Add a function to validate an x509 RSA key pair

Add a function to validate an x509 RSA certificate and key pair, as
commonly used for TLS certificates.

The rationale behind this is that we store our TLS certificates and
private keys in Hiera YAML files, and poor indentation or formatting in
the YAML file could cause a valid certificate to be considered invalid.

Will cause the Puppet run to fail if:

- an invalid certificate is detected
- an invalid RSA key is detected
- the certificate does not match the key, i.e. the certificate
  has not been signed by the supplied key

The test certificates I've used in the spec tests were generated using
the Go standard library:

    $ go run $GOROOT/src/crypto/tls/generate_cert.go -host localhost

Example output:

    ==> cache-1.router: Error: Not a valid RSA key: Neither PUB key nor PRIV key:: nested asn1 error at /var/govuk/puppet/modules/nginx/manifests/config/ssl.pp:30 on node cache-1.router.dev.gov.uk

1 files changed, 47 insertions, 0 deletions

diff --git a/lib/puppet/parser/functions/validate_x509_rsa_key_pair.rb b/lib/puppet/parser/functions/validate_x509_rsa_key_pair.rb
new file mode 100644
index 0000000..fc9f23f
--- /dev/null
+++ b/lib/puppet/parser/functions/validate_x509_rsa_key_pair.rb
@@ -0,0 +1,47 @@
+module Puppet::Parser::Functions
+
+  newfunction(:validate_x509_rsa_key_pair, :doc => <<-ENDHEREDOC
+    Validates a PEM-formatted X.509 certificate and RSA private key using
+    OpenSSL. Verifies that the certficate's signature was created from the
+    supplied key.
+
+    Fail compilation if any value fails this check.
+
+    validate_x509_rsa_key_pair($cert, $key)
+
+    ENDHEREDOC
+  ) do |args|
+
+    require 'openssl'
+
+    NUM_ARGS = 2 unless defined? NUM_ARGS
+
+    unless args.length == NUM_ARGS then
+      raise Puppet::ParseError,
+        ("validate_x509_rsa_key_pair(): wrong number of arguments (#{args.length}; must be #{NUM_ARGS})")
+    end
+
+    args.each do |arg|
+      unless arg.is_a?(String)
+        raise Puppet::ParseError, "#{arg.inspect} is not a string."
+      end
+    end
+
+    begin
+      cert = OpenSSL::X509::Certificate.new(args[0])
+    rescue OpenSSL::X509::CertificateError => e
+      raise Puppet::ParseError, "Not a valid x509 certificate: #{e}"
+    end
+
+    begin
+      key = OpenSSL::PKey::RSA.new(args[1])
+    rescue OpenSSL::PKey::RSAError => e
+      raise Puppet::ParseError, "Not a valid RSA key: #{e}"
+    end
+
+    unless cert.verify(key)
+      raise Puppet::ParseError, "Certificate signature does not match supplied key"
+    end
+  end
+
+end