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547 lines
12 KiB
Go
547 lines
12 KiB
Go
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// Copyright 2017 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// Package knownhosts implements a parser for the OpenSSH
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// known_hosts host key database.
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package knownhosts
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import (
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"bufio"
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"bytes"
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"crypto/hmac"
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"crypto/rand"
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"crypto/sha1"
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"encoding/base64"
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"errors"
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"fmt"
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"io"
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"net"
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"os"
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"strings"
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"golang.org/x/crypto/ssh"
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)
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// See the sshd manpage
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// (http://man.openbsd.org/sshd#SSH_KNOWN_HOSTS_FILE_FORMAT) for
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// background.
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type addr struct{ host, port string }
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func (a *addr) String() string {
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h := a.host
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if strings.Contains(h, ":") {
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h = "[" + h + "]"
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}
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return h + ":" + a.port
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}
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type matcher interface {
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match([]addr) bool
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}
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type hostPattern struct {
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negate bool
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addr addr
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}
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func (p *hostPattern) String() string {
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n := ""
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if p.negate {
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n = "!"
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}
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return n + p.addr.String()
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}
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type hostPatterns []hostPattern
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func (ps hostPatterns) match(addrs []addr) bool {
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matched := false
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for _, p := range ps {
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for _, a := range addrs {
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m := p.match(a)
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if !m {
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continue
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}
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if p.negate {
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return false
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}
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matched = true
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}
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}
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return matched
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}
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// See
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// https://android.googlesource.com/platform/external/openssh/+/ab28f5495c85297e7a597c1ba62e996416da7c7e/addrmatch.c
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// The matching of * has no regard for separators, unlike filesystem globs
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func wildcardMatch(pat []byte, str []byte) bool {
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for {
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if len(pat) == 0 {
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return len(str) == 0
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}
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if len(str) == 0 {
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return false
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}
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if pat[0] == '*' {
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if len(pat) == 1 {
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return true
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}
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for j := range str {
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if wildcardMatch(pat[1:], str[j:]) {
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return true
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}
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}
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return false
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}
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if pat[0] == '?' || pat[0] == str[0] {
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pat = pat[1:]
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str = str[1:]
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} else {
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return false
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}
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}
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}
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func (l *hostPattern) match(a addr) bool {
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return wildcardMatch([]byte(l.addr.host), []byte(a.host)) && l.addr.port == a.port
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}
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type keyDBLine struct {
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cert bool
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matcher matcher
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knownKey KnownKey
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}
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func serialize(k ssh.PublicKey) string {
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return k.Type() + " " + base64.StdEncoding.EncodeToString(k.Marshal())
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}
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func (l *keyDBLine) match(addrs []addr) bool {
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return l.matcher.match(addrs)
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}
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type hostKeyDB struct {
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// Serialized version of revoked keys
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revoked map[string]*KnownKey
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lines []keyDBLine
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}
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func newHostKeyDB() *hostKeyDB {
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db := &hostKeyDB{
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revoked: make(map[string]*KnownKey),
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}
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return db
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}
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func keyEq(a, b ssh.PublicKey) bool {
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return bytes.Equal(a.Marshal(), b.Marshal())
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}
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// IsAuthorityForHost can be used as a callback in ssh.CertChecker
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func (db *hostKeyDB) IsHostAuthority(remote ssh.PublicKey, address string) bool {
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h, p, err := net.SplitHostPort(address)
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if err != nil {
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return false
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}
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a := addr{host: h, port: p}
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for _, l := range db.lines {
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if l.cert && keyEq(l.knownKey.Key, remote) && l.match([]addr{a}) {
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return true
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}
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}
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return false
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}
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// IsRevoked can be used as a callback in ssh.CertChecker
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func (db *hostKeyDB) IsRevoked(key *ssh.Certificate) bool {
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_, ok := db.revoked[string(key.Marshal())]
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return ok
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}
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const markerCert = "@cert-authority"
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const markerRevoked = "@revoked"
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func nextWord(line []byte) (string, []byte) {
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i := bytes.IndexAny(line, "\t ")
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if i == -1 {
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return string(line), nil
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}
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return string(line[:i]), bytes.TrimSpace(line[i:])
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}
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func parseLine(line []byte) (marker, host string, key ssh.PublicKey, err error) {
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if w, next := nextWord(line); w == markerCert || w == markerRevoked {
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marker = w
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line = next
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}
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host, line = nextWord(line)
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if len(line) == 0 {
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return "", "", nil, errors.New("knownhosts: missing host pattern")
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}
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// ignore the keytype as it's in the key blob anyway.
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_, line = nextWord(line)
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if len(line) == 0 {
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return "", "", nil, errors.New("knownhosts: missing key type pattern")
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}
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keyBlob, _ := nextWord(line)
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keyBytes, err := base64.StdEncoding.DecodeString(keyBlob)
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if err != nil {
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return "", "", nil, err
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}
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key, err = ssh.ParsePublicKey(keyBytes)
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if err != nil {
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return "", "", nil, err
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}
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return marker, host, key, nil
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}
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func (db *hostKeyDB) parseLine(line []byte, filename string, linenum int) error {
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marker, pattern, key, err := parseLine(line)
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if err != nil {
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return err
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}
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if marker == markerRevoked {
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db.revoked[string(key.Marshal())] = &KnownKey{
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Key: key,
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Filename: filename,
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Line: linenum,
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}
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return nil
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}
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entry := keyDBLine{
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cert: marker == markerCert,
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knownKey: KnownKey{
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Filename: filename,
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Line: linenum,
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Key: key,
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},
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}
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if pattern[0] == '|' {
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entry.matcher, err = newHashedHost(pattern)
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} else {
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entry.matcher, err = newHostnameMatcher(pattern)
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}
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if err != nil {
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return err
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}
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db.lines = append(db.lines, entry)
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return nil
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}
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func newHostnameMatcher(pattern string) (matcher, error) {
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var hps hostPatterns
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for _, p := range strings.Split(pattern, ",") {
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if len(p) == 0 {
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continue
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}
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var a addr
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var negate bool
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if p[0] == '!' {
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negate = true
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p = p[1:]
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}
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if len(p) == 0 {
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return nil, errors.New("knownhosts: negation without following hostname")
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}
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var err error
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if p[0] == '[' {
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a.host, a.port, err = net.SplitHostPort(p)
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if err != nil {
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return nil, err
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}
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} else {
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a.host, a.port, err = net.SplitHostPort(p)
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if err != nil {
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a.host = p
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a.port = "22"
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}
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}
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hps = append(hps, hostPattern{
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negate: negate,
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addr: a,
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})
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}
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return hps, nil
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}
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// KnownKey represents a key declared in a known_hosts file.
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type KnownKey struct {
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Key ssh.PublicKey
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Filename string
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Line int
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}
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func (k *KnownKey) String() string {
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return fmt.Sprintf("%s:%d: %s", k.Filename, k.Line, serialize(k.Key))
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}
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// KeyError is returned if we did not find the key in the host key
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// database, or there was a mismatch. Typically, in batch
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// applications, this should be interpreted as failure. Interactive
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// applications can offer an interactive prompt to the user.
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type KeyError struct {
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// Want holds the accepted host keys. For each key algorithm,
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// there can be one hostkey. If Want is empty, the host is
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// unknown. If Want is non-empty, there was a mismatch, which
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// can signify a MITM attack.
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Want []KnownKey
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}
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func (u *KeyError) Error() string {
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if len(u.Want) == 0 {
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return "knownhosts: key is unknown"
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}
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return "knownhosts: key mismatch"
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}
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// RevokedError is returned if we found a key that was revoked.
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type RevokedError struct {
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Revoked KnownKey
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}
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func (r *RevokedError) Error() string {
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return "knownhosts: key is revoked"
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}
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// check checks a key against the host database. This should not be
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// used for verifying certificates.
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func (db *hostKeyDB) check(address string, remote net.Addr, remoteKey ssh.PublicKey) error {
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if revoked := db.revoked[string(remoteKey.Marshal())]; revoked != nil {
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return &RevokedError{Revoked: *revoked}
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}
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host, port, err := net.SplitHostPort(remote.String())
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if err != nil {
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return fmt.Errorf("knownhosts: SplitHostPort(%s): %v", remote, err)
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}
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addrs := []addr{
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{host, port},
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}
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if address != "" {
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host, port, err := net.SplitHostPort(address)
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if err != nil {
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return fmt.Errorf("knownhosts: SplitHostPort(%s): %v", address, err)
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}
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addrs = append(addrs, addr{host, port})
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}
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return db.checkAddrs(addrs, remoteKey)
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}
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// checkAddrs checks if we can find the given public key for any of
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// the given addresses. If we only find an entry for the IP address,
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// or only the hostname, then this still succeeds.
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func (db *hostKeyDB) checkAddrs(addrs []addr, remoteKey ssh.PublicKey) error {
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// TODO(hanwen): are these the right semantics? What if there
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// is just a key for the IP address, but not for the
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// hostname?
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// Algorithm => key.
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knownKeys := map[string]KnownKey{}
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for _, l := range db.lines {
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if l.match(addrs) {
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typ := l.knownKey.Key.Type()
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if _, ok := knownKeys[typ]; !ok {
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knownKeys[typ] = l.knownKey
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}
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}
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}
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keyErr := &KeyError{}
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for _, v := range knownKeys {
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keyErr.Want = append(keyErr.Want, v)
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}
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// Unknown remote host.
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if len(knownKeys) == 0 {
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return keyErr
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}
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// If the remote host starts using a different, unknown key type, we
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// also interpret that as a mismatch.
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if known, ok := knownKeys[remoteKey.Type()]; !ok || !keyEq(known.Key, remoteKey) {
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return keyErr
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}
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return nil
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}
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// The Read function parses file contents.
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func (db *hostKeyDB) Read(r io.Reader, filename string) error {
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scanner := bufio.NewScanner(r)
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lineNum := 0
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for scanner.Scan() {
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lineNum++
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line := scanner.Bytes()
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line = bytes.TrimSpace(line)
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if len(line) == 0 || line[0] == '#' {
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continue
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}
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if err := db.parseLine(line, filename, lineNum); err != nil {
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return fmt.Errorf("knownhosts: %s:%d: %v", filename, lineNum, err)
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}
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}
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return scanner.Err()
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}
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// New creates a host key callback from the given OpenSSH host key
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// files. The returned callback is for use in
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// ssh.ClientConfig.HostKeyCallback. Hashed hostnames are not supported.
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func New(files ...string) (ssh.HostKeyCallback, error) {
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db := newHostKeyDB()
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for _, fn := range files {
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f, err := os.Open(fn)
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if err != nil {
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return nil, err
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}
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defer f.Close()
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if err := db.Read(f, fn); err != nil {
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return nil, err
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}
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}
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var certChecker ssh.CertChecker
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certChecker.IsHostAuthority = db.IsHostAuthority
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certChecker.IsRevoked = db.IsRevoked
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certChecker.HostKeyFallback = db.check
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return certChecker.CheckHostKey, nil
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}
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// Normalize normalizes an address into the form used in known_hosts
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func Normalize(address string) string {
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host, port, err := net.SplitHostPort(address)
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if err != nil {
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host = address
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port = "22"
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}
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entry := host
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if port != "22" {
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entry = "[" + entry + "]:" + port
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} else if strings.Contains(host, ":") && !strings.HasPrefix(host, "[") {
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entry = "[" + entry + "]"
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}
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return entry
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}
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||
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// Line returns a line to add append to the known_hosts files.
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||
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func Line(addresses []string, key ssh.PublicKey) string {
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||
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var trimmed []string
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||
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for _, a := range addresses {
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||
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trimmed = append(trimmed, Normalize(a))
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}
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return strings.Join(trimmed, ",") + " " + serialize(key)
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}
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// HashHostname hashes the given hostname. The hostname is not
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||
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// normalized before hashing.
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||
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func HashHostname(hostname string) string {
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||
|
// TODO(hanwen): check if we can safely normalize this always.
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||
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salt := make([]byte, sha1.Size)
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||
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|
||
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_, err := rand.Read(salt)
|
||
|
if err != nil {
|
||
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panic(fmt.Sprintf("crypto/rand failure %v", err))
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||
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}
|
||
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|
||
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hash := hashHost(hostname, salt)
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||
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return encodeHash(sha1HashType, salt, hash)
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}
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|
func decodeHash(encoded string) (hashType string, salt, hash []byte, err error) {
|
||
|
if len(encoded) == 0 || encoded[0] != '|' {
|
||
|
err = errors.New("knownhosts: hashed host must start with '|'")
|
||
|
return
|
||
|
}
|
||
|
components := strings.Split(encoded, "|")
|
||
|
if len(components) != 4 {
|
||
|
err = fmt.Errorf("knownhosts: got %d components, want 3", len(components))
|
||
|
return
|
||
|
}
|
||
|
|
||
|
hashType = components[1]
|
||
|
if salt, err = base64.StdEncoding.DecodeString(components[2]); err != nil {
|
||
|
return
|
||
|
}
|
||
|
if hash, err = base64.StdEncoding.DecodeString(components[3]); err != nil {
|
||
|
return
|
||
|
}
|
||
|
return
|
||
|
}
|
||
|
|
||
|
func encodeHash(typ string, salt []byte, hash []byte) string {
|
||
|
return strings.Join([]string{"",
|
||
|
typ,
|
||
|
base64.StdEncoding.EncodeToString(salt),
|
||
|
base64.StdEncoding.EncodeToString(hash),
|
||
|
}, "|")
|
||
|
}
|
||
|
|
||
|
// See https://android.googlesource.com/platform/external/openssh/+/ab28f5495c85297e7a597c1ba62e996416da7c7e/hostfile.c#120
|
||
|
func hashHost(hostname string, salt []byte) []byte {
|
||
|
mac := hmac.New(sha1.New, salt)
|
||
|
mac.Write([]byte(hostname))
|
||
|
return mac.Sum(nil)
|
||
|
}
|
||
|
|
||
|
type hashedHost struct {
|
||
|
salt []byte
|
||
|
hash []byte
|
||
|
}
|
||
|
|
||
|
const sha1HashType = "1"
|
||
|
|
||
|
func newHashedHost(encoded string) (*hashedHost, error) {
|
||
|
typ, salt, hash, err := decodeHash(encoded)
|
||
|
if err != nil {
|
||
|
return nil, err
|
||
|
}
|
||
|
|
||
|
// The type field seems for future algorithm agility, but it's
|
||
|
// actually hardcoded in openssh currently, see
|
||
|
// https://android.googlesource.com/platform/external/openssh/+/ab28f5495c85297e7a597c1ba62e996416da7c7e/hostfile.c#120
|
||
|
if typ != sha1HashType {
|
||
|
return nil, fmt.Errorf("knownhosts: got hash type %s, must be '1'", typ)
|
||
|
}
|
||
|
|
||
|
return &hashedHost{salt: salt, hash: hash}, nil
|
||
|
}
|
||
|
|
||
|
func (h *hashedHost) match(addrs []addr) bool {
|
||
|
for _, a := range addrs {
|
||
|
if bytes.Equal(hashHost(Normalize(a.String()), h.salt), h.hash) {
|
||
|
return true
|
||
|
}
|
||
|
}
|
||
|
return false
|
||
|
}
|