Sign xml with XADES-EPES using PHP - php

Question

I need to sign a document using XADES-EPES and I am using PHP for that purpose.
Using the xml below, I have been able to calculate the digest value <ds:DigestValue>ql0urtXTsc9W0GMIhTdzYHXnQYfnieoIttOBn9fGw7A=</ds:DigestValue> in the example given below, but I wonder how the other <ds:DigestValue>5JVZPTwN5Lj0sGTfFzaUeMKCo/xbCAj7fw6TLUFtZIk=</ds:DigestValue> is calculated.
I am using this XML as a test case:
<?xml version="1.0" encoding="UTF-8" xs:xmlns="https://tribunet.hacienda.go.cr/docs/esquemas/2016/v4.2/FacturaElectronica_V.4.2.xsd"?>
<FacturaElectronica>
<Clave>1</Clave>
<NumeroConsecutivo>1</NumeroConsecutivo>
<FechaEmision>1</FechaEmision>
<Emisor>1</Emisor>
<Receptor>1</Receptor>
<CondicionVenta>1</CondicionVenta>
<CondicionVenta>1</CondicionVenta>
<MedioPago>1</MedioPago>
<DetalleServicio>1</DetalleServicio>
<ResumenFactura>1</ResumenFactura>
<Normativa>1</Normativa>
</FacturaElectronica>
I am supposed to sign it with something similar to this (note: the SignatureValue and the X509Certificate have been truncated):
<ds:Signature Id="id-e34ffbff277e8d1432e864436aa11882" xmlns:ds="http://www.w3.org/2000/09/xmldsig#">
<ds:SignedInfo>
<ds:CanonicalizationMethod Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"/>
<ds:SignatureMethod Algorithm="http://www.w3.org/2001/04/xmldsig-more#rsa-sha256"/>
<ds:Reference Id="r-id-1" Type="" URI="">
<ds:Transforms>
<ds:Transform Algorithm="http://www.w3.org/TR/1999/REC-xpath-19991116">
<ds:XPath>not(ancestor-or-self::ds:Signature)</ds:XPath>
</ds:Transform>
<ds:Transform Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"/>
</ds:Transforms>
<ds:DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256"/>
<ds:DigestValue>ql0urtXTsc9W0GMIhTdzYHXnQYfnieoIttOBn9fGw7A=</ds:DigestValue>
</ds:Reference>
<ds:Reference Type="http://uri.etsi.org/01903#SignedProperties" URI="#xades-ide34ffbff277e8d1432e864436aa11882">
<ds:Transforms>
<ds:Transform Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"/>
</ds:Transforms>
<ds:DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256"/>
<ds:DigestValue>5JVZPTwN5Lj0sGTfFzaUeMKCo/xbCAj7fw6TLUFtZIk=</ds:DigestValue>
</ds:Reference>
</ds:SignedInfo>
<ds:SignatureValue Id="value-ide34ffbff277e8d1432e864436aa11882">Mt1TUuPK3W8/0eRtJX5t45GV9bHvMjw....</ds:SignatureValue>
<ds:KeyInfo>
<ds:X509Data>
<ds:X509Certificate>MIIFpTCCBI2gAwIBAgIKK+...</ds:X509Certificate>
</ds:X509Data>
</ds:KeyInfo>
<ds:Object>
<xades:QualifyingProperties xmlns:xades="http://uri.etsi.org/01903/v1.3.2#" Target="#ide34ffbff277e8d1432e864436aa11882">
<xades:SignedProperties Id="xades-id-e34ffbff277e8d1432e864436aa11882">
<xades:SignedSignatureProperties>
<xades:SigningTime>2016-11-25T16:35:06Z</xades:SigningTime>
<xades:SigningCertificate>
<xades:Cert>
<xades:CertDigest>
<ds:DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
<ds:DigestValue>LoXZC86JwDL7zWC35qj7Q4AzrRQ=</ds:DigestValue>
</xades:CertDigest>
<xades:IssuerSerial>
<ds:X509IssuerName>CN=CA SINPE - PERSONA FISICA,OU=DIVISION DE SERVICIOS FINANCIEROS,O=BANCO CENTRAL DE COSTA RICA,C=CR,2.5.4.5=#130c342d3030302d303034303137</ds:X509IssuerName>
<ds:X509SerialNumber>207422209224813750547132</ds:X509SerialNumber>
</xades:IssuerSerial>
</xades:Cert>
</xades:SigningCertificate>
<xades:SignaturePolicyIdentifier>
<xades:SignaturePolicyId>
<xades:SigPolicyId>
<xades:Identifier>https://tribunet.hacienda.go.cr/docs/esquemas/2016/v4.1/Resolucion_Comprobantes_Electronicos_DGT-R-48-2016.pdf</xades:Identifier>
</xades:SigPolicyId>
<xades:SigPolicyHash>
<ds:DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256"/>
<ds:DigestValue>NmI5Njk1ZThkNzI0MmIzMGJmZDAyNDc4YjUwNzkzODM2NTBiOWUxNTBkMmI2YjgzYzZjM2I5NTZlNDQ4OWQzMQ==</ds:DigestValue>
</xades:SigPolicyHash>
</xades:SignaturePolicyId>
</xades:SignaturePolicyIdentifier>
</xades:SignedSignatureProperties>
<xades:SignedDataObjectProperties>
<xades:DataObjectFormat ObjectReference="#r-id-1">
<xades:MimeType>application/octet-stream</xades:MimeType>
</xades:DataObjectFormat>
</xades:SignedDataObjectProperties>
</xades:SignedProperties>
</xades:QualifyingProperties>
</ds:Object>
</ds:Signature>

Answer 1

From what I read here and here, I stand by what I described previously: it's the hash of the node SignedProperties. This hash is part of the SignedInfo node, which is the one that gets signed afterwards. The first link is a blog in polish. It's not perfect, but Google Translate is doing a quite decent job and the info in the blog helped me to understand more clearly.
Make sure to get the node canonicalized and base64_encoded
P.S.: Suerte con la implementación que está desarrollando para la factura electrónica en Costa Rica ;-D

Answer 2

I just started reading into a similar topic, so it's just an idea and I have not tried it, but I believe the second hash comes from the xades:SignedProperties node that is part of the ds:Object.
At least the id (xades-id-e34ff...) in the object and in the reference are identical.
Hope this gives you (and me :-D ) a hint.
Regards,
Sebastian