Cryptographically strong pseudorandom data used for overwriting is created using the ISAAC (Indirection, Shift, Accumulate, Add and Count) algorithm by Bob Jenkins. The ISAAC generator is reseeded before each task using Eraser's own multi-source polling random number generator.
The random data generated using ISAAC is guaranteed to have a period length of 2^40 numbers, and the average cycle is 2^8295 32-bit (4-byte) numbers. Therefore, the average amount of data provided by the generator before the sequence starts from the beginning is 4.12e2488 gigabytes (and is at least 4096 gigabytes), which is more than enough for overwriting even the largest hard drives.
The number of overwriting passes for this method is user selectable, the maximum being ((2^16) - 1) = 65535 passes.
For more information, see the source code.
Because the random data is highly incompressible, this is the only method that should be used on compressed drives.
References:
ISAAC: a fast cryptographic random number generator
Donald Ervin Knuth, The Art of Computer Programming: Seminumerical Algorithms, Volume 2, 3rdEdition
[