Use salt when generating ids from ips

Implements #6

add-comment/add_comment.go

@@ -167,6 +167,7 @@ func getId(ip string) string {
 	ip = strings.ReplaceAll(ip, "_", ":") // Use real IP address, not sanitized
 	h := sha256.New()
 	h.Write([]byte(ip))
+	shared.WriteIPSalt(h)
 	// First 8 chars
 	return fmt.Sprintf("%x", h.Sum(nil))[:8]
 }

example-config.toml

@@ -7,6 +7,18 @@ data = "/home/username/.local/share/gemlikes"
 # It must be accessible to the user that the CGI script runs under.
 # Directories will be created as needed, don't create them yourself.
 
+# Where to store IP salt:
+ip_salt = "auto"                              # default
+#ip_salt = "disabled"                         # disable IP salting (old behavior)
+#ip_salt = "/usr/local/etc/gemlikes/ip_salt"  # use existing salt
+# If unset, the default is "auto", which places the salt in the data directory,
+# generating it the first time if needed. "disabled" restores the old behavior,
+# which can potentially expose IP addresses to sufficiently-motivated attackers.
+# Specifying an absolute path uses the contents of the given file as a salt,
+# which must already exist. This allows multiple instances of gemlikes (e.g. in
+# a shared environment) to present identical identifiers for the same IP. The
+# path should be somewhere the server won't serve.
+
 # The list of directories where files exist that can be liked and commented on:
 dirs = ["/var/gemini", "/var/gemini/my_gemlog"]
 # IMPORTANT: There can't be any files with same name in any of these directories,

shared/shared.go

@@ -2,8 +2,11 @@
 package shared
 
 import (
+	"crypto/rand"
 	"errors"
 	"fmt"
+	"io"
+	"io/ioutil"
 	"net/url"
 	"os"
 	"path/filepath"
@@ -15,6 +18,8 @@ import (
 var ErrConfigDir = errors.New("config dir invalid or not set")
 var LikesDisabled = false
 
+var ipSaltValue []byte
+
 func PathExists(path string) bool {
 	_, err := os.Stat(path)
 	if err == nil {
@@ -93,6 +98,12 @@ func SafeInit() error {
 	if config.Get("disable_likes") != nil {
 		LikesDisabled = config.Get("disable_likes").(bool)
 	}
+	// Load IP->ID salt
+	ipSaltValue, err = loadIPSalt(config, data.(string))
+	if err != nil {
+		return err
+	}
+
 	return nil
 }
 
@@ -176,6 +187,35 @@ func GetTmpDir() string {
 	return filepath.Join(getDataDir(), "tmp")
 }
 
+func WriteIPSalt(w io.Writer) (int, error) {
+	return w.Write(ipSaltValue)
+}
+
+func loadIPSalt(config *toml.Tree, dataDir string) ([]byte, error) {
+	ipSaltMethod := "auto"
+	if key := config.Get("ip_salt"); key != nil {
+		ipSaltMethod = key.(string)
+	}
+
+	ipSaltPath := ipSaltMethod
+	switch ipSaltMethod {
+	case "disabled":
+		return nil, nil
+	case "auto":
+		ipSaltPath = filepath.Join(dataDir, "ip_salt")
+		f, err := os.OpenFile(ipSaltPath, os.O_RDWR|os.O_CREATE|os.O_EXCL, 0o644)
+		if err == nil {
+			// First time, create random data
+			defer f.Close()
+			if _, err := io.CopyN(f, rand.Reader, 16); err != nil {
+				return nil, err
+			}
+		}
+	}
+
+	return ioutil.ReadFile(ipSaltPath)
+}
+
 func getConfigDir() string {
 	e, _ := os.Executable()
 	return filepath.Dir(e)