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## Creating dynamic encryption templates. The encryption template has structure similar to the XML Encryption structure as it is described in specification. The only difference is that some nodes (for example, <enc:CipherValue/>) are empty. The XML Security Library sets the content of these nodes after doing necessary calculations. Figure 4. XML Encryption structure
    <enc:EncryptedData Id? Type? MimeType? Encoding?>
        <enc:EncryptionMethod Algorithm />?
        (<dsig:KeyInfo>
    	<dsig:KeyName>?
    	<dsig:KeyValue>?
    	<dsig:RetrievalMethod>?
    	<dsig:X509Data>?
    	<dsig:PGPData>?
    	<enc:EncryptedKey>?
    	<enc:AgreementMethod>?
    	<dsig:KeyName>?
    	<dsig:RetrievalMethod>?
    	<*>?
        </dsig:KeyInfo>)?
        <enc:CipherData>
    	<enc:CipherValue>?
    	<enc:CipherReference URI?>?
        </enc:CipherData>
        <enc:EncryptionProperties>?
    </enc:EncryptedData>
Example 13. Creating dynamic encrytion template.
    /**
     * encrypt_file:
     * @xml_file:		the encryption template file name.
     * @key_file:		the Triple DES key file.
     *
     * Encrypts #xml_file using a dynamicaly created template and DES key from
     * #key_file.
     *
     * Returns 0 on success or a negative value if an error occurs.
     */
    int
    encrypt_file(const char* xml_file, const char* key_file) {
        xmlDocPtr doc = NULL;
        xmlNodePtr encDataNode = NULL;
        xmlNodePtr keyInfoNode = NULL;
        xmlSecEncCtxPtr encCtx = NULL;
        int res = -1;

        assert(xml_file);
        assert(key_file);

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

        /* create encryption template to encrypt XML file and replace
         * its content with encryption result */
        encDataNode = xmlSecTmplEncDataCreate(doc, xmlSecTransformDes3CbcId,
    				NULL, xmlSecTypeEncElement, NULL, NULL);
        if(encDataNode == NULL) {
    	fprintf(stderr, "Error: failed to create encryption template\n");
    	goto done;
        }

        /* we want to put encrypted data in the <enc:CipherValue/> node */
        if(xmlSecTmplEncDataEnsureCipherValue(encDataNode) == NULL) {
    	fprintf(stderr, "Error: failed to add CipherValue node\n");
    	goto done;
        }

        /* add <dsig:KeyInfo/> and <dsig:KeyName/> nodes to put key name in the signed document */
        keyInfoNode = xmlSecTmplEncDataEnsureKeyInfo(encDataNode, 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 encryption context, we don't need keys manager in this example */
        encCtx = xmlSecEncCtxCreate(NULL);
        if(encCtx == NULL) {
            fprintf(stderr,"Error: failed to create encryption context\n");
    	goto done;
        }

        /* load DES key, assuming that there is not password */
        encCtx->encKey = xmlSecKeyReadBinaryFile(xmlSecKeyDataDesId, key_file);
        if(encCtx->encKey == NULL) {
            fprintf(stderr,"Error: failed to load des key from binary file \"%s\"\n", key_file);
    	goto done;
        }

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

        /* encrypt the data */
        if(xmlSecEncCtxXmlEncrypt(encCtx, encDataNode, xmlDocGetRootElement(doc)) < 0) {
            fprintf(stderr,"Error: encryption failed\n");
    	goto done;
        }

        /* we template is inserted in the doc */
        encDataNode = NULL;

        /* print encrypted data with document to stdout */
        xmlDocDump(stdout, doc);

        /* success */
        res = 0;

    done:

        /* cleanup */
        if(encCtx != NULL) {
    	xmlSecEncCtxDestroy(encCtx);
        }

        if(encDataNode != NULL) {
    	xmlFreeNode(encDataNode);
        }

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