formalize token munging code

irc/channel.go

@@ -777,12 +777,6 @@ func stripMaskFromNick(nickMask string) (nick string) {
 	return nickMask[0:index]
 }
 
-// munge the msgid corresponding to a replayable event,
-// yielding a consistent msgid for the fake PRIVMSG from HistServ
-func mungeMsgidForHistserv(token string) (result string) {
-	return fmt.Sprintf("_%s", token)
-}
-
 func (channel *Channel) replayHistoryItems(rb *ResponseBuffer, items []history.Item, autoreplay bool) {
 	chname := channel.Name()
 	client := rb.target
@@ -823,7 +817,7 @@ func (channel *Channel) replayHistoryItems(rb *ResponseBuffer, items []history.I
 				} else {
 					message = fmt.Sprintf(client.t("%[1]s [account: %[2]s] joined the channel"), nick, item.AccountName)
 				}
-				rb.AddFromClient(item.Message.Time, mungeMsgidForHistserv(item.Message.Msgid), "HistServ", "*", nil, "PRIVMSG", chname, message)
+				rb.AddFromClient(item.Message.Time, utils.MungeSecretToken(item.Message.Msgid), "HistServ", "*", nil, "PRIVMSG", chname, message)
 			}
 		case history.Part:
 			if eventPlayback {
@@ -833,14 +827,14 @@ func (channel *Channel) replayHistoryItems(rb *ResponseBuffer, items []history.I
 					continue // #474
 				}
 				message := fmt.Sprintf(client.t("%[1]s left the channel (%[2]s)"), nick, item.Message.Message)
-				rb.AddFromClient(item.Message.Time, mungeMsgidForHistserv(item.Message.Msgid), "HistServ", "*", nil, "PRIVMSG", chname, message)
+				rb.AddFromClient(item.Message.Time, utils.MungeSecretToken(item.Message.Msgid), "HistServ", "*", nil, "PRIVMSG", chname, message)
 			}
 		case history.Kick:
 			if eventPlayback {
 				rb.AddFromClient(item.Message.Time, item.Message.Msgid, item.Nick, item.AccountName, nil, "HEVENT", "KICK", chname, item.Params[0], item.Message.Message)
 			} else {
 				message := fmt.Sprintf(client.t("%[1]s kicked %[2]s (%[3]s)"), nick, item.Params[0], item.Message.Message)
-				rb.AddFromClient(item.Message.Time, mungeMsgidForHistserv(item.Message.Msgid), "HistServ", "*", nil, "PRIVMSG", chname, message)
+				rb.AddFromClient(item.Message.Time, utils.MungeSecretToken(item.Message.Msgid), "HistServ", "*", nil, "PRIVMSG", chname, message)
 			}
 		case history.Quit:
 			if eventPlayback {
@@ -850,14 +844,14 @@ func (channel *Channel) replayHistoryItems(rb *ResponseBuffer, items []history.I
 					continue // #474
 				}
 				message := fmt.Sprintf(client.t("%[1]s quit (%[2]s)"), nick, item.Message.Message)
-				rb.AddFromClient(item.Message.Time, mungeMsgidForHistserv(item.Message.Msgid), "HistServ", "*", nil, "PRIVMSG", chname, message)
+				rb.AddFromClient(item.Message.Time, utils.MungeSecretToken(item.Message.Msgid), "HistServ", "*", nil, "PRIVMSG", chname, message)
 			}
 		case history.Nick:
 			if eventPlayback {
 				rb.AddFromClient(item.Message.Time, item.Message.Msgid, item.Nick, item.AccountName, nil, "HEVENT", "NICK", item.Params[0])
 			} else {
 				message := fmt.Sprintf(client.t("%[1]s changed nick to %[2]s"), nick, item.Params[0])
-				rb.AddFromClient(item.Message.Time, mungeMsgidForHistserv(item.Message.Msgid), "HistServ", "*", nil, "PRIVMSG", chname, message)
+				rb.AddFromClient(item.Message.Time, utils.MungeSecretToken(item.Message.Msgid), "HistServ", "*", nil, "PRIVMSG", chname, message)
 			}
 		}
 	}

irc/utils/crypto.go

@@ -28,6 +28,29 @@ func GenerateSecretToken() string {
 	return B32Encoder.EncodeToString(buf[:])
 }
 
+// "munge" a secret token to a new value. requirements:
+// 1. MUST be roughly as unlikely to collide with `GenerateSecretToken` outputs
+// as those outputs are with each other
+// 2. SHOULD be deterministic (motivation: if a JOIN line has msgid x,
+// create a deterministic msgid y for the fake HistServ PRIVMSG that "replays" it)
+// 3. SHOULD be in the same "namespace" as `GenerateSecretToken` outputs
+// (same length and character set)
+func MungeSecretToken(token string) (result string) {
+	bytes, err := B32Encoder.DecodeString(token)
+	if err != nil {
+		// this should never happen
+		return GenerateSecretToken()
+	}
+	// add 1 with carrying
+	for i := len(bytes) - 1; 0 <= i; i -= 1 {
+		bytes[i] += 1
+		if bytes[i] != 0 {
+			break
+		} // else: overflow, carry to the next place
+	}
+	return B32Encoder.EncodeToString(bytes)
+}
+
 // securely check if a supplied token matches a stored token
 func SecretTokensMatch(storedToken string, suppliedToken string) bool {
 	// XXX fix a potential gotcha: if the stored token is uninitialized,

irc/utils/crypto_test.go

@@ -47,8 +47,37 @@ func TestTokenCompare(t *testing.T) {
 	}
 }
 
+func TestMunging(t *testing.T) {
+	count := 131072
+	set := make(map[string]bool)
+	var token string
+	for i := 0; i < count; i++ {
+		token = GenerateSecretToken()
+		set[token] = true
+	}
+	// all tokens generated thus far should be unique
+	assertEqual(len(set), count, t)
+
+	// iteratively munge the last generated token an additional `count` times
+	mungedToken := token
+	for i := 0; i < count; i++ {
+		mungedToken = MungeSecretToken(mungedToken)
+		assertEqual(len(mungedToken), len(token), t)
+		set[mungedToken] = true
+	}
+	// munged tokens should not collide with generated tokens, or each other
+	assertEqual(len(set), count*2, t)
+}
+
 func BenchmarkGenerateSecretToken(b *testing.B) {
 	for i := 0; i < b.N; i++ {
 		GenerateSecretToken()
 	}
 }
+
+func BenchmarkMungeSecretToken(b *testing.B) {
+	t := GenerateSecretToken()
+	for i := 0; i < b.N; i++ {
+		t = MungeSecretToken(t)
+	}
+}