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The signature template has structure similar to the XML Digital Signature structure as it is described in specification. The only difference is that some nodes (for example, <dsig:DigestValue/> or <SignatureValue/>) are empty. The XML Security Library sets the content of these nodes after doing necessary calculations.
Figure 3. XML Digital Signature structure
<dsig:Signature ID?>
<dsig:SignedInfo>
<dsig:CanonicalizationMethod Algorithm />
<dsig:SignatureMethod Algorithm />
(<dsig:Reference URI? >
(<dsig:Transforms>
(<dsig:Transform Algorithm />)+
</dsig:Transforms>)?
<dsig:DigestMethod Algorithm >
<dsig:DigestValue>
</dsig:Reference>)+
</dsig:SignedInfo>
<dsig:SignatureValue>
(<dsig:KeyInfo>
<dsig:KeyName>?
<dsig:KeyValue>?
<dsig:RetrievalMethod>?
<dsig:X509Data>?
<dsig:PGPData>?
<enc:EncryptedKey>?
<enc:AgreementMethod>?
<dsig:KeyName>?
<dsig:RetrievalMethod>?
<*>?
</dsig:KeyInfo>)?
(<dsig:Object ID?>)*
</dsig:Signature>
Example 12. Creating dynamic signature template.
/**
* sign_file:
* @xml_file: the XML file name.
* @key_file: the PEM private key file name.
*
* Signs the #xml_file using private key from #key_file and dynamicaly
* created enveloped signature template.
*
* Returns 0 on success or a negative value if an error occurs.
*/
int
sign_file(const char* xml_file, const char* key_file) {
xmlDocPtr doc = NULL;
xmlNodePtr signNode = NULL;
xmlNodePtr refNode = NULL;
xmlNodePtr keyInfoNode = NULL;
xmlSecDSigCtxPtr dsigCtx = NULL;
int res = -1;
assert(xml_file);
assert(key_file);
/* load doc file */
doc = xmlParseFile(xml_file);
if ((doc == NULL) || (xmlDocGetRootElement(doc) == NULL)){
fprintf(stderr, "Error: unable to parse file \"%s\"\n", xml_file);
goto done;
}
/* create signature template for RSA-SHA1 enveloped signature */
signNode = xmlSecTmplSignatureCreate(doc, xmlSecTransformExclC14NId,
xmlSecTransformRsaSha1Id, 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 */
refNode = xmlSecTmplSignatureAddReference(signNode, xmlSecTransformSha1Id,
NULL, NULL, NULL);
if(refNode == NULL) {
fprintf(stderr, "Error: failed to add reference to signature template\n");
goto done;
}
/* add enveloped transform */
if(xmlSecTmplReferenceAddTransform(refNode, xmlSecTransformEnvelopedId) == NULL) {
fprintf(stderr, "Error: failed to add enveloped transform to reference\n");
goto done;
}
/* add <dsig:KeyInfo/> and <dsig:KeyName/> nodes to put key name in the signed document */
keyInfoNode = xmlSecTmplSignatureEnsureKeyInfo(signNode, 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 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 not password */
dsigCtx->signKey = xmlSecCryptoAppKeyLoad(key_file, xmlSecKeyDataFormatPem, NULL, NULL, NULL);
if(dsigCtx->signKey == NULL) {
fprintf(stderr,"Error: failed to load private pem key from \"%s\"\n", key_file);
goto done;
}
/* set key name to the file name, this is just an example! */
if(xmlSecKeySetName(dsigCtx->signKey, 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);
}