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Satellite API

The Blockstream Satellite API provides developers with an easy-to-use RESTful API that can be used to broadcast messages globally using the Blockstream Satellite network.

As illustrated in the diagram below, the process starts with a sender application, which requests the transmission of a particular file or text message. This transmission order gets queued up in the API server. Once the order is paid over the Bitcoin Lightning Network, the API server sends the message to the Blockstream Satellite Teleport (ground station). From there, the message is broadcast globally through the Blockstream Satellite network.

Blockstream Satellite API architecture

The blocksat-cli command-line interface (CLI) provides a range of commands for the interaction with the Satellite API. This guide clarifies these commands.

To install the CLI, please refer to the installation instructions.

The API support on the CLI is available starting from version 0.3.0. Please refer to instructions regarding how to check and upgrade your CLI version.

For details regarding the RESTful API, please refer to the Satellite API’s repository.

Table of Contents

Encryption Keys

To start, we recommend setting up a GPG key pair for the encryption of messages sent through the satellite API. To do so, run:

blocksat-cli api cfg

After filling the requested information, this command will generate the key pair (public and private keys) and create a new keyring, by default, located at the ~/.blocksat/ directory.

Satellite API Transmission

To send a text message over the Blockstream Satellite network, run:

blocksat-cli api send

Alternatively, you can send a file by running:

blocksat-cli api send -f [file]

where [file] should be replaced with the path to the file.

The application asks for the bid in millisatoshis (msats) and suggests the minimum acceptable bid. To accept the suggested bid, simply press enter and continue. Otherwise, if you prefer to fill in the bid manually, make sure to satisfy the two requirements below:

  1. The total bid must be greater than at least 1000 msats.
  2. The ratio between the bid in msats and the number of bytes used for transmission (the so-called bid/byte ratio) must be at least 1 msat/byte.

After confirming the bid, get the Lightning Invoice Number printed on the console or the QR code and pay it using Bitcoin Lightning (refer to the list of Lightning wallet apps). Once the payment is confirmed, the transmission will start as soon as the transmission queue serves your message.

By default, the above commands encrypt your message or file using the GPG key you set up in the beginning. With that, only you (the owner of the private key) can decrypt the message on reception (see GPG’s manual for further information). Other users listening for messages broadcast over the Blockstream Satellite network will still receive your encrypted message. However, they will not be able to decrypt it.

Choosing the Recipient

You can also define a specific recipient for your transmission. To do so, use argument -r, as follows:

blocksat-cli api send -r [fingerprint]

where [fingerprint] is the public key fingerprint corresponding to the target recipient. In other words, the fingerprint defines who is going to be able to decrypt the message.

In case you want to skip the validation of the recipient’s public key and assume it is fully trusted, you can use argument --trust. For example:

blocksat-cli api send -r [fingerprint] --trust

These commands assume that the recipient’s public key is available on the local GPG keyring. Thus, you need to import the public key into the keyring located at ~/.blocksat/.gnupg. Assuming the recipient has shared its public key with you on a file named recipient.gpg, you can do so using:

gpg --import recipent.gpg --homedir $HOME/.blocksat/.gnupg

Signing the Messages

You can also digitally sign a message so that the recipient can verify it was really generated by you and not altered in any way. To do so, use argument --sign. For example:

blocksat-cli api send --sign

By default, messages will be signed using the default private key from your keyring, but you can specify the key as follows:

blocksat-cli api send --sign --sign-key [fingerprint]

where [fingerprint] is the fingerprint of the private key you wish to use for signing.

Satellite API Reception

To receive messages sent over satellite through the Satellite API, first of all, you need to have your satellite receiver running. If you do not have a real receiver, you can experiment with the demo receiver explained in the sequel.

The satellite receiver continuously receives Bitcoin data and API messages broadcast over satellite. To listen for the API messages, run:

blocksat-cli api listen

Once an API message is received, this application first tries to decrypt it. If it succeeds, it then validates the integrity of the data and saves the final result into the download directory (by default at ~/.blocksat/api/downloads/).

Note that the incoming data stream multiplexes transmissions from all users of the Satellite API. Hence, the application is expected to fail decryption in most cases except when it finds a message it is supposed to receive.

Also, by default, this app saves both files and text messages into the download directory. You can also configure it to print a received (successfully decrypted) text message to the console by running with:

blocksat-cli api listen --echo

Choosing the Sender

By default, the listener application processes any file that is successfully decrypted. In other words, it processes any message encrypted using your public key.

You can also filter the messages by the sender, as follows:

blocksat-cli api listen --sender [fingerprint]

where [fingerprint] is the public key fingerprint corresponding to the target sender.

In this case, the listener application only processes the messages that are digitally signed by the specific sender of interest.

Demo Receiver

The demo receiver application imitates the output of a real Blockstream Satellite receiver. It outputs UDP packets containing the API messages, just like the regular receiver does. The difference is that it fetches these messages directly through the internet, rather than receiving via satellite.

You can run the demo receiver with:

blocksat-cli api demo-rx

Then, on another terminal session, you can listen for API messages coming from the demo receiver by running:

blocksat-cli api listen --demo

NOTE: the demo receiver sends packets with API data to the loopback interface. When the listener application is launched with option --demo, it will correspondingly listen to the loopback interface.

Further Information

Lightning Wallets

Here are some options of Lightning Wallets that can be used to pay for API transmissions:

You can also set up a Lightning node and use the lightning-cli to handle payments.

Plaintext Mode

In addition to encrypted messages, you can also send and receive plaintext messages, i.e., unencrypted messages. In this case, all users listening to Satellite API messages will be able to see your message. Correspondingly, you can receive all messages sent by other users in plaintext format.

To send a plaintext message, run:

blocksat-cli api send --plaintext

To receive plaintext messages, run:

blocksat-cli api listen --plaintext

In this case, please be aware that all API transmissions will be saved to the download directory. In contrast, in normal mode (with encryption), the listener application only saves the messages addressed to you (i.e., the messages you can decrypt).

Alternatively, you can filter plaintext messages by the sender, using the --sender option explained earlier. In this case, the application retains only the clearsigned messages sent by the specific sender of choice.

Receiving Messages Sent from the Browser

If you want to receive a file uploaded directly on the Satellite Queue page, from the browser, run the API listener application as follows:

blocksat-cli api listen --plaintext --save-raw

The rationale is that files uploaded to the Satellite Queue are sent in plaintext (i.e., without encryption). Furthermore, the browser transmission tool does not encapsulate the data in the same way as the CLI sender tool (i.e., command blocksat-cli api send). Argument --save-raw accounts for this missing encapsulation.

The CLI can also reproduce the message transmission format used by the Satellite Queue tool on the browser. To do so, run the sender application as follows:

blocksat-cli api send --plaintext --send-raw

Reliable Transmissions

The API messages sent over satellite are not guaranteed to be received by all satellite receivers. Each receiver experiences a unique reception quality, depending primarily on its location, weather conditions, and the adopted receiver hardware. When the signal quality is low, it becomes more likely for the receiver to fail on the reception of packets.

One way to increase the chances of successful reception is to use forward error correction (FEC). In essence, FEC adds redundancy to the transmit data so that receivers can recover the original message even if some parts are missing. This is the mechanism that is used, for instance, in Bitcoin Satellite (see further details in the project’s Wiki).

To send an API message using FEC encoding, run:

blocksat-cli api send --fec

The api listen command detects and decodes FEC-encoded messages automatically.

In general, the higher the number of extra (redundant) pieces of data sent over satellite, the better the protection to data loss over the satellite link. The user can tune this parameter using the command-line argument --fec-overhead. By default, this argument is 0.1 (equivalent to 10%), such that, for a message that originally occupies 10 packets, the application sends one extra packet.

Transmission over Selected Regions

By default, API messages are broadcast worldwide through the six satellite beams currently composing the Blockstream Satellite network. However, you can also explicitly select the regions for each transmission.

For example, to send over regions 0 (G18) and 1 (E113) only, call the sender application as follows:

blocksat-cli api send --regions 0 1

The regions are numbered from 0 to 5 according to the following mapping:

Region Satellite Beam
0 Galaxy 18 (G18)
1 Eutelsat 113 (E113)
2 Telstar 11N Africa (T11N AFR)
3 Telstar 11N Europe (T11N EU)
4 Telstar 18V C band (T18V C)
5 Telstar 18V Ku band (T18V Ku)

Running on Testnet

The API commands described thus far interact with the API server that handles live broadcasting via the Blockstream Satellite network. This server operates on Bitcoin’s Mainnet network and, therefore, the payment requires actual bitcoins.

Nevertheless, there is an alternative API server that operates on the Bitcoin Testnet network. You can interact with this Testnet server using argument --net test. For example:

blocksat-cli api --net test send

However, note 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. Thus, you need to use the demo receiver to receive API messages sent through the Testnet server.

In this case, run the demo receiver as follows:

blocksat-cli api --net test demo-rx

Bump and Delete API orders

When users send messages to the Satellite API, these messages first go into the Satellite Queue. From there, the satellite transmitter serves the transmission orders with the highest bid (per byte) first.

If your message is still waiting for transmission due to other messages with a higher bid per byte, you can bump your bid by running:

blocksat-cli api bump

You can also delete (cancel) a transmission order by running:

blocksat-cli api del

Both of these commands will ask for the UUID and the authorization token of the order. These were printed to the console when the message was first sent.

Password-protected GPG keyring

By default, it is assumed that the private key in your keyring is password-protected. In case you want to use a private key that is not password-protected for signing a transmission or decrypting incoming messages, run the apps with option --no-password, as follows:

blocksat-cli api send --sign --no-password
blocksat-cli api listen --no-password

Automating Lightning Payments

The command used to send API messages (blocksat-cli api send) is by default interactive. It prompts for the bid and prints the Lightning invoice number. The user, in turn, has to choose the bid and pay the invoice manually.

Nevertheless, there are options to automate the payment. You can specify the bid using the command-line argument --bid. Additionally, you can run an arbitrary command with the invoice number. For example, you can use the lightning-cli to pay the invoice automatically as follows:

blocksat-cli api send -f [file] --bid [bid] \
    --invoice-exec "lightning-cli pay {}"

This command will send the transmission order to the server directly with the given bid. Subsequently, it will execute the lightning-cli pay command while substituting {} with the invoice number (i.e., the bolt11 payment request string).

Executing Commands with Received Files

The API data listener application provides the command-line option --exec, which configures an arbitrary command to be executed for each file received through the satellite API stream. For example, the option can be used as follows:

blocksat-cli api listen --exec 'cat {}' --sender [fingerprint]

where [fingerprint] should be the public key fingerprint corresponding to the target sender.

In this case, the application will execute the given command (cat) for each downloaded file while substituting {} with the path to the file in the download directory. In other words, this example will print the contents of every incoming file on the standard output (similar to option --echo).

Due to security reasons, this option requires two safety layers:

  • Encryption: it only runs the specified command for successfully decrypted messages.

  • Digital signature: it requires digitally signed messages signed by a specific sender of choice and verified.

The encryption requirement guarantees that the --exec command only gets executed for files explicitly addressed to your node. In other words, the command applies to messages sent by someone who has your public key and encrypted specifically to your node as the recipient. Meanwhile, the digital signature guarantees that the sender is a selected (trustworthy) source and not an unintended one (e.g., a malicious user who has your public key).

If you don’t want to specify the sender, you can still run the command with option --insecure, as follows:

blocksat-cli api listen --exec 'cat {}' --insecure

In this case, make sure to avoid unsafe commands and use at your own risk.

Satellite API Channels

The Satellite API messages are sent over satellite through multiple channels. Each channel is identified by a corresponding number. For example, channel 1 is the default channel used for user transmissions. Meanwhile, there are other active channels for applications described in the sequel.

When receiving API messages via satellite, you can tune to a specific channel. To do so, use argument --channel on the api listen command. For example, to listen to messages coming through channel 4 instead of the default channel 1, run the listener app as follows:

blocksat-cli api listen --channel 4

In most cases, however, it is not necessary to specify the channel number. The application configures the appropriate channel automatically based on other command-line arguments. For example, this is the case with the Lightning Gossip and Bitcoin source code options described next.

Lightning Gossip Snapshots

The satellite API has a channel dedicated to Lightning gossip messages, namely messages carrying snapshots of the gossip synchronization mechanism available for the Lightning Network. To receive such messages, simply run the listener application using argument --gossip, as follows:

blocksat-cli api listen --gossip

When this argument is specified, the listener application tunes to the appropriate channel for gossip messages (channel 4). Furthermore, it automatically applies other required configurations to receive the gossip messages. For example, it automatically invokes the historian-cli tool in order to load gossip snapshots downloaded via satellite.

Bitcoin Source Code Messages

The satellite API also has a channel dedicated to messages carrying the Bitcoin Satellite and Bitcoin Core source codes. To receive such messages, run the listener application using argument --btc-src, as follows:

blocksat-cli api listen --btc-src

This argument configures the listener application to tune to channel 5 and applies other required configurations.