PHP：从私钥字符串/文件获取RSA公钥

Question

I'm building a basic encrypted chat application with laravel. Im trying to authenticate a user given a private key that was generated when the user signed up.

The users' public key is stored in the DB. When the user wants to log in, the need to upload the private key that was given to them. I want to compare the public key in the DB and the public key from the original private key.

/**
 * Validate user key file for login
 */
public function validateKey($attribute, $value, $parameters){
    if (!($value instanceof UploadedFile)) return false;

    // Get user to match key
    $user = $this->data['username'];

    // Get public key from file
    $privateKey = File::get($value->getRealPath());

    dd($privateKey);
}

I am able to get the private key in a string format. From here I cannot seem to generate or extract the public key to compare to the DB value.

Answer 1

They should also never upload their private key to your server. You should never have your private key at a remote location.

The point of having two keys is one key can decrypt any message encrypted by the other key. The major difference between the public key and the private key in most algorithms would be more or less which one you keep private and which one you share. In RSA, the private key is designated, but you still do not want to upload it or share it.

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Redacted: ~That's not how public/private keys work. You can't extract one from the other. If you could, that would defeat the purpose of the asymmetric encryption.~

As @iainn pointed out, you can extract a public key from an RSA private key, however I still do not recommend having users upload their private key.