ssh.go

package xfer

import (
	"crypto/x509"
	"encoding/pem"
	"errors"
	"fmt"
	"net"
	"os"
	"path/filepath"
	"strings"
	"time"

	"golang.org/x/crypto/ssh"
)

func signerFromPem(pemBytes []byte, password []byte) (ssh.Signer, error) {

	// read pem block
	err := errors.New("cannot decode pem block, no key found")
	pemBlock, _ := pem.Decode(pemBytes)
	if pemBlock == nil {
		return nil, err
	}

	// handle key encrypted with password
	if x509.IsEncryptedPEMBlock(pemBlock) {
		// decrypt PEM
		pemBlock.Bytes, err = x509.DecryptPEMBlock(pemBlock, []byte(password))
		if err != nil {
			return nil, fmt.Errorf("cannot decrypt PEM block %s", err.Error())
		}

		// get RSA, EC or DSA key
		key, err := parsePemBlock(pemBlock)
		if err != nil {
			return nil, err
		}

		// generate signer instance from key
		signer, err := ssh.NewSignerFromKey(key)
		if err != nil {
			return nil, fmt.Errorf("cannot create signer from encrypted key %s", err.Error())
		}

		return signer, nil
	}

	// generate signer instance from plain key
	signer, err := ssh.ParsePrivateKey(pemBytes)
	if err != nil {
		return nil, fmt.Errorf("cannot parsie plain private key %s", err.Error())
	}

	return signer, nil
}

func parsePemBlock(block *pem.Block) (any, error) {
	switch block.Type {
	case "RSA PRIVATE KEY":
		key, err := x509.ParsePKCS1PrivateKey(block.Bytes)
		if err != nil {
			return nil, fmt.Errorf("cannot parse PKCS private key %s", err.Error())
		} else {
			return key, nil
		}
	case "EC PRIVATE KEY":
		key, err := x509.ParseECPrivateKey(block.Bytes)
		if err != nil {
			return nil, fmt.Errorf("cannot parse EC private key %s", err.Error())
		} else {
			return key, nil
		}
	case "DSA PRIVATE KEY":
		key, err := ssh.ParseDSAPrivateKey(block.Bytes)
		if err != nil {
			return nil, fmt.Errorf("cannot parse DSA private key %s", err.Error())
		} else {
			return key, nil
		}
	default:
		return nil, fmt.Errorf("cannot parse private key, unsupported key type %s", block.Type)
	}
}

func NewSshClientConfig(user string, privateKeyPath string, privateKeyPassword string) (*ssh.ClientConfig, error) {
	keyPath := privateKeyPath
	if strings.HasPrefix(keyPath, "~/") {
		homeDir, _ := os.UserHomeDir()
		keyPath = filepath.Join(homeDir, keyPath[2:])
	}
	pemBytes, err := os.ReadFile(keyPath)
	if err != nil {
		return nil, fmt.Errorf("cannot read private key file %s: %s", keyPath, err.Error())
	}

	signer, err := signerFromPem(pemBytes, []byte(privateKeyPassword))
	if err != nil {
		return nil, err
	}

	return &ssh.ClientConfig{
		Timeout: time.Duration(10 * time.Second),
		User:    user,
		Auth: []ssh.AuthMethod{
			ssh.PublicKeys(signer),
		},
		HostKeyCallback: func(hostname string, remote net.Addr, key ssh.PublicKey) error {
			// use known_hosts file if you care about host validation
			return nil
		},
	}, nil
}