Magento – Magento 2 Oauth request token – oauth_problem=Invalid+signature

Did you managed to Encode the names and values of parameters?

I was having this issue too and this helped me.

You have to make sure that your signature base string parameters are in alphabetic order by parameter name

This is what i'm using to encode parameters in my signature base.

var result = new StringBuilder();
string webUnreservedChars = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789-_.~";
foreach (char symbol in url)
{
    if (webUnreservedChars.IndexOf(symbol) != -1)
        result.Append(symbol);
    else
        result.Append('%' + ((int)symbol).ToString("X2"));
}
return result.ToString();

Hope this helps :)

If all credentials and URLs are correct and existing, the issue might happen because of time synchronization issues of your local with remote (it is important for signatures validation).