ISAAC says it is based on RC4 (even if it is better than RC4), so AES-CTR will be more secure. Plain RC4 has been disallowed in all secure communication for good reasons. Also, RC4 is only 0.460GB/sec/core, but it shouldn't be used even if it's fast.
On my laptop, AES-128-CTR can run at 4.8 GB/sec/core (
openssl speed -evp aes-128-ctr) and ChaCha20 can run at 2.5 GB/sec/core (
openssl speed -evp chacha20). Both are random enough for any purpose, though in theory ChaCha20 is more secure.
My linux 5.0.0 kernel gives me 0.161 GB/sec/core read from
/dev/urandom (reading 1MB at a time), even though it uses ChaCha. Investigation below.
Disks have block remapping and SSD disks have very complicated garbage collection, overwriting the user-visible blocks might not be enough. So you should try to use the disk's firmware's "secure erase" features. For example, some disks actually use AES encryption even if it is not enabled by the user, just to ensure data stored on the disk has about equal number of 1s and 0s. Then, clearing just the AES key erases all data (you just have to make sure it really cleared the key, which might be complicated).
I tried perf to see what linux is doing:
- 99.83% read
- 99.81% entry_SYSCALL_64_after_hwframe
- 99.81% __x64_sys_read
- 99.81% ksys_read
- 99.80% vfs_read
- 99.77% __vfs_read
- 99.26% urandom_read
- 90.55% extract_crng
- 89.44% _extract_crng
- 39.99% chacha_block
- 1.62% __check_object_size
_extract_crng is spending all its time waiting for rdrand (called arch_get_random_long).
chacha_permute, which takes less than 40% of the time, uses the slow non-vectorized version for some reason, when a vectorized version is available (and even an AVX2 version). Loading the module chacha_x86_64 didn't help. My guess was that the random char device driver is linked with the generic chacha implementation and will only use the fast chacha implementation if that is built linked into the kernel and not as a module, but after compiling a 5.3.0 kernel with vectorized chacha (
CONFIG_CRYPTO_CHACHA20_X86_64=y) no dice - urandom is still using slow chacha. I don't know why.