## Verifying a signature with a single key.

### verify1.c

    /**
     * XML Security Library example: Verifying a file using a single key.
     *
     * Verifies a file using a key from PEM file.
     *
     * Usage:
     *      verify1 <signed-file> <pem-key>
     *
     * Example:
     *      ./verify1 sign1-res.xml rsapub.pem
     *      ./verify1 sign2-res.xml rsapub.pem
     *
     * This is free software; see Copyright file in the source
     * distribution for preciese wording.
     *
     * Copyright (C) 2002-2022 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>

    int verify_file(const char* xml_file, const char* key_file);

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

        assert(argv);

        if(argc != 3) {
            fprintf(stderr, "Error: wrong number of arguments.\n");
            fprintf(stderr, "Usage: %s <xml-file> <key-file>\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);
        }

        if(verify_file(argv[1], argv[2]) < 0) {
            return(-1);
        }

        /* 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);
    }

    /**
     * verify_file:
     * @xml_file:           the signed XML file name.
     * @key_file:           the PEM public key file name.
     *
     * Verifies XML signature in #xml_file using public key from #key_file.
     *
     * Returns 0 on success or a negative value if an error occurs.
     */
    int
    verify_file(const char* xml_file, const char* key_file) {
        xmlDocPtr doc = NULL;
        xmlNodePtr node = NULL;
        xmlSecDSigCtxPtr dsigCtx = NULL;
        int res = -1;

        assert(xml_file);
        assert(key_file);

        /* load file */
        doc = xmlParseFile(xml_file);
        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, we don't need keys manager in this example */
        dsigCtx = xmlSecDSigCtxCreate(NULL);
        if(dsigCtx == NULL) {
            fprintf(stderr,"Error: failed to create signature context\n");
            goto done;
        }

        /* load public key */
        dsigCtx->signKey = xmlSecCryptoAppKeyLoadEx(key_file, xmlSecKeyDataTypePrivate | xmlSecKeyDataTypePublic, xmlSecKeyDataFormatPem, NULL, NULL, NULL);
        if(dsigCtx->signKey == NULL) {
            fprintf(stderr,"Error: failed to load public pem key from \"%s\"\n", key_file);
            goto done;
        }

        /* set key name to the file name, this is just an example! */
        if(xmlSecKeySetName(dsigCtx->signKey, BAD_CAST key_file) < 0) {
            fprintf(stderr,"Error: failed to set key name for key from \"%s\"\n", key_file);
            goto done;
        }

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

        /* print verification result to stdout */
        if(dsigCtx->status == xmlSecDSigStatusSucceeded) {
            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);
    }