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How to: Envelope a Message for Multiple Recipients 

This example creates a CMS/PKCS #7 enveloped message by using System.Security.Cryptography.Pkcs. The message is encrypted for multiple recipients. The message is then decrypted for each recipient using that recipient's private key. This example uses the EnvelopedCms object, which allows messages to be encrypted for one or more recipients, or enveloped.

Example

This example uses the following classes:

To run on a single computer, the following example requires that two public key certificates with the subject names "Recipient1" and "Recipient2" be contained in both the AddressBook and My certificate stores. This example also requires that the associated private keys be stored on that computer. The example code first acts as the message sender, and then as the message recipient. The same public key credentials are used in each role. As such, the example requires that the public key certificate be in two stores. As the sender, the example searches the AddressBook certificate store for the recipient's certificate and uses it to encrypt the message. As the recipient, it searches the My certificate store for the certificate, and uses the associated private key to decrypt the message.

NoteNote

This example is only for illustrative purposes. Production environments might use a different model in which the sender and the recipient of the message execute in different processes with their unique public key credentials.

Set up this example by using the Makecert.exe utility, one of several ways to do so. Certificate Creation Tool (Makecert.exe) is a convenient utility for testing certificates. In a production environment, certificates are generated by a certification authority.

The following Makecert commands generate the required public key certificates and private keys.

Makecert -n "CN=Recipient1" -ss My

Makecert -n "CN=Recipient2" -ss My

These commands place the appropriate public key certificates in the My certificate store. At this point, you need to export the public key certificates, and then import them into the AddressBook certificate store by following the procedure in How to: Export and Import a Public Key Certificate for each recipient.

// Copyright (c) Microsoft Corporation.  All rights reserved.
#region Using directives

using System;
using System.Security.Cryptography;
using System.Security.Cryptography.Pkcs;
using System.Security.Cryptography.X509Certificates;
using System.Text;

#endregion

namespace EnvelopAMessageForMultipleRecipients
{
    class EnvelopedCmsMultipleRecipients
    {
        const String recipientName1 = "Recipient1";
        const String recipientName2 = "Recipient2";

        static void Main(string[] args)
        {
            Console.WriteLine("System.Security.Cryptography.Pkcs " +
                "Sample: Multiple-recipient " +
                "encrypted and decrypted message");

            //  Original message.
            const String msg = "To all department heads: The following " +
                "organizational changes will be announced next week:";

            Console.WriteLine("\nOriginal message (len {0}): {1}  ",
                msg.Length, msg);

            //  Convert message to array of Unicode bytes for signing.
            UnicodeEncoding unicode = new UnicodeEncoding();
            byte[] msgBytes = unicode.GetBytes(msg);

            Console.WriteLine("\n\n------------------------------");
            Console.WriteLine("     SETUP OF CREDENTIALS     ");
            Console.WriteLine("------------------------------\n");

            //  The recipients' certificates are necessary to encrypt
            //  the message for those recipients.
            X509Certificate2Collection recipientCerts = GetRecipientCerts();

            Console.WriteLine("\n\n----------------------");
            Console.WriteLine("     SENDER SIDE      ");
            Console.WriteLine("----------------------\n");

            byte[] encodedEnvelopedCms = EncryptMsg(msgBytes,
                recipientCerts);

            Console.Write("\nMessage after encryption (len {0}):  ",
                encodedEnvelopedCms.Length);
            foreach (byte b in encodedEnvelopedCms)
            {
                Console.Write("{0:x}", b);
            }
            Console.WriteLine();

            Console.WriteLine("\n\n------------------------");
            Console.WriteLine("     RECIPIENT SIDE     ");
            Console.WriteLine("------------------------\n");

            //  Decrypts the message for one of the recipients. 
            //  Return the decrypted message to display it.
            Byte[] decryptedMsg = DecryptMsg(encodedEnvelopedCms);

            //  Convert Unicode bytes to the original message string.
            Console.WriteLine("\nDecrypted Message: {0}",
                unicode.GetString(decryptedMsg));
        }

        //  Open the AddressBook (called Other in Internet Explorer) 
        //  certificate store and search for recipient
        //  certificates with which to encrypt the message. It must
        //  include two certificates: one with the subject name  
        //  "Recipient1", and one with the subject name "Recipient2".
        static public X509Certificate2Collection GetRecipientCerts()
        {
            //  Open the AddressBook local user X509 certificate store.
            X509Store storeAddressBook = new X509Store(StoreName.
                AddressBook, StoreLocation.CurrentUser);
            storeAddressBook.Open(OpenFlags.ReadOnly);

            //  Display certificates to help troubleshoot the 
            //  example's setup.
            Console.WriteLine(
                "Found certs with the following subject names in the " +
                "{0} store:", storeAddressBook.Name);
            foreach (X509Certificate2 cert in storeAddressBook.Certificates)
            {
                Console.WriteLine("\t{0}", cert.SubjectName.Name);
            }

            //  Get recipient certificates.
            //  For purposes of this sample, do not validate the
            //  certificates. Note that in a production environment,
            //  validating the certificates will probably be necessary.

            //  Get first recipient certificate.
            X509Certificate2Collection certColl = storeAddressBook.
                Certificates.Find(X509FindType.FindBySubjectName,
                recipientName1, false);
            Console.WriteLine(
                "Found {0} certificates in the {1} store with name {2}",
                certColl.Count, storeAddressBook.Name, recipientName1);
            X509Certificate2Collection recipientCerts =
                new X509Certificate2Collection();

            //  Check to see if the certificate suggested by the example
            //  requirements is not present.
            if (certColl.Count == 0)
            {
                Console.WriteLine(
                    "A suggested certificate to use for this example " +
                    "is not in the certificate store. Select " +
                    "an alternate certificate to use for " +
                    "signing the message.");
            }

            recipientCerts.Add(certColl[0]);

            //  Get second recipient certificate.
            certColl = storeAddressBook.
                Certificates.Find(X509FindType.FindBySubjectName,
                recipientName2, false);
            Console.WriteLine(
                "Found {0} certificates in the {1} store with name {2}",
                certColl.Count, storeAddressBook.Name, recipientName2);

            //  Check to see if the certificate suggested by the example
            //  requirements is not present.
            if (certColl.Count == 0)
            {
                Console.WriteLine(
                    "A suggested certificate to use for this example " +
                    "is not in the certificate store. Select " +
                    "an alternate certificate to use for " +
                    "signing the message.");
            }

            recipientCerts.Add(certColl[0]);

            storeAddressBook.Close();

            return recipientCerts;
        }

        //  Encrypt the message for each recipient by using the public
        //  key of that recipient. This is done by 
        //  enveloping the message by using an EnvelopedCms object.
        static public byte[] EncryptMsg(
            Byte[] msg,
            X509Certificate2Collection recipientCerts)
        {
            //  Place message in a ContentInfo object.
            //  This is required to build an EnvelopedCms object.
            ContentInfo contentInfo = new ContentInfo(msg);

            //  Instantiate EnvelopedCms object with the ContentInfo
            //  above.
            //  Has default SubjectIdentifierType IssuerAndSerialNumber.
            //  Has default ContentEncryptionAlgorithm property value
            //  RSA_DES_EDE3_CBC.
            EnvelopedCms envelopedCms = new EnvelopedCms(contentInfo);

            //  Formulate a CmsRecipientCollection object that
            //  represents information about the set of recipients 
            //  to encrypt the message for.
            CmsRecipientCollection recips =
                new CmsRecipientCollection(
                SubjectIdentifierType.IssuerAndSerialNumber,
                recipientCerts);

            Console.WriteLine("\nEncrypting data for multiple " +
                "recipients with subject names: ");
            foreach (CmsRecipient recip in recips)
            {
                Console.WriteLine("\t" +
                    recip.Certificate.SubjectName.Name);
            }

            //  Encrypt the message for the collection of recipients.
            envelopedCms.Encrypt(recips);
            Console.WriteLine("Done.");

            //  The encoded EnvelopedCms message contains the message
            //  ciphertext and the information about each recipient 
            //  that the message was enveloped for.
            return envelopedCms.Encode();
        }

        //  Decrypt the encoded EnvelopedCms message for one of the
        //  recipients.
        static public Byte[] DecryptMsg(byte[] encodedEnvelopedCms)
        {
            //  Prepare object in which to decode and decrypt.
            EnvelopedCms envelopedCms = new EnvelopedCms();

            //  Decode the message.
            envelopedCms.Decode(encodedEnvelopedCms);

            //  Display the number of recipients the message is
            //  enveloped for; it should be 2 for this example.
            DisplayEnvelopedCms(envelopedCms, false);

            //  Decrypt the message.
            //  The message is decrypted for the recipient that
            //  you find the first matching private key for.
            //  A line similar to the following, however, 
            //  decrypts the message for a specified recipient. In
            //  in this case, the first recipient.
            //  envelopedCms.Decrypt(envelopedCms.RecipientInfos[0]);
            Console.Write("Decrypting Data for one recipient ... ");
            envelopedCms.Decrypt();
            Console.WriteLine("Done.");

            //  The decrypted message occupies the ContentInfo property
            //  after the Decrypt method is invoked.
            return envelopedCms.ContentInfo.Content;
        }

        //  Display the ContentInfo property of an EnvelopedCms object.
        static private void DisplayEnvelopedCmsContent(String desc,
            EnvelopedCms envelopedCms)
        {
            Console.WriteLine(desc + " (length {0}):  ",
                envelopedCms.ContentInfo.Content.Length);
            foreach (byte b in envelopedCms.ContentInfo.Content)
            {
                Console.Write(b.ToString() + " ");
            }
            Console.WriteLine();
        }

        //  Display some properties of an EnvelopedCms object.
        static private void DisplayEnvelopedCms(EnvelopedCms e,
            Boolean displayContent)
        {
            Console.WriteLine("\nEnveloped PKCS #7 Message Information:");
            Console.WriteLine(
                "\tThe number of recipients for the Enveloped PKCS #7 " +
                "is:  {0}", e.RecipientInfos.Count);
            for (int i = 0; i < e.RecipientInfos.Count; i++)
            {
                Console.WriteLine(
                    "\tRecipient #{0} has type {1}.",
                    i + 1,
                    e.RecipientInfos[i].RecipientIdentifier.Type);
            }
            if (displayContent)
            {
                DisplayEnvelopedCmsContent("Enveloped PKCS #7 Content", e);
            }

            Console.WriteLine();
        }
    }
}

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