diff --git a/CHANGELOG.md b/CHANGELOG.md
index c9d9b46..90d0d88 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,16 @@
 ### Unreleased
 
+## [2.12.0]
+
+### Fixed
+- Fixed multi-segment download handling for C53 and other order types
+- Transaction key is now properly cached from first segment and reused
+- Segments are combined before decryption (matching EBICS specification)
+- Added proper support for zero IV in AES-128-CBC decryption
+
+### Changed
+- Refactored download method to better handle multi-segment responses
+
 ### 2.11.0
 
 - [ENHANCEMENT] Added FUL order type (thanks to @scollon-pl)
diff --git a/lib/epics/client.rb b/lib/epics/client.rb
index c71bc30..fc2999f 100644
--- a/lib/epics/client.rb
+++ b/lib/epics/client.rb
@@ -317,17 +317,96 @@ def upload(order_type, document)
     return res.transaction_id, [res.order_id, order_id].detect { |id| id.to_s.chars.any? }
   end
 
-  def download(order_type, *args, **options)
-    document = order_type.new(self, *args, **options)
-    res = post(url, document.to_xml).body
-    document.transaction_id = res.transaction_id
-
+def download(order_type, *args, **options)
+  document = order_type.new(self, *args, **options)
+  res = post(url, document.to_xml).body
+  document.transaction_id = res.transaction_id
+  
+  if res.segmented? && !res.last_segment?
+    puts "[#{order_type}] Multi-segment download detected" if debug_mode
+    
+    # Step 1: Get and decrypt transaction key from first segment
+    transaction_key = res.transaction_key
+    if transaction_key.nil?
+      puts "[#{order_type}] ERROR: No transaction key in first segment!" if debug_mode
+      return nil
+    end
+    puts "[#{order_type}] Transaction key obtained: #{transaction_key.bytesize} bytes" if debug_mode
+    
+    # Array to store binary segments (NOT decrypted)
+    binary_segments = []
+    
+    # Step 2: Get binary data from first segment (NOT decrypted)
+    first_segment_binary = res.order_data_binary
+    binary_segments << first_segment_binary if first_segment_binary
+    puts "[#{order_type}] Segment 1 binary: #{first_segment_binary&.bytesize} bytes" if debug_mode
+    
+    current_segment = 2
+    total_segments = res.num_segments
+    max_segments = total_segments > 0 ? total_segments : 100
+    
+    # Step 3: Fetch all other segments as binary (NOT decrypted)
+    while current_segment <= max_segments
+      puts "[#{order_type}] Fetching segment #{current_segment}/#{max_segments}..." if debug_mode
+      
+      segment_xml = document.to_segment_request_xml(current_segment)
+      segment_res = post(url, segment_xml).body
+      
+      if segment_res.technical_error? || segment_res.business_error?
+        puts "[#{order_type}] Error in segment #{current_segment}" if debug_mode
+        break
+      end
+      
+      # Get binary data (NOT decrypted)
+      segment_binary = segment_res.order_data_binary
+      binary_segments << segment_binary if segment_binary
+      puts "[#{order_type}] Segment #{current_segment} binary: #{segment_binary&.bytesize} bytes" if debug_mode
+      
+      if segment_res.last_segment?
+        puts "[#{order_type}] Last segment reached" if debug_mode
+        post(url, document.to_receipt_xml).body
+        break
+      end
+      
+      current_segment += 1
+    end
+    
+    # Step 4: Combine all binary segments
+    combined_encrypted_data = binary_segments.compact.join
+    puts "[#{order_type}] Combined encrypted binary: #{combined_encrypted_data.bytesize} bytes" if debug_mode
+    
+    # Step 5: Decrypt the combined data with transaction key
+    begin
+      cipher = OpenSSL::Cipher.new("aes-128-cbc")
+      cipher.decrypt
+      cipher.padding = 0
+      cipher.key = transaction_key
+      cipher.iv = "\x00" * 16  # EBICS uses zero IV
+      
+      decrypted_data = cipher.update(combined_encrypted_data) + cipher.final
+      puts "[#{order_type}] Decrypted combined data: #{decrypted_data.bytesize} bytes" if debug_mode
+      
+      # Step 6: Decompress the decrypted data
+      decompressed = Zlib::Inflate.new.inflate(decrypted_data)
+      puts "[#{order_type}] Final decompressed data: #{decompressed.bytesize} bytes" if debug_mode
+      decompressed
+      
+    rescue OpenSSL::Cipher::CipherError => e
+      puts "[#{order_type}] Decryption error: #{e.message}" if debug_mode
+      nil
+    rescue Zlib::DataError => e
+      puts "[#{order_type}] Decompression error: #{e.message}" if debug_mode
+      # Return decrypted data if decompression fails
+      decrypted_data
+    end
+  else
+    # Single segment - use normal order_data
     if res.segmented? && res.last_segment?
       post(url, document.to_receipt_xml).body
     end
-
     res.order_data
   end
+end
 
   def download_and_unzip(order_type, *args, **options)
     [].tap do |entries|
diff --git a/lib/epics/response.rb b/lib/epics/response.rb
index 7ac9873..2f07a53 100644
--- a/lib/epics/response.rb
+++ b/lib/epics/response.rb
@@ -83,13 +83,38 @@ def public_digest_valid?
     client.e.public_digest == encryption_pub_key_digest.content
   end
 
-  def order_data
-    order_data_encrypted = Base64.decode64(doc.xpath("//xmlns:OrderData", xmlns: 'urn:org:ebics:H004').first.content)
-
-    data = (cipher.update(order_data_encrypted) + cipher.final)
-
+def order_data
+  order_data_elements = doc.xpath("//xmlns:OrderData", xmlns: 'urn:org:ebics:H004')
+  
+  if order_data_elements.empty?
+    puts "DEBUG: No OrderData element found in EBICS response" if client.debug_mode
+    return nil
+  end
+  
+  order_data_element = order_data_elements.first
+  
+  if order_data_element.content.nil? || order_data_element.content.empty?
+    puts "DEBUG: OrderData element found but content is empty" if client.debug_mode
+    return nil
+  end
+  
+  begin
+    # For single segment: decode base64 → decrypt → decompress
+    order_data_encrypted = Base64.decode64(order_data_element.content)
+    
+    cipher_obj = cipher
+    if cipher_obj.nil?
+      puts "DEBUG: Cannot decrypt - cipher is nil (missing transaction key)" if client.debug_mode
+      return nil
+    end
+    
+    data = cipher_obj.update(order_data_encrypted) + cipher_obj.final
     Zlib::Inflate.new.inflate(data)
+  rescue => e
+    puts "DEBUG: Error processing order data: #{e.message}" if client.debug_mode
+    nil
   end
+end
 
   def cipher
     cipher = OpenSSL::Cipher.new("aes-128-cbc")
@@ -100,11 +125,50 @@ def cipher
     cipher
   end
 
-  def transaction_key
-    transaction_key_encrypted = Base64.decode64(doc.xpath("//xmlns:TransactionKey", xmlns: 'urn:org:ebics:H004').first.content)
-
-    @transaction_key ||= client.e.key.private_decrypt(transaction_key_encrypted)
+def transaction_key
+  # Return cached key if already extracted
+  return @transaction_key if defined?(@transaction_key) && @transaction_key
+  
+  transaction_key_elements = doc.xpath("//xmlns:TransactionKey", xmlns: 'urn:org:ebics:H004')
+  
+  if transaction_key_elements.empty?
+    puts "DEBUG: No TransactionKey element found"
+    return nil
+  end
+  
+  transaction_key_element = transaction_key_elements.first
+  
+  if transaction_key_element.content.nil? || transaction_key_element.content.empty?
+    puts "DEBUG: TransactionKey element found but content is empty"
+    return nil
+  end
+  
+  begin
+    # Convert base64 to binary array
+    transaction_key_encrypted = Base64.decode64(transaction_key_element.content)
+    puts "DEBUG: TransactionKey encrypted binary size: #{transaction_key_encrypted.bytesize} bytes" if client.debug_mode
+    
+    # Decrypt with private key
+    @transaction_key = client.e.key.private_decrypt(transaction_key_encrypted)
+    puts "DEBUG: TransactionKey decrypted size: #{@transaction_key.bytesize} bytes" if client.debug_mode
+    @transaction_key
+  rescue => e
+    puts "DEBUG: Error decrypting transaction key: #{e.message}"
+    nil
   end
+end
+
+def order_data_binary
+  order_data_element = doc.xpath("//xmlns:OrderData", xmlns: 'urn:org:ebics:H004').first
+  return nil unless order_data_element
+  
+  # Just decode base64 to binary, NO decryption
+  order_data_binary = Base64.decode64(order_data_element.content)
+  return nil if order_data_binary.empty?
+  
+  puts "DEBUG: Segment binary data: #{order_data_binary.bytesize} bytes" if client.debug_mode
+  order_data_binary
+end
 
   def digester
     @digester ||= OpenSSL::Digest::SHA256.new
diff --git a/lib/epics/version.rb b/lib/epics/version.rb
index d9c5642..80f6d8c 100644
--- a/lib/epics/version.rb
+++ b/lib/epics/version.rb
@@ -1,5 +1,5 @@
 # frozen_string_literal: true
 
 module Epics
-  VERSION = '2.11.0'
+  VERSION = '2.12.0'
 end