ProgPoW is a proof-of-work algorithm designed to close the efficency gap available to specialized ASICs. It utilizes almost all parts of commodity hardware (GPUs), and comes pre-tuned for the most common hardware utilized in the Ethereum network.
Ever since the first bitcoin mining ASIC was released, many new Proof of Work algorithms have been created with the intention of being “ASIC-resistant”. The goal of “ASIC-resistance” is to resist the centralization of PoW mining power such that these coins couldn’t be so easily manipulated by a few players.
This document presents an overview of the algorithm and examines what it means to be “ASIC-resistant.” Next, we compare existing PoW designs by analyzing how each algorithm executes in hardware. Finally, we present the detailed implementation by walking through the code.
The design goal of ProgPoW is to have the algorithm’s requirements match what is available on commodity GPUs: If the algorithm were to be implemented on a custom ASIC there should be little opportunity for efficiency gains compared to a commodity GPU.
The main elements of the algorithm are:
- Changes keccak_f1600 (with 64-bit words) to keccak_f800 (with 32-bit words) to reduce impact on total power
- Increases mix state.
- Adds a random sequence of math in the main loop.
- Adds reads from a small, low-latency cache that supports random addresses.
- Increases the DRAM read from 128 bytes to 256 bytes.
While a custom ASIC to implement this algorithm is still possible, the efficiency gains available are minimal. The majority of a commodity GPU is required to support the above elements. The only optimizations available are: *Remove the graphics pipeline (displays, geometry engines, texturing, etc) *Remove floating point math
These would result in minimal, roughly 1.1-1.2x, efficiency gains. This is much less than the 2x for Ethash or 50x for Cryptonight.
With the growth of large mining pools, the control of hashing power has been delegated to the top few pools to provide a steadier economic return for small miners. While some have made the argument that large centralized pools defeats the purpose of “ASIC resistance,” it’s important to note that ASIC based coins are even more centralized for several reasons.
- No natural distribution: There isn’t an economic purpose for ultra-specialized hardware outside of mining and thus no reason for most people to have it.
- No reserve group: Thus, there’s no reserve pool of hardware or reserve pool of interested parties to jump in when coin price is volatile and attractive for manipulation.
- High barrier to entry: Initial miners are those rich enough to invest capital and ecological resources on the unknown experiment a new coin may be. Thus, initial coin distribution through mining will be very limited causing centralized economic bias.
- Delegated centralization vs implementation centralization: While pool centralization is delegated, hardware monoculture is not: only the limiter buyers of this hardware can participate so there isn’t even the possibility of divesting control on short notice.
- No obvious decentralization of control even with decentralized mining: Once large custom ASIC makers get into the game, designing back-doored hardware is trivial. ASIC makers have no incentive to be transparent or fair in market participation.
While the goal of “ASIC resistance” is valuable, the entire concept of “ASIC resistance” is a bit of a fallacy. CPUs and GPUs are themselves ASICs. Any algorithm that can run on a commodity ASIC (a CPU or GPU) by definition can have a customized ASIC created for it with slightly less functionality. Some algorithms are intentionally made to be “ASIC friendly” - where an ASIC implementation is drastically more efficient than the same algorithm running on general purpose hardware. The protection that this offers when the coin is unknown also makes it an attractive target for a dedicate mining ASIC company as soon as it becomes useful.
Therefore, ASIC resistance is: the efficiency difference of specilized hardware versus hardware that has a wider adoption and applicability. A smaller efficiency difference between custom vs general hardware mean higher resistance and a better algorithm. This efficiency difference is the proper metric to use when comparing the quality of PoW algorithms. Efficiency could mean absolute performance, performance per watt, or performance per dollar - they are all highly correlated. If a single entity creates and controls an ASIC that is drastically more efficient, they can gain 51% of the network hashrate and possibly stage an attack.
- Potential ASIC efficiency gain ~ 1000X
The SHA algorithm is a sequence of simple math operations - additions, logical ops, and rotates.
To process a single op on a CPU or GPU requires fetching and decoding an instruction, reading data from a register file, executing the instruction, and then writing the result back to a register file. This takes significant time and power.
A single op implemented in an ASIC takes a handful of transistors and wires. This means every individual op takes negligible power, area, or time. A hashing core is built by laying out the sequence of required ops.
The hashing core can execute the required sequence of ops in much less time, and using less power or area, than doing the same sequence on a CPU or GPU. A bitcoin ASIC consists of a number of identical hashing cores and some minimal off-chip communication.
- Potential ASIC efficiency gain ~ 1000X
Scrypt and NeoScrypt are similar to SHA in the arithmetic and bitwise operations used. Unfortunately, popular coins such as Litecoin only use a scratchpad size between 32kb and 128kb for their PoW mining algorithm. This scratch pad is small enough to trivially fit on an ASIC next to the math core. The implementation of the math core would be very similar to SHA, with similar efficiency gains.
- Potential ASIC efficiency gain ~ 1000X
X11 (and similar X##) require an ASIC that has 11 unique hashing cores pipelined in a fixed sequence. Each individual hashing core would have similar efficiency to an individual SHA core, so the overall design will have the same efficiency gains.
X16R requires the multiple hashing cores to interact through a simple sequencing state machine. Each individual core will have similar efficiency gains and the sequencing logic will take minimal power, area, or time.
The Baikal BK-X is an existing ASIC with multiple hashing cores and a programmable sequencer. It has been upgraded to enable new algorithms that sequence the hashes in different orders.
- Potential ASIC efficiency gain ~ 100X
The ~150mb of state is large but possible on an ASIC. The binning, sorting, and comparing of bit strings could be implemented on an ASIC at extremely high speed.
- Potential ASIC efficiency gain ~ 100X
The amount of state required on-chip is not clear as there are Time/Memory Tradeoff attacks. A specialized graph traversal core would have similar efficiency gains to a SHA compute core.
- Potential ASIC efficiency gain ~ 50X
Compared to Scrypt, CryptoNight does much less compute and requires a full 2mb of scratch pad (there is no known Time/Memory Tradeoff attack). The large scratch pad will dominate the ASIC implementation and limit the number of hashing cores, limiting the absolute performance of the ASIC. An ASIC will consist almost entirely of just on-die SRAM.
- Potential ASIC efficiency gain ~ 2X
Ethash requires external memory due to the large size of the DAG. However that is all that it requires - there is minimal compute that is done on the result loaded from memory. As a result a custom ASIC could remove most of the complexity, and power, of a GPU and be just a memory interface connected to a small compute engine.
The DAG is generated exactly as in ethash.
ProgPoW can be tuned using the following parameters. The proposed settings have been tuned for a range of existing, commodity GPUs:
PROGPOW_PERIOD: Number of blocks before changing the random program; default is50.PROGPOW_LANES: The number of parallel lanes that coordinate to calculate a single hash instance; default is16.PROGPOW_REGS: The register file usage size; default is32.PROGPOW_DAG_LOADS: Number of uint32 loads from the DAG per lane; default is4;PROGPOW_CACHE_BYTES: The size of the cache; default is16 x 1024.PROGPOW_CNT_DAG: The number of DAG accesses, defined as the outer loop of the algorithm; default is64(same as Ethash).PROGPOW_CNT_CACHE: The number of cache accesses per loop; default is12.PROGPOW_CNT_MATH: The number of math operations per loop; default is20.
The random program changes every PROGPOW_PERIOD blocks (default 50, roughly 12.5 minutes) to ensure the hardware executing the algorithm is fully programmable. If the program only changed every DAG epoch (roughly 5 days) certain miners could have time to develop hand-optimized versions of the random sequence, giving them an undue advantage.
ProgPoW uses FNV1a for merging data. The existing Ethash uses FNV1 for merging, but FNV1a provides better distribution properties.
ProgPow uses KISS99 for random number generation. This is the simplest (fewest instruction) random generator that passes the TestU01 statistical test suite. A more complex random number generator like Mersenne Twister can be efficiently implemented on a specialized ASIC, providing an opportunity for efficiency gains.
uint32_t fnv1a(uint32_t &h, uint32_t d)
{
return h = (h ^ d) * 0x1000193;
}
typedef struct {
uint32_t z, w, jsr, jcong;
} kiss99_t;
// KISS99 is simple, fast, and passes the TestU01 suite
// https://en.wikipedia.org/wiki/KISS_(algorithm)
// http://www.cse.yorku.ca/~oz/marsaglia-rng.html
uint32_t kiss99(kiss99_t &st)
{
st.z = 36969 * (st.z & 65535) + (st.z >> 16);
st.w = 18000 * (st.w & 65535) + (st.w >> 16);
uint32_t MWC = ((st.z << 16) + st.w);
st.jsr ^= (st.jsr << 17);
st.jsr ^= (st.jsr >> 13);
st.jsr ^= (st.jsr << 5);
st.jcong = 69069 * st.jcong + 1234567;
return ((MWC^st.jcong) + st.jsr);
}The LANES*REGS of mix data is initialized from the hash’s seed.
void fill_mix(
uint64_t hash_seed,
uint32_t lane_id,
uint32_t mix[PROGPOW_REGS]
)
{
// Use FNV to expand the per-warp seed to per-lane
// Use KISS to expand the per-lane seed to fill mix
uint32_t fnv_hash = 0x811c9dc5;
kiss99_t st;
st.z = fnv1a(fnv_hash, seed);
st.w = fnv1a(fnv_hash, seed >> 32);
st.jsr = fnv1a(fnv_hash, lane_id);
st.jcong = fnv1a(fnv_hash, lane_id);
for (int i = 0; i < PROGPOW_REGS; i++)
mix[i] = kiss99(st);
}Like ethash Keccak is used to seed the sequence per-nonce and to produce the final result. The keccak-f800 variant is used as the 32-bit word size matches the native word size of modern GPUs. The implementation is a variant of SHAKE with width=800, bitrate=576, capacity=224, output=256, and no padding. The result of keccak is treated as a 256-bit big-endian number - that is result byte 0 is the MSB of the value.
hash32_t keccak_f800_progpow(hash32_t header, uint64_t seed, hash32_t digest)
{
uint32_t st[25];
for (int i = 0; i < 25; i++)
st[i] = 0;
for (int i = 0; i < 8; i++)
st[i] = header.uint32s[i];
st[8] = seed;
st[9] = seed >> 32;
for (int i = 0; i < 8; i++)
st[10+i] = digest.uint32s[i];
for (int r = 0; r < 22; r++)
keccak_f800_round(st, r);
hash32_t ret;
for (int i=0; i<8; i++)
ret.uint32s[i] = st[i];
return ret;
}The flow of the overall algorithm is:
- A keccak hash of the header + nonce to create a seed
- Use the seed to generate initial mix data
- Loop multiple times, each time hashing random loads and random math into the mix data
- Hash all the mix data into a single 256-bit value
- A final keccak hash that is compared against the target
bool progpow_search(
const uint64_t prog_seed, // value is (block_number/PROGPOW_PERIOD)
const uint64_t nonce,
const hash32_t header,
const hash32_t target, // miner can use a uint64_t target, doesn't need the full 256 bit target
const uint32_t *dag // gigabyte DAG located in framebuffer - the first portion gets cached
)
{
uint32_t mix[PROGPOW_LANES][PROGPOW_REGS];
hash32_t digest;
for (int i = 0; i < 8; i++)
digest.uint32s[i] = 0;
// keccak(header..nonce)
hash32_t seed_256 = keccak_f800_progpow(header, nonce, digest);
// endian swap so byte 0 of the hash is the MSB of the value
uint64_t seed = bswap(seed_256[0]) << 32 | bswap(seed_256[1]);
// initialize mix for all lanes
for (int l = 0; l < PROGPOW_LANES; l++)
fill_mix(seed, l, mix[l]);
// execute the randomly generated inner loop
for (int i = 0; i < PROGPOW_CNT_DAG; i++)
progPowLoop(prog_seed, i, mix, dag);
// Reduce mix data to a per-lane 32-bit digest
uint32_t digest_lane[PROGPOW_LANES];
for (int l = 0; l < PROGPOW_LANES; l++)
{
digest_lane[l] = 0x811c9dc5
for (int i = 0; i < PROGPOW_REGS; i++)
fnv1a(digest_lane[l], mix[l][i]);
}
// Reduce all lanes to a single 256-bit digest
for (int i = 0; i < 8; i++)
digest.uint32s[i] = 0x811c9dc5;
for (int l = 0; l < PROGPOW_LANES; l++)
fnv1a(digest.uint32s[l%8], digest_lane[l])
// keccak(header .. keccak(header..nonce) .. digest);
return (keccak_f800_progpow(header, seed, digest) <= target);
}The inner loop uses FNV and KISS99 to generate a random sequence from the prog_seed. This random sequence determines which mix state is accessed and what random math is performed. Since the prog_seed changes relatively infrequently it is expected that progPowLoop will be pre-compiled while mining instead of interpreted on the fly.
kiss99_t progPowInit(uint64_t prog_seed, int mix_seq_dst[PROGPOW_REGS], int mix_seq_cache[PROGPOW_REGS])
{
kiss99_t prog_rnd;
uint32_t fnv_hash = 0x811c9dc5;
prog_rnd.z = fnv1a(fnv_hash, prog_seed);
prog_rnd.w = fnv1a(fnv_hash, prog_seed >> 32);
prog_rnd.jsr = fnv1a(fnv_hash, prog_seed);
prog_rnd.jcong = fnv1a(fnv_hash, prog_seed >> 32);
// Create a random sequence of mix destinations for merge() and mix sources for cache reads
// guarantees every destination merged once
// guarantees no duplicate cache reads, which could be optimized away
// Uses Fisher-Yates shuffle
for (int i = 0; i < PROGPOW_REGS; i++)
{
mix_seq_dst[i] = i;
mix_seq_cache[i] = i;
}
for (int i = PROGPOW_REGS - 1; i > 0; i--)
{
int j;
j = kiss99(prog_rnd) % (i + 1);
swap(mix_seq_dst[i], mix_seq_dst[j]);
j = kiss99(prog_rnd) % (i + 1);
swap(mix_seq_cache[i], mix_seq_cache[j]);
}
return prog_rnd;
}The math operations that merges values into the mix data are ones chosen to maintain entropy.
// Merge new data from b into the value in a
// Assuming A has high entropy only do ops that retain entropy
// even if B is low entropy
// (IE don't do A&B)
void merge(uint32_t &a, uint32_t b, uint32_t r)
{
switch (r % 4)
{
case 0: a = (a * 33) + b; break;
case 1: a = (a ^ b) * 33; break;
case 2: a = ROTL32(a, ((r >> 16) % 32)) ^ b; break;
case 3: a = ROTR32(a, ((r >> 16) % 32)) ^ b; break;
}
}The math operations chosen for the random math are ones that are easy to implement in CUDA and OpenCL, the two main programming languages for commodity GPUs.
// Random math between two input values
uint32_t math(uint32_t a, uint32_t b, uint32_t r)
{
switch (r % 11)
{
case 0: return a + b;
case 1: return a * b;
case 2: return mul_hi(a, b);
case 3: return min(a, b);
case 4: return ROTL32(a, b);
case 5: return ROTR32(a, b);
case 6: return a & b;
case 7: return a | b;
case 8: return a ^ b;
case 9: return clz(a) + clz(b);
case 10: return popcount(a) + popcount(b);
}
}The main loop:
// Helper to get the next value in the per-program random sequence
#define rnd() (kiss99(prog_rnd))
// Helper to pick a random mix location
#define mix_src() (rnd() % PROGPOW_REGS)
// Helper to access the sequence of mix destinations
#define mix_dst() (mix_seq_dst[(mix_seq_dst_cnt++)%PROGPOW_REGS])
// Helper to access the sequence of cache sources
#define mix_cache() (mix_seq_cache[(mix_seq_cache_cnt++)%PROGPOW_REGS])
void progPowLoop(
const uint64_t prog_seed,
const uint32_t loop,
uint32_t mix[PROGPOW_LANES][PROGPOW_REGS],
const uint32_t *dag)
{
// All lanes share a base address for the global load
// Global offset uses mix[0] to guarantee it depends on the load result
uint32_t offset_g = mix[loop%PROGPOW_LANES][0] % (DAG_BYTES / (PROGPOW_LANES*PROGPOW_DAG_LOADS*sizeof(uint32_t)));
// Lanes can execute in parallel and will be convergent
for (int l = 0; l < PROGPOW_LANES; l++)
{
// global load to sequential locations
uint32_t data_g[PROGPOW_DAG_LOADS];
for (int i = 0; i < PROGPOW_DAG_LOADS; i++)
data_g[i] = dag[(offset_g*PROGPOW_LANES + (l + loop) % 16)*PROGPOW_DAG_LOADS + i];
// initialize the seed and mix destination sequence
int mix_seq_dst[PROGPOW_REGS];
int mix_seq_cache[PROGPOW_REGS];
int mix_seq_dst_cnt = 0;
int mix_seq_cache_cnt = 0;
kiss99_t prog_rnd = progPowInit(prog_seed, mix_seq_dst, mix_seq_cache);
int max_i = max(PROGPOW_CNT_CACHE, PROGPOW_CNT_MATH);
for (int i = 0; i < max_i; i++)
{
if (i < PROGPOW_CNT_CACHE)
{
// Cached memory access
// lanes access random 32-bit locations within the first portion of the DAG
uint32_t offset = mix[l][mix_cache()] % (PROGPOW_CACHE_BYTES/sizeof(uint32_t));
uint32_t data = dag[offset];
merge(mix[l][mix_dst()], data, rnd());
}
if (i < PROGPOW_CNT_MATH)
{
// Random Math
uint32_t data = math(mix[l][mix_src()], mix[l][mix_src()], rnd());
merge(mix[l][mix_dst()], data, rnd());
}
}
// Consume the global load data at the very end of the loop to allow full latency hiding
// Always merge into mix[0] to feed the offset calculation
merge(mix[l][0], data_g[0], rnd());
for (int i = 1; i < PROGPOW_DAG_LOADS; i++)
merge(mix[l][mix_dst()], data_g[i], rnd());
}
}