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Example Applications for Data Transmission via API

This directory contains example scripts for sending and receiving data through the Blockstream Satellite API. The first example illustrates how a user can create its own user/application-specific protocol for sending data through the API. In particular, it illustrates a case in which data is transmitted alongside user-defined metadata fields. The second example, in turn, illustrates the transmission of a file to the Satellite API server. Lastly, the third example illustrates how one can simulate the output of the Blockstream Satellite receiver while fetching data directly from the Satellite API via the Internet, rather than receiving data via the satellite link.

Blockstream Satellite API Architecture



The first step in order to use the examples that follow is to prepare the environment.

For Python, create a virtual environment with the packages listed in the requirements.txt file of this directory. For example, if using virtualenvwrapper, run the following:

mkvirtualenv --python=`which python2` -r requirements.txt blocksat-api

Note this virtual environment will be required for all example scripts described in this page. Hence, once you open a new terminal session in order to launch one of the example applications, ensure to activate the environment again. For example, assuming you are using virtualenvwrapper, run the following on every new terminal session:

workon blocksat-api

NOTE: for a quick introduction to virtualenvwrapper visit their introduction page. Also, after installing virtualenvwrapper, make sure to follow the shell startup instructions on their documentation.

Next, generate a key pair for encryption (prior to transmission) and decryption (on reception). You can do so by running the helper script below:


Note that by default this will create the .gnupg directory in the local (examples) directory. This directory is where the GPG public and secret keyrings are stored.

If you could not use the above helper script successfully, or if you prefer, you can also generate keys on a local .gnupg directory with:

mkdir .gnupg
gpg --full-generate-key --homedir .gnupg

NOTE: you may need to use an absolute path for the --homedir argument above.

Alternatively, you can use your own pre-existing GPG keys.

Example 1: Sending data in a user-defined protocol

This example uses two scripts of the examples directory:

  1. API data sender
  2. API data reader

The API data sender by default places a user-specified file into a data structure and then sends the entire structure to the API. The structure carries the file name as a string and also contains a CRC32 checksum that can be used for data integrity check on the receiver side. The entire structure is first encrypted using GnuPG and then posted via HTTPS to the Blockstream Satellite API.

Meanwhile, the API data reader application waits for data written by the Blockstream Satellite receiver into the pipe file at /tmp/blocksat/api. It continuously reads this named pipe, decrypts the incoming data, validates the integrity of the data and then saves the unpacked files. The integrity validation is done by computing the CRC32 checksum of the received data and comparing it with the checksum that is advertised on the header of the incoming data structure. Ultimately, the incoming file is saved in the downloads/ folder with the name that is given in the header.

In order to run the example, first ensure that the Blocksat receiver (or the demo receiver of Example 3) is running. Next, ensure that you are using the correct Python virtual environment. If using virtualenvwrapper, run:

workon blocksat-api

Then, launch the API data reader as follows:


The reader will wait for data to appear in the API named pipe (at /tmp/blocksat/api).

Next, send some data. On another terminal session, activate the Python virtual environment once again (e.g. with workon blocksat-api). Then, post a file of your choice for transmission via Blockstream Satellite:

./ -f filename

where filename is the path to the file you want to send.

Note: the script will encrypt the data structure prior to posting to the API, so there is no need to encrypt the data before calling it.

Subsequently, get the Lightning Invoice Number that was printed by the API data sender on the console and pay.

By default, the above command sends the transmission request to the API server that handles live broadcasting via the Blockstream Satellite network. This server operates in Mainnet and thus the payment requires actual bitcoins. However, for developers/testers/experimenters, there is an alternative API server that operates in Testnet. You can send the transmission request to the Testnet server, using:

./ --net test -f filename

Do note however that the Testnet server does not transmit data through the satellite network. It only broadcasts the data to clients that are connected directly to the server through the Internet. Hence, the way to receive Testnet data is with the demo receiver of Example 3).

Once the API server effectively transmits your data, the data is expected to pop at the API data reader application. In the end, the received file will be saved in the downloads folder.

For further understanding, you can compare the received file with the one that was sent. For example, by running md5sum on both files. Also, you can test what happens when trying to decrypt with wrong keys. For example, generate another key pair by creating a second GnuPG home directory:

./ --gnupghome .gnupg-alt

Then, send the file using one GnuPG home and try to decrypt using another. For example, send with the alternative GnuPG home:

./ -f filename --gnupghome .gnupg-alt

Assuming the API data reader is still running with the default GnuPG home directory, you should expect decryption to fail in this case.

Note that, in practice, data written by the Blocksat Receiver in the API named pipe (at /tmp/blocksat/api) multiplexes transmissions from all users of the Satellite API. Hence, the application is expected to fail decryption several times until it finds the data for which it is actually a recipient of.

Example 2: Sending files directly

In this example, the goal is to send a file directly to the API, without placing it on any user-specific protocol.

The same two scripts of Example 1 are used, except for different command-line arguments:

  1. API data sender
  2. API data reader

In this case, launch the API data reader as follows:

./ --save-raw

Next, send some data using:

./ -f filename --send-raw

Note that the --send-raw flag means that the data is sent as it is, without any additional protocol framing.

Again, to use the Testnet server instead, run:

./ --net test -f filename --send-raw

Once the Blocksat receiver outputs your data into the API output pipe, the reader will receive this data and retrieve the file.

Note that, just like in Example 1, this example also handles encryption internally. That is, you can point to a non-encrypted file and the data sender will encrypt it internally. The reader will then decrypt the data.

This use case is also useful when sending a file directly via the form in the API web page. In this case, the file can be retrieved on the Blocksat receiver side by running the API data reader as above. The only difference is that in this case you will need to encrypt the file offline, before uploading to the form in the API web page. This is because the API data reader application by default assumes the incoming data is encrypted with the keys that are available in the local GnupG home directory.

To encrypt a file offline, you can run for example:

gpg --encrypt --recipient pub_key_id_or_email filename

where pub_key_id_or_email can be either the public key ID of the target recipient or its e-mail.

Alternatively, you can run the API data reader in "plaintext mode". This will allow you to receive plaintext files uploaded directly via the API website, or any other plaintext transmission broadcast via the satellite network. However, please be aware that in this case all API transmissions will be saved to the downloads folder, rather than solely the ones that can be decrypted with the GPG keys you possess. To run in this mode, execute:

./ --plaintext

Example 3: Testing the API while receiving data directly via Internet

This example illustrates the scenario in which instead of receiving data with the actual Blockstream Satellite receiver (i.e. the blocksat-rx application), you fetch data directly from the API through the Internet.

Now, you will need three scripts from the examples directory:

  1. API data sender
  2. API data reader
  3. Demo receiver

You can choose to use the API data sender and API data reader either as in Example 1 or as in Example 2. The important difference here is that the API data reader will read data from a named pipe that is filled by the demo receiver, rather than the actual Blockstream Satellite receiver.

Start by activating the blocksat-api Python virtual environment. Then, run the demo receiver:


This application will continuously wait for data broadcast directly by the API over the Internet and then it will output the data to the same named pipe that the Blockstream Satellite receiver would use, namely the pipe file at /tmp/blocksat/api.

NOTE: in case you want to concurrently run both the actual blocksat-rx receiver application and the demo receiver, you will need to use another named pipe for the demo receiver. Otherwise, the two applications would try to use the same named pipe. To do so, run ./ -f pipe_file, where pipe_file is the name of the other named pipe file to be used for the demo receiver.

Next, assuming for the explanation that the approach of Example 1 is adopted, you can leave the data reader running with:


Finally, send a file with the API data sender application and wait until it pops in the data reader.

./ -f filename

If instead of sending to the Mainnet server, the goal is to send to the Testnet server, run:

./ --net test -f filename

In this case, the Demo receiver also needs to be listening to the Testnet server. That is, it needs to be launched as follows:

./ --net test

Further Information

The API data sender script also supports bumping and deletion of orders sent to the API.

For bumping, you can run:

./ --bump

To delete an order, run:

./ --delete

Both of these commands will ask for the UUID and the authorization token of the order. These were originally printed to the console by the API data sender, when the latter was used to send the data to the API.

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