Converting an ssh.PublicKey to an *rsa.PublicKey in Go

main.go

package main

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

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

func main() {
	// First, generate the test RSA keypair in SSH format
	priv, err := rsa.GenerateKey(rand.Reader, 2048)
	if err != nil {
		log.Fatal(err)
	}
	rsaPub := priv.PublicKey
	sshPub, err := ssh.NewPublicKey(&rsaPub)
	if err != nil {
		log.Fatal(err)
	}
	sshPubBytes := sshPub.Marshal()

	// Now we can convert it back to PEM format
	// Remember: if you're reading the public key from a file, you probably
	// want ssh.ParseAuthorizedKey.
	parsed, err := ssh.ParsePublicKey(sshPubBytes)
	if err != nil {
		log.Fatal(err)
	}
	// To get back to an *rsa.PublicKey, we need to first upgrade to the
	// ssh.CryptoPublicKey interface
	parsedCryptoKey := parsed.(ssh.CryptoPublicKey)

	// Then, we can call CryptoPublicKey() to get the actual crypto.PublicKey
	pubCrypto := parsedCryptoKey.CryptoPublicKey()

	// Finally, we can convert back to an *rsa.PublicKey
	pub := pubCrypto.(*rsa.PublicKey)

	// After this, it's encoding to PEM - same as always
	encoded := pem.EncodeToMemory(&pem.Block{
		Type:  "RSA PUBLIC KEY",
		Bytes: x509.MarshalPKCS1PublicKey(pub),
	})
	fmt.Printf("%s", encoded)
}