session: implement function for receiving session public key bytes

After recent changes PrivateToken cannot directly return public key
bytes. In order to provide this ability, this commit implements
a function over PrivateToken interface.

session/errors.go

@@ -13,3 +13,7 @@ const ErrNilGPRCClientConn = internal.Error("gRPC client connection is nil")
 // ErrPrivateTokenNotFound is returned when addressed private token was
 // not found in storage.
 const ErrPrivateTokenNotFound = internal.Error("private token not found")
+
+// ErrNilPrivateToken is returned by functions that expect a non-nil
+// PrivateToken, but received nil.
+const ErrNilPrivateToken = internal.Error("private token is nil")

session/private.go

@@ -4,6 +4,8 @@ import (
 	"crypto/ecdsa"
 	"crypto/elliptic"
 	"crypto/rand"
+
+	crypto "github.com/nspcc-dev/neofs-crypto"
 )
 
 type pToken struct {
@@ -28,7 +30,24 @@ func NewPrivateToken(validUntil uint64) (PrivateToken, error) {
 	}, nil
 }
 
-// PrivateKey returns a binary representation of the session public key.
+// PublicSessionToken returns a binary representation of session public key.
+//
+// If passed PrivateToken is nil, ErrNilPrivateToken returns.
+// If passed PrivateToken carries nil private key, crypto.ErrEmptyPrivateKey returns.
+func PublicSessionToken(pToken PrivateToken) ([]byte, error) {
+	if pToken == nil {
+		return nil, ErrNilPrivateToken
+	}
+
+	sk := pToken.PrivateKey()
+	if sk == nil {
+		return nil, crypto.ErrEmptyPrivateKey
+	}
+
+	return crypto.MarshalPublicKey(&sk.PublicKey), nil
+}
+
+// PrivateKey is a session private key getter.
 func (t *pToken) PrivateKey() *ecdsa.PrivateKey {
 	return t.sessionKey
 }

session/private_test.go

@@ -3,6 +3,7 @@ package session
 import (
 	"testing"
 
+	crypto "github.com/nspcc-dev/neofs-crypto"
 	"github.com/stretchr/testify/require"
 )
 
@@ -49,3 +50,27 @@ func TestPrivateTokenKey_SetTokenID(t *testing.T) {
 
 	require.Equal(t, tokenID, s.token)
 }
+
+func TestPublicSessionToken(t *testing.T) {
+	var err error
+
+	// nil PrivateToken
+	_, err = PublicSessionToken(nil)
+	require.EqualError(t, err, ErrNilPrivateToken.Error())
+
+	// empty private key
+	var pToken PrivateToken = new(pToken)
+	_, err = PublicSessionToken(pToken)
+	require.EqualError(t, err, crypto.ErrEmptyPrivateKey.Error())
+
+	// correct PrivateToken
+	pToken, err = NewPrivateToken(0)
+	require.NoError(t, err)
+
+	key := pToken.PrivateKey()
+	require.NotNil(t, key)
+
+	res, err := PublicSessionToken(pToken)
+	require.NoError(t, err)
+	require.Equal(t, res, crypto.MarshalPublicKey(&key.PublicKey))
+}