diff --git a/cert.go b/cert.go
index c2c0c5e..dd8a1f5 100644
--- a/cert.go
+++ b/cert.go
@@ -30,7 +30,7 @@ type CertificateDir struct {
 	CertificateStore
 	dir  bool
 	path string
-	mu   sync.Mutex
+	mu   sync.RWMutex
 }
 
 // Add adds a certificate for the given scope to the store.
diff --git a/tofu.go b/tofu.go
index 44b4648..9fa08eb 100644
--- a/tofu.go
+++ b/tofu.go
@@ -31,7 +31,7 @@ type KnownHosts map[string]Fingerprint
 type KnownHostsFile struct {
 	KnownHosts
 	out io.Writer
-	mu  sync.Mutex
+	mu  sync.RWMutex
 }
 
 // SetOutput sets the output to which new known hosts will be written to.