Retrieve public SSH key from private key file using Go - go

Question

I have a private key file id_rsa (starts with -----BEGIN RSA PRIVATE KEY-----). With the tool ssh-keygen I am able to generate an SSH public key using the following command:
ssh-keygen -y -f ~/.ssh/id_rsa > ~/.ssh/id_rsa.pub
The generated file will have the following content:
ssh-rsa
AAAAB3NzaC1yc2EAAAABIwAAAQEAklOUpkDHrfHY17SbrmTIpNLTGK9Tjom/BWDSU
GPl+nafzlHDTYW7hdI4yZ5ew18JH4JW9j...
I am trying to achieve the same within my Go code. The code will be executed on AWS Lambda so I want to avoid executing system commands with os.exec since I don't have control over the underlying environment.
Given that I have a variable private_key, how can I extract the ssh public key from it?

Answer 1

This is my solution to extract the public key in OpenSSH format, using packages rsa, pem and x509 from the standard library and golang.org/x/crypto/ssh
import (
"crypto/rsa"
"crypto/x509"
"encoding/base64"
"encoding/pem"
"errors"
"os"
"golang.org/x/crypto/ssh"
)
func check(e error) {
if e != nil {
panic(e)
}
}
func ParseRsaPrivateKeyFromPemStr(privPEM string) (*rsa.PrivateKey, error) {
block, _ := pem.Decode([]byte(privPEM))
if block == nil {
return nil, errors.New("failed to parse PEM block containing the key")
}
priv, err := x509.ParsePKCS1PrivateKey(block.Bytes)
if err != nil {
return nil, err
}
return priv, nil
}
func PublicPEMtoOpenSSH(rsaPubKey *rsa.PublicKey) (string, error) {
pub, err := ssh.NewPublicKey(rsaPubKey)
if err != nil {
return "", err
}
sshPubKey := ssh.MarshalAuthorizedKey(pub)
return string(sshPubKey), nil
}
func main() {
private_PEM, err := os.ReadFile("path/to/file.pem")
check(err)
priv_parsed, err := ParseRsaPrivateKeyFromPemStr(string(private_PEM))
check(err)
openssh_key, err := PublicPEMtoOpenSSH(&priv_parsed.PublicKey)
check(err)
print(openssh_key) // ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEA...
}