Issue #108: Mark ApiGateway certificates as CA certs

AWS API Gateway certficates are self-signed but don't have
the IsCA flag set to true and also don't mark the certificate
as to be used for certificate signing. This prevents Go to
accept the certificate for client-cert authentication. Since
it isn't possible to use a custom certificate for client authentication
in the AWS API Gateway we need to patch the certificate on the fly.

This is a preliminary patch to verify that the approach works but
has the the certificate detection hard-coded and permanent. This
should be configurable and off by default.

listen.go

@@ -3,7 +3,7 @@ package main
 import (
 	"crypto/tls"
 	"crypto/x509"
-	"errors"
+	"encoding/pem"
 	"io/ioutil"
 	"log"
 	"net"
@@ -89,10 +89,23 @@ func newServer(l config.Listen, h http.Handler) (*http.Server, error) {
 			if err != nil {
 				return nil, err
 			}
+
+			// Issue #108: Temp patch to allow generated AWS API Gateway certs to be
+			// used for client cert authentication
 			pool := x509.NewCertPool()
-			if !pool.AppendCertsFromPEM(pemBlock) {
-				return nil, errors.New("failed to add client auth certs")
+			for p, rest := pem.Decode(pemBlock); p != nil; p, rest = pem.Decode(rest) {
+				cert, err := x509.ParseCertificate(p.Bytes)
+				if err != nil {
+					return nil, err
+				}
+				if cert.Issuer.CommonName == "ApiGateway" {
+					cert.BasicConstraintsValid = true
+					cert.IsCA = true
+					cert.KeyUsage = x509.KeyUsageCertSign
+				}
+				pool.AddCert(cert)
 			}
+
 			srv.TLSConfig.ClientCAs = pool
 			srv.TLSConfig.ClientAuth = tls.RequireAndVerifyClientCert
 		}