Decrypt and verify PKCS7 message in C#

Question

I am trying to decrypt and verify PKCS7 response I am getting from client using C#. Initially I tried Enveloping and signing my payload and followed the answer mentioned here.

Now I am getting response in PKCS7 again and having trouble in Decrypting and verifying the response.

I tried using EnvelopedCMS:

        ecms.Decode(Convert.FromBase64String(payloadContent));
        ecms.Decrypt(new X509.X509Certificate2Collection { _signerCert });
        string decodedContent = Encoding.UTF8.GetString(ecms.ContentInfo.Content);

Here _signerCert is my own certificate with private key.

I could see my required response in the decodedContent along with some of the client's information and some unknown ASCII characters.

Does anyone know how can I achieve decryption and verification of incoming response?

Solution found:

I used SignedCms like this as suggested by @bartonjs

SignedCms signedCMS = new SignedCms();

signedCMS.Decode(ecms.ContentInfo.Content);

If you ran it through sign then encrypt when building it, you'd run it through decrypt then verify when receiving it. Here you've only run it through decrypt. — bartonjs, Dec 05 '21 at 16:38
Assuming you did what the linked post said, now you’d send the content into SignedCms.Decode. — bartonjs, Dec 05 '21 at 17:48
`SignedCms signedCMS = new SignedCms();` `signedCMS.Decode(ecms.ContentInfo.Content);` It worked. Thankyou so much. — Rocky Mishra, Dec 05 '21 at 18:00

score 1 · Answer 1 · answered Dec 04 '21 at 11:13

1

ICryptoManager objCM = new CryptoManager();

ICryptoContext objContext = objCM.OpenContext( "", true, Missing.Value );

ICryptoMessage objMsg = objContext.CreateMessage( true );

// Obtain encryption certificate
ICryptoCert objCert = objCM.ImportCertFromFile( @"c:\path\mycert.cer" );
objMsg.AddRecipientCert( objCert );


txtResult.Text = objMsg.EncryptText("my secret phrase");

answered Dec 04 '21 at 11:13

A.M

119
4

How do it decrypts and verifies the incoming PKCS#7 payload? – Rocky Mishra Dec 04 '21 at 11:39

Decrypt and verify PKCS7 message in C#

1 Answers1

Linked