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forgejo/models/asymkey/ssh_key_parse.go
zeripath 2914c5299b
Improve error report when user passes a private key (#22726)
The error reported when a user passes a private ssh key as their ssh
public key is not very nice.

This PR improves this slightly.

Ref #22693

Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: delvh <dev.lh@web.de>
2023-02-02 18:25:54 +00:00

312 lines
9.3 KiB
Go

// Copyright 2021 The Gitea Authors. All rights reserved.
// SPDX-License-Identifier: MIT
package asymkey
import (
"crypto/rsa"
"crypto/x509"
"encoding/asn1"
"encoding/base64"
"encoding/binary"
"encoding/pem"
"fmt"
"math/big"
"os"
"strconv"
"strings"
"code.gitea.io/gitea/models/db"
"code.gitea.io/gitea/modules/log"
"code.gitea.io/gitea/modules/process"
"code.gitea.io/gitea/modules/setting"
"code.gitea.io/gitea/modules/util"
"golang.org/x/crypto/ssh"
)
// ____ __. __________
// | |/ _|____ ___.__. \______ \_____ _______ ______ ___________
// | <_/ __ < | | | ___/\__ \\_ __ \/ ___// __ \_ __ \
// | | \ ___/\___ | | | / __ \| | \/\___ \\ ___/| | \/
// |____|__ \___ > ____| |____| (____ /__| /____ >\___ >__|
// \/ \/\/ \/ \/ \/
//
// This file contains functions for parsing ssh-keys
//
// TODO: Consider if these functions belong in models - no other models function call them or are called by them
// They may belong in a service or a module
const ssh2keyStart = "---- BEGIN SSH2 PUBLIC KEY ----"
func extractTypeFromBase64Key(key string) (string, error) {
b, err := base64.StdEncoding.DecodeString(key)
if err != nil || len(b) < 4 {
return "", fmt.Errorf("invalid key format: %w", err)
}
keyLength := int(binary.BigEndian.Uint32(b))
if len(b) < 4+keyLength {
return "", fmt.Errorf("invalid key format: not enough length %d", keyLength)
}
return string(b[4 : 4+keyLength]), nil
}
// parseKeyString parses any key string in OpenSSH or SSH2 format to clean OpenSSH string (RFC4253).
func parseKeyString(content string) (string, error) {
// remove whitespace at start and end
content = strings.TrimSpace(content)
var keyType, keyContent, keyComment string
if strings.HasPrefix(content, ssh2keyStart) {
// Parse SSH2 file format.
// Transform all legal line endings to a single "\n".
content = strings.NewReplacer("\r\n", "\n", "\r", "\n").Replace(content)
lines := strings.Split(content, "\n")
continuationLine := false
for _, line := range lines {
// Skip lines that:
// 1) are a continuation of the previous line,
// 2) contain ":" as that are comment lines
// 3) contain "-" as that are begin and end tags
if continuationLine || strings.ContainsAny(line, ":-") {
continuationLine = strings.HasSuffix(line, "\\")
} else {
keyContent += line
}
}
t, err := extractTypeFromBase64Key(keyContent)
if err != nil {
return "", fmt.Errorf("extractTypeFromBase64Key: %w", err)
}
keyType = t
} else {
if strings.Contains(content, "-----BEGIN") {
// Convert PEM Keys to OpenSSH format
// Transform all legal line endings to a single "\n".
content = strings.NewReplacer("\r\n", "\n", "\r", "\n").Replace(content)
block, _ := pem.Decode([]byte(content))
if block == nil {
return "", fmt.Errorf("failed to parse PEM block containing the public key")
}
if strings.Contains(block.Type, "PRIVATE") {
return "", ErrKeyIsPrivate
}
pub, err := x509.ParsePKIXPublicKey(block.Bytes)
if err != nil {
var pk rsa.PublicKey
_, err2 := asn1.Unmarshal(block.Bytes, &pk)
if err2 != nil {
return "", fmt.Errorf("failed to parse DER encoded public key as either PKIX or PEM RSA Key: %v %w", err, err2)
}
pub = &pk
}
sshKey, err := ssh.NewPublicKey(pub)
if err != nil {
return "", fmt.Errorf("unable to convert to ssh public key: %w", err)
}
content = string(ssh.MarshalAuthorizedKey(sshKey))
}
// Parse OpenSSH format.
// Remove all newlines
content = strings.NewReplacer("\r\n", "", "\n", "").Replace(content)
parts := strings.SplitN(content, " ", 3)
switch len(parts) {
case 0:
return "", util.NewInvalidArgumentErrorf("empty key")
case 1:
keyContent = parts[0]
case 2:
keyType = parts[0]
keyContent = parts[1]
default:
keyType = parts[0]
keyContent = parts[1]
keyComment = parts[2]
}
// If keyType is not given, extract it from content. If given, validate it.
t, err := extractTypeFromBase64Key(keyContent)
if err != nil {
return "", fmt.Errorf("extractTypeFromBase64Key: %w", err)
}
if len(keyType) == 0 {
keyType = t
} else if keyType != t {
return "", fmt.Errorf("key type and content does not match: %s - %s", keyType, t)
}
}
// Finally we need to check whether we can actually read the proposed key:
_, _, _, _, err := ssh.ParseAuthorizedKey([]byte(keyType + " " + keyContent + " " + keyComment))
if err != nil {
return "", fmt.Errorf("invalid ssh public key: %w", err)
}
return keyType + " " + keyContent + " " + keyComment, nil
}
// CheckPublicKeyString checks if the given public key string is recognized by SSH.
// It returns the actual public key line on success.
func CheckPublicKeyString(content string) (_ string, err error) {
if setting.SSH.Disabled {
return "", db.ErrSSHDisabled{}
}
content, err = parseKeyString(content)
if err != nil {
return "", err
}
content = strings.TrimRight(content, "\n\r")
if strings.ContainsAny(content, "\n\r") {
return "", util.NewInvalidArgumentErrorf("only a single line with a single key please")
}
// remove any unnecessary whitespace now
content = strings.TrimSpace(content)
if !setting.SSH.MinimumKeySizeCheck {
return content, nil
}
var (
fnName string
keyType string
length int
)
if setting.SSH.StartBuiltinServer {
fnName = "SSHNativeParsePublicKey"
keyType, length, err = SSHNativeParsePublicKey(content)
} else {
fnName = "SSHKeyGenParsePublicKey"
keyType, length, err = SSHKeyGenParsePublicKey(content)
}
if err != nil {
return "", fmt.Errorf("%s: %w", fnName, err)
}
log.Trace("Key info [native: %v]: %s-%d", setting.SSH.StartBuiltinServer, keyType, length)
if minLen, found := setting.SSH.MinimumKeySizes[keyType]; found && length >= minLen {
return content, nil
} else if found && length < minLen {
return "", fmt.Errorf("key length is not enough: got %d, needs %d", length, minLen)
}
return "", fmt.Errorf("key type is not allowed: %s", keyType)
}
// SSHNativeParsePublicKey extracts the key type and length using the golang SSH library.
func SSHNativeParsePublicKey(keyLine string) (string, int, error) {
fields := strings.Fields(keyLine)
if len(fields) < 2 {
return "", 0, fmt.Errorf("not enough fields in public key line: %s", keyLine)
}
raw, err := base64.StdEncoding.DecodeString(fields[1])
if err != nil {
return "", 0, err
}
pkey, err := ssh.ParsePublicKey(raw)
if err != nil {
if strings.Contains(err.Error(), "ssh: unknown key algorithm") {
return "", 0, ErrKeyUnableVerify{err.Error()}
}
return "", 0, fmt.Errorf("ParsePublicKey: %w", err)
}
// The ssh library can parse the key, so next we find out what key exactly we have.
switch pkey.Type() {
case ssh.KeyAlgoDSA:
rawPub := struct {
Name string
P, Q, G, Y *big.Int
}{}
if err := ssh.Unmarshal(pkey.Marshal(), &rawPub); err != nil {
return "", 0, err
}
// as per https://bugzilla.mindrot.org/show_bug.cgi?id=1647 we should never
// see dsa keys != 1024 bit, but as it seems to work, we will not check here
return "dsa", rawPub.P.BitLen(), nil // use P as per crypto/dsa/dsa.go (is L)
case ssh.KeyAlgoRSA:
rawPub := struct {
Name string
E *big.Int
N *big.Int
}{}
if err := ssh.Unmarshal(pkey.Marshal(), &rawPub); err != nil {
return "", 0, err
}
return "rsa", rawPub.N.BitLen(), nil // use N as per crypto/rsa/rsa.go (is bits)
case ssh.KeyAlgoECDSA256:
return "ecdsa", 256, nil
case ssh.KeyAlgoECDSA384:
return "ecdsa", 384, nil
case ssh.KeyAlgoECDSA521:
return "ecdsa", 521, nil
case ssh.KeyAlgoED25519:
return "ed25519", 256, nil
case ssh.KeyAlgoSKECDSA256:
return "ecdsa-sk", 256, nil
case ssh.KeyAlgoSKED25519:
return "ed25519-sk", 256, nil
}
return "", 0, fmt.Errorf("unsupported key length detection for type: %s", pkey.Type())
}
// writeTmpKeyFile writes key content to a temporary file
// and returns the name of that file, along with any possible errors.
func writeTmpKeyFile(content string) (string, error) {
tmpFile, err := os.CreateTemp(setting.SSH.KeyTestPath, "gitea_keytest")
if err != nil {
return "", fmt.Errorf("TempFile: %w", err)
}
defer tmpFile.Close()
if _, err = tmpFile.WriteString(content); err != nil {
return "", fmt.Errorf("WriteString: %w", err)
}
return tmpFile.Name(), nil
}
// SSHKeyGenParsePublicKey extracts key type and length using ssh-keygen.
func SSHKeyGenParsePublicKey(key string) (string, int, error) {
tmpName, err := writeTmpKeyFile(key)
if err != nil {
return "", 0, fmt.Errorf("writeTmpKeyFile: %w", err)
}
defer func() {
if err := util.Remove(tmpName); err != nil {
log.Warn("Unable to remove temporary key file: %s: Error: %v", tmpName, err)
}
}()
stdout, stderr, err := process.GetManager().Exec("SSHKeyGenParsePublicKey", setting.SSH.KeygenPath, "-lf", tmpName)
if err != nil {
return "", 0, fmt.Errorf("fail to parse public key: %s - %s", err, stderr)
}
if strings.Contains(stdout, "is not a public key file") {
return "", 0, ErrKeyUnableVerify{stdout}
}
fields := strings.Split(stdout, " ")
if len(fields) < 4 {
return "", 0, fmt.Errorf("invalid public key line: %s", stdout)
}
keyType := strings.Trim(fields[len(fields)-1], "()\r\n")
length, err := strconv.ParseInt(fields[0], 10, 32)
if err != nil {
return "", 0, err
}
return strings.ToLower(keyType), int(length), nil
}