Through the Looking Glass

Entropy Sharing

Posted on October 9, 2014

For running small single-purpose applications, I tend to fire up new Digital Ocean VPSes. However, VPSes tend to have poor and disfunctional PRNGs. I needed a way to get entropy from a higher-quality PRNG to these VPSes, particularly since one runs an instance of cfssl that’s used for generating keys. Fortunately, the CryptoCape has a TPM with a HWRNG, and several other fun hardware crypto chips that can be mixed into the operating system PRNG, and this can be used to my advantage.

I ended up building a system to use the CryptoCape and a Beaglebone to distribute randomness to the VPSes, which run a server program (the “sink”) that listens for incoming connections; the BBB runs a dæmon (the “source”) that periodically (every six hours) sends a packet of random data to the VPSes. The BBB has a signing RSA key, which I plan on migrating to a TPM signature key later, and each VPS has a Curve25519 key pair to which entropy packets are encrypted.

The randomness packets are defined as

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

When the source prepares a new randomness packet, it signs the packet with its signature key, generates an ephemeral encryption keypair, and encrypts the packet to the appropriate sink. Naturally, a new packet is prepared for each sink that the source is tracking. A 16-bit unsigned integer indicating the length of the packet is written to the sink first, followed by the encrypted packet. The sink decrypts the packet and checks its signature, ensures the counter hasn’t regressed, and verifies that the timestamp is within an acceptable drift range. If all goes well, the chunk of randomness is written to the system PRNG. A packet containing 1K of random data is serialised to 1041 bytes; after being signed and encrypted, it is 1381 bytes.

The source runs a copy of the Fortuna PRNG, stirring in entropy occasionally from the system PRNG and the TPM. In the future, I’d like to add a Fortuna source that reads data from the ATSHA204 on the cape.

The first proof-of-concept version of this code is available on Github. This proof of concept works, but there are some improvements planned:

The project has a page on my homepage, and I’m keeping track of other projects as well.