Scrypt proof of work

Scrypt hashing

Here's the source code in C for the Scrypt hashing function: https://github.com/coblee/litecoin/blob/master/src/scrypt.c

Litecoin uses these values for the call to Scrypt:

• N = 1024;
• r = 1;
• p = 1;
• salt is the same 80 bytes as the input
• output is 256 bits (32 bytes)

And here's a Java implementation: https://github.com/wg/scrypt/blob/master/src/main/java/com/lambdaworks/crypto/SCrypt.java

Example dbdump of block 29255

./dbdump.py --datadir=/home/mining/.litecoin/ --block 29255
Block height: 29255
BLOCK adf6e2e56df692822f5e064a8b6404a05d67cccd64bc90f57f65b46805e9a54b
Next block: 0000000000000000000000000000000000000000000000000000000000000000
Time: Wed Nov  9 16:15:52 2011 Nonce: 3562614017
nBits: 0x0x1d018ea7
hashMerkleRoot: 0x066b2a758399d5f19b5c6073d09b500d925982adc4b3edd352efe14667a8ca9f
Previous block: 279f6330ccbbb9103b9e3a5350765052081ddbae898f1ef6b8c64f3bcef715f6
1 transactions:
1 tx in, 1 out
TxIn: COIN GENERATED coinbase:04b217bb4e022309
TxOut: value: 50.000000 pubkey: 1HXG8MWvUFNU3pLpQUJueSC4kHcrNepuwC Script: 65:0448...b8cd CHECKSIG

Raw block header: 01000000f615f7ce3b4fc6b8f61e8f89aedb1d0852507650533a9e3b10b9bbcc30639f279fcaa86746e1ef52d3edb3c4ad8259920d509bd073605c9bf1d59983752a6b06b817bb4ea78e011d012d59d4

Python script to check hashing

import hashlib
import ltc_scrypt    # Packaged with litecoin p2pool - https://github.com/coblee/p2pool

header_hex = "01000000f615f7ce3b4fc6b8f61e8f89aedb1d0852507650533a9e3b10b9bbcc30639f279fcaa86746e1ef52d3edb3c4ad8259920d509bd073605c9bf1d59983752a6b06b817bb4ea78e011d012d59d4"
header_bin = header_hex.decode('hex')

hash = hashlib.sha256(hashlib.sha256(header_bin).digest()).digest()
hash.encode('hex_codec')
hash[::-1].encode('hex_codec')    # convert from big-endian to little-endian

scrypt = ltc_scrypt.getPoWHash(header_bin)
scrypt.encode('hex_codec')
scrypt[::-1].encode('hex_codec')    # convert from big-endian to little-endian

After running this script, it will print out these results:
hash = adf6e2e56df692822f5e064a8b6404a05d67cccd64bc90f57f65b46805e9a54b
scrypt = 0000000110c8357966576df46f3b802ca897deb7ad18b12f1c24ecff6386ebd9

You can see that the hash matches the dbdump BLOCK line above. And the scrypt hash is less than the target:
target = 000000018ea70000000000000000000000000000000000000000000000000000

Breakdown of data and header

(See https://en.bitcoin.it/wiki/Block_hashing_algorithm)

~/litecoind getwork
{
    "midstate" : "40fd268321efcf60e625707d4e31f9deadd13157e228985de8a10a057b98ed4d",
    "data" : "0000000105e9a54b7f65b46864bc90f55d67cccd8b6404a02f5e064a6df69282adf6e2e5f7f953b0632b25b099858b717bb7b24084148cfa841a89f106bc6b655b18d2ed4ebb191a1d018ea700000000000000800000000000000000000000000000000000000000000000000000000000000000000000000000000080020000",
    "hash1" : "00000000000000000000000000000000000000000000000000000000000000000000008000000000000000000000000000000000000000000000000000010000",
    "target" : "0000000000000000000000000000000000000000000000000000a78e01000000"
}

The data field is stored in big-endian format. We need to cover that to little-endian for each of the fields in the data because we can pass it to the hashing function.

Data is broken down to:

• Version - 00000001 (4 bytes)
• Previous hash - 05e9a54b7f65b46864bc90f55d67cccd8b6404a02f5e064a6df69282adf6e2e5 (32 bytes)
• Merkle root - f7f953b0632b25b099858b717bb7b24084148cfa841a89f106bc6b655b18d2ed (32 bytes)
• Timestamp - 4ebb191a (4 bytes)
• Bits (target in compact form) - 1d018ea7 (4 bytes)
• Nonce - 00000000 (4 bytes)

You need covert these from big-endian to little-endian. This is done 2 characters at a time because each byte is represented by 2 hex chars. (each hex char is 4 bits)

• Version becomes 01000000
• Previous hash becomes e5e2f6.....a5e905
• Merkle root becomes edd218...53f9f7
• Timestamp becomes 1a19bb4e
• Bits becomes a78e011d
• And Nonce is a 32-bit integer you choose that will make the Scrypt hash be less than the target.

Remember that you will need to convert the 32-bit nonce to hex and little-endian also. So if you are trying the nonce 2504433986. The hex version is 9546a142 in big-endian and 42a14695 in little-endian.

You then concatenate these little-endian hex strings together to get the header string (80 bytes) you input into Scrypt:
01000000 e5e2f6.....a5e905 edd218...53f9f7 1a19bb4e a78e011d 42a14695