Decrypting an encrypted document: XML Security Library Reference Manual

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## Decrypting an encrypted document The typical decryption process includes following steps: Load keys, X509 certificates, etc. in the keys manager . Create encryption context xmlSecEncCtx using xmlSecEncCtxCreate or xmlSecEncCtxInitialize functions. Select start decryption <enc:EncryptedData> node. Decrypt by calling xmlSecencCtxDecrypt function. Check returned value and if necessary consume encrypted data. Destroy encryption context xmlSecEncCtx using xmlSecEncCtxDestroy or xmlSecEncCtxFinalize functions. Example 15. Decrypting a document.
    int
    decrypt_file(const char* enc_file, const char* key_file) {
        xmlDocPtr doc = NULL;
        xmlNodePtr node = NULL;
        xmlSecEncCtxPtr encCtx = NULL;
        int res = -1;

        assert(enc_file);
        assert(key_file);

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

        /* find start node */
        node = xmlSecFindNode(xmlDocGetRootElement(doc), xmlSecNodeEncryptedData, xmlSecEncNs);
        if(node == NULL) {
    	fprintf(stderr, "Error: start node not found in \"%s\"\n", enc_file);
    	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 */
        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;
        }

        /* decrypt the data */
        if((xmlSecEncCtxDecrypt(encCtx, node) < 0) || (encCtx->result == NULL)) {
            fprintf(stderr,"Error: decryption failed\n");
    	goto done;
        }

        /* print decrypted data to stdout */
        if(encCtx->resultReplaced != 0) {
    	fprintf(stdout, "Decrypted XML data:\n");
    	xmlDocDump(stdout, doc);
        } else {
    	fprintf(stdout, "Decrypted binary data (%d bytes):\n", xmlSecBufferGetSize(encCtx->result));
    	if(xmlSecBufferGetData(encCtx->result) != NULL) {
    	    fwrite(xmlSecBufferGetData(encCtx->result),
    	          1,
    	          xmlSecBufferGetSize(encCtx->result),
    	          stdout);
    	}
        }
        fprintf(stdout, "\n");

        /* success */
        res = 0;

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

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