Xmldsig

unsigned_xml = <<-XML
<?xml version="1.0" encoding="UTF-8"?>
<foo:Foo ID="foo" xmlns:foo="http://example.com/foo#" xmlns:ds="http://www.w3.org/2000/09/xmldsig#" xmlns:ec="http://www.w3.org/2001/10/xml-exc-c14n#">
  <foo:Bar>bar</foo:Bar>
  <ds:Signature>
    <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 URI="#foo">
        <ds:Transforms>
          <ds:Transform Algorithm="http://www.w3.org/2000/09/xmldsig#enveloped-signature"/>
          <ds:Transform Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#">
            <ec:InclusiveNamespaces PrefixList="foo"/>
          </ds:Transform>
        </ds:Transforms>
        <ds:DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256"/>
        <ds:DigestValue></ds:DigestValue>
      </ds:Reference>
    </ds:SignedInfo>
    <ds:SignatureValue></ds:SignatureValue>
  </ds:Signature>
</foo:Foo>
XML

private_key = OpenSSL::PKey::RSA.new(File.read("key.pem"))
certificate = OpenSSL::X509::Certificate.new(File.read("certificate.cer"))

unsigned_document = Xmldsig::SignedDocument.new(unsigned_xml)
signed_xml = unsigned_document.sign(private_key)

# With block
signed_xml = unsigned_document.sign do |data|
  private_key.sign(OpenSSL::Digest::SHA256.new, data)
end

# Validation

signed_document = Xmldsig::SignedDocument.new(signed_xml)
signed_document.validate(certificate)

# With block
signed_document = Xmldsig::SignedDocument.new(signed_xml)
signed_document.validate do |signature_value, data|
  certificate.public_key.verify(OpenSSL::Digest::SHA256.new, signature_value, data)
end