Random Number Generator
The VeraCrypt random number generator (RNG) is used to generate the master encryption key, the
secondary key (XTS mode), salt, and keyfiles. It creates a pool of random values in RAM
(memory). The pool, which is 320 bytes long, is filled with data from the following sources:
Mouse movements
Keystrokes
Mac OS X and Linux: Values generated by the built-in RNG (both /dev/random and
/dev/urandom)
MS Windows only: MS Windows CryptoAPI (collected regularly at 500-ms interval)
MS Windows only: Network interface statistics (NETAPI32)
MS Windows only: Various Win32 handles, time variables, and counters (collected regularly
at 500-ms interval)
Before a value obtained from any of the above-mentioned sources is written to the pool, it is divided
into individual bytes (e.g., a 32-bit number is divided into four bytes). These bytes are then
individually written to the pool with the modulo 2
8
addition operation (not by replacing the old
values in the pool) at the position of the pool cursor. After a byte is written, the pool cursor position
is advanced by one byte. When the cursor reaches the end of the pool, its position is set to the
beginning of the pool. After every 16
th
byte written to the pool, the pool mixing function is
automatically applied to the entire pool (see below).
Pool Mixing Function
The purpose of this function is to perform diffusion [2]. Diffusion spreads the influence of
individual “raw” input bits over as much of the pool state as possible, which also hides statistical
relationships. After every 16
th
byte written to the pool, this function is applied to the entire pool.
Description of the pool mixing function:
1. Let R be the randomness pool.
2. Let H be the hash function selected by the user (SHA-512, RIPEMD-160, or Whirlpool).
3. l = byte size of the output of the hash function H (i.e., if H is RIPEMD-160, then l = 20;
if H is SHA-512, l = 64)
4. z = byte size of the randomness pool R (320 bytes)
5. q = z / l – 1 (e.g., if H is Whirlpool, then q = 4)
6. R is divided into l-byte blocks B
0
...B
q
.
For 0 i q (i.e., for each block B) the following steps are performed:
a. M = H (B
0
|| B
1
|| ... || B
q
) [i.e., the randomness pool is hashed using the hash function H,
which produces a hash M]
b. B
i
= B
i
^ M
7. R = B
0
|| B
1
|| ... || B
q
For example, if q = 1, the randomness pool would be mixed as follows:
1. (B
0
|| B
1
) = R
2. B
0
= B
0
^ H(B
0
|| B
1
)
3. B
1
= B
1
^ H(B
0
|| B
1
)
4. R = B
0
|| B
1
