diff --git a/client.go b/client.go
index dd38f67..bd4cbd4 100644
--- a/client.go
+++ b/client.go
@@ -163,14 +163,14 @@ func (resp *Response) read(r *bufio.Reader) error {
 }
 
 // Client represents a Gemini client.
-type Client struct {
-	// VerifyCertificate, if not nil, will be called to verify the server certificate.
+type Client interface {
+	// VerifyCertificate will be called to verify the server certificate.
 	// If error is not nil, the connection will be aborted.
-	VerifyCertificate func(cert *x509.Certificate, req *Request) error
+	VerifyCertificate(cert *x509.Certificate, req *Request) error
 }
 
 // Send sends a Gemini request and returns a Gemini response.
-func (c *Client) Send(req *Request) (*Response, error) {
+func Send(c Client, req *Request) (*Response, error) {
 	// Connect to the host
 	config := &tls.Config{
 		InsecureSkipVerify: true,
diff --git a/examples/client/client.go b/examples/client/client.go
index 89bdb12..6887ebf 100644
--- a/examples/client/client.go
+++ b/examples/client/client.go
@@ -14,12 +14,12 @@ import (
 )
 
 var (
-	client = &gemini.Client{
-		VerifyCertificate: func(cert *x509.Certificate, req *gemini.Request) error {
-			return nil
+	client = &gemini.TOFUClient{
+		Trusts: func(cert *x509.Certificate, req *gemini.Request) bool {
+			// Trust all certificates
+			return true
 		},
 	}
-
 	cert tls.Certificate
 )
 
@@ -29,7 +29,7 @@ func init() {
 	//
 	//     openssl genrsa -out client.key 2048
 	//     openssl ecparam -genkey -name secp384r1 -out client.key
-	//     openssl req -new -x509 -sha256 -key client.key -out client.crt -days 3650
+	//     openssl req -new -x509 -sha512 -key client.key -out client.crt -days 365
 	//
 	var err error
 	cert, err = tls.LoadX509KeyPair("examples/client/client.crt", "examples/client/client.key")
@@ -45,13 +45,11 @@ func makeRequest(url string) {
 	}
 	req.Certificate = cert
 
-	resp, err := client.Send(req)
+	resp, err := gemini.Send(client, req)
 	if err != nil {
 		log.Fatal(err)
 	}
 
-	fmt.Println(gemini.Fingerprint(resp.TLS.PeerCertificates[0]))
-
 	fmt.Println("Status code:", resp.Status)
 	fmt.Println("Meta:", resp.Meta)
 
diff --git a/examples/server/server.go b/examples/server/server.go
index 02e8643..f99c6cd 100644
--- a/examples/server/server.go
+++ b/examples/server/server.go
@@ -15,7 +15,7 @@ func main() {
 	//
 	//     openssl genrsa -out server.key 2048
 	//     openssl ecparam -genkey -name secp384r1 -out server.key
-	//     openssl req -new -x509 -sha256 -key server.key -out server.crt -days 3650
+	//     openssl req -new -x509 -sha512 -key server.key -out server.crt -days 365
 	//
 	cert, err := tls.LoadX509KeyPair("examples/server/server.crt", "examples/server/server.key")
 	if err != nil {
@@ -27,9 +27,6 @@ func main() {
 		rw.WriteHeader(gemini.StatusSuccess, "text/gemini")
 		rw.Write([]byte("You requested " + req.URL.String()))
 		log.Printf("Request from %s for %s", req.RemoteAddr.String(), req.URL)
-		if len(req.TLS.PeerCertificates) != 0 {
-			log.Print("Client certificate: ", gemini.Fingerprint(req.TLS.PeerCertificates[0]))
-		}
 	})
 
 	server := gemini.Server{
diff --git a/gemini.go b/gemini.go
index ccdb5df..5b95b6a 100644
--- a/gemini.go
+++ b/gemini.go
@@ -1,5 +1,13 @@
 package gemini
 
+import (
+	"crypto/x509"
+	"errors"
+	"log"
+	"os"
+	"path/filepath"
+)
+
 // Status codes.
 const (
 	StatusInput                     = 10
@@ -35,3 +43,47 @@ const (
 var (
 	crlf = []byte("\r\n")
 )
+
+// TOFUClient is a client that implements Trust-On-First-Use.
+type TOFUClient struct {
+	// Trusts, if not nil, will be called to determine whether the client should
+	// trust the provided certificate.
+	Trusts func(cert *x509.Certificate, req *Request) bool
+}
+
+func (t *TOFUClient) VerifyCertificate(cert *x509.Certificate, req *Request) error {
+	if knownHosts.Has(req.URL.Host, cert) {
+		return nil
+	}
+	if t.Trusts != nil && t.Trusts(cert, req) {
+		host := NewKnownHost(cert)
+		knownHosts = append(knownHosts, host)
+		knownHostsFile, err := os.OpenFile(knownHostsPath, os.O_CREATE|os.O_APPEND|os.O_WRONLY, 0644)
+		if err != nil {
+			log.Print(err)
+		}
+		if _, err := host.Write(knownHostsFile); err != nil {
+			log.Print(err)
+		}
+		return nil
+	}
+	return errors.New("gemini: certificate not trusted")
+}
+
+var (
+	knownHosts     KnownHosts
+	knownHostsPath string
+	knownHostsFile *os.File
+)
+
+func init() {
+	configDir, err := os.UserConfigDir()
+	knownHostsPath = filepath.Join(configDir, "gemini")
+	os.MkdirAll(knownHostsPath, 0755)
+	knownHostsPath = filepath.Join(knownHostsPath, "known_hosts")
+	knownHostsFile, err = os.OpenFile(knownHostsPath, os.O_CREATE|os.O_RDONLY, 0644)
+	if err != nil {
+		return
+	}
+	knownHosts = ParseKnownHosts(knownHostsFile)
+}
diff --git a/tofu.go b/tofu.go
index df0bec9..30ec6a2 100644
--- a/tofu.go
+++ b/tofu.go
@@ -71,9 +71,18 @@ type KnownHost struct {
 	Expires     int64  // unix time of certificate notAfter date
 }
 
+func NewKnownHost(cert *x509.Certificate) KnownHost {
+	return KnownHost{
+		Hostname:    cert.Subject.CommonName,
+		Algorithm:   "SHA-512",
+		Fingerprint: Fingerprint(cert),
+		Expires:     cert.NotAfter.Unix(),
+	}
+}
+
 // Write writes the known host to the provided io.Writer.
 func (k KnownHost) Write(w io.Writer) (int, error) {
-	s := fmt.Sprintf("\n%s %s %s %d", k.Hostname, k.Algorithm, k.Fingerprint, k.Expires)
+	s := fmt.Sprintf("%s %s %s %d\n", k.Hostname, k.Algorithm, k.Fingerprint, k.Expires)
 	return w.Write([]byte(s))
 }