Feature request: allow decryption on low memory systems

[coderb]

Debian release version 1.16 seems to error out if there is not enough memory available to decrypt a file. From what I understand, it should be possible to do the decryption, albeit slower.

[cperciva]

It is theoretically possible to use less memory and more CPU time, but I haven't written the code necessary to implement this. If anyone is interested in working on this, please contact me to discuss how this would be done before you start writing code.

[coderb]

I thought I needed this, but I was able to work around it. So not a high priority for me.

[DavidBurkett]

I'd be interested in giving this a shot @cperciva. Let me know whatever I should be aware of before I get started. Email is on my github profile, if you prefer that.

[cperciva]

@DavidBurkett Great! Not too much to be aware of; in step 3 of smix you want to record some of the X values into V[] rather than all, and in step 8 you want to look up the closest preceding V[] value and then iterate blockmix_salsa8 on it some number of times.

I'd suggest making this an explicit command-line option to the scrypt utility rather than turning it on automatically based on the available memory -- the available-memory estimator isn't very accurate so there will likely be times when people want to use -f rather than --low-memory.