Go: how to read the information of an SSL certificate

SSL (Secure Socket Layer) certificates are a crucial tool for ensuring the security of Internet communications. They are used to encrypt data between your web browser and a remote server, ensuring that the transmitted information is safe from prying eyes. When you visit a website, your browser checks the site's SSL certificate to make sure it is valid and legitimate. In this article, we'll explore how you can use the Go programming language to read the main information of an SSL certificate from a remote site.

To get started, you'll need to import the libraries needed to work with SSL certificates in Go. The main library we'll be using is crypto/tls. You can do this in your Go file with the following import:

package main

import (
"crypto/tls"
"fmt"
"net"
)

To obtain a remote site's SSL certificate, you must establish a TCP connection with the remote server using its address and the appropriate port. You can do this with the following code:

func main() {
    // Address and port of the remote server
    serverAddr := "example.com:443"

    // TCP connection to the server
    conn, err := net.Dial("tcp", serverAddr)
    if err != nil {
        fmt.Println("Error connecting to the server:", err)
        return
    }
    defer conn.Close()

    // Initialize a TLS connection
    config := tls.Config{ServerName: "example.com"}
    tlsConn := tls.Client(conn, &config)

    // TLS handshake
    err = tlsConn.Handshake()
    if err != nil {
        fmt.Println("Error during TLS handshake:", err)
        return
    }

    // Now you can read the SSL certificate
}

Make sure to replace "example.com" with the name of the website whose SSL certificate you want to obtain.

Once the TLS connection is established with the remote server, you can read the SSL certificate information. The SSL certificate is available as part of the TLS connection and you can access its information like this:

     // Read the SSL certificate
     certificate := tlsConn.ConnectionState().PeerCertificates[0]

     // Print the main information of the certificate
     fmt.Println("Common Name):", certificate.Subject.CommonName)
     fmt.Println("Issuer):", certificate.Issuer.CommonName)
     fmt.Println("Expires (Valid until):", certificate.NotAfter)

This code reads the SSL certificate from the remote server and prints some key information such as the certificate's Common Name, Issuer, and expiration date.

Conclusion

In this article, we saw how to use Go to read the main information of an SSL certificate from a remote website. This is useful for debugging purposes, security monitoring, and to gain insight into the authenticity of a website. Always make sure you use this information responsibly and respectfully of privacy and online security.