From 13dd306e69b26ab8b7aedcd6ed915df4b6672a01 Mon Sep 17 00:00:00 2001
From: Rasmus Dahlberg <rasmus.dahlberg@kau.se>
Date: Tue, 27 Oct 2020 16:22:13 +0100
Subject: isolated chain and signature verification

---
 reqres.go | 41 +++++++++++------------------------------
 x509.go   | 35 +++++++++++++++++++++++++++++++++++
 2 files changed, 46 insertions(+), 30 deletions(-)

diff --git a/reqres.go b/reqres.go
index a725363..e1f97ee 100644
--- a/reqres.go
+++ b/reqres.go
@@ -4,8 +4,6 @@ import (
 	"fmt"
 	"strconv"
 
-	"crypto/ecdsa"
-	"crypto/rsa"
 	"crypto/x509"
 	"encoding/base64"
 	"encoding/json"
@@ -186,50 +184,33 @@ func VerifyAddEntryRequest(ld *LogParameters, r AddEntryRequest) ([]byte, []byte
 	leaf, err := tls.Marshal(item)
 	if err != nil {
 		return nil, nil, fmt.Errorf("failed tls marshaling StItem: %v", err)
-	}
+	} // leaf is the serialized data that should be added to the tree
 
-	certificate, err := base64.StdEncoding.DecodeString(r.Certificate)
+	c, err := base64.StdEncoding.DecodeString(r.Certificate)
 	if err != nil {
 		return nil, nil, fmt.Errorf("failed decoding certificate: %v", err)
 	}
-	c, err := x509.ParseCertificate(certificate)
+	certificate, err := x509.ParseCertificate(c)
 	if err != nil {
 		return nil, nil, fmt.Errorf("failed decoding certificate: %v", err)
-	}
-	opts := x509.VerifyOptions{
-		Roots: ld.AnchorPool,
-	}
-	chains, err := c.Verify(opts)
+	} // certificate is the end-entity certificate that signed leaf
+
+	chain, err := VerifyChain(ld, certificate)
 	if err != nil {
 		return nil, nil, fmt.Errorf("chain verification failed: %v", err)
-	}
-	if len(chains) == 0 {
-		return nil, nil, fmt.Errorf("chain verification failed: no chain")
-	}
-	x509chain := chains[0]
+	} // chain is a valid path to some trust anchor
 
 	signature, err := base64.StdEncoding.DecodeString(r.Signature)
 	if err != nil {
 		return nil, nil, fmt.Errorf("failed decoding signature: %v", err)
 	}
+	if err := VerifySignature(leaf, signature, certificate); err != nil {
+		return nil, nil, fmt.Errorf("signature verification failed: %v", err)
+	} // signature is valid for certificate
 
-	var algo x509.SignatureAlgorithm
-	switch t := c.PublicKey.(type) {
-	case *rsa.PublicKey:
-		algo = x509.SHA256WithRSA
-	case *ecdsa.PublicKey:
-		algo = x509.ECDSAWithSHA256
-	default:
-		return nil, nil, fmt.Errorf("unsupported public key algorithm: %v", t)
-	}
-
-	if err := c.CheckSignature(algo, leaf, signature); err != nil {
-		return nil, nil, fmt.Errorf("invalid signature: %v", err)
-	}
 	// TODO: update doc of what signature "is", i.e., w/e x509 does
 	// TODO: doc in markdown/api.md what signature schemes we expect
-
-	appendix, err := tls.Marshal(NewAppendix(x509chain, signature))
+	appendix, err := tls.Marshal(NewAppendix(chain, signature))
 	if err != nil {
 		return nil, nil, fmt.Errorf("failed tls marshaling appendix: %v", err)
 	}
diff --git a/x509.go b/x509.go
index cdcd523..4e5a4d6 100644
--- a/x509.go
+++ b/x509.go
@@ -3,6 +3,8 @@ package stfe
 import (
 	"fmt"
 
+	"crypto/ecdsa"
+	"crypto/rsa"
 	"crypto/x509"
 	"encoding/pem"
 	"io/ioutil"
@@ -41,3 +43,36 @@ func LoadTrustAnchors(path string) ([]*x509.Certificate, *x509.CertPool, error)
 	}
 	return anchors, pool, nil
 }
+
+func VerifyChain(ld *LogParameters, certificate *x509.Certificate) ([]*x509.Certificate, error) {
+	opts := x509.VerifyOptions{
+		Roots:     ld.AnchorPool,
+		KeyUsages: []x509.ExtKeyUsage{x509.ExtKeyUsageAny}, // TODO: move to ld
+	} // TODO: add intermediates
+
+	chains, err := certificate.Verify(opts)
+	if err != nil {
+		return nil, fmt.Errorf("chain verification failed: %v", err)
+	}
+	if len(chains) == 0 {
+		return nil, fmt.Errorf("chain verification failed: no chain")
+	}
+	return chains[0], nil // if we found multiple paths just pick the first one
+}
+
+func VerifySignature(leaf, signature []byte, certificate *x509.Certificate) error {
+	var algo x509.SignatureAlgorithm
+	switch t := certificate.PublicKey.(type) {
+	case *rsa.PublicKey:
+		algo = x509.SHA256WithRSA
+	case *ecdsa.PublicKey:
+		algo = x509.ECDSAWithSHA256
+	default:
+		return fmt.Errorf("unsupported public key algorithm: %v", t)
+	}
+
+	if err := certificate.CheckSignature(algo, leaf, signature); err != nil {
+		return fmt.Errorf("invalid signature: %v", err)
+	}
+	return nil
+}
-- 
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