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Block Header Specifications

The block header is the first component of a valid block generated by a Proof-of-Work (PoW) miner. It consists of a summary of all of the information contained in the block, as well as metadata such as the time and difficulty when the block was mined, the Merkle root of all the transactions, and the nonce. Also present in a block header is the hash of the previous block, which enables blocks to be linked together in a contiguous chain.

The header of a block is what PoW miners repeatedly hash in an attempt to find a hash which is at or below the current difficulty requirement.

Block header format

Decred block headers occupy 180 bytes when serialized. The serialization format for a block header is displayed below:

Field Description Size
Version Block header version 4 bytes
PrevBlock Hash of the previous block 32 bytes
MerkleRoot Merkle tree hash calculated using all transactions in the block 32 bytes
StakeRoot Merkle tree hash calculated using all stake transactions in the block 32 bytes
VoteBits Bit flags. Currently used to signify votes on the previous Merkle root and vote on proposed consensus rule changes 2 bytes
FinalState Commitment to the final state of the PRNG (for lottery purposes) 6 bytes
Voters Number of participating voters in the block 2 bytes
FreshStake Number of new tickets in the block 1 byte
Revocations Number of revocations present in the block 1 byte
PoolSize Size of the ticket pool 4 bytes
Bits Difficulty target for the block 4 bytes
SBits Stake difficulty target for the block 8 bytes
Height The number of blocks that precede the block in the blockchain 4 bytes
Size Number of bytes that the serialized block occupies 4 bytes
Timestamp Time that the block was created 4 bytes
Nonce The nonce used to generate the block 4 bytes
ExtraData Any other data (may be used later for consensus purposes) 32 bytes
StakeVersion The stake version used for voting on the created block 4 bytes

Example encoded block header

        0x01, 0x00, 0x00, 0x00, // Version
        0x6f, 0xe2, 0x8c, 0x0a, 0xb6, 0xf1, 0xb3, 0x72, // PrevBlock
        0xc1, 0xa6, 0xa2, 0x46, 0xae, 0x63, 0xf7, 0x4f,
        0x93, 0x1e, 0x83, 0x65, 0xe1, 0x5a, 0x08, 0x9c,
        0x68, 0xd6, 0x19, 0x00, 0x00, 0x00, 0x00, 0x00,
        0x3b, 0xa3, 0xed, 0xfd, 0x7a, 0x7b, 0x12, 0xb2, // MerkleRoot
        0x7a, 0xc7, 0x2c, 0x3e, 0x67, 0x76, 0x8f, 0x61,
        0x7f, 0xc8, 0x1b, 0xc3, 0x88, 0x8a, 0x51, 0x32,
        0x3a, 0x9f, 0xb8, 0xaa, 0x4b, 0x1e, 0x5e, 0x4a,
        0x3b, 0xa3, 0xed, 0xfd, 0x7a, 0x7b, 0x12, 0xb2, // StakeRoot
        0x7a, 0xc7, 0x2c, 0x3e, 0x67, 0x76, 0x8f, 0x61,
        0x7f, 0xc8, 0x1b, 0xc3, 0x88, 0x8a, 0x51, 0x32,
        0x3a, 0x9f, 0xb8, 0xaa, 0x4b, 0x1e, 0x5e, 0x4a,
        0x00, 0x00,                                     // VoteBits
        0x00, 0x00, 0x00, 0x00, 0x00, 0x00,             // FinalState
        0x00, 0x00,                                     // Voters
        0x00,                                           // FreshStake
        0x00,                                           // Revocations
        0x00, 0x00, 0x00, 0x00,                         // PoolSize
        0xff, 0xff, 0x00, 0x1d,                         // Bits
        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, // SBits
        0x00, 0x00, 0x00, 0x00,                         // Height
        0x00, 0x00, 0x00, 0x00,                         // Size
        0x29, 0xab, 0x5f, 0x49,                         // Timestamp
        0xf3, 0xe0, 0x01, 0x00,                         // Nonce
        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, // ExtraData
        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
        0x03, 0x00, 0x00, 0x00,                         // StakeVersion

Example encoded block header as raw bytes

        010000006fe28c0ab6f1b372c1a6a246ae63f74f931e8365e15a089c68d61900
        000000003ba3edfd7a7b12b27ac72c3e67768f617fc81bc3888a51323a9fb8aa
        4b1e5e4a3ba3edfd7a7b12b27ac72c3e67768f617fc81bc3888a51323a9fb8aa
        4b1e5e4a0000000000000000000000000000000ffff001d00000000000000000
        00000000000000029ab5f49f3e00100000000000000000000000000000000000
        000000000000000000000000000000003000000

Mining details

Both getwork and getblocktemplate are implemented, but neither is precisely the same as in Bitcoin. getwork is very similar, but returns a non-int32 reversed byte string to work off of. The ‘data’ field refers to the properly padded BLAKE-256 input of 3x64 byte chunks. Only the last chunk needs to be modified when mining.

Example getwork response:

{
  "data": "00000000bf9d58482135c4d8d9445af7604b24ccf1d31b33df33abf21a4e11f52c
000000ef9d462a20d734def40076f0027faa8ef8d9c52d68c46741d03b1732377a556ed9cbf3b
694e65c1b287e8fd2be2e2243a74356d1ef95413d9fc74ffb8a8a07530100e11754a6af500500
03000115000000096f1d28d1354d000000000a0d0000591300009329a55600000000000000000
00000000000000000000000000000000000000000000000000000000000000080000001000000
00000005a0",
  "target": "000000000000000000000000000000000000000000000000000000096f000000"
}

getblocktemplate also returns a full header and data about the transactions that are included in the block. An example:

{
  "header": "...",
  "sigoplimit": 20000,
  "sizelimit": 1000000,
  "transactions": [
    {
      "data": "...",
      "hash": "9e72fdd4f0ac763da72ef672e6a4a9ec157412439fc578f979a6643fb9ad3bb6",
      "depends": [],
      "fee": 1000,
      "sigops": 2,
      "txtype": "regular"
    }
  ],
  "stransactions": [
    {
      "data": "...",
      "hash": "53b4dd7e041f492d9b2de54215b8fff107885313e4556eadabec7784b936f96e",
      "depends": [],
      "fee": 0,
      "sigops": 1,
      "txtype": "vote"
    },
    {
      "data": "...",
      "hash": "316819cd5ab8a67d56e81cb02f2386a52ce9e88e2984c8fb773b9a7435a843fe",
      "depends": [],
      "fee": 0,
      "sigops": 1,
      "txtype": "vote"
    },
    {
      "data": "...",
      "hash": "ec478ed0572049dea679feeb2cfac70740644dde18ad9241d9075501ea293f25",
      "depends": [],
      "fee": 0,
      "sigops": 1,
      "txtype": "vote"
    },
    {
      "data": "...",
      "hash": "c83befd244e25cc79b79956d1046003c605aaee672da14c03f4a12a42ba85126",
      "depends": [],
      "fee": 0,
      "sigops": 1,
      "txtype": "vote"
    },
    {
      "data": "...",
      "hash": "17e7cad2a56540c64c6a655263de22bf45c3ed4d43d059f808038212e3c7d840",
      "depends": [],
      "fee": 0,
      "sigops": 1,
      "txtype": "vote"
    },
    {
      "data": "...",
      "hash": "e4f17fd4f487fdbf8c24cd1bde1b621203561e9edce975c9f649961160819afb",
      "depends": [],
      "fee": 1000,
      "sigops": 3,
      "txtype": "ticket"
    },
    {
      "data": "...",
      "hash": "e214c31c466a60772cfc463b856c29236c7db10126d0a7a4da000aba0e2a58bb",
      "depends": [],
      "fee": 1000,
      "sigops": 2,
      "txtype": "ticket"
    },
    {
      "data": "...",
      "hash": "2dd201d2960a8e44cbe33caae34d8cdb4c7a9bcfcfc397e88fb5e4afd9e3e425",
      "depends": [],
      "fee": 1000,
      "sigops": 2,
      "txtype": "ticket"
    },
    {
      "data": "...",
      "hash": "b00a34879e4dffacd79f1adae07cf78a415c733695cb309290843f81b860a570",
      "depends": [],
      "fee": 1000,
      "sigops": 3,
      "txtype": "ticket"
    }
  ],
  "coinbaseaux": {
    "flags": "062f646372642f"
  },
  "coinbasevalue": 247524752,
  "longpollid": "0000005de504d40790bbcccdbe7c693b5bbc0e4871c2d9375625e5248be2f645-1453664716",
  "target": "0000006f09000000000000000000000000000000000000000000000000000000",
  "maxtime": 1453671916,
  "mintime": 1453664454,
  "mutable": [
    "time",
    "transactions/add",
    "prevblock",
    "coinbase/append"
  ],
  "noncerange": "00000000ffffffff",
  "capabilities": [
    "proposal"
  ]
}

Mining is performed by incrementing the nonce until the block header is below the target. ExtraData is allowed to be used as an extra nonce for fast hashing devices like ASICs, but a soft fork may change this at some point (to allow it to be used for other consensus data structures).