diff --git a/CHANGELOG.adoc b/CHANGELOG.adoc
index b973bbc7..1652be86 100644
--- a/CHANGELOG.adoc
+++ b/CHANGELOG.adoc
@@ -5,7 +5,7 @@
 Enhancements::
 
 * Issue #423: Add support for loading JSyntrax via the `asciidoctor-diagram-jsyntrax` gem (@inponomarev)
-* Issue #464: Replace usage of 'base64' gem with 'radix_encoding' to resolve Ruby 3.3+ compatibility issues
+* Issue #464: Remove usage of 'base64' gem to resolve Ruby 3.3+ compatibility issues
 
 Bugfixes::
 
diff --git a/asciidoctor-diagram.gemspec b/asciidoctor-diagram.gemspec
index 588f009d..d6df1315 100644
--- a/asciidoctor-diagram.gemspec
+++ b/asciidoctor-diagram.gemspec
@@ -34,6 +34,5 @@ Gem::Specification.new do |s|
   s.add_runtime_dependency 'asciidoctor', '>= 1.5.7', '< 3.x'
   s.add_runtime_dependency 'asciidoctor-diagram-ditaamini', '~> 1.0'
   s.add_runtime_dependency 'asciidoctor-diagram-plantuml', '~> 1.2021'
-  s.add_runtime_dependency 'radix_encoding', '0.2.0'
   s.add_runtime_dependency 'rexml'
 end
diff --git a/lib/asciidoctor-diagram/http/converter.rb b/lib/asciidoctor-diagram/http/converter.rb
index 48a96e9b..3485b052 100644
--- a/lib/asciidoctor-diagram/http/converter.rb
+++ b/lib/asciidoctor-diagram/http/converter.rb
@@ -1,7 +1,7 @@
 require_relative '../diagram_converter'
+require_relative '../util/base64'
 require_relative '../util/platform'
 
-require 'radix_encoding'
 require 'net/http'
 require 'uri'
 require 'zlib'
@@ -12,18 +12,6 @@ module Diagram
     class HttpConverter
       DEFAULT_MAX_GET_SIZE = 1024
 
-      PLANTUML_ENCODING = RadixEncoding::Encoding.new(
-        alphabet: "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz-_",
-        radix: 64,
-        padding: "="
-      )
-
-      BASE64_URL_SAFE = RadixEncoding::Encoding.new(
-        alphabet: "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_",
-        radix: 64,
-        padding: "="
-      )
-
       include DiagramConverter
 
       def initialize(base_uri, type, converter)
@@ -59,16 +47,20 @@ def convert(source, format, options)
           compressed = deflate.deflate(code, Zlib::FINISH)
           deflate.close
 
-
-
-          data = PLANTUML_ENCODING.encode(compressed)
+          data = Base64.urlsafe_encode(compressed)
+          # See https://plantuml.com/text-encoding
+          # PlantUML uses a different alphabet than the one from RFC 4648
+          data.tr!(
+            'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789',
+            '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz'
+          )
 
           path = uri.path.dup
           path << '/' unless path.end_with? '/'
           path << format.to_s
         when :kroki_io
           compressed = Zlib.deflate(code, Zlib::BEST_COMPRESSION)
-          data = BASE64_URL_SAFE.encode(compressed)
+          data = Base64.urlsafe_encode(compressed)
 
           path = uri.path.dup
           path << '/' unless path.end_with? '/'
diff --git a/lib/asciidoctor-diagram/http/server.rb b/lib/asciidoctor-diagram/http/server.rb
index 61ad19ac..b51f37c6 100644
--- a/lib/asciidoctor-diagram/http/server.rb
+++ b/lib/asciidoctor-diagram/http/server.rb
@@ -4,6 +4,7 @@
 
 require_relative '../diagram_source'
 require_relative '../graphviz/converter'
+require_relative '../util/base64'
 require_relative '../util/which'
 
 module Asciidoctor
@@ -13,7 +14,7 @@ class Server < Sinatra::Base
         type = params['type']
         accepts = lambda { |t| params['format'].downcase.to_sym == t }
         raw_source = params['source']
-        decoded_source = Asciidoctor::Diagram::HttpConverter::BASE64_URL_SAFE.decode(raw_source)
+        decoded_source = Base64.urlsafe_decode(raw_source)
         decompressed_source = Zlib::Inflate.inflate(decoded_source)
         source = decompressed_source
         render_diagram(type, accepts, source, {})
diff --git a/lib/asciidoctor-diagram/util/base64.rb b/lib/asciidoctor-diagram/util/base64.rb
new file mode 100644
index 00000000..20d52566
--- /dev/null
+++ b/lib/asciidoctor-diagram/util/base64.rb
@@ -0,0 +1,25 @@
+module Asciidoctor
+  module Diagram
+    module Base64
+      def self.urlsafe_encode(bin, padding: true)
+        str = [bin].pack("m0")
+        str.chomp!("==") or str.chomp!("=") unless padding
+        str.tr!("+/", "-_")
+        str
+      end
+
+      def self.urlsafe_decode(str)
+        # NOTE: RFC 4648 does say nothing about unpadded input, but says that
+        # "the excess pad characters MAY also be ignored", so it is inferred that
+        # unpadded input is also acceptable.
+        if !str.end_with?("=") && str.length % 4 != 0
+          str = str.ljust((str.length + 3) & ~3, "=")
+          str.tr!("-_", "+/")
+        else
+          str = str.tr("-_", "+/")
+        end
+        str.unpack1("m0")
+      end
+    end
+  end
+end
\ No newline at end of file