[feature] add TOTP two-factor authentication (2FA) (#3960)

* [feature] add TOTP two-factor authentication (2FA)

* use byteutil.S2B to avoid allocations when comparing + generating password hashes

* don't bother with string conversion  for consts

* use io.ReadFull

* use MustGenerateSecret for backup codes

* rename util functions

internal/util/secret.go

@@ -0,0 +1,47 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package util
+
+import (
+	"crypto/rand"
+	"encoding/base32"
+	"io"
+)
+
+// crockfordBase32 is an encoding alphabet that misses characters I,L,O,U,
+// to avoid confusion and abuse. See: http://www.crockford.com/wrmg/base32.html
+const crockfordBase32 = "0123456789ABCDEFGHJKMNPQRSTVWXYZ"
+
+// base32enc is a pre-initialized CrockfordBase32 encoding without any padding.
+var base32enc = base32.NewEncoding(crockfordBase32).WithPadding(base32.NoPadding)
+
+// MustGenerateSecret returns a cryptographically-secure,
+// CrockfordBase32-encoded string of 32 chars in length
+// (ie., 20-bytes/160 bits of entropy), or panics on error.
+//
+// The source of randomness is crypto/rand.
+func MustGenerateSecret() string {
+	// Crockford base32 with no padding
+	// encodes 20 bytes to 32 characters.
+	const blen = 20
+	b := make([]byte, blen)
+	if _, err := io.ReadFull(rand.Reader, b); err != nil {
+		panic(err)
+	}
+	return base32enc.EncodeToString(b)
+}