add dns package

Mostly imported from sigsum-log-go.  Added strict domain hint parsing,
two tests, and an example.  This should be part of the sigsum-go lib
because some client tooling may also want to check domain hints.

2 files changed, 164 insertions, 0 deletions

diff --git a/pkg/dns/dns.go b/pkg/dns/dns.go
new file mode 100644
index 0000000..3bc0583
--- /dev/null
+++ b/pkg/dns/dns.go
@@ -0,0 +1,62 @@
+// package dns checks if a domain name is aware of a hashed public key.  A
+// look-up is performed if the specified domain name matches "^_sigsum_v0.*".
+package dns
+
+import (
+	"context"
+	"fmt"
+	"net"
+	"strings"
+
+	"git.sigsum.org/sigsum-lib-go/pkg/hex"
+	"git.sigsum.org/sigsum-lib-go/pkg/types"
+)
+
+const (
+	prefix = "_sigsum_v0."
+)
+
+// Verifier can verify that a domain name is aware of a public key
+type Verifier interface {
+	Verify(ctx context.Context, name string, key *types.PublicKey) error
+}
+
+// DefaultResolver implements the Verifier interface with Go's default resolver
+type DefaultResolver struct {
+	resolver net.Resolver
+}
+
+func NewDefaultResolver() Verifier {
+	return &DefaultResolver{}
+}
+
+func (dr *DefaultResolver) Verify(ctx context.Context, name string, pub *types.PublicKey) error {
+	if err := validPrefix(name); err != nil {
+		return fmt.Errorf("dns: %s", err)
+	}
+	rsps, err := dr.resolver.LookupTXT(ctx, name)
+	if err != nil {
+		return fmt.Errorf("dns: look-up failed: %s", name)
+	}
+	if err := validResponse(pub, rsps); err != nil {
+		return fmt.Errorf("dns: %s", err)
+	}
+	return nil
+}
+
+func validResponse(pub *types.PublicKey, rsps []string) error {
+	keyHash := hex.Serialize(types.HashFn(pub[:])[:])
+	for _, rsp := range rsps {
+		if rsp == keyHash {
+			return nil
+		}
+	}
+	return fmt.Errorf("unknown key hash %s", keyHash)
+}
+
+func validPrefix(name string) error {
+	if !strings.HasPrefix(name, prefix) {
+		return fmt.Errorf("domain name prefix must be %s", prefix)
+	}
+	return nil
+}
diff --git a/pkg/dns/dns_test.go b/pkg/dns/dns_test.go
new file mode 100644
index 0000000..e8e2c01
--- /dev/null
+++ b/pkg/dns/dns_test.go
@@ -0,0 +1,102 @@
+package dns
+
+import (
+	"context"
+	"log"
+	"testing"
+	"time"
+
+	"git.sigsum.org/sigsum-lib-go/pkg/hex"
+	"git.sigsum.org/sigsum-lib-go/pkg/types"
+)
+
+func Example() {
+	name := "_sigsum_v0.testonly.sigsum.org"
+	pub := mustDecodePublicKey("cda2517e17dcba133eb0e71bf77473f94a77d7e61b1de4e1e64adfd0938d6182")
+
+	timeout := 10 * time.Second
+	ctx, cancel := context.WithTimeout(context.Background(), timeout)
+	defer cancel()
+
+	dr := NewDefaultResolver()
+	if err := dr.Verify(ctx, name, pub); err != nil {
+		log.Fatal(err.Error())
+	}
+
+	// Output:
+}
+
+func TestValidResponse(t *testing.T) {
+	pub := mustDecodePublicKey("cda2517e17dcba133eb0e71bf77473f94a77d7e61b1de4e1e64adfd0938d6182")
+	for _, table := range []struct {
+		desc   string
+		rsps   []string
+		wantOK bool
+	}{
+		{
+			desc: "invalid: upper-case hex-encoding",
+			rsps: []string{
+				"",
+				"abc",
+				"C522D929B261241EEF174B51B8472FA5D5F961892089A7B85FD25CE73271ABCA",
+				"defghi",
+			},
+		},
+		{
+			desc: "valid",
+			rsps: []string{
+				"",
+				"abc",
+				"c522d929b261241eef174b51b8472fa5d5f961892089a7b85fd25ce73271abca",
+				"defghi",
+			},
+			wantOK: true,
+		},
+	} {
+		err := validResponse(pub, table.rsps)
+		if got, want := err == nil, table.wantOK; got != want {
+			t.Errorf("got error but wanted none in test %q: %v", table.desc, err)
+		}
+	}
+}
+
+func TestValidPrefix(t *testing.T) {
+	for _, table := range []struct {
+		desc   string
+		name   string
+		wantOK bool
+	}{
+		{
+			desc: "invalid: bad prefix (1/2)",
+			name: "x_sigsum_v0.sigsum.org",
+		},
+		{
+			desc: "invalid: bad prefix (2/2)",
+			name: "_sigsum_v0x.sigsum.org",
+		},
+		{
+			desc:   "valid",
+			name:   "_sigsum_v0.sigsum.org",
+			wantOK: true,
+		},
+	} {
+		err := validPrefix(table.name)
+		if got, want := err == nil, table.wantOK; got != want {
+			t.Errorf("got error but wanted none in test %q: %v", table.desc, err)
+		}
+	}
+}
+
+func mustDecodePublicKey(str string) *types.PublicKey {
+	b, err := hex.Deserialize(str)
+	if err != nil {
+		log.Fatal(err.Error())
+	}
+	if len(b) != types.PublicKeySize {
+		log.Fatal("invalid key size")
+	}
+
+	var pub types.PublicKey
+	copy(pub[:], b)
+	return &pub
+}