_footer.html.erb

<footer>
  <div class="mycontainer">
    <div class="ui stackable grid">
    <div class="left floated six wide column">
      &copy; <%= Time.now.year %> blablabla
    </div>
    <div id="imprint" class="right floated six wide column">
      <nav>
        <% if current_user %>
          <%= link_to t('footer.signin'), new_session_path %>
        <% else %>
          <%= link_to t('footer.signout'), session_path, method: :delete %>
        <% end %>
            | <%= link_to t('footer.imprint'), imprint_path %>
      </nav>
    </div>
  </div>
    </div>
</footer>

application_helper.rb

module ApplicationHelper

  def current_user
     User.find(session[:user_id]) if session[:user_id]
  end
 
  ...
  <snip>
  ...  
  
end

sessions_controller.rb

class SessionsController < ApplicationController

  def new

  end

  def create
    if user = User.authenticate(params[:username], params[:password])
      session[:user_id] = user.id
      flash[:notice] = t('sessions.success_html')
      redirect_to messages_path
    else
      flash[:error] = t('sessions.failure')
      render :new
    end
  end

  def destroy
    session[:user_id] = nil
    redirect_to root_path, notice: t('sessions.signedout')
  end
end

user.rb

class User < ApplicationRecord
  has_secure_password

  def self.authenticate(username, password)
    user = User.find_by(username: username)
    user && user.authenticate(password)
  end
end