Writing a custom keys manager.: XML Security Library Reference Manual

## Writing a custom keys manager.

### decrypt3.c

    /**
     * XML Security Library example: Decrypting an encrypted file using a custom keys manager.
     *
     * Decrypts encrypted XML file using a custom files based keys manager.
     * We assume that key's name in <dsig:KeyName/> element is just
     * key's file name in the current folder.
     *
     * Usage:
     *      ./decrypt3 <xml-enc>
     *
     * Example:
     *      ./decrypt3 encrypt1-res.xml
     *      ./decrypt3 encrypt2-res.xml
     *
     * 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 <ctype.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/xmlenc.h>
    #include <xmlsec/crypto.h>

    xmlSecKeyStoreId  files_keys_store_get_klass(void);
    xmlSecKeysMngrPtr create_files_keys_mngr(void);
    int decrypt_file(xmlSecKeysMngrPtr mngr, const char* enc_file);

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

        assert(argv);

        if(argc != 2) {
            fprintf(stderr, "Error: wrong number of arguments.\n");
            fprintf(stderr, "Usage: %s <enc-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);
        }

        /* create keys manager and load keys */
        mngr = create_files_keys_mngr();
        if(mngr == NULL) {
            return(-1);
        }

        if(decrypt_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);
    }

    /**
     * decrypt_file:
     * @mngr:               the pointer to keys manager.
     * @enc_file:           the encrypted XML  file name.
     *
     * Decrypts the XML file #enc_file using DES key from #key_file and
     * prints results to stdout.
     *
     * Returns 0 on success or a negative value if an error occurs.
     */
    int
    decrypt_file(xmlSecKeysMngrPtr mngr, const char* enc_file) {
        xmlDocPtr doc = NULL;
        xmlNodePtr node = NULL;
        xmlSecEncCtxPtr encCtx = NULL;
        int res = -1;

        assert(mngr);
        assert(enc_file);

        /* load template */
        doc = xmlReadFile(enc_file, NULL, XML_PARSE_PEDANTIC | XML_PARSE_NONET);
        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 */
        encCtx = xmlSecEncCtxCreate(mngr);
        if(encCtx == NULL) {
            fprintf(stderr,"Error: failed to create encryption context\n");
            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 (" XMLSEC_SIZE_FMT " 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);
    }

    /**
     * create_files_keys_mngr:
     *
     * Creates a files based keys manager: we assume that key name is
     * the key file name,
     *
     * Returns pointer to newly created keys manager or NULL if an error occurs.
     */
    xmlSecKeysMngrPtr
    create_files_keys_mngr(void) {
        xmlSecKeyStorePtr keysStore;
        xmlSecKeysMngrPtr mngr;

        /* create files based keys store */
        keysStore = xmlSecKeyStoreCreate(files_keys_store_get_klass());
        if(keysStore == NULL) {
            fprintf(stderr, "Error: failed to create keys store.\n");
            return(NULL);
        }

        /* create keys manager */
        mngr = xmlSecKeysMngrCreate();
        if(mngr == NULL) {
            fprintf(stderr, "Error: failed to create keys manager.\n");
            xmlSecKeyStoreDestroy(keysStore);
            return(NULL);
        }

        /* add store to keys manager, from now on keys manager destroys the store if needed */
        if(xmlSecKeysMngrAdoptKeysStore(mngr, keysStore) < 0) {
            fprintf(stderr, "Error: failed to add keys store to keys manager.\n");
            xmlSecKeyStoreDestroy(keysStore);
            xmlSecKeysMngrDestroy(mngr);
            return(NULL);
        }

        /* initialize crypto library specific data in keys manager */
        if(xmlSecCryptoKeysMngrInit(mngr) < 0) {
            fprintf(stderr, "Error: failed to initialize crypto data in keys manager.\n");
            xmlSecKeysMngrDestroy(mngr);
            return(NULL);
        }

        /* set the get key callback */
        mngr->getKey = xmlSecKeysMngrGetKey;
        return(mngr);
    }

    /****************************************************************************
     *
     * Files Keys Store: we assume that key's name (content of the
     * <dsig:KeyName/> element is a name of the file with a key (in the
     * current folder).
     * Attention: this probably not a good solution for high traffic systems.
     *
     ***************************************************************************/
    static xmlSecKeyPtr             files_keys_store_find_key       (xmlSecKeyStorePtr store,
                                                                     const xmlChar* name,
                                                                     xmlSecKeyInfoCtxPtr keyInfoCtx);
    static xmlSecKeyStoreKlass files_keys_store_klass = {
        sizeof(xmlSecKeyStoreKlass),
        sizeof(xmlSecKeyStore),
        BAD_CAST "files-based-keys-store",  /* const xmlChar* name; */
        NULL,                               /* xmlSecKeyStoreInitializeMethod initialize; */
        NULL,                               /* xmlSecKeyStoreFinalizeMethod finalize; */
        files_keys_store_find_key,          /* xmlSecKeyStoreFindKeyMethod findKey; */
        NULL,                               /* xmlSecKeyStoreFindKeyFromX509DataMethod findKeyFromX509Data; */

        /* reserved for the future */
        NULL,                               /* void* reserved0; */
    };

    /**
     * files_keys_store_get_klass:
     *
     * The files based keys store klass: we assume that key name is the
     * key file name,
     *
     * Returns files based keys store klass.
     */
    xmlSecKeyStoreId
    files_keys_store_get_klass(void) {
        return(&files_keys_store_klass);
    }

    /**
     * files_keys_store_find_key:
     * @store:              the pointer to simple keys store.
     * @name:               the desired key name.
     * @keyInfoCtx:         the pointer to <dsig:KeyInfo/> node processing context.
     *
     * Lookups key in the @store. The caller is responsible for destroying
     * returned key with #xmlSecKeyDestroy function.
     *
     * Returns pointer to key or NULL if key not found or an error occurs.
     */
    static xmlSecKeyPtr
    files_keys_store_find_key(xmlSecKeyStorePtr store, const xmlChar* name, xmlSecKeyInfoCtxPtr keyInfoCtx) {
        xmlSecKeyPtr key;
        const xmlChar* p;

        assert(store);
        assert(keyInfoCtx);

        /* it's possible to do not have the key name or desired key type
         * but we could do nothing in this case */
        if((name == NULL) || (keyInfoCtx->keyReq.keyId == xmlSecKeyDataIdUnknown)){
            return(NULL);
        }

        /* we don't want to open files in a folder other than "current";
         * to prevent it limit the characters in the key name to alpha/digit,
         * '.', '-' or '_'.
         */
        for(p = name; (*p) != '\0'; ++p) {
            if(!isalnum((*p)) && ((*p) != '.') && ((*p) != '-') && ((*p) != '_')) {
                return(NULL);
            }
        }

        if((keyInfoCtx->keyReq.keyId == xmlSecKeyDataDsaId) || (keyInfoCtx->keyReq.keyId == xmlSecKeyDataRsaId)) {
            /* load key from a pem file, if key is not found then it's an error (is it?) */
            key = xmlSecCryptoAppKeyLoadEx((const char*)name, xmlSecKeyDataTypePrivate, xmlSecKeyDataFormatPem, NULL, NULL, NULL);
            if(key == NULL) {
                fprintf(stderr,"Error: failed to load public pem key from \"%s\"\n", name);
                return(NULL);
            }
        } else {
            /* otherwise it's a binary key, if key is not found then it's an error (is it?) */
            key = xmlSecKeyReadBinaryFile(keyInfoCtx->keyReq.keyId, (const char*)name);
            if(key == NULL) {
                fprintf(stderr,"Error: failed to load key from binary file \"%s\"\n", name);
                return(NULL);
            }
        }

        /* set key name */
        if(xmlSecKeySetName(key, name) < 0) {
            fprintf(stderr,"Error: failed to set key name for key from \"%s\"\n", (const char*)name);
            xmlSecKeyDestroy(key);
            return(NULL);
        }

        return(key);
    }