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Date: Fri, 4 Sep 2015 23:50:28 -0700
From: Bill Cox <>
To: "" <>
Subject: Bandwidth hardened PoW with rapid verification (now GPUHog)

I see that my previous emails never stated this algorithm clearly, so I
just wanted to restate it here.  Also, I've concluded that the defender is
better off using a GPU to do the mining, since it has high bandwidth and
low cost, improving ASIC defense considerably.  Thus the change from CPUHog
go GPUHog.


The crypto-currency LiteCoin became popular after BitCoin switched to ASIC
mining.  LiteCoin democratised mining, enabling anyone with a graphics card
to participate, while BitCoin mining has become more and more centralized.

The "Proof of Work" used in LiteCoin is a memory-hard hashing algorithm
(Scrypt) which is more difficult to speed up using custom ASICs.
Unfortunately, it also makes verifying the block chain much more compute
intensive. Because of this, LiteCoin  uses only 128 KiB of memory, trading
off block-chain verification speed against ASIC resistance. This memory
size is small enough that ASICs are already being used to take over
LiteCoin mining.  Like BitCoin, LiteCoin mining is becoming centralized.

So, the idea here is to bring back the fun days, when GPU mining ruled.
The ideal PoW would be a fast memory-hard hashing algorithm which can
somehow be verified more rapidly than by simply rerunning the entire
hashing algorithm.

GPUHog Algorithm:

First, fill a large memory  on a GPU with say 1 GiB, with many "large
blocks", which might be 1 MiB each.  Generate the data in each large block
using a fast memory hard hashing function. TwoCats is the fastest I know
of, and would be the best choice, IMO.  Large blocks are generated in
counter mode so that each large block can be generated independently of
each other, from their index.  This memory hashing needs to be done in a
GPU friendly manner to defend well against ASICs (meaning no small random
memory access - this is already an option in TwoCats).

Once memory has been initialized with this large ROM:

- Start with the hash of a new "crypto-currency block" which we want to
"solve".  Call this hash M.
- Iterate many times, choosing a new nonce N each time, trying to solve the
- Pick two pseudo-random "small blocks", of say 1KiB each, using a hash
function H, such as SHA-256.
- Address a1 = H(N || M) mod l, and a2 = H(a1) mod l, where l is the number
of small blocks in memory.
- Compute a fast hash of these 2 blocks, possibly again with TwoCats.  If
this is < difficulty, then the block is solved.

Low memory verification can be done using only 2 MiB, by generating the two
large blocks containing the two small blocks.  This is basically 2 1MiB
TwoCats hashes.  It can be done in a small fraction of a millisecond on a
desktop computer.  This should be OK if done every 10-ish minutes by a

For mining systems that have all the large block data in GPU memory, they
only do the simple hash of 2 KiB per attempt to solve the block.

Comparison to LiteCoin's use of 128KiB Scrypt hashes:

- ASIC attack on LiteCoin can put 32 hashing cores on an ASIC with 4MiB of
RAM, each running about 16X faster for a total of 512X speedup per ASIC,
assuming the ASIC is made in the same process as the defenders CPU.
- ASIC attack on GPUHog uses external DRAM for the 1 GiB hash data.  The
ASIC speedup will be equal to its improvement in memory bandwidth.
However, GPUs already have cost efficient memory bandwidth.  The ASIC might
be lucky to get a 2X cost improvement.

In short, LiteCoin is totally PWN-able by ASIC mining because of it's low
memory size.  With GPUHog, the advantage of an ASIC should be reduced from
something like 512X to something like 2X.

In short, we can increase memory from 128 KiB to 1 GiB, while reducing the
hashing time for full block-chain verification from a 128 KiB slow hash to
an 8 KiB fast hash.  ASIC defense is increase something like 256X.

The ASIC defense of GPUHog should be far higher than Momentum as well.
Momentum is heavily RAM latency limited on GPUs, while an ASIC attack can
mount a parallel radix sort attack, eliminating the latency problem.  The
best GPU algorithms for Momentum still seem 100X-ish worse in speed than a
custom ASIC could do.

GPUHash seems like the best choice I've run across for a LiteCoin successor.

BTW, this is not a memory-hard PoW.  It uses a memory-hard hashing
primitive, but can be attacked using a distributed ROM attack.  For each
hash computation, a small block would have to be transmitted from one node
in the network to another, causing a memory bandwidth bottleneck.  Thus,
GPUHog is "bandwidth hard", not "memory hard"

Did I finally describe this well this time?  :)


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