Refactor TOFU

README.md

@@ -33,11 +33,41 @@ A quick overview of the Gemini protocol:
 The way this is implemented in this package is like so:
 
 1. Client makes a request with `NewRequest`. The client then sends the request
-	with `Send(*Request) (*Response, error)`. The client can optionally verify
-	the server certificate with `VerifyCertificate(*x509.Certificate, *Request)`
+	with `(*Client).Send(*Request) (*Response, error)`. The client then determines whether
+	to trust the certificate in `TrustCertificte(*x509.Certificate, *KnownHosts) bool`.
+	(See [TOFU](#tofu)).
 2. Server recieves the request and constructs a response.
 	The server calls the `Serve(*ResponseWriter, *Request)` method on the
 	`Handler` field. The handler writes the response. The server then closes
 	the connection.
 3. Client recieves the response as a `*Response`. The client then handles the
 	response.
+
+## TOFU
+
+This package provides an easy way to implement Trust-On-First-Use in your
+clients. Here is a simple example client using TOFU to authenticate
+certificates:
+
+```go
+client := &gemini.Client{
+	KnownHosts: gemini.LoadKnownHosts(".local/share/gemini/known_hosts"),
+	TrustCertificate: func(cert *x509.Certificate, knownHosts *gemini.KnownHosts) bool {
+		// If the certificate is in the known hosts list, allow the connection
+		if knownHosts.Has(cert) {
+			return true
+		}
+		// Prompt the user
+		if userTrustsCertificateTemporarily() {
+			// Temporarily trust the certificate
+			return true
+		} else if userTrustsCertificatePermanently() {
+			// Add the certificate to the known hosts file
+			knownHosts.Add(cert)
+			return true
+		}
+		// User does not trust the certificate
+		return false
+	},
+}
+```

client.go

@@ -10,12 +10,15 @@ import (
 	"net"
 	"net/url"
 	"strconv"
+	"strings"
 )
 
 // Errors.
 var (
-	ErrProtocol   = errors.New("gemini: protocol error")
-	ErrInvalidURL = errors.New("gemini: requested URL is invalid")
+	ErrProtocol              = errors.New("gemini: protocol error")
+	ErrInvalidURL            = errors.New("gemini: requested URL is invalid")
+	ErrCertificateNotValid   = errors.New("gemini: certificate is invalid")
+	ErrCertificateNotTrusted = errors.New("gemini: certificate is not trusted")
 )
 
 // Request represents a Gemini request.
@@ -163,24 +166,40 @@ func (resp *Response) read(r *bufio.Reader) error {
 }
 
 // Client represents a Gemini client.
-type Client interface {
-	// VerifyCertificate will be called to verify the server certificate.
-	// If error is not nil, the connection will be aborted.
-	VerifyCertificate(cert *x509.Certificate, req *Request) error
+type Client struct {
+	// KnownHosts is a list of known hosts that the client trusts.
+	KnownHosts *KnownHosts
+
+	// TrustCertificate, if not nil, will be called to determine whether the
+	// client should trust the given certificate.
+	TrustCertificate func(cert *x509.Certificate, knownHosts *KnownHosts) bool
 }
 
 // Send sends a Gemini request and returns a Gemini response.
-func Send(c Client, req *Request) (*Response, error) {
+func (c *Client) Send(req *Request) (*Response, error) {
 	// Connect to the host
 	config := &tls.Config{
 		InsecureSkipVerify: true,
 		Certificates:       []tls.Certificate{req.Certificate},
 		VerifyPeerCertificate: func(rawCerts [][]byte, _ [][]*x509.Certificate) error {
+			// Parse the certificate
 			cert, err := x509.ParseCertificate(rawCerts[0])
 			if err != nil {
 				return err
 			}
-			return c.VerifyCertificate(cert, req)
+			// Check that the certificate is valid for the hostname
+			if cert.Subject.CommonName != hostname(req.Host) {
+				return ErrCertificateNotValid
+			}
+			// Check that the client trusts the certificate
+			if c.TrustCertificate == nil {
+				if c.KnownHosts == nil || !c.KnownHosts.Has(cert) {
+					return ErrCertificateNotTrusted
+				}
+			} else if !c.TrustCertificate(cert, c.KnownHosts) {
+				return ErrCertificateNotTrusted
+			}
+			return nil
 		},
 	}
 	conn, err := tls.Dial("tcp", req.Host, config)
@@ -206,3 +225,12 @@ func Send(c Client, req *Request) (*Response, error) {
 	}
 	return resp, nil
 }
+
+// hostname extracts the host name from a valid host or host:port
+func hostname(host string) string {
+	i := strings.LastIndexByte(host, ':')
+	if i != -1 {
+		return host[:i]
+	}
+	return host
+}

examples/client/client.go

@@ -14,8 +14,8 @@ import (
 )
 
 var (
-	client = &gemini.TOFUClient{
-		Trusts: func(cert *x509.Certificate, req *gemini.Request) bool {
+	client = &gemini.Client{
+		TrustCertificate: func(cert *x509.Certificate, knownHosts *gemini.KnownHosts) bool {
 			// Trust all certificates
 			return true
 		},
@@ -45,7 +45,7 @@ func makeRequest(url string) {
 	}
 	req.Certificate = cert
 
-	resp, err := gemini.Send(client, req)
+	resp, err := client.Send(req)
 	if err != nil {
 		log.Fatal(err)
 	}

gemini.go

@@ -1,13 +1,5 @@
 package gemini
 
-import (
-	"crypto/x509"
-	"errors"
-	"log"
-	"os"
-	"path/filepath"
-)
-
 // Status codes.
 const (
 	StatusInput                     = 10
@@ -43,47 +35,3 @@ 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)
-}

tofu.go

@@ -5,37 +5,75 @@ import (
 	"bytes"
 	"crypto/sha512"
 	"crypto/x509"
-	"errors"
 	"fmt"
 	"io"
+	"os"
+	"path/filepath"
 	"strconv"
 	"strings"
 	"time"
 )
 
-// Errors.
-var (
-	ErrInvalidKnownHosts = errors.New("gemini: invalid known hosts")
-)
-
 // KnownHosts represents a list of known hosts.
-type KnownHosts []KnownHost
+type KnownHosts struct {
+	hosts []KnownHost
+	file  *os.File
+}
 
-// Has reports whether the given hostname and certificate are in the list.
-func (k KnownHosts) Has(hostname string, cert *x509.Certificate) bool {
+// LoadKnownHosts loads the known hosts from the provided path.
+// It creates the path and any of its parent directories if they do not exist.
+// The returned KnownHosts appends to the file whenever a certificate is added.
+func LoadKnownHosts(path string) (*KnownHosts, error) {
+	if dir := filepath.Dir(path); dir != "." {
+		err := os.MkdirAll(dir, 0755)
+		if err != nil {
+			return nil, err
+		}
+	}
+	f, err := os.OpenFile(path, os.O_CREATE|os.O_RDONLY, 0644)
+	if err != nil {
+		return nil, err
+	}
+	k := &KnownHosts{}
+	k.Parse(f)
+	f.Close()
+	// Open the file for append-only use
+	f, err = os.OpenFile(path, os.O_CREATE|os.O_WRONLY|os.O_APPEND, 0644)
+	if err != nil {
+		return nil, err
+	}
+	k.file = f
+	return k, nil
+}
+
+// Add adds a certificate to the KnownHosts.
+// If KnownHosts was loaded from a file, Add will append to the file.
+func (k *KnownHosts) Add(cert *x509.Certificate) {
+	host := NewKnownHost(cert)
+	k.hosts = append(k.hosts, host)
+	// Append to the file
+	if k.file != nil {
+		host.Write(k.file)
+	}
+}
+
+// Has reports whether the provided certificate is in the list.
+func (k *KnownHosts) Has(cert *x509.Certificate) bool {
 	now := time.Now().Unix()
+	hostname := cert.Subject.CommonName
 	fingerprint := Fingerprint(cert)
-	for i := range k {
-		if k[i].Expires > now && k[i].Hostname == hostname && k[i].Fingerprint == fingerprint {
+	for i := range k.hosts {
+		if k.hosts[i].Expires > now && k.hosts[i].Hostname == hostname &&
+			k.hosts[i].Fingerprint == fingerprint {
 			return true
 		}
 	}
 	return false
 }
 
-// ParseKnownHosts parses and returns a list of known hosts from the provided io.Reader.
+// Parse parses the provided reader and adds the parsed known hosts to the list.
 // Invalid lines are ignored.
-func ParseKnownHosts(r io.Reader) (hosts KnownHosts) {
+func (k *KnownHosts) Parse(r io.Reader) {
 	scanner := bufio.NewScanner(r)
 	for scanner.Scan() {
 		text := scanner.Text()
@@ -53,14 +91,13 @@ func ParseKnownHosts(r io.Reader) (hosts KnownHosts) {
 			continue
 		}
 
-		hosts = append(hosts, KnownHost{
+		k.hosts = append(k.hosts, KnownHost{
 			Hostname:    hostname,
 			Algorithm:   algorithm,
 			Fingerprint: fingerprint,
 			Expires:     expires,
 		})
 	}
-	return
 }
 
 // KnownHost represents a known host.
@@ -71,6 +108,7 @@ type KnownHost struct {
 	Expires     int64  // unix time of certificate notAfter date
 }
 
+// NewKnownHost creates a new known host from a certificate.
 func NewKnownHost(cert *x509.Certificate) KnownHost {
 	return KnownHost{
 		Hostname:    cert.Subject.CommonName,