Add verifying snapshots book entry (#5885)

book/src/SUMMARY.md

@@ -63,6 +63,7 @@
   - [Cross-Program Invocation](cross-program-invocation.md)
   - [Rent](rent.md)
   - [Inter-chain Transaction Verification](interchain-transaction-verification.md)
+  - [Snapshot Verification](snapshot-verification.md)
 
 - [Implemented Design Proposals](implemented-proposals.md)
   - [Blocktree](blocktree.md)

book/src/snapshot-verification.md

@@ -0,0 +1,50 @@
+# Verifying Snapshots and Account state proof
+
+## Problem
+
+When a validator boots up from a snapshot, it needs a way to verify the account set matches what the rest of the network sees quickly. A potential attacker
+could give the validator an incorrect state, and then try to convince it to accept a transaction that would otherwise be rejected.
+
+## Solution
+
+Currently the bank hash is derived from hashing the delta state of the accounts in a slot which is then combined with the previous bank hash value. The problem with this is that the list of hashes
+will grow on the order of the number of slots processed by the chain and become a burden to both transmit and verify successfully.
+
+Another naive method could be to create a merkle tree of the account state. This has the downside that with each account update which removes an account state from the tree, the merkle
+tree would have to be recomputed from the entire account state of all live accounts in the system.
+
+To verify the snapshot, we propose the following:
+
+On account store hash the following data:
+* Account owner
+* Account data
+* Account pubkey
+* Account lamports balance
+* Fork the account is stored on
+
+Use this resulting hash value as input to an expansion function which expands the hash value into an image value.
+The function will create a 440 byte block of data where the first 32 bytes are the hash value, and the next 440 - 32 bytes
+are generated from a Chacha RNG with the hash as the seed.
+
+The account images are then combined with xor. The previous account value will be xored into the state and the new
+account value also xored into the state.
+
+Voting and sysvar hash values occur with the hash of the resulting full image value.
+
+On validator boot, when it loads from a snapshot, it would verify the hash value with the accounts set. It would
+then use SPV to display the percentage of the network that voted for the hash value given.
+
+The resulting value can be verified by a validator to be the result of xoring all current account states together.
+
+An attack on the xor state could be made to influence its value:
+
+Thus the 440 byte image size comes from this paper, avoiding xor collision with 0 (or thus any other given bit pattern):
+[https://link.springer.com/content/pdf/10.1007%2F3-540-45708-9_19.pdf]
+
+The math provides 128 bit security in this case:
+```ignore
+O(k * 2^(n/(1+lg(k)))
+k=2^40 accounts
+n=440
+2^(40) * 2^(448 * 8 / 41) ~= O(2^(128))
+```