# Assessment 7

#### 64 transactions were timestamped into block 700. If you are in possession of an up to date set of block headers, what will be the minimum amount of additional data that is required to verify an Unspent Transaction Output (UTXO) as valid from this set of transactions 699 300 blocks deep?

* [x] 1192
* [ ] 192
* [ ] 80

**Notes to Learners**

Let's break down what data is needed to verify a UTXO from block 700 when you're currently at block 700,000.

### Given Information

* **Block in question:** Block 700
* **Current block height:** 700,000
* **Transactions in block 700:** 64
* **You already have:** All block headers up to block 700,000

### What You Need to Verify the UTXO

Since you already have the block headers (including block 700's header with its Merkle root), you need:

#### 1. **The Transaction itself**

Average transaction size: **\~1,000 bytes** (1 KB as stated in the previous question)

#### 2. **Merkle Proof for the transaction**

With 64 transactions in the block, the Merkle tree has:

* 64 leaves (transactions)
* Height = log₂(64) = 6 levels

**Merkle path length = 6 hashes** **Size = 6 × 32 bytes = 192 bytes**

### Total Minimum Data Required

**Total = 1,000 + 192 = 1,192 bytes**

***

**Answer: \~1,192 bytes (or approximately 1.2 KB)**

### Why This Works

1. You use the Merkle proof to verify the transaction is in block 700
2. You verify the Merkle root matches the one in block 700's header
3. Since you have all headers, you've already verified the chain of work from block 700 to 700,000
4. This proves the transaction is valid and part of the longest chain

This is the essence of SPV - verifying specific transactions without downloading the entire blockchain!


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