# Signing a node by ID with X509 certificate

Signs a specific XML node referenced by ID attribute using an X509 certificate.

## Source: `sign4.c`

```c
/**
 * XML Security Library (http://www.aleksey.com/xmlsec).
 *
 * This is free software; see the Copyright file in the source distribution for precise wording.
 *
 * Copyright (C) 2002-2026 Aleksey Sanin <aleksey@aleksey.com>. All Rights Reserved.
 */
/**
 * @brief XML Security Library example: Signing a file with a dynamically created template and an X509 certificate.
 * @details Signs a file using a dynamically created template, a key from a PEM file, and
 * an X509 certificate. The signature has one reference using the "ID" attribute
 * of the node to be signed. The key certificate is written in the <dsig:X509Data/> node.
 *
 * This example was developed and tested with OpenSSL crypto library. The
 * certificates management policies for another crypto library may break it.
 *
 * Usage:
 *
 * \code{.sh}
 *      sign4 <xml-doc> <id-attribute-to-sign> <pem-key-file> <pem-cert-file>
 * \endcode
 *
 * Example:
 *
 * \code{.sh}
 *      ./sign4 sign4-doc.xml "data" rsakey.pem rsacert.pem > sign4-res.xml
 * \endcode
 *
 * The resulting signature can be validated using the verify3 example:
 *
 * \code{.sh}
 *      ./verify3 sign4-res.xml ca2cert.pem cacert.pem
 * \endcode
 */
#include <stdlib.h>
#include <string.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/xmldsig.h>
#include <xmlsec/templates.h>
#include <xmlsec/crypto.h>

int sign_file(const char* xml_file, const char* id_attr, const char* key_file, const char* cert_file);

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

    assert(argv);

    if(argc != 5) {
        fprintf(stderr, "Error: wrong number of arguments.\n");
        fprintf(stderr, "Usage: %s <xml-file> <id-attr> <key-file> <cert-file>\n", argv[0]);
        return(1);
    }

    /* Init LibXML2 */
    xmlInitParser();
    LIBXML_TEST_VERSION

    /* Init LibXSLT */
#ifndef XMLSEC_NO_XSLT
    /* disable all XSLT file and network access */
    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 */
    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);
    }

    if(sign_file(argv[1], argv[2], argv[3], argv[4]) < 0) {
        return(-1);
    }

    /* Shutdown xmlsec-crypto library */
    xmlSecCryptoShutdown();

    /* Shutdown crypto library */
    xmlSecCryptoAppShutdown();

    /* Shutdown XMLSec */
    xmlSecShutdown();

    /* Shutdown LibXSLT / LibXML2*/
#ifndef XMLSEC_NO_XSLT
    xsltFreeSecurityPrefs(xsltSecPrefs);
    xsltCleanupGlobals();
#endif /* XMLSEC_NO_XSLT */
    xmlCleanupParser();

    return(0);
}

/**
 * @brief Signs an XML file using a private key, X.509 certificate, and ID-based reference.
 * @details Signs #xml_file using the private key from #key_file and a
 * dynamically created enveloped signature template. The certificate from #cert_file
 * is placed in the <dsig:X509Data/> node.
 * @param xml_file the XML file name.
 * @param id_attr the ID attribute of the node to sign.
 * @param key_file the PEM private key file name.
 * @param cert_file the x509 certificate PEM file.
 * @return 0 on success or a negative value if an error occurs.
 */
int
sign_file(const char* xml_file, const char* id_attr, const char* key_file, const char* cert_file) {
    xmlDocPtr doc = NULL;
    xmlNodePtr signNode = NULL;
    xmlNodePtr refNode = NULL;
    xmlNodePtr keyInfoNode = NULL;
    xmlNodePtr x509DataNode = NULL;
    xmlSecDSigCtxPtr dsigCtx = NULL;
    char uri[1024];
    const xmlChar* id_attributes[] = { BAD_CAST "id", BAD_CAST "ID", NULL };
    int res = -1;

    assert(xml_file);
    assert(id_attr);
    assert(key_file);
    assert(cert_file);

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

    /* create signature template for RSA-SHA256 signature */
    signNode = xmlSecTmplSignatureCreate(doc, xmlSecTransformExclC14NId, xmlSecTransformRsaSha256Id, 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 */
    if (strlen(id_attr) + 2 > sizeof(uri)) {
        fprintf(stderr, "Error: id attribute is too long\n");
        goto done;
    }
    sprintf(uri, "#%s", id_attr);

    refNode = xmlSecTmplSignatureAddReference(signNode, xmlSecTransformSha256Id,
                                        NULL, BAD_CAST uri, NULL);
    if(refNode == NULL) {
        fprintf(stderr, "Error: failed to add reference to signature template\n");
        goto done;
    }

    /* add <dsig:KeyInfo/> and <dsig:X509Data/> */
    keyInfoNode = xmlSecTmplSignatureEnsureKeyInfo(signNode, NULL);
    if(keyInfoNode == NULL) {
        fprintf(stderr, "Error: failed to add key info\n");
        goto done;
    }

    x509DataNode = xmlSecTmplKeyInfoAddX509Data(keyInfoNode);
    if(x509DataNode == NULL) {
        fprintf(stderr, "Error: failed to add X509Data node\n");
        goto done;
    }

    if(xmlSecTmplX509DataAddSubjectName(x509DataNode) == NULL) {
        fprintf(stderr, "Error: failed to add X509SubjectName node\n");
        goto done;
    }

    if(xmlSecTmplX509DataAddCertificate(x509DataNode) == NULL) {
        fprintf(stderr, "Error: failed to add X509Certificate node\n");
        goto done;
    }

    /* add ID attributes to the doc context since we don't have DTDs */
    xmlSecAddIDs(doc, NULL, id_attributes);

    /* 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 no password */
    dsigCtx->signKey = xmlSecCryptoAppKeyLoadEx(key_file,
        xmlSecKeyDataTypePrivate,
        xmlSecKeyDataFormatPem,
        NULL,
        NULL,
        NULL);
    if(dsigCtx->signKey == NULL) {
        fprintf(stderr,"Error: failed to load private pem key from \"%s\"\n", key_file);
        goto done;
    }

    /* load certificate and add to the key */
    if(xmlSecCryptoAppKeyCertLoad(dsigCtx->signKey, cert_file, xmlSecKeyDataFormatPem) < 0) {
        fprintf(stderr,"Error: failed to load pem certificate \"%s\"\n", cert_file);
        goto done;
    }

    /* set the key name to the file name; this is only an example */
    if(xmlSecKeySetName(dsigCtx->signKey, BAD_CAST 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);
}
```

## Input Document: `sign4-doc.xml`

```xml
<?xml version="1.0" encoding="UTF-8"?>
<!-- 
XML Security Library example: Original XML doc file for sign3 example. 
-->
<Envelope xmlns="urn:envelope">
  <Data ID="data">
	Hello, World!
  </Data>
</Envelope>
```

## Result: `sign4-res.xml`

```xml
<?xml version="1.0" encoding="UTF-8"?>
<!-- 
XML Security Library example: Original XML doc file for sign3 example. 
-->
<Envelope xmlns="urn:envelope">
  <Data ID="data">
	Hello, World!
  </Data>
<Signature xmlns="http://www.w3.org/2000/09/xmldsig#">
<SignedInfo>
<CanonicalizationMethod Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"/>
<SignatureMethod Algorithm="http://www.w3.org/2001/04/xmldsig-more#rsa-sha256"/>
<Reference URI="#data">
<DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256"/>
<DigestValue>eZuEhA6xeEm+T9S3q5P/NQ8Mphp1zSAFhnBbiHN4IeE=</DigestValue>
</Reference>
</SignedInfo>
<SignatureValue>OlxLIdZJDjjgYak2F7udevdsY2fjCksQ8UXsB9qXLyDUvG9UcDqhbKwtukVUTzE7
H84ofXx2RnJiF5Bcq4arsA==</SignatureValue>
<KeyInfo>
<X509Data>
<X509SubjectName>emailAddress=xmlsec@aleksey.com,CN=Aleksey Sanin,OU=Test Third Level RSA Certificate,O=XML Security Library (http://www.aleksey.com/xmlsec),ST=California,C=US</X509SubjectName>
<X509Certificate>MIIDpzCCA1GgAwIBAgIJAK+ii7kzrdqvMA0GCSqGSIb3DQEBBQUAMIGcMQswCQYD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</X509Certificate>
</X509Data>
</KeyInfo>
</Signature></Envelope>
```

