tenderlove · Nov 30, 2021 · Jul 7, 2021 · Jul 7, 2021 · Jul 7, 2021 · Jul 8, 2021
diff --git a/lib/bcrypt/engine.rb b/lib/bcrypt/engine.rb
@@ -7,6 +7,14 @@ class Engine
     MIN_COST        = 4
     # The maximum cost supported by the algorithm.
     MAX_COST = 31
+    # Maximum possible size of bcrypt() secrets.
+    # Older versions of the bcrypt library would truncate passwords longer
+    # than 72 bytes, but newer ones do not. We truncate like the old library for
+    # forward compatibility. This way users upgrading from Ubuntu 18.04 to 20.04
+    # will not have their user passwords invalidated, for example.
+    # A max secret length greater than 255 leads to bcrypt returning nil.
+    # https://github.com/bcrypt-ruby/bcrypt-ruby/issues/225#issuecomment-875908425
+    MAX_SECRET_BYTESIZE = 72
     # Maximum possible size of bcrypt() salts.
     MAX_SALT_LENGTH = 16
 
@@ -43,14 +51,16 @@ def self.cost=(cost)
     end
 
     # Given a secret and a valid salt (see BCrypt::Engine.generate_salt) calculates
-    # a bcrypt() password hash.
+    # a bcrypt() password hash. Secrets longer than 72 bytes are truncated.
     def self.hash_secret(secret, salt, _ = nil)
       if valid_secret?(secret)
         if valid_salt?(salt)
           if RUBY_PLATFORM == "java"
             Java.bcrypt_jruby.BCrypt.hashpw(secret.to_s.to_java_bytes, salt.to_s)
           else
-            __bc_crypt(secret.to_s, salt)
+            secret = secret.to_s
+            secret = secret.byteslice(0, MAX_SECRET_BYTESIZE) if secret && secret.bytesize > MAX_SECRET_BYTESIZE
+            __bc_crypt(secret, salt)
           end
         else
           raise Errors::InvalidSalt.new("invalid salt")

diff --git a/spec/bcrypt/engine_spec.rb b/spec/bcrypt/engine_spec.rb
@@ -1,4 +1,5 @@
 require File.expand_path(File.join(File.dirname(__FILE__), "..", "spec_helper"))
+require 'securerandom'
 
 describe 'BCrypt::Engine' do
   describe '.calibrate(upper_time_limit_in_ms)' do
@@ -157,4 +158,19 @@ class MyInvalidSecret
       expect(BCrypt::Engine.hash_secret(secret, salt)).to eql(test_vector)
     end
   end
+
+  specify "should truncate 1-byte character secrets to 72 bytes" do
+    # 'b' as a base triggers the failure at 256 characters, but 'a' does not.
+    too_long_secret = 'b'*(BCrypt::Engine::MAX_SECRET_BYTESIZE + 1)
+    just_right_secret = 'b'*BCrypt::Engine::MAX_SECRET_BYTESIZE
+    expect(BCrypt::Engine.hash_secret(too_long_secret, @salt)).to eq(BCrypt::Engine.hash_secret(just_right_secret, @salt))
+  end
+
+  specify "should truncate multi-byte character secrets to 72 bytes" do
+    # 256 times causes bcrypt to return nil for libxcrypt > 4.4.18-4.
+    too_long_secret = '𐐷'*256
+    # 𐐷 takes 4 bytes in UTF-8. 18 times is 72 bytes
+    just_right_secret = '𐐷'*18
+    expect(BCrypt::Engine.hash_secret(too_long_secret, @salt)).to eq(BCrypt::Engine.hash_secret(just_right_secret, @salt))
+  end
 end
diff --git a/spec/bcrypt/password_spec.rb b/spec/bcrypt/password_spec.rb
@@ -1,4 +1,5 @@
 require File.expand_path(File.join(File.dirname(__FILE__), "..", "spec_helper"))
+require 'securerandom'
 
 describe "Creating a hashed password" do
 
@@ -26,6 +27,28 @@
     expect { BCrypt::Password.create( ""         ) }.not_to raise_error
     expect { BCrypt::Password.create( String.new ) }.not_to raise_error
   end
+
+  context 'with a 256 character password' do
+    before :each do
+      @secret = SecureRandom.hex(256)
+      @password = BCrypt::Password.create(@secret, :cost => 4)
+    end
+
+    specify "should return a BCrypt::Password" do
+      expect(@password).to be_an_instance_of(BCrypt::Password)
+    end
+  end
+
+  context 'with a 255 character password' do
+    before :each do
+      @secret = SecureRandom.hex(255)
+      @password = BCrypt::Password.create(@secret, :cost => 4)
+    end
+
+    specify "should return a BCrypt::Password" do
+      expect(@password).to be_an_instance_of(BCrypt::Password)
+    end
+  end
 end
 
 describe "Reading a hashed password" do