### Flavio Poletti

Irreducible Perler.

# Telegram Keyboard Button Encoding

Telegram custom keyboards are a handy feature to provide a cleaner interface to your users. But unlike HTML, these buttons are doomed to send their text instead of a configurable value. Or are they?

## What’s The Problem, Exactly?

The buttons in a keyboard for Telegram have only one mandatory field text that has the following description (as of March 1st, 2017 at least):

Text of the button. If none of the optional fields are used, it will be sent to the bot as a message when the button is pressed.

The optional fields, if you’re curious, are available only to send either a location or a phone number, not some generic, programmer-defined value.

So, the bottom line is that, in the general case, a button is stuck to sending whatever text it also displays. As anticipated in the introductory teaser, this is unlike HTML buttons, where you can set independently the text that is shown on the button and the value that is associated to the button itself (which, in most cases, is also sent to the server).

## Why Is That A Problem, Exactly?

Suppose you want to implement a keyboard to track two different variables, Happyness and Relax. You want a button for showing their current value, and some buttons to increase those values. It might be something like this:

+-----------+-----------+-----------+-----------+
| Happyness |    +1     |    +2     |    +3     |
+-----------+-----------+-----------+-----------+
|   Relax   |    +1     |    +2     |    +3     |
+-----------+-----------+-----------+-----------+

Do you see the problem? There are two versions of button +1, +2 and +3, which means that you will not be able to figure out which exact button the user pressed, because in both +1 cases you would just be getting… +1.

## Unicode Zero-Width Rescuers

It turns out that Telegram keyboard buttons actually support Unicode strings as texts, and most clients seem to display them correctly. So… can we ab*COUGH*use this? Sure!

Unicode has a few characters that you can put there but that will not appear in the text. Some of them are:

• U+200B, a.k.a. ZERO WIDTH SPACE
• U+200C, a.k.a. ZERO WIDTH NON-JOINER
• U+200D, a.k.a. ZERO WIDTH JOINER

The ZERO WIDTH property is what interests us here: printing a zero-width character basically means printing… nothing! So, the following strings:

"Hello"
"Hello\x{200B}"
"\x{200D}He\x{200C}llo\x{200B}\x{200C}"

will render exactly the same in the client, but will result in very different strings sent from the client to the bot when pressed. Exactly the mechanism we were after!

## Possible Formal Uses

The suggestion, at this point, is to figure out an encoding scheme for attaching a custom (and unique) value to each button, that will allow us to understand what button was pressed even though multiple buttons expose the same visible text.

If you have only a few of them, you can for example keep a counter that increases at each new button, and append that number of zero-width characters, like this:

state $count = 0; # ...$button{text} .= "\x{200B}" x ++$count; Upon reception of anything from the client, we can just extract the number of these characters and we have our code back: my ($empties) = $text =~ m{(\x{200B}+)\z}mxs; my$button_id = length $empties; Another alternative is to turn the numeric code into a binary sequence and encode it using two caracters, one for the 1 and another one for the 0: state$count = 0;
# ...
my $binary = unpack 'B32', pack 'N', ++$count;
$button{text} .= join '', map {$_ ? "\x{200C}" : "\x{200D}" }
split //, \$binary;

You get the idea. Once you successfully attach an integer value to the button, you can keep a translation table on the bot side to associate some “intended value” for the button, e.g.:

my %value_for = (
1 => '/command-one',
2 => '/command-one with-arg',
3 => '/stop',
# ...
);

Hence, you can follow this workflow:

• decide the text and the value for the button
• associate a unique integer code to this pair
• generate the encoded text from text and unique integer, using one of the techniques shown above
• set the text in the keyboard button to the encoded text - it will render just like text in the client
• when the button is pressed in the client, you will receive the encoded text back
• get the unique integer back from the encoded text
• use the unique integer to find the associated text/value pair
• use the value from the pair

and it will be like you put text in the button… but actually received value.

## Perl Anyone?

The technique above has been used to create the new class Bot::ChatBots::Telegram::Keyboard, if you are interested you are more than welcome to try and use it!

The binary encoding explained in the previous section has been slightly changed and extended to cope with:

• introducing a keyboard identifier, so that you can manage multiple keyboards and be able to figure out which keyboard was used in association to a received command;
• optimizing the space removing leading 0 characters.

For each button, you can define both the text that you want to be shown, and the _value that you want to get back. The field name starts with an underscore to cope for possible future extensions of the Telegram API where they might introduce a real value field… making this blog post so obsolete!

The introduction of the keyboard identifier and the space optimization rely on the usage of all three characters introduced before:

• U+200B represents the 1
• U+200C represents the 0
• U+200D is used for marking the boundaries of the codes

## Wrap-up

Short article this time, but hopefully useful!

We saw that Telegram custom keyboards can be annoying in their overlapping of the text printed on the button and the value that is sent back to the bot when the button is pressed. We also saw that it is possible to work around this limitation thanks to a few Unicode characters that have zero width, so they don’t appear when printed but are anyway still there and are also sent by the client when the button is pressed. By means of some not-so-clever encoding we can then associated a unique identifier to each button in a keyboard, and ultimately a value of our choice.

Leave comments below if you have questions, until next time have fun!