About

Entropy is a simple go pkg and cmdline interface for manipulating entropy in the linux kernel.

It achieves this by using the the IOCTL interface to /dev/(u)random.

Some basic theory

The linux kernel simulates randomness by keeping a pool of data, estimating its information entropy, and generating data out of it using SHA hashes.

When information is added to the pool it gets "mixed" into the pool with a CRC-like algorithm; it doesn't actually add the raw data.

Optionally, when information is added, the entropy count of the pool can be incrememnted. This isn't a literal add, but rather an asymptotic algorithm that approaches the pool size.

Information can be added to the pool by writing to /dev/random or /dev/urandom, but this will not increment the entropy count. This package/command provide an interface to the IOCTLs that provide extended userspace functionality for manipulating randomness. Of particular note, the AddToEntCnt() function adds bits to the entropy count and the AddEntropy() function adds bytes to the pool while also incrementing entropy bits.

Command

The entropy command has the following usage information:

Usage: entropy <command> [<opts>...]

Commands:

        get(entropy)                    - get the current system entropy.
        addto(entcnt) <num>             - (superuser) Add <num> bits to the current entropy count.
                                                                          Note: this does not literally increase entropy count by <num>.  The kernel adds using an asymptotic algorithm.  
                                                                          See <drivers/char/random.c> for details.
        add(entropy) <file> [<quality>] - (superuser) Add the contents of <file> to entropy, incrementing entropy by the byte-length of the file. 
                                                                          The optional <quality> specifies the percentage of total data to count as Shannon entropy (default: 1, which is highly unlikely).
        zap(entcnt)                     - (superuser) Clear the kernel entropy count.
        clear(pool)                     - (superuser) Clear the entropy pool and counters (on modern linux, this just does zapentcnt).
        reseed(crng)                    - (superuser) Reseed the CRNG.

Package

The entropy package provides a basic wrapper for all IOCTL functions provided by the kernel.

Authors

• J. Lowell Wofford lowell@lanl.gov