diff --git a/pki.go b/pki.go
index b63cb4e3..1dc4a68b 100644
--- a/pki.go
+++ b/pki.go
@@ -238,11 +238,15 @@ func (cs *CertState) getCertificate(v cert.Version) cert.Certificate {
 	return nil
 }
 
-func newCipherSuite(curve cert.Curve, pkcs11backed bool, cipher string) (noise.CipherSuite, error) {
+// newCipherSuite builds the noise.CipherSuite for the given curve and cipher.
+// When fips140Enforced is true (FIPS 140-only mode), non-approved algorithms
+// (Curve25519 and ChaChaPoly) are rejected with an error. Callers pass
+// fips140.Enforced() for fips140Enforced.
+func newCipherSuite(curve cert.Curve, pkcs11backed bool, cipher string, fips140Enforced bool) (noise.CipherSuite, error) {
 	var dhFunc noise.DHFunc
 	switch curve {
 	case cert.Curve_CURVE25519:
-		if fips140.Enforced() {
+		if fips140Enforced {
 			return nil, errors.New("pki: use of Curve25519 is not allowed in FIPS 140-only mode")
 		}
 		dhFunc = noise.DH25519
@@ -257,7 +261,7 @@ func newCipherSuite(curve cert.Curve, pkcs11backed bool, cipher string) (noise.C
 	}
 
 	if cipher == "chachapoly" {
-		if fips140.Enforced() {
+		if fips140Enforced {
 			return nil, errors.New("pki: use of ChaChaPoly is not allowed in FIPS 140-only mode")
 		}
 		return noise.NewCipherSuite(dhFunc, noise.CipherChaChaPoly, noise.HashSHA256), nil
@@ -420,7 +424,7 @@ func newCertState(dv cert.Version, v1, v2 cert.Certificate, pkcs11backed bool, p
 		if err != nil {
 			return nil, fmt.Errorf("error marshalling v1 certificate for handshake: %w", err)
 		}
-		ncs, err := newCipherSuite(v1.Curve(), pkcs11backed, cipher)
+		ncs, err := newCipherSuite(v1.Curve(), pkcs11backed, cipher, fips140.Enforced())
 		if err != nil {
 			return nil, err
 		}
@@ -445,7 +449,7 @@ func newCertState(dv cert.Version, v1, v2 cert.Certificate, pkcs11backed bool, p
 		if err != nil {
 			return nil, fmt.Errorf("error marshalling v2 certificate for handshake: %w", err)
 		}
-		ncs, err := newCipherSuite(v2.Curve(), pkcs11backed, cipher)
+		ncs, err := newCipherSuite(v2.Curve(), pkcs11backed, cipher, fips140.Enforced())
 		if err != nil {
 			return nil, err
 		}
diff --git a/pki_test.go b/pki_test.go
new file mode 100644
index 00000000..f90ff2ca
--- /dev/null
+++ b/pki_test.go
@@ -0,0 +1,93 @@
+package nebula
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/slackhq/nebula/cert"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestNewCipherSuite(t *testing.T) {
+	tests := []struct {
+		name            string
+		curve           cert.Curve
+		cipher          string
+		fips140Enforced bool
+		wantErr         string
+		// wantName is a substring expected in the resulting CipherSuite name
+		// (e.g. "P256" or "AESGCM"), only checked when wantErr is empty.
+		wantName string
+	}{
+		{
+			name:     "curve25519 aesgcm, not enforced",
+			curve:    cert.Curve_CURVE25519,
+			cipher:   "aesgcm",
+			wantName: "AESGCM",
+		},
+		{
+			name:     "curve25519 chachapoly, not enforced",
+			curve:    cert.Curve_CURVE25519,
+			cipher:   "chachapoly",
+			wantName: "ChaChaPoly",
+		},
+		{
+			name:     "p256 aesgcm, not enforced",
+			curve:    cert.Curve_P256,
+			cipher:   "aesgcm",
+			wantName: "P256",
+		},
+		{
+			name:            "p256 aesgcm, enforced is allowed",
+			curve:           cert.Curve_P256,
+			cipher:          "aesgcm",
+			fips140Enforced: true,
+			wantName:        "P256",
+		},
+		{
+			name:            "curve25519 rejected when enforced",
+			curve:           cert.Curve_CURVE25519,
+			cipher:          "aesgcm",
+			fips140Enforced: true,
+			wantErr:         "pki: use of Curve25519 is not allowed in FIPS 140-only mode",
+		},
+		{
+			name:            "chachapoly rejected when enforced",
+			curve:           cert.Curve_P256,
+			cipher:          "chachapoly",
+			fips140Enforced: true,
+			wantErr:         "pki: use of ChaChaPoly is not allowed in FIPS 140-only mode",
+		},
+		{
+			// Curve is checked before cipher, so a Curve25519+ChaChaPoly
+			// request reports the Curve25519 rejection.
+			name:            "curve25519 chachapoly rejected on curve when enforced",
+			curve:           cert.Curve_CURVE25519,
+			cipher:          "chachapoly",
+			fips140Enforced: true,
+			wantErr:         "pki: use of Curve25519 is not allowed in FIPS 140-only mode",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			cs, err := newCipherSuite(tt.curve, false, tt.cipher, tt.fips140Enforced)
+			if tt.wantErr != "" {
+				require.EqualError(t, err, tt.wantErr)
+				assert.Nil(t, cs)
+				return
+			}
+			require.NoError(t, err)
+			require.NotNil(t, cs)
+			assert.Contains(t, string(cs.Name()), tt.wantName)
+		})
+	}
+}
+
+func TestNewCipherSuiteUnsupportedCurve(t *testing.T) {
+	cs, err := newCipherSuite(cert.Curve(99), false, "aesgcm", false)
+	require.Error(t, err)
+	assert.True(t, strings.HasPrefix(err.Error(), "unsupported curve:"), "got: %v", err)
+	assert.Nil(t, cs)
+}