fix(tlv_conversion): Convert to Characters Rather than Hex

The original implementation was converting to hex, whereas all we needed to do was to convert to characters and ensure ASCII-8BIT encoding.

ZATCA expects ASCII, the original implementation was assuming they were expecting hexadecimal versions of numbers, but the desired values are in ASCII.

This change ensures that the whole thing is forced to be ASCII-8BIT.

It was unclear from ZATCA if the `value` should be left as Unicode or not, but this presents a challenge because we would have to concatenate an ASCII string with a UTF-8 string and that's not possible.

We use `String.force_encoding` to ASCII-8BIT to get around those errors and avoid `String#encode` because it will throw an exception because you can't convert UTF-8 to ASCII-8BIT because there is no point of reference for what each character maps to.

This approach generates the same output as [ZATCA's SDK](https://zatca.gov.sa/en/E-Invoicing/SystemsDevelopers/ComplianceEnablementToolbox/Pages/DownloadSDK.aspx) does.

lib/zatca/tag.rb

@@ -22,10 +22,13 @@ def to_h
 
     def to_tlv
       # TLV should be concatenated together without any separator in the following
-      # format: hex_value_of_id hex_value_of_length value_itself
-      # where hex_values should be padded with a 0 if containing only a single digit.
+      # format: character_value_of_id character_value_of_value_length value_itself
+      # All of this should be in 8-bit ASCII.
+      tlv = @id.chr + @value.length.chr + value
 
-      sprintf("%02X%02X%s", @id, @value.length, @value)
+      # We need to use force_encoding because encode will raise errors when
+      # trying to encode a string with utf-8 characters.
+      tlv.force_encoding("ASCII-8BIT")
     end
   end
 end

spec/lib/zatca/qr_code_generator_spec.rb

@@ -3,7 +3,7 @@
 
   describe "#render" do
     it "returns a base64 data_url" do
-      expect(generator.render).to eq("data:image/png;base64,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")
+      expect(generator.render).to eq("data:image/png;base64,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")
     end
   end
 end

spec/lib/zatca/tag_spec.rb

@@ -1,5 +1,6 @@
 describe ZATCA::Tag do
   let(:tag) { ZATCA::Tag.new(key: :seller_name, value: "Mrsool") }
+  let(:unicode_tag) { ZATCA::Tag.new(key: :seller_name, value: "مرسول") }
 
   describe "#id" do
     it "gets mapped from seller_name to 1" do
@@ -31,7 +32,13 @@
 
   describe "#to_tlv" do
     it "generates a valid TLV" do
-      expect(tag.to_tlv).to eq("0106Mrsool")
+      expect(tag.to_tlv).to eq("\x01\x06Mrsool")
+    end
+
+    it "generates a valid TLV when passed unicode input" do
+      expected_output = "\x01\x05\xD9\x85\xD8\xB1\xD8\xB3\xD9\x88\xD9\x84".force_encoding("ASCII-8BIT")
+
+      expect(unicode_tag.to_tlv).to eq(expected_output)
     end
   end
 end

spec/lib/zatca/tags_spec.rb

@@ -3,7 +3,7 @@
 
   describe "#to_base64" do
     it "generates valid Base64" do
-      expect(tags.to_base64).to eq("MDEwNk1yc29vbDAyMEYzMTAyMjg4MzM0MDAwMDMwMzE5MjAyMS0xMC0yMFQxOToyOTozMiswMzowMDA0MDMxMTUwNTAyMTU=")
+      expect(tags.to_base64).to eq("AQZNcnNvb2wCDzMxMDIyODgzMzQwMDAwMwMZMjAyMS0xMC0yMFQxOToyOTozMiswMzowMAQDMTE1BQIxNQ==")
     end
   end
 end