Use some syntactic sugar that Ruby provides.

lib/sirp/client.rb

@@ -22,13 +22,13 @@ def process_challenge(username, password, xsalt, xbb)
       bb = xbb.to_i(16)
 
       # SRP-6a safety check
-      return false if (bb % @N) == 0
+      return false if (bb % @N).zero?
 
       x = calc_x(username, password, xsalt, hash)
       u = calc_u(@A, xbb, @N, hash)
 
       # SRP-6a safety check
-      return false if u == 0
+      return false if u.zero?
 
       # calculate session key
       @S = num_to_hex(calc_client_S(bb, @a, @k, x, u, @N, @g))

lib/sirp/sirp.rb

@@ -58,7 +58,7 @@ def H(hash_klass, n, *a)
 
     hashin = a.map do |s|
       next unless s
-      shex = (s.class == String) ? s : num_to_hex(s)
+      shex = s.is_a?(String) ? s : num_to_hex(s)
       if shex.length > nlen
         raise 'Bit width does not match - client uses different prime'
       end

lib/sirp/verifier.rb

@@ -24,7 +24,8 @@ def generate_userauth(username, password)
     # Parameters should be given in hex.
     def get_challenge_and_proof(username, xverifier, xsalt, xaa)
       # SRP-6a safety check
-      return false if (xaa.to_i(16) % @N) == 0
+      return false if (xaa.to_i(16) % @N).zero?
+
       generate_B(xverifier)
 
       {
@@ -45,7 +46,7 @@ def verify_session(proof, client_M)
       u = calc_u(@A, @B, @N, hash)
 
       # SRP-6a safety check
-      return false if u == 0
+      return false if u.zero?
 
       # calculate session key
       @S = num_to_hex(calc_server_S(@A.to_i(16), @b, v, u, @N))