class SIRP::Client

def initialize(group = 2048)
  raise ArgumentError, 'must be an Integer' unless group.is_a?(Integer)
  raise ArgumentError, 'must be a known group size' unless [1024, 1536, 2048, 3072, 4096, 6144, 8192].include?(group)
  @N, @g, @hash = Ng(group)
  @k = calc_k(@N, @g, hash)
end

def process_challenge(username, password, xsalt, xbb, is_password_encrypted: false)
  raise ArgumentError, 'username must be a string' unless username.is_a?(String) && !username.empty?
  raise ArgumentError, 'password must be a string' unless password.is_a?(String) && !password.empty?
  raise ArgumentError, 'xsalt must be a string' unless xsalt.is_a?(String)
  raise ArgumentError, 'xsalt must be a hex string' unless xsalt =~ /^[a-fA-F0-9]+$/
  raise ArgumentError, 'xbb must be a string' unless xbb.is_a?(String)
  raise ArgumentError, 'xbb must be a hex string' unless xbb =~ /^[a-fA-F0-9]+$/
  # Convert the 'B' hex value to an Integer
  bb = xbb.to_i(16)
  # SRP-6a safety check
  return false if (bb % @N).zero?
  if is_password_encrypted
    x = calc_x_hex(password, xsalt, hash)
  else
    x = calc_x(username, password, xsalt, hash)
  end
  u = calc_u(@A, xbb, @N, hash)
  # SRP-6a safety check
  return false if u.zero?
  # Calculate session key 'S' and secret key 'K'
  @S = num_to_hex(calc_client_S(bb, @a, @k, x, u, @N, @g))
  @K = sha_hex(@S, hash)
  # Calculate the 'M' matcher
  @M = calc_M(@N, @g, username, xsalt, @A, xbb, @K, hash)
  # Calculate the H(A,M,K) verifier
  @H_AMK = num_to_hex(calc_H_AMK(@A, @M, @K, hash))
  # Return the 'M' matcher to be sent to the server
  @M
end

def start_authentication
  @a ||= SecureRandom.hex(256).hex
  @A = num_to_hex(calc_A(@a, @N, @g))
end

def verify(server_HAMK)
  return false unless @H_AMK && server_HAMK
  return false unless server_HAMK.is_a?(String)
  return false unless server_HAMK =~ /^[a-fA-F0-9]+$/
  # Hash the comparison params to ensure that both strings
  # being compared are equal length 32 Byte strings.
  secure_compare(Digest::SHA256.hexdigest(@H_AMK), Digest::SHA256.hexdigest(server_HAMK))
end