Added function to generate vkey from skey (#199)

This is scoped only to Ed25519 keys. This comes up in many implementations where an operator wants to sign new messages, but also authenticate previous messages they may have made (e.g. witnesses, logs).

Author: mhutchinson

note/note_verifier.go

@@ -17,17 +17,60 @@ package note
 
 import (
 	"crypto/ecdsa"
+	"crypto/ed25519"
 	"crypto/sha256"
 	"crypto/x509"
 	"encoding/base64"
 	"encoding/binary"
+	"errors"
 	"fmt"
 	"strconv"
 	"strings"
 
 	"golang.org/x/mod/sumdb/note"
 )
 
+// NewEd25519SignerVerifier returns a note Signer and Verifier given an
+// Ed25519 private key in the standard note-formatted form, e.g.
+// `PRIVATE+KEY+logandmap+38581672+AXJ0FKWOcO2ch6WC8kP705Ed3Gxu7pVtZLhfHAQwp+FE`.
+func NewEd25519SignerVerifier(skey string) (note.Signer, note.Verifier, error) {
+	const algEd25519 = 1
+	s, err := note.NewSigner(skey)
+	if err != nil {
+		return nil, nil, err
+	}
+	parts := strings.SplitN(skey, "+", 5)
+	if n := len(parts); n != 5 {
+		return nil, nil, fmt.Errorf("expected 5 parts but got %d", n)
+	}
+	if parts[0] != "PRIVATE" || parts[1] != "KEY" {
+		return nil, nil, fmt.Errorf("expected first tokens to be [PRIVATE, KEY]")
+	}
+	key, err := base64.StdEncoding.DecodeString(parts[4])
+	if err != nil {
+		return nil, nil, fmt.Errorf("failed to decode base64: %v", err)
+	}
+
+	alg, key := key[0], key[1:]
+	if alg != algEd25519 {
+		return nil, nil, errors.New("unsupported algorithm")
+	}
+	if l := len(key); l != ed25519.SeedSize {
+		return nil, nil, fmt.Errorf("expected key seed of size %d but got %d", ed25519.SeedSize, l)
+	}
+	publicKey := ed25519.NewKeyFromSeed(key).Public().(ed25519.PublicKey)
+	vkey, err := note.NewEd25519VerifierKey(s.Name(), publicKey)
+	if err != nil {
+		return nil, nil, fmt.Errorf("failed to generate verifier from key: %v", err)
+
+	}
+	v, err := note.NewVerifier(vkey)
+	if err != nil {
+		return nil, nil, fmt.Errorf("failed to create verifier from vkey: %v", err)
+	}
+	return s, v, err
+}
+
 // NewVerifier returns a verifier for the given key, if the key's algo is known.
 func NewVerifier(key string) (note.Verifier, error) {
 	parts := strings.SplitN(key, "+", 3)

note/note_verifier_test.go

@@ -33,6 +33,42 @@ const (
 	pixelKey         = "pixel6_transparency_log" + "+" + pixelKeyHash + "+" + pixelKeyMaterial
 )
 
+func TestSignerVerifierFromEd25519Key(t *testing.T) {
+	for _, test := range []struct {
+		name    string
+		key     string
+		wantErr bool
+	}{
+		{
+			name: "Ed25519 private key",
+			key:  "PRIVATE+KEY+logandmap+38581672+AXJ0FKWOcO2ch6WC8kP705Ed3Gxu7pVtZLhfHAQwp+FE",
+		},
+		{
+			name:    "Ed25519 public key",
+			key:     "logandmap+38581672+Ab/PCr1eCclRPRMBqw/r5An1xO71MCnImLiospEq6b4l",
+			wantErr: true,
+		},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			s, v, err := NewEd25519SignerVerifier(test.key)
+			if gotErr := err != nil; gotErr != test.wantErr {
+				t.Fatalf("NewVerifier: %v, wantErr %t", err, test.wantErr)
+			}
+			if test.wantErr {
+				return
+			}
+			msg := []byte("hello")
+			sig, err := s.Sign(msg)
+			if err != nil {
+				t.Fatal(err)
+			}
+			if !v.Verify(msg, sig) {
+				t.Fatal("Failed to verify signature from signer")
+			}
+		})
+	}
+}
+
 func TestNewVerifier(t *testing.T) {
 	for _, test := range []struct {
 		name    string