Implementing a custom keys store.

In many cases, a default built-in list based keys store is not good enough. For example, XML Security Library (and the built-in default keys store) have no synchronization and you'll need to implement a custom keys store if you want to add or remove keys while other threads use the store.
Example 4. Creating a custom keys manager.
/**
 * 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.
 * 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; */

    /* reserved for the future */
    NULL,				/* void* reserved0; */
    NULL,				/* void* reserved1; */
};

/**
 * 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 default keys store.
 * @name:		the desired key name.
 * @keyInfoCtx:		the pointer to <dsig:KeyInfo/> node processing context.
 *  
 * Lookups key in the @store.
 *
 * 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 = xmlSecCryptoAppKeyLoad(name, xmlSecKeyDataFormatPem, NULL, NULL, NULL);
	if(key == NULL) {
    	    fprintf(stderr,"Error: failed to load 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, 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", name);
        xmlSecKeyDestroy(key);
        return(NULL);	
    }

    return(key);
}
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