golang ssh host key

SSH host key in Golang

Introduction:

A Secure Shell (SSH) host key is an important component of secure communication between a client and a server. It is used to verify the authenticity of the host during the SSH handshake process. In this article, we will explore how to work with SSH host keys in Golang.

Generating SSH host keys:

In Golang, we can use the crypto/ssh package to generate SSH host keys. The host key can be generated using the GenerateKeyPair function from the rsa or ecdsa package.

package main

import (
    "crypto/rand"
    "crypto/rsa"
    "crypto/x509"
    "encoding/pem"
    "fmt"
    "io/ioutil"
    "log"
)

func main() {
    // Generate RSA private key
    privateKey, err := rsa.GenerateKey(rand.Reader, 2048)
    if err != nil {
        log.Fatal(err)
    }

    // Encode private key to PEM format
    privateKeyPEM := &pem.Block{
        Type:  "RSA PRIVATE KEY",
        Bytes: x509.MarshalPKCS1PrivateKey(privateKey),
    }
    privateKeyPEMBytes := pem.EncodeToMemory(privateKeyPEM)

    // Save private key to file
    err = ioutil.WriteFile("private_key.pem", privateKeyPEMBytes, 0600)
    if err != nil {
        log.Fatal(err)
    }

    // Generate public key from private key
    publicKey := &privateKey.PublicKey

    // Encode public key to authorized_keys format
    publicKeyBytes, err := ssh.NewPublicKey(publicKey)
    if err != nil {
        log.Fatal(err)
    }

    // Save public key to file
    err = ioutil.WriteFile("authorized_keys", ssh.MarshalAuthorizedKey(publicKeyBytes), 0600)
    if err != nil {
        log.Fatal(err)
    }

    fmt.Println("SSH host key generated successfully!")
}

In this example, we generate an RSA private key with a length of 2048 bits. We then encode the private key to PEM format and save it to a file named "private_key.pem". Next, we extract the public key from the private key and encode it to the authorized_keys format, which is commonly used for SSH host keys. The encoded public key is saved to a file named "authorized_keys".

Verifying SSH host keys:

To verify an SSH host key in Golang, we can use the crypto/ssh package. The PublicKeyCallback function can be used to define a callback function that will be called during the SSH handshake process. This function can be used to compare the received host key with a known host key.

package main

import (
    "crypto/Subtle"
    "crypto/x509"
    "encoding/pem"
    "fmt"
    "io/ioutil"
    "log"
    "os"

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

func main() {
    // Read the known host key from file
    knownHostKeyBytes, err := ioutil.ReadFile("known_host_key.pem")
    if err != nil {
        log.Fatal(err)
    }

    knownHostKeyBlock, _ := pem.Decode(knownHostKeyBytes)
    if knownHostKeyBlock == nil {
        log.Fatal("Failed to decode known host key")
    }

    knownHostKey, err := x509.ParsePKIXPublicKey(knownHostKeyBlock.Bytes)
    if err != nil {
        log.Fatal(err)
    }

    // Configure the SSH client
    config := &ssh.ClientConfig{
        User: "sshuser",
        Auth: []ssh.AuthMethod{
            ssh.Password("sshpassword"),
        },
        HostKeyCallback: func(hostname string, remote net.Addr, key ssh.PublicKey) error {
            if subtle.ConstantTimeCompare(knownHostKey.(*rsa.PublicKey).N.Bytes(), key.(*rsa.PublicKey).N.Bytes()) == 1 {
                return nil
            }
            return fmt.Errorf("untrusted host")
        },
    }

    // Connect to the SSH server
    client, err := ssh.Dial("tcp", "sshserver:22", config)
    if err != nil {
        log.Fatal(err)
    }

    // Close the SSH connection
    defer client.Close()

    fmt.Println("SSH connection established!")
}

In this example, we read the known host key from a file named "known_host_key.pem". We decode the PEM block and parse the key into an x509.PublicKey object. We then configure the SSH client using ssh.ClientConfig, setting the user, authentication method, and using the HostKeyCallback function to compare the received host key with the known host key. If the two keys match, the connection is established successfully.

Conclusion:

In this article, we explored how to work with SSH host keys in Golang. We learned how to generate SSH host keys using the crypto/ssh package and how to verify SSH host keys during the SSH handshake process. By understanding and using SSH host keys correctly, we can ensure secure and authenticated communication between clients and servers.