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## Creating dynamic signature templates. The signature template has structure similar to the XML Digital Signature structure as it is described in specification. The only difference is that some nodes (for example, <dsig:DigestValue/> or <SignatureValue/>) are empty. The XML Security Library sets the content of these nodes after doing necessary calculations. Figure 3. XML Digital Signature structure
    <dsig:Signature ID?>
        <dsig:SignedInfo>
            <dsig:CanonicalizationMethod Algorithm />
            <dsig:SignatureMethod Algorithm />
            (<dsig:Reference URI? >
        	    (<dsig:Transforms>
    		(<dsig:Transform Algorithm />)+
    	     </dsig:Transforms>)?
    	    <dsig:DigestMethod Algorithm >
    	    <dsig:DigestValue>
    	</dsig:Reference>)+
        </dsig:SignedInfo>
        <dsig:SignatureValue>
        (<dsig:KeyInfo>
    	<dsig:KeyName>?
    	<dsig:KeyValue>?
    	<dsig:RetrievalMethod>?
    	<dsig:X509Data>?
    	<dsig:PGPData>?
    	<enc:EncryptedKey>?
    	<enc:AgreementMethod>?
    	<dsig:KeyName>?
    	<dsig:RetrievalMethod>?
    	<*>?
        </dsig:KeyInfo>)?
        (<dsig:Object ID?>)*
    </dsig:Signature>
Example 12. Creating dynamic signature template.
    /**
     * sign_file:
     * @xml_file:		the XML file name.
     * @key_file:		the PEM private key file name.
     *
     * Signs the #xml_file using private key from #key_file and dynamicaly
     * created enveloped signature template.
     *
     * Returns 0 on success or a negative value if an error occurs.
     */
    int
    sign_file(const char* xml_file, const char* key_file) {
        xmlDocPtr doc = NULL;
        xmlNodePtr signNode = NULL;
        xmlNodePtr refNode = NULL;
        xmlNodePtr keyInfoNode = NULL;
        xmlSecDSigCtxPtr dsigCtx = NULL;
        int res = -1;

        assert(xml_file);
        assert(key_file);

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

        /* create signature template for RSA-SHA1 enveloped signature */
        signNode = xmlSecTmplSignatureCreate(doc, xmlSecTransformExclC14NId,
    				         xmlSecTransformRsaSha1Id, NULL);
        if(signNode == NULL) {
    	fprintf(stderr, "Error: failed to create signature template\n");
    	goto done;
        }

        /* add <dsig:Signature/> node to the doc */
        xmlAddChild(xmlDocGetRootElement(doc), signNode);

        /* add reference */
        refNode = xmlSecTmplSignatureAddReference(signNode, xmlSecTransformSha1Id,
    					NULL, NULL, NULL);
        if(refNode == NULL) {
    	fprintf(stderr, "Error: failed to add reference to signature template\n");
    	goto done;
        }

        /* add enveloped transform */
        if(xmlSecTmplReferenceAddTransform(refNode, xmlSecTransformEnvelopedId) == NULL) {
    	fprintf(stderr, "Error: failed to add enveloped transform to reference\n");
    	goto done;
        }

        /* add <dsig:KeyInfo/> and <dsig:KeyName/> nodes to put key name in the signed document */
        keyInfoNode = xmlSecTmplSignatureEnsureKeyInfo(signNode, NULL);
        if(keyInfoNode == NULL) {
    	fprintf(stderr, "Error: failed to add key info\n");
    	goto done;
        }

        if(xmlSecTmplKeyInfoAddKeyName(keyInfoNode, NULL) == NULL) {
    	fprintf(stderr, "Error: failed to add key name\n");
    	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 private key, assuming that there is not password */
        dsigCtx->signKey = xmlSecCryptoAppKeyLoad(key_file, xmlSecKeyDataFormatPem, NULL, NULL, NULL);
        if(dsigCtx->signKey == NULL) {
            fprintf(stderr,"Error: failed to load private 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, key_file) < 0) {
        	fprintf(stderr,"Error: failed to set key name for key from \"%s\"\n", key_file);
    	goto done;
        }

        /* sign the template */
        if(xmlSecDSigCtxSign(dsigCtx, signNode) < 0) {
            fprintf(stderr,"Error: signature failed\n");
    	goto done;
        }

        /* print signed document to stdout */
        xmlDocDump(stdout, doc);

        /* success */
        res = 0;

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

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