XPath evaluation on XML file

gistfile1.lua

#!/usr/bin/env luajit

local xmlua = require("xmlua")

xml = [[<?xml version="1.0" encoding="utf-8"?>
<EntityDescriptor xmlns="urn:oasis:names:tc:SAML:2.0:metadata" xmlns:alg="urn:oasis:names:tc:SAML:metadata:algsupport" ID="_21f12cc0-a7f7-4e65-8b21-a0bba35e39c5" entityID="urn:federation:MicrosoftOnline">
  <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/2000/09/xmldsig#rsa-sha1"/>
      <Reference URI="#_21f12cc0-a7f7-4e65-8b21-a0bba35e39c5">
        <Transforms>
          <Transform Algorithm="http://www.w3.org/2000/09/xmldsig#enveloped-signature"/>
          <Transform Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"/>
        </Transforms>
        <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
        <DigestValue>kPo8d5mfBABNtUVhLyCgLcwHT3I=</DigestValue>
      </Reference>
    </SignedInfo>
    <SignatureValue>EiRoxVsH8GIUoAl8o2prFZMb/+fuWxrrNa6OtVVI0wGCCuZ2md1fzs7puD8A8KRDLIETCC7fRzT01ZtxUa9LqVOEnpcMgJRDDRXgJk1Asm5tXHHtT3ZDBLr9u4NIPYb/zwd5E+wUCwM0o5c8TDwHHXjGS9B4HfT2zz3ryAoGyVeDn5uldZhMZ9WFPxN1QrfotmZv79oMqKyTw43o95v9tzVpXiKxOiB7MmX2umR2qltj/Gik204C1nNbOpnFLoc5Y1WKWV+2g5zyfsb4yHMp5KRnZA4EvoWAEBE+jeGfA19ujs+Y/MVsqSlXDbyff5M2ZtlGHjtV15R8ItDR5upBKg==</SignatureValue>
    <KeyInfo>
      <X509Data>
        <X509Certificate>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</X509Certificate>
      </X509Data>
    </KeyInfo>
  </Signature>
  <SPSSODescriptor WantAssertionsSigned="true" protocolSupportEnumeration="urn:oasis:names:tc:SAML:2.0:protocol">
    <KeyDescriptor use="signing">
      <KeyInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
        <X509Data>
          <X509Certificate>MIIC/TCCAeWgAwIBAgIQbgDHfi3t1JNGVqwD5/7lmjANBgkqhkiG9w0BAQsFADApMScwJQYDVQQDEx5MaXZlIElEIFNUUyBTaWduaW5nIFB1YmxpYyBLZXkwHhcNMjAxMjIxMDAwMDAwWhcNMjUxMjIxMDAwMDAwWjApMScwJQYDVQQDEx5MaXZlIElEIFNUUyBTaWduaW5nIFB1YmxpYyBLZXkwggEiMA0GCSqGSIb3DQEBAQUAA4IBDwAwggEKAoIBAQDFT0/0/2qQurnYa0LbJHF9YYozhEH6r9mCxVDBYbewSG4tGgrWpsewQ/96pcczGMQctMvU+h2eX38Hx/f9JAIDbuRQzQlsPhQS7DDZ6WlTXU+t8d/g2C7fpSoLs4KVdJih4xyjLUWj+BK/ijsRjBt4Riw9VbJH/DdWKyoSMbECEiE+s1RtLP/eYoMmNfxyQGqWirCNqVNBTlqzYQp4dgF0foYy4ktoxwmQOVoTcIMFYp1I4pFPI7CxuMLkfK0X7aTbM7YGphvMfJxJkjrQdyI7G5d1t4DNi3zkEbBT7FGAr6qPt3Kn9ralpqJKHdpEBA9N0vNwQo5XTYIhUbPQ16IRAgMBAAGjITAfMB0GA1UdDgQWBBRs7tPmfkksSr67KtElHjYZbeaCTjANBgkqhkiG9w0BAQsFAAOCAQEAJqwMZSjQJ36x+1sty6EeLKQLQewQwPaEC47Zut+8bXed6Q8jMZ0bfa/MM7XquEcabaMZLQuKLft44YXwXXQOfQrI2qjQr3eToJFlDT9hR0rfp9wQqttDxd6Aa6RWwDTgo5oKUQCTKLHhEy8uWzScK0eGt2d7TWTaDXjRSwNq6tM7fRhZs07tKBV3xfi9EQy/mlavAMFRBVm86NSo7AsOG1IOMq03U3ooCWAXh9PdvvHNfHhH19futAnC/HeOjwRF1Qc527aBMphYFQLdiThfmfmiE/AhQqCwZ2oE7uCJhBtR+Kb1ZGhjI35pHfsSqGiFa7Kr+5ave822PDcke89Mvg==</X509Certificate>
        </X509Data>
      </KeyInfo>
    </KeyDescriptor>
    <KeyDescriptor use="signing">
      <KeyInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
        <X509Data>
          <X509Certificate>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</X509Certificate>
        </X509Data>
      </KeyInfo>
    </KeyDescriptor>
    <SingleLogoutService Binding="urn:oasis:names:tc:SAML:2.0:bindings:HTTP-POST" Location="https://login.microsoftonline.com/login.srf"/>
    <NameIDFormat>urn:oasis:names:tc:SAML:1.1:nameid-format:emailAddress</NameIDFormat>
    <NameIDFormat>urn:mace:shibboleth:1.0:nameIdentifier</NameIDFormat>
    <NameIDFormat>urn:oasis:names:tc:SAML:1.1:nameid-format:unspecified</NameIDFormat>
    <NameIDFormat>urn:oasis:names:tc:SAML:2.0:nameid-format:transient</NameIDFormat>
    <NameIDFormat>urn:oasis:names:tc:SAML:2.0:nameid-format:persistent</NameIDFormat>
    <AssertionConsumerService Binding="urn:oasis:names:tc:SAML:2.0:bindings:HTTP-POST" Location="https://login.microsoftonline.com/login.srf" index="0" isDefault="true"/>
    <AssertionConsumerService Binding="urn:oasis:names:tc:SAML:2.0:bindings:HTTP-POST-SimpleSign" Location="https://login.microsoftonline.com/login.srf" index="1"/>
    <AssertionConsumerService Binding="urn:oasis:names:tc:SAML:2.0:bindings:PAOS" Location="https://login.microsoftonline.com/login.srf" index="2"/>
  </SPSSODescriptor>
  <Extensions>
    <alg:DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
    <alg:SigningMethod Algorithm="http://www.w3.org/2000/09/xmldsig#rsa-sha1"/>
  </Extensions>
</EntityDescriptor>
]]

local namespaces = {
  {
    prefix = "metadata",
    href = "urn:oasis:names:tc:SAML:2.0:metadata",
  },
  {
    prefix = "alg",
    href = "urn:oasis:names:tc:SAML:metadata:algsupport",
  },
  {
    prefix = "xmldsig",
    href = "http://www.w3.org/2000/09/xmldsig#",
  },
}

local function find_value(searchable, path)
  local value = searchable:search(path, namespaces)[1]
  if value then
    return value:text()
  end
end

local document = xmlua.XML.parse(xml)

print(find_value(document, "/metadata:EntityDescriptor/xmldsig:Signature/xmldsig:KeyInfo/xmldsig:X509Data/xmldsig:X509Certificate"))
if not find_value(document, "/metadata:NonExistent") then
  print("not found, expectedly")
end