## Verifying a signature with X509 certificates.

### verify3.c

    /**
     * XML Security Library example: Verifying a file signed with X509 certificate
     *
     * Verifies a file signed with X509 certificate.
     *
     * This example was developed and tested with OpenSSL crypto library. The
     * certificates management policies for another crypto library may break it.
     *
     * Usage:
     *      verify3 <signed-file> <trusted-cert-pem-file1> [<trusted-cert-pem-file2> [...]]
     *
     * Example:
     *      ./verify3 sign3-res.xml ca2cert.pem cacert.pem
     *
     * This is free software; see Copyright file in the source
     * distribution for preciese wording.
     *
     * Copyright (C) 2002-2024 Aleksey Sanin <aleksey@aleksey.com>. All Rights Reserved.
     */
    #include <stdlib.h>
    #include <string.h>
    #include <assert.h>

    #include <libxml/tree.h>
    #include <libxml/xmlmemory.h>
    #include <libxml/parser.h>

    #ifndef XMLSEC_NO_XSLT
    #include <libxslt/xslt.h>
    #include <libxslt/security.h>
    #endif /* XMLSEC_NO_XSLT */

    #include <xmlsec/xmlsec.h>
    #include <xmlsec/xmltree.h>
    #include <xmlsec/xmldsig.h>
    #include <xmlsec/crypto.h>

    xmlSecKeysMngrPtr load_trusted_certs(char** files, int files_size);
    int verify_file(xmlSecKeysMngrPtr mngr, const char* xml_file);
    int verify_signature_results(xmlSecDSigCtxPtr dsigCtx);

    int
    main(int argc, char **argv) {
    #ifndef XMLSEC_NO_XSLT
        xsltSecurityPrefsPtr xsltSecPrefs = NULL;
    #endif /* XMLSEC_NO_XSLT */
        xmlSecKeysMngrPtr mngr;

        assert(argv);

        if(argc < 3) {
            fprintf(stderr, "Error: wrong number of arguments.\n");
            fprintf(stderr, "Usage: %s <xml-file> <cert-file1> [<cert-file2> [...]]\n", argv[0]);
            return(1);
        }

        /* Init libxml and libxslt libraries */
        xmlInitParser();
        LIBXML_TEST_VERSION
        xmlLoadExtDtdDefaultValue = XML_DETECT_IDS | XML_COMPLETE_ATTRS;
        xmlSubstituteEntitiesDefault(1);
    #ifndef XMLSEC_NO_XSLT
        xmlIndentTreeOutput = 1;
    #endif /* XMLSEC_NO_XSLT */

        /* Init libxslt */
    #ifndef XMLSEC_NO_XSLT
        /* disable everything */
        xsltSecPrefs = xsltNewSecurityPrefs();
        xsltSetSecurityPrefs(xsltSecPrefs,  XSLT_SECPREF_READ_FILE,        xsltSecurityForbid);
        xsltSetSecurityPrefs(xsltSecPrefs,  XSLT_SECPREF_WRITE_FILE,       xsltSecurityForbid);
        xsltSetSecurityPrefs(xsltSecPrefs,  XSLT_SECPREF_CREATE_DIRECTORY, xsltSecurityForbid);
        xsltSetSecurityPrefs(xsltSecPrefs,  XSLT_SECPREF_READ_NETWORK,     xsltSecurityForbid);
        xsltSetSecurityPrefs(xsltSecPrefs,  XSLT_SECPREF_WRITE_NETWORK,    xsltSecurityForbid);
        xsltSetDefaultSecurityPrefs(xsltSecPrefs);
    #endif /* XMLSEC_NO_XSLT */

        /* Init xmlsec library */
        if(xmlSecInit() < 0) {
            fprintf(stderr, "Error: xmlsec initialization failed.\n");
            return(-1);
        }

        /* Check loaded library version */
        if(xmlSecCheckVersion() != 1) {
            fprintf(stderr, "Error: loaded xmlsec library version is not compatible.\n");
            return(-1);
        }

        /* Load default crypto engine if we are supporting dynamic
         * loading for xmlsec-crypto libraries. Use the crypto library
         * name ("openssl", "nss", etc.) to load corresponding
         * xmlsec-crypto library.
         */
    #ifdef XMLSEC_CRYPTO_DYNAMIC_LOADING
        if(xmlSecCryptoDLLoadLibrary(NULL) < 0) {
            fprintf(stderr, "Error: unable to load default xmlsec-crypto library. Make sure\n"
                            "that you have it installed and check shared libraries path\n"
                            "(LD_LIBRARY_PATH and/or LTDL_LIBRARY_PATH) environment variables.\n");
            return(-1);
        }
    #endif /* XMLSEC_CRYPTO_DYNAMIC_LOADING */

        /* Init crypto library */
        if(xmlSecCryptoAppInit(NULL) < 0) {
            fprintf(stderr, "Error: crypto initialization failed.\n");
            return(-1);
        }

        /* Init xmlsec-crypto library */
        if(xmlSecCryptoInit() < 0) {
            fprintf(stderr, "Error: xmlsec-crypto initialization failed.\n");
            return(-1);
        }

        /* create keys manager and load trusted certificates */
        mngr = load_trusted_certs(&(argv[2]), argc - 2);
        if(mngr == NULL) {
            return(-1);
        }

        /* verify file */
        if(verify_file(mngr, argv[1]) < 0) {
            xmlSecKeysMngrDestroy(mngr);
            return(-1);
        }

        /* destroy keys manager */
        xmlSecKeysMngrDestroy(mngr);

        /* Shutdown xmlsec-crypto library */
        xmlSecCryptoShutdown();

        /* Shutdown crypto library */
        xmlSecCryptoAppShutdown();

        /* Shutdown xmlsec library */
        xmlSecShutdown();

        /* Shutdown libxslt/libxml */
    #ifndef XMLSEC_NO_XSLT
        xsltFreeSecurityPrefs(xsltSecPrefs);
        xsltCleanupGlobals();
    #endif /* XMLSEC_NO_XSLT */
        xmlCleanupParser();

        return(0);
    }

    /**
     * load_trusted_certs:
     * @files:              the list of filenames.
     * @files_size:         the number of filenames in #files.
     *
     * Creates simple keys manager and load trusted certificates from PEM #files.
     * The caller is responsible for destroying returned keys manager using
     * @xmlSecKeysMngrDestroy.
     *
     * Returns the pointer to newly created keys manager or NULL if an error
     * occurs.
     */
    xmlSecKeysMngrPtr
    load_trusted_certs(char** files, int files_size) {
        xmlSecKeysMngrPtr mngr;
        int i;

        assert(files);
        assert(files_size > 0);

        /* create and initialize keys manager, we use a simple list based
         * keys manager, implement your own xmlSecKeysStore klass if you need
         * something more sophisticated
         */
        mngr = xmlSecKeysMngrCreate();
        if(mngr == NULL) {
            fprintf(stderr, "Error: failed to create keys manager.\n");
            return(NULL);
        }
        if(xmlSecCryptoAppDefaultKeysMngrInit(mngr) < 0) {
            fprintf(stderr, "Error: failed to initialize keys manager.\n");
            xmlSecKeysMngrDestroy(mngr);
            return(NULL);
        }

        for(i = 0; i < files_size; ++i) {
            assert(files[i]);

            /* load trusted cert */
            if(xmlSecCryptoAppKeysMngrCertLoad(mngr, files[i], xmlSecKeyDataFormatPem, xmlSecKeyDataTypeTrusted) < 0) {
                fprintf(stderr,"Error: failed to load pem certificate from \"%s\"\n", files[i]);
                xmlSecKeysMngrDestroy(mngr);
                return(NULL);
            }
        }

        return(mngr);
    }

    /**
     * verify_file:
     * @mngr:               the pointer to keys manager.
     * @xml_file:           the signed XML file name.
     *
     * Verifies XML signature in #xml_file.
     *
     * Returns 0 on success or a negative value if an error occurs.
     */
    int
    verify_file(xmlSecKeysMngrPtr mngr, const char* xml_file) {
        xmlDocPtr doc = NULL;
        xmlNodePtr node = NULL;
        xmlSecDSigCtxPtr dsigCtx = NULL;
        int res = -1;

        assert(mngr);
        assert(xml_file);

        /* load file */
        doc = xmlReadFile(xml_file, NULL, XML_PARSE_PEDANTIC | XML_PARSE_NONET);
        if ((doc == NULL) || (xmlDocGetRootElement(doc) == NULL)){
            fprintf(stderr, "Error: unable to parse file \"%s\"\n", xml_file);
            goto done;
        }

        /* find start node */
        node = xmlSecFindNode(xmlDocGetRootElement(doc), xmlSecNodeSignature, xmlSecDSigNs);
        if(node == NULL) {
            fprintf(stderr, "Error: start node not found in \"%s\"\n", xml_file);
            goto done;
        }

        /* create signature context */
        dsigCtx = xmlSecDSigCtxCreate(mngr);
        if(dsigCtx == NULL) {
            fprintf(stderr,"Error: failed to create signature context\n");
            goto done;
        }

        /* Verify signature */
        if(xmlSecDSigCtxVerify(dsigCtx, node) < 0) {
            fprintf(stderr,"Error: signature verify\n");
            goto done;
        }

        /* verif results and print outcome to stdout */
        if(verify_signature_results(dsigCtx) == 0) {
            fprintf(stdout, "Signature is OK\n");
        } else {
            fprintf(stdout, "Signature is INVALID\n");
        }

        /* success */
        res = 0;

    done:
        /* cleanup */
        if(dsigCtx != NULL) {
            xmlSecDSigCtxDestroy(dsigCtx);
        }

        if(doc != NULL) {
            xmlFreeDoc(doc);
        }
        return(res);
    }

    /**
     * verify_signature_results:
     * @dsigCtx:            the XMLDSig context
     *
     * Verifies XML signature results to ensure that signature was applied
     * to the expected data.
     *
     * Returns 0 on success or a negative value if an error occurs.
     */
    int
    verify_signature_results(xmlSecDSigCtxPtr dsigCtx) {
        xmlSecDSigReferenceCtxPtr dsigRefCtx;
        xmlSecTransformPtr transform;

        assert(dsigCtx);

        /* check that signature verification succeeded */
        if(dsigCtx->status != xmlSecDSigStatusSucceeded) {
            fprintf(stderr,"Error: Signature verificaton result is not SUCCESS\n");
            return(-1);
        }

        /* in this example we expect exactly ONE reference with URI="" and
        *  exactly ONE enveloped signature transform (i.e. the whole document is signed)*/
        if(xmlSecPtrListGetSize(&(dsigCtx->signedInfoReferences)) != 1) {
            fprintf(stderr,"Error: Exactly one Reference is expected\n");
            return(-1);
        }
        dsigRefCtx = (xmlSecDSigReferenceCtxPtr)xmlSecPtrListGetItem(&(dsigCtx->signedInfoReferences), 0);
        if((dsigRefCtx == NULL) || (dsigRefCtx->status != xmlSecDSigStatusSucceeded)) {
            fprintf(stderr,"Error: Reference verification result is not SUCCESS\n");
            return(-1);
        }

        /* check URI */
        if(!xmlStrEqual(dsigRefCtx->uri, BAD_CAST "")) {
            fprintf(stderr,"Error: Reference URI value doesn't match expected one\n");
            return(-1);
        }

        /* check transforms: we expect only one "enveloped signature" transform */
        transform = dsigRefCtx->transformCtx.first;
        if((transform == NULL) || (!xmlStrEqual(transform->id->name, xmlSecNameEnveloped))) {
            fprintf(stderr,"Error: First Transform name '%s' doesn't match expected '%s'\n", (transform != NULL ? transform->id->name : BAD_CAST "NULL"), xmlSecNameEnveloped);
            return(-1);
        }

        /* all other transforms should be inserted by XMLSec */
        transform = transform->next;
        while(transform != NULL) {
            if((transform->flags & XMLSEC_TRANSFORM_FLAGS_USER_SPECIFIED) != 0) {
                fprintf(stderr,"Error: Found unexpected Transform name '%s'\n", transform->id->name);
                return(-1);
            }
            transform = transform->next;
        }

        /* all good! */
        return(0);
    }