diff --git a/lib/webauthn/attestation_statement/base.rb b/lib/webauthn/attestation_statement/base.rb
index 6c5e07d4..a65e1ee1 100644
--- a/lib/webauthn/attestation_statement/base.rb
+++ b/lib/webauthn/attestation_statement/base.rb
@@ -46,7 +46,7 @@ def attestation_certificate
       end
 
       def attestation_certificate_key_id
-        attestation_certificate.subject_key_identifier&.unpack("H*")&.[](0)
+        attestation_certificate.subject_key_identifier&.unpack1("H*")
       end
 
       private
diff --git a/webauthn.gemspec b/webauthn.gemspec
index dac6b913..6a22cd23 100644
--- a/webauthn.gemspec
+++ b/webauthn.gemspec
@@ -46,7 +46,7 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "byebug", "~> 11.0"
   spec.add_development_dependency "rake", "~> 13.0"
   spec.add_development_dependency "rspec", "~> 3.8"
-  spec.add_development_dependency "rubocop", "~> 1.9.1"
-  spec.add_development_dependency "rubocop-rake", "~> 0.5.1"
-  spec.add_development_dependency "rubocop-rspec", "~> 2.2.0"
+  spec.add_development_dependency "rubocop", "~> 1"
+  spec.add_development_dependency "rubocop-rake", "~> 0.5"
+  spec.add_development_dependency "rubocop-rspec", ">= 2.2", "< 4.0"
 end