.GetBytes(plainText);
// Do 128-bit AES encryption:
byte[] cipherText = crypt.EncryptBytes(myText);
return Convert.ToBase64String(cipherText);
}
public string mcrypt_decrypt(string cipher_text)
{
byte[] cipher_dec = Convert.FromBase64String(cipher_text);
byte[] plainBytes = crypt.DecryptBytes(cipher_dec);
string decrypted = Encoding.ASCII.GetString(plainBytes);
string plain_text = decrypted.Substring(16, decrypted.Length - 16);
return plain_text.TrimEnd('\0');
}
C# Chilkat init:
// AES is also known as Rijndael.
crypt.CryptAlgorithm = "aes";
// CipherMode may be "ecb" or "cbc"
crypt.CipherMode = "cbc";
// KeyLength may be 128, 192, 256
crypt.KeyLength = 128;
// Pad with NULL bytes (PHP pads with NULL bytes)
crypt.PaddingScheme = 3;
// EncodingMode specifies the encoding of the output for
// encryption, and the input for decryption.
// It may be "hex", "url", "base64", or "quoted-printable".
crypt.EncodingMode = "hex";
// The secret key must equal the size of the key. For
// 256-bit encryption, the binary secret key is 32 bytes.
// For 128-bit encryption, the binary secret key is 16 bytes.
string keyAscii = #"&=*FS6wksG#Zs3qG";
crypt.SecretKey = Encoding.UTF8.GetBytes(keyAscii);
crypt.Charset = "ASCII";
crypt.SetEncodedIV("\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0", "ascii");

You have a variety of different problems here. The issue causing the problem you seeing in not being able to decrypt in PHP data that was encrypted in .NET is that in the PHP version you're performing a substr on the ciphertext prior to decryption. The comments in the code indicate that you're removing the IV, except that that seems to have only been relevant in a prior version of your code when you were (correctly) using a random IV each time - now you're just discarding the first 16 bytes of ciphertext which due to the mode of operation corrupts the subsequent data block.
The other problem (though masked by the fact that you're discarding the first 16 bytes of the plaintext data when decrypting in .NET) is that the IV you're using in .NET (16 bytes of 0x00) is not the same as the IV you're using in PHP (16 '0' characters = 16 bytes of 0x30).
I would suggest reverting to using a random IV for every encryption and prepending the IV to the ciphertext after encryption When decrypting, read the IV from the first bytes of the ciphertext then decrypt the remainder. This is much more secure than having a static IV, especially when the data being encrypted is likely to often be the same.

It looks like it's a simple character encoding issue. Your C# code is getting the ASCII representation of your string and encrypting that, but your PHP code is decrypting it and expecting it to be UTF-8.
Try swapping your Encoding.ASCII calls for Encoding.UTF8 and make sure your crypt.Charset and crypt.SetEncodedIV are UTF8 as well

input too large for RSA cipher with BouncyCastle

I'm trying to write a service to SFTP to a server on a given interval, download all files in a directory, and then decrypt them for processing.
The decrypt method was working at one point, and I have no modified it since. I am still using the same keys as when it did successfully work.
I am using the PGPEncrypt, PGPDecrypt, and PGPEncryptionKeys classes for BouncyCastle found here: https://github.com/sledwith/PGP-Decryption-With-C-Sharp
Additionally, I have modified the code as shown here: Exception on decrypting file using BouncyCastle PGP
If you noticed, I commented on how that fixed my code and the decryption worked.
Now, it isn't.
Does anyone have some insight why this might have happened, or how I could go about fixing it?

I am going to make a guess that you are using a 1024 bit RSA public/private key and trying to encrypt 128 bytes of something with it? If I am right, use a larger RSA key (RSA 4096 will allow you to encrypt up to ~500 bytes).
I note in the linked post you say this is sporadic. Some googling indicates that if you have leading zeros in the to-be-encrypted bytes it may be able to handle a full 128 bytes.
Also - if you are just encrypting "data" with the keypair, you need to be certain it will not overrun these limitations (240 or so bytes for RSA 2048 and 500 or so for RSA 4096 accounting for padding). Good practice and future proofing would be to implement a symmetric key encryption of all your data and just use the RSA key to encrypt / decrypt your keys.

If you're not dead-set on using the PGP process explicitly, you might use my library here:
https://github.com/zenith-nz/ObscurCore
Its "banner feature" is not yet fully active (creating encrypted archives of a kind), but it does everything that you want it for, it appears - it does key derivation with a ECDHC scheme, UM1, and implements a large variety of block and stream ciphers. It's built on top of BouncyCastle.

I think I resolved this problem; please try this code.
public string StringToDecrypt(string text)
{
byte[] toDecrypt = Convert.FromBase64String(text);
AsymmetricCipherKeyPair keyPair;
using (var reader = File.OpenText(#"Private Key File Path"))
{
keyPair = (AsymmetricCipherKeyPair) new PemReader(reader).ReadObject();
}
var engine = new RsaEngine();
engine.Init(false, keyPair.Private);
return Encoding.UTF8.GetString(engine.ProcessBlock(toDecrypt, 0, toDecrypt.Length));
}

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.