An example of how to verify SAML/XML signatures, execute in Ruby. Adapted from https://github.com/zendesk/samlr/.

README.md

It's hard to find a good example of how to verify SAML signatures online. There are plenty magic that "just calls a Java method" -- but few clear step-by-step guide.

This gist covers the signature check of a SAML response in Ruby, and as such it's also an example of how to verify an XML Secure.

The code here is lifted entirely from Morten Primdahls and Zendesks awesome SAMLR library.

verify_saml_signature.rb

require "nokogiri"
require "openssl"
require "base64"

# Constants
C14N    = Nokogiri::XML::XML_C14N_EXCLUSIVE_1_0
NS_MAP  = {
  "c14n"  => "http://www.w3.org/2001/10/xml-exc-c14n#",
  "ds"    => "http://www.w3.org/2000/09/xmldsig#",
  "saml"  => "urn:oasis:names:tc:SAML:2.0:assertion",
  "samlp" => "urn:oasis:names:tc:SAML:2.0:protocol",
  "md"    => "urn:oasis:names:tc:SAML:2.0:metadata",
  "xsi"   => "http://www.w3.org/2001/XMLSchema-instance",
  "xs"    => "http://www.w3.org/2001/XMLSchema"
}
SHA_MAP = {
  1    => OpenSSL::Digest::SHA1,
  256  => OpenSSL::Digest::SHA256,
  384  => OpenSSL::Digest::SHA384,
  512  => OpenSSL::Digest::SHA512
}

# Set up the certificate
certificate = OpenSSL::X509::Certificate.new(File.read("cert.pem"))

# Read the document
original = Nokogiri::XML(File.read("login.xml"))
document = original.dup
prefix = "/samlp:Response"

# Read, then clear,  the signature
signature = document.at("#{prefix}/ds:Signature", NS_MAP)
signature.remove

# Verify the document digests to ensure that the document hasn't been modified
original.xpath("#{prefix}/ds:Signature/ds:SignedInfo/ds:Reference[@URI]", NS_MAP).each do |ref|
  digest_value = ref.at("./ds:DigestValue", NS_MAP).text
  decoded_digest_value = Base64.decode64(digest_value);
  
  reference_id = ref["URI"][1..-1]
  reference_node = document.xpath("//*[@ID='#{reference_id}']").first
  reference_canoned = reference_node.canonicalize(C14N)

  # Figure out which method has been used to the sign the node
  digest_method = OpenSSL::Digest::SHA1
  if ref.at("./ds:DigestMethod/@Algorithm", NS_MAP).text =~ /sha(\d+)$/
    digest_method = SHA_MAP[$1.to_i]
  end

  # Verify the digest
  digest = digest_method.digest(reference_canoned)
  if digest == decoded_digest_value
    print "Digest verified for #{reference_id}\n"
  else
    print "Digest check mismatch for #{reference_id}\n"
  end
end

# Canonicalization: Stringify the node in a nice way
node = original.at("#{prefix}/ds:Signature/ds:SignedInfo", NS_MAP)
canoned = node.canonicalize(C14N)

# Figure out which method has been used to the sign the node
signature_method = OpenSSL::Digest::SHA1
if signature.at("./ds:SignedInfo/ds:SignatureMethod/@Algorithm", NS_MAP).text =~ /sha(\d+)$/
  signature_method = SHA_MAP[$1.to_i]
end

# Read the signature
signature_value = signature.at("./ds:SignatureValue", NS_MAP).text
decoded_signature_value = Base64.decode64(signature_value);

# Finally, verify that the signature is correct
verify = certificate.public_key.verify(signature_method.new, decoded_signature_value, canoned)
if verify
  print "Document signature is correct\n"
else
  print "Document signature is incorrect\n"
end

Thanks much. We found this very useful.

Thanks for that!