Entropy sharing system for the Beaglebone Black.
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entropy distribution infrastructure

entropyshare is a system designed to provide additional, high-quality entropy to virtual servers via a Beaglebone Black on a residential connection (in fact, currently, this runs over a hotspot).

In entropyshare, sources generate packets of entropy, sending them to sinks.

Packets are defined as

packet ::= SEQUENCE {
       timestamp INTEGER        -- int64
       counter   INTEGER        -- int64
       chunk     OCTET STRING   -- [1024]byte

ASN.1 was selected because it was in the Go standard library, and it results in a packet that is significantly smaller than either JSON-encoded or gob-encoded packets (the only other serialisation formats that really made sense in the standard library encoders). Running the common package test code with the -sizes flag will print the sizes of packets:

              ASN.1 packet length: 1041
               JSON packet length: 1416
                Gob packet length: 1099
Signed and encrypted ASN.1 length: 1381
 Signed and encrypted JSON length: 1756
  Signed and encrypted gob length: 1439

Having a small packet size may not seem like a big deal, but the BBB runs on a hotspot connection whose connection speed is often measured in hundreds of bytes per second.

When a source needs to send a new packet to a sink, it generates a fresh packet, signs it with it's signature RSA private key, and encrypts it to the sink's Curve25519 public key. The sink will decrypt the packet, verify the signature, check the packet's timestamp to ensure it is within an acceptable drift range, and ensure the counter hasn't regressed.


This system requires a working Go installation.

$ go get github.com/kisom/entropyshare/...

This will install six binaries in $GOPATH/bin:

  • entropy-config
  • entropy-sink
  • entropy-source
  • entropy-target
  • rsagen
  • curve25519gen

Running a source

A source node takes two parameters on startup:

  • the signature key (see the rsagen section)
  • a JSON file containing an array of sinks, which looks like
        "Address": "vps.example.net",
        "Counter": 13,
        "Next": 1411351662,
        "Public": "MI...AB"

The "Public" field has been truncated for clarity, but each sink entry has four fields, only two of which are required for a new entry:

  • Address contains the host:port address for the sink; this is required.
  • Public contains the sink's public encryption key; this is required. This should be the base64-encoded public key.
  • Counter is the packet counter for the sink; if not provided, it will be filled in with an initial value of 0.
  • Next contains the time that the sink should be sent a new packet, stored as a Unix timestamp.

The targets file is re-read on each run, and written once the run is complete to update the counter and timestamp values.

The entropy-target command can be used to generate a new target entry.

Running a sink

A sink takes a JSON configuration file in the form:

    "Address": ":4141",
    "Counter": 14,
    "Drift": 120,
    "Private": "MI...AB",
    "Signer": "MI...AB"

The fields are:

  • Address is the address the server should listen on.
  • Counter is a 64-bit integer storing the current counter value; only packets with a higher counter number than this will be accepted. When the counter rolls over, the counter will have to be manually reset (and the encryption key should be rotated, as well). If this isn't provided initially, it will be set to 0.
  • Drift stores the allowed range for the timestamp's drift, in seconds. If not provided, this is set to 0, which will require the clocks of the source and sink to be kept in precise sync.
  • Private: the base64-encoded Curve25519 private key for decryption used to decrypt incoming packets.
  • Signer: the signer's base64-encoded PKIX public key to verify the signatures on incoming packets.

The entropy-config command can be used to generate a new configuration file.


The rsagen utility is used to generate RSA keypairs. For example, to generate the signature keys:

rsagen -s 4096


The curve25519 utility is used to generate Curve25519 keypairs.

Planned improvements:

  • Set up client auth TLS and an HTTP API
  • Use TPM for signing packets