1 files changed, 100 insertions, 0 deletions
diff --git a/hosts/hosts.go b/hosts/hosts.go
new file mode 100644
index 0000000..d31ed5e
--- /dev/null
+++ b/hosts/hosts.go
@@ -0,0 +1,100 @@
+package hosts
+
+import (
+	"bufio"
+	"encoding/hex"
+	"errors"
+	"fmt"
+	"github.com/adrg/xdg"
+	"net"
+	"net/netip"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+var knownHostsFile = filepath.Join(xdg.DataHome, "hose", "known_hosts")
+
+// Set sets the public key of a remote host.
+// It replaces or creates an entry in the known hosts file.
+func Set(host net.Addr, pubkey [32]byte) error {
+	addr, err := netip.ParseAddr(host.String())
+	if err != nil {
+		return err
+	}
+
+	hosts, err := Load()
+	if err != nil {
+		return err
+	}
+
+	hosts[addr] = pubkey
+
+	return Store(hosts)
+}
+
+// Load loads the set of known hosts and their associated public keys
+// from disc.
+func Load() (map[netip.Addr][32]byte, error) {
+	hosts := make(map[netip.Addr][32]byte)
+
+	f, err := os.Open(knownHostsFile)
+	if errors.Is(err, os.ErrNotExist) {
+		return hosts, nil // no known hosts yet.
+	} else if err != nil {
+		return hosts, err
+	}
+	defer f.Close()
+
+	scanner := bufio.NewScanner(f)
+	for line := 1; scanner.Scan(); line++ {
+		host, pubkey, err := parseHostKeyPair(scanner.Text())
+		if err != nil {
+			return hosts, fmt.Errorf("error parsing known hosts file: %s:%d: %v", err)
+		}
+		if _, ok := hosts[host]; ok {
+			return hosts, fmt.Errorf("duplicate entry in known hosts file: %s", host)
+		}
+		hosts[host] = pubkey
+	}
+	return hosts, scanner.Err()
+}
+
+// parseHostKeyPair parses a line of the known hosts file.
+func parseHostKeyPair(s string) (netip.Addr, [32]byte, error) {
+	fields := strings.Fields(s)
+	if len(fields) != 2 {
+		return netip.Addr{}, [32]byte{}, fmt.Errorf("expected 2 fields; got %d", len(fields))
+	}
+
+	addr, err := netip.ParseAddr(fields[0])
+	if err != nil {
+		return netip.Addr{}, [32]byte{}, err
+	}
+
+	var key [32]byte
+	if hex.DecodedLen(len(fields[1])) != len(key) {
+		return netip.Addr{}, [32]byte{}, fmt.Errorf("malformed key: %s", fields[1])
+	}
+	if _, err := hex.Decode(key[:], []byte(fields[1])); err != nil {
+		return netip.Addr{}, [32]byte{}, err
+	}
+
+	return addr, key, nil
+}
+
+// Store stores the set of known hosts and their associated public keys
+// to disc. It overwrites the entire file.
+func Store(hosts map[netip.Addr][32]byte) error {
+	f, err := os.Create(knownHostsFile)
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+
+	for host, key := range hosts {
+		fmt.Fprintf(f, "%s %x", host, key)
+	}
+
+	return nil
+}