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+Block chain
+
+Tomasz Kowalczyk
+https://joind.in/event/php-uk-conference-2019/dive-deep-into-blockchain
+
+- Block chain is a database
+- Bespoke data structure
+
+"Not crpyto"
+
+General Purpose block chain:
+
+Why?
+- Data in block is immutable
+- Don't edit or remove, you append
+- traceability (Auditing / logging)
+- Verifiability (Data must be fully valid)
+- reproducibility
+- decentralisation
+- Trust-less (Removes need for trust)
+- transparency
+
+How?
+- What type?
+    - public - will be hacked
+    - shared - shared ledger
+    - private
+- Cryptography
+    - pub/rivate keys
+    - sign information
+    - verifablity
+    - Elliptic Curves
+    - openssl ecparam -list_curves
+    - Addressing
+        - can shorten public keys massively
+    - Hash algorithm
+        - Integrity
+        - 2x sha256 (sha256(sha256($data)))
+        - 4B version, 32B hash of  last Block, 32B  hash of data block, 4B timestamp, 4B difficulty, 4B nonce
+        - Merkle tree - binary hash tree
+        - Merkle path - list of hashes through the tree
+        - Genesis block - First block in the chain
+            - Special as it's first
+            - Prevents exploitation by being hardcoded
+
+- consensus
+    - Confidence
+    - chain forking needs resolution
+        - bitcoin users longest chain wins
+        - proof of work / proof of stake
+        - difficulty is no of zeros  that need to be at the start of the hash
+        - others (proof of burn, proof of time, proof of ....)
+
+
+- Smart contracts
+    - Programs that are included in the chain
+    - e.g. A loan paid by the chain
+
+- Lighting network (drawback off blockhain is it focuses on integrity not perf)
+    - Off chain transactions with on-chain settlement
+
+- Not all blockchains are chains
+    - IOTA Tangle
+
+- Challenges
+    - Fault tolerance
+        - You lose control of public/shared blockchains
+        - Now way to enforce software updates
+        - Conflict resolution
+    - Integration with current systems
+    - Security
+        - Cryptography is first and last line of defence
+
+- Not perfect for:
+    - scalability
+    - performance
+    - throughput
+
+- Incentives
+    - Mining in currencies
+        - Power for money
+- Privacy
+    - Protect data
+
+- Hard forks
+- 51% attacks
+
+- Right solution for the right kind of problem
+
+- Event sourcing on steroids
+
+QUESTIONS
+- Any php recommendations?
+    - No.
+- security?
+    - Store data not values somewhere else
+    - Encrypt
+- Why private blocks?
+    - Can give public list of hashes for verification
+    - keep data private