/**
         * Validates an attribute certificate with the given certificate path.
         *
         * <p>
         * <code>params</code> must be an instance of
         * <code>ExtendedPkixParameters</code>.
         * </p><p>
         * The target constraints in the <code>params</code> must be an
         * <code>X509AttrCertStoreSelector</code> with at least the attribute
         * certificate criterion set. Obey that also target informations may be
         * necessary to correctly validate this attribute certificate.
         * </p><p>
         * The attribute certificate issuer must be added to the trusted attribute
         * issuers with {@link ExtendedPkixParameters#setTrustedACIssuers(Set)}.
         * </p>
         * @param certPath The certificate path which belongs to the attribute
         *            certificate issuer public key certificate.
         * @param params The PKIX parameters.
         * @return A <code>PKIXCertPathValidatorResult</code> of the result of
         *         validating the <code>certPath</code>.
         * @throws InvalidAlgorithmParameterException if <code>params</code> is
         *             inappropriate for this validator.
         * @throws CertPathValidatorException if the verification fails.
         */
        public virtual PkixCertPathValidatorResult Validate(
            PkixCertPath certPath,
            PkixParameters pkixParams)
        {
            IX509Selector certSelect = pkixParams.GetTargetConstraints();

            if (!(certSelect is X509AttrCertStoreSelector))
            {
                throw new ArgumentException(
                          "TargetConstraints must be an instance of " + typeof(X509AttrCertStoreSelector).FullName,
                          "pkixParams");
            }
            IX509AttributeCertificate attrCert = ((X509AttrCertStoreSelector)certSelect).AttributeCert;

            PkixCertPath holderCertPath            = Rfc3281CertPathUtilities.ProcessAttrCert1(attrCert, pkixParams);
            PkixCertPathValidatorResult result     = Rfc3281CertPathUtilities.ProcessAttrCert2(certPath, pkixParams);
            X509Certificate             issuerCert = (X509Certificate)certPath.Certificates[0];

            Rfc3281CertPathUtilities.ProcessAttrCert3(issuerCert, pkixParams);
            Rfc3281CertPathUtilities.ProcessAttrCert4(issuerCert, pkixParams);
            Rfc3281CertPathUtilities.ProcessAttrCert5(attrCert, pkixParams);
            // 6 already done in X509AttrCertStoreSelector
            Rfc3281CertPathUtilities.ProcessAttrCert7(attrCert, certPath, holderCertPath, pkixParams);
            Rfc3281CertPathUtilities.AdditionalChecks(attrCert, pkixParams);
            DateTime date;

            try
            {
                date = PkixCertPathValidatorUtilities.GetValidCertDateFromValidityModel(pkixParams, null, -1);
            }
            catch (Exception e)
            {
                throw new PkixCertPathValidatorException(
                          "Could not get validity date from attribute certificate.", e);
            }
            Rfc3281CertPathUtilities.CheckCrls(attrCert, pkixParams, issuerCert, date, certPath.Certificates);
            return(result);
        }
        protected virtual PkixCertPathBuilderResult Build(
            X509Certificate tbvCert,
            PkixBuilderParameters pkixParams,
            IList tbvPath)
        {
            // If tbvCert is readily present in tbvPath, it indicates having run
            // into a cycle in the PKI graph.
            if (tbvPath.Contains(tbvCert))
            {
                return(null);
            }

            // step out, the certificate is not allowed to appear in a certification
            // chain.
            if (pkixParams.GetExcludedCerts().Contains(tbvCert))
            {
                return(null);
            }

            // test if certificate path exceeds maximum length
            if (pkixParams.MaxPathLength != -1)
            {
                if (tbvPath.Count - 1 > pkixParams.MaxPathLength)
                {
                    return(null);
                }
            }

            tbvPath.Add(tbvCert);

//			X509CertificateParser certParser = new X509CertificateParser();
            PkixCertPathBuilderResult builderResult = null;
            PkixCertPathValidator     validator     = new PkixCertPathValidator();

            try
            {
                // check whether the issuer of <tbvCert> is a TrustAnchor
                if (PkixCertPathValidatorUtilities.IsIssuerTrustAnchor(tbvCert, pkixParams.GetTrustAnchors()))
                {
                    // exception message from possibly later tried certification
                    // chains
                    PkixCertPath certPath = null;
                    try
                    {
                        certPath = new PkixCertPath(tbvPath);
                    }
                    catch (Exception e)
                    {
                        throw new Exception(
                                  "Certification path could not be constructed from certificate list.",
                                  e);
                    }

                    PkixCertPathValidatorResult result = null;
                    try
                    {
                        result = (PkixCertPathValidatorResult)validator.Validate(
                            certPath, pkixParams);
                    }
                    catch (Exception e)
                    {
                        throw new Exception(
                                  "Certification path could not be validated.", e);
                    }

                    return(new PkixCertPathBuilderResult(certPath, result.TrustAnchor,
                                                         result.PolicyTree, result.SubjectPublicKey));
                }
                else
                {
                    // add additional X.509 stores from locations in certificate
                    try
                    {
                        PkixCertPathValidatorUtilities.AddAdditionalStoresFromAltNames(
                            tbvCert, pkixParams);
                    }
                    catch (CertificateParsingException e)
                    {
                        throw new Exception(
                                  "No additiontal X.509 stores can be added from certificate locations.",
                                  e);
                    }

                    // try to get the issuer certificate from one of the stores
                    HashSet issuers = new HashSet();
                    try
                    {
                        issuers.AddAll(PkixCertPathValidatorUtilities.FindIssuerCerts(tbvCert, pkixParams));
                    }
                    catch (Exception e)
                    {
                        throw new Exception(
                                  "Cannot find issuer certificate for certificate in certification path.",
                                  e);
                    }

                    if (issuers.IsEmpty)
                    {
                        throw new Exception("No issuer certificate for certificate in certification path found.");
                    }

                    foreach (X509Certificate issuer in issuers)
                    {
                        builderResult = Build(issuer, pkixParams, tbvPath);

                        if (builderResult != null)
                        {
                            break;
                        }
                    }
                }
            }
            catch (Exception e)
            {
                certPathException = e;
            }

            if (builderResult == null)
            {
                tbvPath.Remove(tbvCert);
            }

            return(builderResult);
        }