Golang: Convert Public key PEM bytes to Open SSH format

public_pem_to_openssh.go

const (
	samplePEMKeyA = `
-----BEGIN PUBLIC KEY-----
MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEApIBaVZ8A6HraVxIVGEU+
psJx8S4nFQQykrJ751aicMUNFYThT/tl5Gvvl4H9eimB06iNf4zqnrvKwgtCYjFp
a5rJoaSxA675FBzYpzsHeygnPkt98mFGLRS6SJdNhvuE5QT6ostiLBufdpIyvSUP
cKdwL7ti8QyX3v8Lm7JrJ+DiGW8y3XYWm5kzlHdbah3DB2z2xh9+sQ1orXcxFTf7
CzVmfoTTO+QkeeTz/HG0gRu+EH1cxkMKP/BS8qOrFBFGWt3B/Vv0/lG4AILag+Dn
x4FfMxhlPtIXW4eUc5/SGaNHarJVN4y6mz4I3NvL3abMAWqpX1U9p/iO5kMLlFQX
qQIDAQAB
-----END PUBLIC KEY-----`

	expectedSSHKeyA = `AAAAB3NzaC1yc2EAAAADAQABAAABAQCkgFpVnwDoetpXEhUYRT6mwnHxLicVBDKSsnvnVqJwxQ0VhOFP+2Xka++Xgf16KYHTqI1/jOqeu8rCC0JiMWlrmsmhpLEDrvkUHNinOwd7KCc+S33yYUYtFLpIl02G+4TlBPqiy2IsG592kjK9JQ9wp3Avu2LxDJfe/wubsmsn4OIZbzLddhabmTOUd1tqHcMHbPbGH36xDWitdzEVN/sLNWZ+hNM75CR55PP8cbSBG74QfVzGQwo/8FLyo6sUEUZa3cH9W/T+UbgAgtqD4OfHgV8zGGU+0hdbh5Rzn9IZo0dqslU3jLqbPgjc28vdpswBaqlfVT2n+I7mQwuUVBep`
)

// Converts PEM public key to OpenSSH format to be used in authorized_keys file
// Similar to: "ssh-keygen", "-i", "-m", "pkcs8", "-f", auth_keys_new_path
func PublicPEMtoOpenSSH(pemBytes []byte) (*string, error) {
	// Decode and get the first block in the PEM file.
	// In our case it should be the Public key block.
	pemBlock, rest := pem.Decode(pemBytes)
	if pemBlock == nil {
		return nil, errors.New("invalid PEM public key passed, pem.Decode() did not find a public key")
	}
	if len(rest) > 0 {
		return nil, errors.New("PEM block contains more than just public key")
	}

	// Confirm we got the PUBLIC KEY block type
	if pemBlock.Type != "PUBLIC KEY" {
		return nil, errors.Errorf("ssh: unsupported key type %q", pemBlock.Type)
	}

	// Convert to rsa
	rsaPubKey, err := x509.ParsePKIXPublicKey(pemBlock.Bytes)
	if err != nil {
		return nil, errors.Wrap(err, "x509.parse pki public key")
	}

	// Confirm we got an rsa public key. Returned value is an interface{}
	sshKey, ok := rsaPubKey.(*rsa.PublicKey)
	if !ok {
		return nil, errors.Wrap(err, "invalid PEM passed in from user")
	}

	// Generate the ssh public key
	pub, err := ssh.NewPublicKey(sshKey)
	if err != nil {
		return nil, errors.Wrap(err, "new ssh public key from pem converted to rsa")
	}

	// Encode to store to file
	sshPubKey := base64.StdEncoding.EncodeToString(pub.Marshal())

	return &sshPubKey, nil
}