How to implement a two-step user verification for increased system security?

Two-factor authentication (2FA) enhances security by requiring two different methods of verifying the user's identity.

A typical implementation of 2FA includes:

1. Password verification (something the user knows).
2. Verification of the second factor — for example, a one-time code from SMS, an authenticator app (Google Authenticator), or a hardware token.

In Go, 2FA can be implemented as follows:

• After successful password verification, generate and send a one-time code to the user.
• Temporarily store the code (e.g., in a database or cache) with a time limit.
• Ask the user to enter the code and verify it.

Example of generating a TOTP code using the github.com/pquerna/otp library:

import (
    "github.com/pquerna/otp/totp"
    "time"
)

// Verify TOTP code
func verifyCode(secret, code string) bool {
    return totp.Validate(code, secret)
}

// Generate a secret key for the user
func generateSecret() (string, error) {
    key, err := totp.Generate(totp.GenerateOpts{
        Issuer:      "MyApp",
        AccountName: "user@example.com",
    })
    if err != nil {
        return "", err
    }
    return key.Secret(), nil
}

Thus, after password verification, the user enters the code from the app, which is verified by the server.