session: Add getter of binary-encoded public key from signature

Signature is used for user authentication so access to public key is
required. Digital signature bytes may still be unexported because they
are only useful for `VerifySignature` method.

Add `IssuerPublicKeyBytes` method returning binary-encoded public key of
the session issuer.

Signed-off-by: Leonard Lyubich <[email protected]>

session/common.go

@@ -336,3 +336,14 @@ func (x commonData) Issuer() user.ID {
 
 	return user.ID{}
 }
+
+// IssuerPublicKeyBytes returns binary-encoded public key of the session issuer.
+//
+// IssuerPublicKeyBytes MUST NOT be called before ReadFromV2 or Sign methods.
+func (x *commonData) IssuerPublicKeyBytes() []byte {
+	if x.sigSet {
+		return x.sig.GetKey()
+	}
+
+	return nil
+}

session/container_test.go

@@ -542,6 +542,15 @@ func TestContainer_Issuer(t *testing.T) {
 		token.SetIssuer(issuer)
 		require.True(t, token.Issuer().Equals(issuer))
 	})
+
+	t.Run("public key", func(t *testing.T) {
+		signer := test.RandomSignerRFC6979(t)
+
+		require.Nil(t, token.IssuerPublicKeyBytes())
+		require.NoError(t, token.Sign(signer))
+
+		require.Equal(t, neofscrypto.PublicKeyBytes(signer.Public()), token.IssuerPublicKeyBytes())
+	})
 }
 
 func TestContainer_Sign(t *testing.T) {

session/object_test.go

@@ -610,12 +610,14 @@ func TestObject_Issuer(t *testing.T) {
 	signer := test.RandomSignerRFC6979(t)
 
 	require.Zero(t, token.Issuer())
+	require.Nil(t, token.IssuerPublicKeyBytes())
 
 	require.NoError(t, token.Sign(signer))
 
 	issuer := signer.UserID()
 
 	require.True(t, token.Issuer().Equals(issuer))
+	require.Equal(t, neofscrypto.PublicKeyBytes(signer.Public()), token.IssuerPublicKeyBytes())
 }
 
 func TestObject_Sign(t *testing.T) {