factor out confirmation codes into utils, change their format

irc/utils/confirmation.go

@@ -0,0 +1,22 @@
+// Copyright (c) 2020 Shivaram Lingamneni <slingamn@cs.stanford.edu>
+// released under the MIT license
+
+package utils
+
+import (
+	"crypto/sha256"
+	"encoding/binary"
+	"time"
+)
+
+// Deterministically generates a confirmation code for some destructive activity;
+// `name` is typically the name of the identity being destroyed (a channel being
+// unregistered, or the server being crashed) and `createdAt` means a different
+// value is required each time.
+func ConfirmationCode(name string, createdAt time.Time) (code string) {
+	buf := make([]byte, len(name)+8)
+	binary.BigEndian.PutUint64(buf, uint64(createdAt.UnixNano()))
+	copy(buf[8:], name[:])
+	out := sha256.Sum256(buf)
+	return B32Encoder.EncodeToString(out[:3])
+}