formalize token munging code

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)
+	}
+}