diff --git a/anonymizer.go b/anonymizer.go
index c90adf5..6f10bc1 100644
--- a/anonymizer.go
+++ b/anonymizer.go
@@ -10,13 +10,16 @@ import (
 	"go4.org/netipx"
 )
 
+// AnonymizerSecretLen is the length of secret key inside Anonymizer.
+const AnonymizerSecretLen = 14
+
 // Anonymizer anonymizes IP addresses and MAC addresses.
 // IPv4 address keeps its leading 24 bits; IPv6 address keeps its leading 48 bits; MAC address keeps its leading 24 bits.
 // Lower bits are XOR'ed with a random value.
 type Anonymizer struct {
 	keepIPs *netipx.IPSet
 	keepMAC bool
-	secret  [14]byte
+	secret  [AnonymizerSecretLen]byte
 }
 
 // AnonymizeIP anonymizes an IP address.
@@ -41,12 +44,16 @@ func (anon *Anonymizer) AnonymizeMAC(mac net.HardwareAddr) {
 }
 
 // NewAnonymizer creates Anonymizer.
-func NewAnonymizer(keepIPs *netipx.IPSet, keepMAC bool) (anon *Anonymizer) {
+func NewAnonymizer(keepIPs *netipx.IPSet, keepMAC bool, secret *[AnonymizerSecretLen]byte) (anon *Anonymizer) {
 	anon = &Anonymizer{
 		keepIPs: keepIPs,
 		keepMAC: keepMAC,
 	}
-	rand.Read(anon.secret[:])
+	if secret == nil {
+		rand.Read(anon.secret[:])
+	} else {
+		anon.secret = *secret
+	}
 	return
 }
 
diff --git a/anonymizer_test.go b/anonymizer_test.go
new file mode 100644
index 0000000..389f65f
--- /dev/null
+++ b/anonymizer_test.go
@@ -0,0 +1,57 @@
+package ndntdump_test
+
+import (
+	"bytes"
+	"net"
+	"net/netip"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"github.com/usnistgov/ndntdump"
+	"go4.org/netipx"
+)
+
+func TestAnonymizer(t *testing.T) {
+	assert, require := assert.New(t), require.New(t)
+	keepIPs, e := ndntdump.ParseIPSet([]string{
+		"10.0.4.32/28",
+		"10.0.10.0/23",
+		"fc44:966b:ce32:f10a::/64",
+	})
+	require.NoError(e)
+	var expectKeepIPsBuilder netipx.IPSetBuilder
+	expectKeepIPsBuilder.AddPrefix(netip.MustParsePrefix("10.0.4.0/24"))
+	expectKeepIPsBuilder.AddPrefix(netip.MustParsePrefix("10.0.10.0/23"))
+	expectKeepIPsBuilder.AddPrefix(netip.MustParsePrefix("fc44:966b:ce32::/48"))
+	expectKeepIPs, _ := expectKeepIPsBuilder.IPSet()
+	assert.True(keepIPs.Equal(expectKeepIPs))
+
+	secret := [ndntdump.AnonymizerSecretLen]byte(bytes.Repeat([]byte{0x01}, ndntdump.AnonymizerSecretLen))
+	anon := ndntdump.NewAnonymizer(keepIPs, false, &secret)
+
+	for _, ipPair := range [][2]string{
+		{"10.0.4.2", "10.0.4.2"},
+		{"10.0.5.2", "10.0.5.3"},
+		{"10.0.11.2", "10.0.11.2"},
+		{"10.0.12.2", "10.0.12.3"},
+		{"fc44:966b:ce32:52c6:74cd:f818:963b:202b", "fc44:966b:ce32:52c6:74cd:f818:963b:202b"},
+		{"fc9b:fd7b:5f42:47d0:78c0:fcb6:85c7:84a3", "fc9b:fd7b:5f42:46d1:79c1:fdb7:84c6:85a2"},
+	} {
+		ip := net.ParseIP(ipPair[0])
+		if ip4 := ip.To4(); ip4 != nil {
+			ip = ip4
+		}
+		anon.AnonymizeIP(ip)
+		assert.Equal(ipPair[1], ip.String())
+	}
+
+	badIP := net.IP{0xF0, 0xF1, 0xF2, 0xF3, 0xF4} // neither IPv4 nor IPv6
+	anon.AnonymizeIP(badIP)
+	assert.Equal(net.IP{0xF0, 0xF1, 0xF2, 0xF3, 0xF4}, badIP) // unchanged
+
+	hwaddr, e := net.ParseMAC("02:bf:8f:44:91:da")
+	require.NoError(e)
+	anon.AnonymizeMAC(hwaddr)
+	assert.Equal("02:bf:8f:45:90:db", hwaddr.String())
+}
diff --git a/cmd/ndntdump/main.go b/cmd/ndntdump/main.go
index 358b1eb..859f06c 100644
--- a/cmd/ndntdump/main.go
+++ b/cmd/ndntdump/main.go
@@ -85,7 +85,7 @@ var app = &cli.App{
 			IsLocal:       input.IsLocal,
 			TCPPort:       c.Int("tcp-port"),
 			WebSocketPort: c.Int("wss-port"),
-			Anonymizer:    ndntdump.NewAnonymizer(keepIPs, c.Bool("keep-mac")),
+			Anonymizer:    ndntdump.NewAnonymizer(keepIPs, c.Bool("keep-mac"), nil),
 			KeepPayload:   c.Bool("keep-payload"),
 		})
 
diff --git a/go.mod b/go.mod
index f7170ae..f359496 100644
--- a/go.mod
+++ b/go.mod
@@ -5,6 +5,7 @@ go 1.21
 require (
 	github.com/google/gopacket v1.1.19
 	github.com/klauspost/compress v1.17.2
+	github.com/stretchr/testify v1.8.4
 	github.com/urfave/cli/v2 v2.25.7
 	github.com/usnistgov/ndn-dpdk v0.0.0-20231012213844-da654608dfe8
 	github.com/zyedidia/generic v1.2.1
@@ -13,9 +14,12 @@ require (
 
 require (
 	github.com/cpuguy83/go-md2man/v2 v2.0.2 // indirect
+	github.com/davecgh/go-spew v1.1.1 // indirect
+	github.com/pmezard/go-difflib v1.0.0 // indirect
 	github.com/russross/blackfriday/v2 v2.1.0 // indirect
 	github.com/xrash/smetrics v0.0.0-20201216005158-039620a65673 // indirect
 	golang.org/x/exp v0.0.0-20231006140011-7918f672742d // indirect
 	golang.org/x/net v0.17.0 // indirect
 	golang.org/x/sys v0.13.0 // indirect
+	gopkg.in/yaml.v3 v3.0.1 // indirect
 )
diff --git a/go.sum b/go.sum
index 7849cee..1824a2d 100644
--- a/go.sum
+++ b/go.sum
@@ -44,5 +44,7 @@ golang.org/x/sys v0.13.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
 golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
 golang.org/x/tools v0.0.0-20200130002326-2f3ba24bd6e7/go.mod h1:TB2adYChydJhpapKDTa4BR/hXlZSLoq2Wpct/0txZ28=
 golang.org/x/xerrors v0.0.0-20191011141410-1b5146add898/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
 gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
 gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=