diff --git a/ct/x509/x509.go b/ct/x509/x509.go
index d6a8ee30..ebc27bfc 100644
--- a/ct/x509/x509.go
+++ b/ct/x509/x509.go
@@ -781,8 +781,10 @@ func parsePublicKey(algo PublicKeyAlgorithm, keyData *publicKeyInfo) (interface{
 		}
 
 		pub := &rsa.PublicKey{
-			E: p.E,
-			N: p.N,
+			// Defensively copy E so callers cannot mutate it through the
+			// parsed wire structure.
+			E: new(big.Int).Set(p.E),
+			N: new(big.Int).Set(p.N),
 		}
 		return pub, nil
 	case DSA:
diff --git a/internal/testenv/testenv.go b/internal/testenv/testenv.go
index d6a2e8b0..7fe48331 100644
--- a/internal/testenv/testenv.go
+++ b/internal/testenv/testenv.go
@@ -33,7 +33,7 @@ func Builder() string {
 	return os.Getenv("GO_BUILDER_NAME")
 }
 
-// HasGoBuild reports whether the current system can build programs with ``go build''
+// HasGoBuild reports whether the current system can build programs with “go build”
 // and then run them with os.StartProcess or exec.Command.
 func HasGoBuild() bool {
 	if os.Getenv("GO_GCFLAGS") != "" {
@@ -50,7 +50,7 @@ func HasGoBuild() bool {
 	return true
 }
 
-// MustHaveGoBuild checks that the current system can build programs with ``go build''
+// MustHaveGoBuild checks that the current system can build programs with “go build”
 // and then run them with os.StartProcess or exec.Command.
 // If not, MustHaveGoBuild calls t.Skip with an explanation.
 func MustHaveGoBuild(t testing.TB) {
@@ -62,13 +62,13 @@ func MustHaveGoBuild(t testing.TB) {
 	}
 }
 
-// HasGoRun reports whether the current system can run programs with ``go run.''
+// HasGoRun reports whether the current system can run programs with “go run.”
 func HasGoRun() bool {
 	// For now, having go run and having go build are the same.
 	return HasGoBuild()
 }
 
-// MustHaveGoRun checks that the current system can run programs with ``go run.''
+// MustHaveGoRun checks that the current system can run programs with “go run.”
 // If not, MustHaveGoRun calls t.Skip with an explanation.
 func MustHaveGoRun(t testing.TB) {
 	if !HasGoRun() {
diff --git a/rsa/rsa_test.go b/rsa/rsa_test.go
index d7775f24..48298ef7 100644
--- a/rsa/rsa_test.go
+++ b/rsa/rsa_test.go
@@ -895,3 +895,32 @@ var testEncryptOAEPData = []testEncryptOAEPStruct{
 		},
 	},
 }
+
+// TestPublicKeyDefensiveCopy verifies that mutating the caller's PublicKey
+// after performing an operation does not affect subsequent operations on the
+// same key via the precomputed FIPS state.
+func TestPublicKeyDefensiveCopy(t *testing.T) {
+	priv, err := GenerateKey(rand.Reader, 2048)
+	if err != nil {
+		t.Fatal(err)
+	}
+	msg := []byte("hello")
+	ct, err := EncryptPKCS1v15(rand.Reader, &priv.PublicKey, msg)
+	if err != nil {
+		t.Fatal(err)
+	}
+	pt, err := DecryptPKCS1v15(rand.Reader, priv, ct)
+	if err != nil || !bytes.Equal(pt, msg) {
+		t.Fatalf("decrypt mismatch: %v / %x", err, pt)
+	}
+	// Mutate the caller's E in place. A correctly defensively-copied internal
+	// state must continue to function.
+	originalE := new(big.Int).Set(priv.PublicKey.E)
+	priv.PublicKey.E.SetInt64(7)
+	pt2, err := DecryptPKCS1v15(rand.Reader, priv, ct)
+	// Restore E so subsequent tests aren't affected.
+	priv.PublicKey.E.Set(originalE)
+	if err != nil || !bytes.Equal(pt2, msg) {
+		t.Fatalf("decrypt after caller-side E mutation failed (defensive copy missing?): %v / %x", err, pt2)
+	}
+}
diff --git a/x509/pkcs1.go b/x509/pkcs1.go
index dc5fb448..f688930f 100644
--- a/x509/pkcs1.go
+++ b/x509/pkcs1.go
@@ -117,8 +117,10 @@ func ParsePKCS1PublicKey(der []byte) (*rsa.PublicKey, error) {
 	//}
 
 	return &rsa.PublicKey{
-		E: pub.E,
-		N: pub.N,
+		// Defensively copy E so callers cannot mutate it through the
+		// parsed wire structure.
+		E: new(big.Int).Set(pub.E),
+		N: new(big.Int).Set(pub.N),
 	}, nil
 }
 
diff --git a/x509/x509.go b/x509/x509.go
index bc64042e..2c41e49d 100644
--- a/x509/x509.go
+++ b/x509/x509.go
@@ -1346,8 +1346,10 @@ func parsePublicKey(algo PublicKeyAlgorithm, keyData *publicKeyInfo) (interface{
 		}
 
 		return &rsa.PublicKey{
-			E: p.E,
-			N: p.N,
+			// Defensively copy E so callers cannot mutate it through the
+			// parsed wire structure.
+			E: new(big.Int).Set(p.E),
+			N: new(big.Int).Set(p.N),
 		}, nil
 	case DSA:
 		var p *big.Int