feat(did): add ToPubKey() and improve crypto tests

did/crypto.go

@@ -37,3 +37,12 @@ func FromPubKey(pubKey crypto.PubKey) (DID, error) {
 		key:  true,
 	}, nil
 }
+
+func ToPubKey(s string) (crypto.PubKey, error) {
+	id, err := Parse(s)
+	if err != nil {
+		return nil, err
+	}
+
+	return id.PubKey()
+}

did/crypto_test.go

@@ -1,33 +1,51 @@
 package did_test
 
 import (
-	"bytes"
-	"encoding/binary"
 	"testing"
 
 	"github.com/libp2p/go-libp2p/core/crypto"
-	"github.com/multiformats/go-multicodec"
 	"github.com/stretchr/testify/require"
 	"github.com/ucan-wg/go-ucan/did"
 )
 
+const (
+	exampleDIDStr    = "did:key:z6MkhaXgBZDvotDkL5257faiztiGiC2QtKLGpbnnEGta2doK"
+	examplePubKeyStr = "Lm/M42cB3HkUiODQsXRcweM6TByfzEHGO9ND274JcOY="
+)
+
 func TestFromPubKey(t *testing.T) {
 	t.Parallel()
 
-	const example = "did:key:z6MkhaXgBZDvotDkL5257faiztiGiC2QtKLGpbnnEGta2doK"
-
-	id, err := did.Parse(example)
+	id, err := did.FromPubKey(examplePubKey(t))
 	require.NoError(t, err)
-
-	buf := bytes.NewBuffer(id.Bytes())
-
-	code, err := binary.ReadUvarint(buf)
-	require.NoError(t, err)
-	require.Equal(t, uint64(multicodec.Ed25519Pub), code)
-
-	pubKey, err := crypto.UnmarshalEd25519PublicKey(buf.Bytes())
-	require.NoError(t, err)
-
-	id2, err := did.FromPubKey(pubKey)
-	require.Equal(t, id, id2)
+	require.Equal(t, exampleDID(t), id)
+}
+
+func TestToPubKey(t *testing.T) {
+	t.Parallel()
+
+	pubKey, err := did.ToPubKey(exampleDIDStr)
+	require.NoError(t, err)
+	require.Equal(t, examplePubKey(t), pubKey)
+}
+
+func exampleDID(t *testing.T) did.DID {
+	t.Helper()
+
+	id, err := did.Parse(exampleDIDStr)
+	require.NoError(t, err)
+
+	return id
+}
+
+func examplePubKey(t *testing.T) crypto.PubKey {
+	t.Helper()
+
+	pubKeyCfg, err := crypto.ConfigDecodeKey(examplePubKeyStr)
+	require.NoError(t, err)
+
+	pubKey, err := crypto.UnmarshalEd25519PublicKey(pubKeyCfg)
+	require.NoError(t, err)
+
+	return pubKey
 }

did/did.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"strings"
 
+	crypto "github.com/libp2p/go-libp2p/core/crypto"
 	mbase "github.com/multiformats/go-multibase"
 	"github.com/multiformats/go-multicodec"
 	varint "github.com/multiformats/go-varint"
@@ -49,6 +50,24 @@ func (d DID) DID() DID {
 	return d
 }
 
+func (d DID) PubKey() (crypto.PubKey, error) {
+	if !d.key {
+		return nil, fmt.Errorf("unsupported did type: %s", d.String())
+	}
+
+	unmarshaler, ok := map[multicodec.Code]crypto.PubKeyUnmarshaller{
+		multicodec.Ed25519Pub:   crypto.UnmarshalEd25519PublicKey,
+		multicodec.RsaPub:       crypto.UnmarshalRsaPublicKey,
+		multicodec.Secp256k1Pub: crypto.UnmarshalSecp256k1PublicKey,
+		multicodec.Es256:        crypto.UnmarshalECDSAPublicKey,
+	}[multicodec.Code(d.code)]
+	if !ok {
+		return nil, fmt.Errorf("unsupported multicodec: %d", d.code)
+	}
+
+	return unmarshaler(d.Bytes()[varint.UvarintSize(d.code):])
+}
+
 // String formats the decentralized identity document (DID) as a string.
 func (d DID) String() string {
 	if d.key {