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Cómo firmar y proteger un mensaje

En este ejemplo se crea un mensaje CMS/PKCS #7 firmado y protegido mediante System.Security.Cryptography.Pkcs. Primero, un solo firmante firma el mensaje que, a continuación, se cifra para un solo destinatario. Se descifra el mensaje mediante la clave privada del destinatario y se verifica la firma. En el ejemplo se utiliza el objeto SignedCms, que permite que un mensaje sea firmado y contrafirmado por uno o varios firmantes. También se utiliza el objeto EnvelopedCms, que permite cifrar o proteger mensajes para uno o varios destinatarios.

Ejemplo

Description

En este ejemplo se utilizan las siguientes clases:

Para la ejecución de este ejemplo en un solo equipo se necesita lo siguiente:

  • Un certificado de clave pública con el nombre de sujeto "MessageSigner1" en el almacén My certificate.

  • Otro certificado con el nombre de sujeto "Recipient1" en los almacenes AddressBook y My certificate.

  • Claves privadas asociadas almacenadas en el equipo.

El código de ejemplo actúa primero como remitente y después como destinatario del mensaje. En el ejemplo se utilizan las mismas credenciales de clave pública en cada función. Por ello, para el ejemplo es necesario que el certificado de clave pública para el nombre de sujeto "Recipient1" se encuentre en dos almacenes. Para la función de remitente, se busca en el almacén de certificados AddressBook el certificado del destinatario, que se utiliza para cifrar el mensaje. Con respecto a la función de destinatario, en el ejemplo se busca el certificado en el almacén My certificate y se utiliza la clave privada asociada para descifrar el mensaje.

NotaNota

Este ejemplo tiene únicamente fines ilustrativos. En los entornos de producción se podría utilizar un modelo distinto, en el que el remitente y el destinatario del mensaje se ejecuten en procesos diferentes con sus credenciales de clave pública exclusivas.

Configure este ejemplo mediante la utilidad Makecert.exe, que ofrece uno de los distintos métodos posibles de hacerlo. La Herramienta Creación de certificados (Makecert.exe) es una práctica utilidad para probar certificados. En un entorno de producción, los certificados son generados por una entidad emisora de certificados.

Mediante los siguientes comandos de Makecert se generan los certificados de clave pública y las claves privadas que se necesitan.

Makecert -n "CN=MessageSigner1" -ss My

Makecert -n "CN=Recipient1" -sky exchange -ss M

Con estos comandos se colocan en el almacén My certificate los certificados de clave pública correspondientes. Para obtener el certificado de clave pública correspondiente al nombre de sujeto "Recipient1" en el almacén de certificados AddressBook, exporte el certificado de clave pública e impórtelo después al almacén AddressBook según el procedimiento que se describe en Cómo exportar e importar un certificado de clave pública.

// 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 SigningAndEnvelopingMessage
{
    class EnvelopedSignedCms
    {
        const String signerName = "MessageSigner1";
        const String recipientName = "Recipient1";

        static void Main(string[] args)
        {
            byte[] origMsg;

            Console.WriteLine("System.Security.Cryptography.Pkcs " +
                "Sample: Encrypted, signed, decrypted, and " +
                "verified message");
            //  Original message.
            const String msg = "Here are the sales figures for the " +
                "upcoming quarterly report to Wall Street.";

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

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

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

            //  The signer's private key, obtained by association with
            //  their signing certificate, is necessary to sign the 
            //  message.
            X509Certificate2 signerCert = GetSignerCert();

            //  The recipient's certificate is necessary to encrypt
            //  the message for that recipient.
            X509Certificate2 recipientCert = GetRecipientCert();

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

            byte[] encodedSignedCms = SignMsg(msgBytes, signerCert);

            //  Encrypt the encoded SignedCms message.
            byte[] encodedEnvelopedCms = EncryptMsg(encodedSignedCms,
                recipientCert);

            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");

            encodedSignedCms = DecryptMsg(encodedEnvelopedCms);

            //  Get the original message back after verification so
            //  it can be displayed.
            if (VerifyMsg(encodedSignedCms, out origMsg))
            {
                Console.WriteLine("\nMessage verified");
            }
            else
            {
                Console.WriteLine("\nMessage failed to verify");
            }

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

        }

        //  Open the My (or Personal) certificate store. Search for
        //  credentials with which to sign the message. The certificate
        //  must have a the subject name "MessageSigner1".
        static public X509Certificate2 GetSignerCert()
        {
            //  Open the My certificate store.
            X509Store storeMy = new X509Store(StoreName.My,
                StoreLocation.CurrentUser);
            storeMy.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:", storeMy.Name);
            foreach (X509Certificate2 cert in storeMy.Certificates)
            {
                Console.WriteLine("\t{0}", cert.SubjectName.Name);
            }

            //  Find the signer's certificate.
            X509Certificate2Collection certColl =
                storeMy.Certificates.Find(X509FindType.FindBySubjectName,
                signerName, false);
            Console.WriteLine(
                "Found {0} certificates in the {1} store with name {2}",
                certColl.Count, storeMy.Name, signerName);

            //  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.");
            }

            storeMy.Close();

            return certColl[0];
        }

        //  Open the AddressBook (called Other in Internet Explorer) 
        //  certificate store and search for a recipient
        //  certificate with which to encrypt the message. The certificate
        //  must have the subject name "Recipient1".
        static public X509Certificate2 GetRecipientCert()
        {
            //  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 certificate.
            //  For purposes of this sample, do not validate the
            //  certificate. Note that in a production environment,
            //  validating the certificate will probably be necessary.
            X509Certificate2Collection certColl = storeAddressBook.
                Certificates.Find(X509FindType.FindBySubjectName,
                recipientName, false);
            Console.WriteLine(
                "Found {0} certificates in the {1} store with name {2}",
                certColl.Count, storeAddressBook.Name, recipientName);

            //  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.");
            }

            storeAddressBook.Close();

            return certColl[0];
        }

        //  Sign the message by the using the private key of the signer.
        //  Note that signer's public key certificate is input here 
        //  because it is used to locate the corresponding private key.
        static public byte[] SignMsg(
            Byte[] msg,
            X509Certificate2 signerCert)
        {
            //  Place message in a ContentInfo object.
            //  This is required to build a SignedCms object.
            ContentInfo contentInfo = new ContentInfo(msg);

            //  Instantiate SignedCms object with the ContentInfo above.
            //  Has default SubjectIdentifierType IssuerAndSerialNumber.
            //  Has default Detached property value false, so message is
            //  included in the encoded SignedCms.
            SignedCms signedCms = new SignedCms(contentInfo);

            //  Formulate a CmsSigner object, which has all the needed
            //  characteristics of the signer.
            CmsSigner cmsSigner = new CmsSigner(signerCert);

            //  Sign the PKCS #7 message.
            Console.Write("Computing signature with signer subject " +
                "name {0} ... ", signerCert.SubjectName.Name);
            signedCms.ComputeSignature(cmsSigner);
            Console.WriteLine("Done.");

            //  Encode the PKCS #7 message.
            return signedCms.Encode();
        }

        //  Verify the encoded SignedCms message and return a Boolean
        //  value that specifies whether the verification was successful.
        //  Also return the original message that was signed, which is
        //  available as part of the SignedCms message after it
        //  is decoded.
        static public bool VerifyMsg(byte[] encodedSignedCms,
            out byte[] origMsg)
        {
            //  Prepare a SignedCms object in which to decode
            //  and verify.
            SignedCms signedCms = new SignedCms();

            signedCms.Decode(encodedSignedCms);

            //  Catch a verification exception in the event you want to
            //  advise the message recipient that security actions
            //  might be appropriate.
            try
            {
                //  Verify signature. Do not validate signer
                //  certificate for the purposes of this example.
                //  Note that in a production environment, validating
                //  the signer certificate chain will probably be
                //  necessary.
                Console.Write("Checking signature on message ... ");
                signedCms.CheckSignature(true);
                Console.WriteLine("Done.");
            }
            catch (System.Security.Cryptography.CryptographicException e)
            {
                Console.WriteLine("VerifyMsg caught exception:  {0}",
                    e.Message);
                Console.WriteLine("The message may have been modified " +
                    "in transit or storage. Authenticity of the " +
                    "message is not guaranteed.");
                origMsg = null;
                return false;
            }

            origMsg = signedCms.ContentInfo.Content;

            return true;
        }

        //  Encrypt the message with the public key of
        //  the recipient. This is done by enveloping the message by
        //  using a EnvelopedCms object.
        static public byte[] EncryptMsg(Byte[] msg,
            X509Certificate2 recipientCert)
        {
            //  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 CmsRecipient object that
            //  represents information about the recipient
            //  to encrypt the message for.
            CmsRecipient recip1 = new CmsRecipient(
                SubjectIdentifierType.IssuerAndSerialNumber,
                recipientCert);

            Console.Write("Encrypting data for a single recipient of " +
                "subject name {0} ... ",
                recip1.Certificate.SubjectName.Name);

            //  Encrypt the message for the recipient.
            envelopedCms.Encrypt(recip1);
            Console.WriteLine("Done.");

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

        //  Decrypt the encoded EnvelopedCms message.
        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 1 for this example.
            DisplayEnvelopedCms(envelopedCms, false);

            //  Decrypt the message for the single recipient.
            //  Note that the following call to the Decrypt method
            //  accomplishes the same result:
            //  envelopedCms.Decrypt();
            Console.Write("Decrypting Data ... ");
            envelopedCms.Decrypt(envelopedCms.RecipientInfos[0]);
            Console.WriteLine("Done.");

            return envelopedCms.Encode();
        }

        //  Display the ContentInfo property of a SignedCms object.
        private void DisplaySignedCmsContent(String desc,
            SignedCms signedCms)
        {
            Console.WriteLine(desc + " (length {0}):  ",
                signedCms.ContentInfo.Content.Length);
            foreach (byte b in signedCms.ContentInfo.Content)
            {
                Console.Write(b.ToString() + " ");
            }
            Console.WriteLine();
        }

        //  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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