From 95c20ce3092936fbfef80e4fb297d000c94c8b8c Mon Sep 17 00:00:00 2001
From: Matt Dainty <matt@bodgit-n-scarper.com>
Date: Tue, 22 Dec 2020 11:40:37 +0000
Subject: [PATCH] Implement sequence tracking

Based on the MIT Kerberos implementation. It tracks the 64 previously seen
sequence numbers for the purposes of out of order delivery and replay
protection.

Fixes #419
---
 v8/gssapi/sequence.go      |  91 +++++++++++++++++++++++++++++
 v8/gssapi/sequence_test.go | 115 +++++++++++++++++++++++++++++++++++++
 2 files changed, 206 insertions(+)
 create mode 100644 v8/gssapi/sequence.go
 create mode 100644 v8/gssapi/sequence_test.go

diff --git a/v8/gssapi/sequence.go b/v8/gssapi/sequence.go
new file mode 100644
index 00000000..96cce70e
--- /dev/null
+++ b/v8/gssapi/sequence.go
@@ -0,0 +1,91 @@
+package gssapi
+
+import (
+	"errors"
+	"math"
+	"sync"
+)
+
+var (
+	errDuplicateToken = errors.New("duplicate per-message token detected")
+	errOldToken       = errors.New("timed-out per-message token detected")
+	errUnseqToken     = errors.New("reordered (early) per-message token detected")
+	errGapToken       = errors.New("skipped predecessor token(s) detected")
+)
+
+// SequenceState tracks previously seen sequence numbers for message replay
+// and/or sequence protection
+type SequenceState struct {
+	m            sync.Mutex
+	doReplay     bool
+	doSequence   bool
+	base         uint64
+	next         uint64
+	receiveMask  uint64
+	sequenceMask uint64
+}
+
+// NewSequenceState returns a new SequenceState seeded with sequenceNumber
+// with doReplay and doSequence controlling replay and sequence protection
+// respectively and wide controlling whether sequence numbers are expected to
+// wrap at a 32- or 64-bit boundary.
+func NewSequenceState(sequenceNumber uint64, doReplay, doSequence, wide bool) *SequenceState {
+	ss := &SequenceState{
+		doReplay:   doReplay,
+		doSequence: doSequence,
+		base:       sequenceNumber,
+	}
+	if wide {
+		ss.sequenceMask = math.MaxUint64
+	} else {
+		ss.sequenceMask = math.MaxUint32
+	}
+	return ss
+}
+
+// Check the next sequence number. Sequence protection requires the sequence
+// number to increase sequentially with no duplicates or out of order delivery.
+// Replay protection relaxes these restrictions to permit limited out of order
+// delivery.
+func (ss *SequenceState) Check(sequenceNumber uint64) error {
+	if !ss.doReplay && !ss.doSequence {
+		return nil
+	}
+
+	ss.m.Lock()
+	defer ss.m.Unlock()
+
+	relativeSequenceNumber := (sequenceNumber - ss.base) & ss.sequenceMask
+
+	if relativeSequenceNumber >= ss.next {
+		offset := relativeSequenceNumber - ss.next
+		ss.receiveMask = ss.receiveMask<<(offset+1) | 1
+		ss.next = (relativeSequenceNumber + 1) & ss.sequenceMask
+
+		if offset > 0 && ss.doSequence {
+			return errGapToken
+		}
+
+		return nil
+	}
+
+	offset := ss.next - relativeSequenceNumber
+
+	if offset > 64 {
+		if ss.doSequence {
+			return errUnseqToken
+		}
+		return errOldToken
+	}
+
+	bit := uint64(1) << (offset - 1)
+	if ss.doReplay && ss.receiveMask&bit != 0 {
+		return errDuplicateToken
+	}
+	ss.receiveMask |= bit
+	if ss.doSequence {
+		return errUnseqToken
+	}
+
+	return nil
+}
diff --git a/v8/gssapi/sequence_test.go b/v8/gssapi/sequence_test.go
new file mode 100644
index 00000000..40f0d2f6
--- /dev/null
+++ b/v8/gssapi/sequence_test.go
@@ -0,0 +1,115 @@
+package gssapi
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func makeRange(min, max uint64) []uint64 {
+	a := make([]uint64, max-min+1)
+	for i := range a {
+		a[i] = min + uint64(i)
+	}
+	return a
+}
+
+func TestSequenceState(t *testing.T) {
+	tables := map[string]struct {
+		base       uint64
+		doReplay   bool
+		doSequence bool
+		wide       bool
+		sequence   []uint64
+		err        error
+	}{
+		"noop": {
+			0,
+			false,
+			false,
+			false,
+			makeRange(0, 64),
+			nil,
+		},
+		"ok": {
+			0,
+			true,
+			true,
+			true,
+			makeRange(0, 64),
+			nil,
+		},
+		"replay skip": {
+			0,
+			true,
+			false,
+			true,
+			append(makeRange(0, 64), 66),
+			nil,
+		},
+		"sequence skip": {
+			0,
+			false,
+			true,
+			true,
+			append(makeRange(0, 64), 66),
+			errGapToken,
+		},
+		"replay too old": {
+			0,
+			true,
+			false,
+			true,
+			append(makeRange(0, 64), 0),
+			errOldToken,
+		},
+		"sequence too old": {
+			0,
+			false,
+			true,
+			true,
+			append(makeRange(0, 64), 0),
+			errUnseqToken,
+		},
+		"replay duplicate": {
+			0,
+			true,
+			false,
+			true,
+			append(makeRange(0, 64), 64),
+			errDuplicateToken,
+		},
+		"sequence duplicate": {
+			0,
+			false,
+			true,
+			true,
+			append(makeRange(0, 64), 64),
+			errUnseqToken,
+		},
+		"replay out of order": {
+			0,
+			true,
+			false,
+			true,
+			append(makeRange(0, 64), 66, 65),
+			nil,
+		},
+	}
+
+	for name, table := range tables {
+		t.Run(name, func(t *testing.T) {
+			ss := NewSequenceState(table.base, table.doReplay, table.doSequence, table.wide)
+
+			var err error
+			for _, next := range table.sequence {
+				err = ss.Check(next)
+				if err != nil {
+					break
+				}
+			}
+
+			assert.Equal(t, table.err, err)
+		})
+	}
+}