diff --git a/internal/catmsg/varint.go b/internal/catmsg/varint.go
new file mode 100644
index 0000000..a2cee2c
--- /dev/null
+++ b/internal/catmsg/varint.go
@@ -0,0 +1,62 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package catmsg
+
+// This file implements varint encoding analogous to the one in encoding/binary.
+// We need a string version of this function, so we add that here and then add
+// the rest for consistency.
+
+import "errors"
+
+var (
+	errIllegalVarint  = errors.New("catmsg: illegal varint")
+	errVarintTooLarge = errors.New("catmsg: varint too large for uint64")
+)
+
+const maxVarintBytes = 10 // maximum length of a varint
+
+// encodeUint encodes x as a variable-sized integer into buf and returns the
+// number of bytes written. buf must be at least maxVarintBytes long
+func encodeUint(buf []byte, x uint64) (n int) {
+	for ; x > 127; n++ {
+		buf[n] = 0x80 | uint8(x&0x7F)
+		x >>= 7
+	}
+	buf[n] = uint8(x)
+	n++
+	return n
+}
+
+func decodeUintString(s string) (x uint64, size int, err error) {
+	i := 0
+	for shift := uint(0); shift < 64; shift += 7 {
+		if i >= len(s) {
+			return 0, i, errIllegalVarint
+		}
+		b := uint64(s[i])
+		i++
+		x |= (b & 0x7F) << shift
+		if b&0x80 == 0 {
+			return x, i, nil
+		}
+	}
+	return 0, i, errVarintTooLarge
+}
+
+func decodeUint(b []byte) (x uint64, size int, err error) {
+	i := 0
+	for shift := uint(0); shift < 64; shift += 7 {
+		if i >= len(b) {
+			return 0, i, errIllegalVarint
+		}
+		c := uint64(b[i])
+		i++
+		x |= (c & 0x7F) << shift
+		if c&0x80 == 0 {
+			return x, i, nil
+		}
+	}
+	return 0, i, errVarintTooLarge
+}
diff --git a/internal/catmsg/varint_test.go b/internal/catmsg/varint_test.go
new file mode 100644
index 0000000..a079c77
--- /dev/null
+++ b/internal/catmsg/varint_test.go
@@ -0,0 +1,123 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package catmsg
+
+import (
+	"fmt"
+	"testing"
+)
+
+func TestEncodeUint(t *testing.T) {
+	testCases := []struct {
+		x   uint64
+		enc string
+	}{
+		{0, "\x00"},
+		{1, "\x01"},
+		{2, "\x02"},
+		{0x7f, "\x7f"},
+		{0x80, "\x80\x01"},
+		{1 << 14, "\x80\x80\x01"},
+		{0xffffffff, "\xff\xff\xff\xff\x0f"},
+		{0xffffffffffffffff, "\xff\xff\xff\xff\xff\xff\xff\xff\xff\x01"},
+	}
+	for _, tc := range testCases {
+		buf := [maxVarintBytes]byte{}
+		got := string(buf[:encodeUint(buf[:], tc.x)])
+		if got != tc.enc {
+			t.Errorf("EncodeUint(%#x) = %q; want %q", tc.x, got, tc.enc)
+		}
+	}
+}
+
+func TestDecodeUint(t *testing.T) {
+	testCases := []struct {
+		x    uint64
+		size int
+		enc  string
+		err  error
+	}{{
+		x:    0,
+		size: 0,
+		enc:  "",
+		err:  errIllegalVarint,
+	}, {
+		x:    0,
+		size: 1,
+		enc:  "\x80",
+		err:  errIllegalVarint,
+	}, {
+		x:    0,
+		size: 3,
+		enc:  "\x80\x80\x80",
+		err:  errIllegalVarint,
+	}, {
+		x:    0,
+		size: 1,
+		enc:  "\x00",
+	}, {
+		x:    1,
+		size: 1,
+		enc:  "\x01",
+	}, {
+		x:    2,
+		size: 1,
+		enc:  "\x02",
+	}, {
+		x:    0x7f,
+		size: 1,
+		enc:  "\x7f",
+	}, {
+		x:    0x80,
+		size: 2,
+		enc:  "\x80\x01",
+	}, {
+		x:    1 << 14,
+		size: 3,
+		enc:  "\x80\x80\x01",
+	}, {
+		x:    0xffffffff,
+		size: 5,
+		enc:  "\xff\xff\xff\xff\x0f",
+	}, {
+		x:    0xffffffffffffffff,
+		size: 10,
+		enc:  "\xff\xff\xff\xff\xff\xff\xff\xff\xff\x01",
+	}, {
+		x:    0xffffffffffffffff,
+		size: 10,
+		enc:  "\xff\xff\xff\xff\xff\xff\xff\xff\xff\x01\x00",
+	}, {
+		x:    0,
+		size: 10,
+		enc:  "\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\x01",
+		err:  errVarintTooLarge,
+	}}
+	forms := []struct {
+		name   string
+		decode func(s string) (x uint64, size int, err error)
+	}{
+		{"decode", func(s string) (x uint64, size int, err error) {
+			return decodeUint([]byte(s))
+		}},
+		{"decodeString", decodeUintString},
+	}
+	for _, f := range forms {
+		for _, tc := range testCases {
+			t.Run(fmt.Sprintf("%s:%q", f.name, tc.enc), func(t *testing.T) {
+				x, size, err := f.decode(tc.enc)
+				if err != tc.err {
+					t.Error("err = %q; want %q", err, tc.err)
+				}
+				if size != tc.size {
+					t.Errorf("size = %d; want %d", size, tc.size)
+				}
+				if x != tc.x {
+					t.Errorf("decode = %#x; want %#x", x, tc.x)
+				}
+			})
+		}
+	}
+}