[ZocoLini]

Closes #1116.

Headers were only checked against the target in their own bits, so past the last checkpoint a peer could serve a chain of minimal difficulty (and, through fork detection, make the client drop stored blocks, #1103).

• Params::next_work_required(window, last_height, time): the bits Dash Core requires, as its GetNextWorkRequired from Dark Gravity Wave on, including testnet's min difficulty rules and regtest's fixed limit. None on devnets (whose difficulty rules are set per devnet), before DGW, or without 24 headers of history.
• BlockHeaderValidator::new(network, ancestors, tip_height): each header's bits must match, checked in the same parallel pass as continuity and PoW. BlockHeadersManager passes the last 24 stored headers, so both initial sync and announced headers are checked. The first headers after a checkpoint start, without full history, are not.
• New ValidationError::InvalidDifficulty.
• Header::mainnet_fixture() in dashcore test utils: 30 real mainnet headers (1999976 to 2000005), hashes checked.

Tests: next_work_required reproduces the bits of mainnet header 2000000 and applies the devnet, pre-DGW, regtest and testnet rules; the validator rejects a header whose hash meets an easier target but not the required difficulty.

Breaking: BlockHeadersManager::new takes the network, and BlockHeaderValidator::new takes the network, the stored headers the batch extends and the height of the last of them.

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Reviewing files that changed from the base of the PR and between 80298d0 and 6383467.

📒 Files selected for processing: 8

• dash-spv/src/client/lifecycle.rs
• dash-spv/src/error.rs
• dash-spv/src/sync/block_headers/manager.rs
• dash-spv/src/validation/header.rs
• dash/src/consensus/params.rs
• dash/src/pow.rs
• dash/src/test_utils/block.rs
• dash/tests/data/mainnet_headers_1999976.txt

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[codecov]

Codecov Report

❌ Patch coverage is 98.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.07%. Comparing base (80298d0) to head (6383467).
⚠️ Report is 12 commits behind head on dev.

Files with missing lines	Patch %	Lines
dash/src/pow.rs	96.55%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #1161      +/-   ##
==========================================
+ Coverage   78.87%   79.07%   +0.20%     
==========================================
  Files         304      304              
  Lines       81405    81527     +122     
==========================================
+ Hits        64206    64467     +261     
+ Misses      17199    17060     -139     

Flag	Coverage Δ
core	77.67% <96.55%> (+0.08%)	⬆️
ffi	52.03% <ø> (+1.24%)	⬆️
rpc	52.05% <ø> (ø)
spv	92.81% <100.00%> (+0.03%)	⬆️
wallet	82.19% <ø> (ø)

Files with missing lines	Coverage Δ
dash-spv/src/client/lifecycle.rs	91.40% <100.00%> (+0.10%)	⬆️
dash-spv/src/error.rs	33.33% <ø> (ø)
dash-spv/src/sync/block_headers/manager.rs	93.68% <100.00%> (+0.21%)	⬆️
dash-spv/src/validation/header.rs	98.05% <100.00%> (+0.87%)	⬆️
dash/src/consensus/params.rs	76.92% <ø> (ø)
dash/src/pow.rs	80.62% <96.55%> (+4.69%)	⬆️

... and 29 files with indirect coverage changes

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[xdustinface]

1. Mainnet below 68589 needs the 50% tolerance. Core only requires exact bits above 68589 (validation.cpp, ContextualCheckBlockHeader). Below that it allows 50%, because early mainnet ran DGW v1/v2. Real headers from 34140 to 68588 fail the exact check. Real mainnet sync with this branch: --start-height 0 rejects 34140 (bits 0x1c00f8a9, required 0x1c00f987) and --start-height 50000 rejects 50024. Skip the check, or use the 50% tolerance, on mainnet up to 68589, and add a test at that boundary.

2. Checkpoint 750000 has the wrong time and bits. It has 1491953700 / 0x1a075a02, but the real block has 1507424630 / 0x1a027884. The hash is correct. Since the anchor now feeds DGW, real mainnet sync with --start-height 750000 rejects 750024 (bits 0x1a02b0d8, required 0x1a08c176). Every wallet born in 750000..799999 hits this. Please fix the entry and add a test that checks every checkpoint's time and bits against the real block.

   In all three runs the client then reports headers Synced at the stall height (32000, 50000, 750000) and never moves again (#1194). The same runs on the base commit sync through.

3. The first 23 headers after the start checkpoint are unchecked, and they seed DGW for everything after. A wallet born after the last checkpoint (2500000) starts there, so a peer can serve 23 minimum-difficulty headers and stay at minimum difficulty from then on. That is the #1116 attack. Either close the gap (for example, start one checkpoint earlier so those headers are pinned by a hash) or change Closes #1116 to Part of #1116 and open a follow-up.

[ZocoLini]

@xdustinface what if a finish #375 first, that would solve the point 2 issue

[ZocoLini]

Also, #375 could store the previous 23, so a checkpoint is now a Vec o headers, and not only one, and, when geenrating the checkpoints, I would be able to validate the chainn in that point

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