publicKey class method

DHKE.rb

class Diffie
    p defined?(Diffie)
    
    def publicKey(base, modPrime, aSecret)
        
        @publicKey = 0
        
#        puts "Enter your shared prime (g): "
#        base = Integer(gets.chomp)
#        puts "Enter your second prime (p): "
#        modPrime = Integer(gets.chomp)
#        puts "Enter your secret prime (a): "
#        aSecret = Integer(gets.chomp)
        
        @publicKey = base ** aSecret % modPrime
            print "Share this with your contact: " + publicKey
    end
end

publicKey output.txt

$ ruby DHKE.rb
"constant"
Are you computing the public key or the shared secret?
public key
"constant"
Enter your shared prime (g): 
7
Enter your second prime (p): 
5
Enter your secret prime (a): 
2
DHKE.rb:17:in `<main>': undefined method `publicKey' for nil:NilClass (NoMethodError)