What is a Block Hash?

In real blockchains, a hash is a unique cryptographic fingerprint of a block's contents. It's created by running all the block data through a complex mathematical function (like SHA-256). Any tiny change in the block would completely change the hash.

For our classroom simulation, we use a simplified version that captures the same concept: the hash depends on which transactions you selected, and different transaction selections produce different hashes.

1
List Your Selected Transactions

Write down all the transaction IDs you included in your block.

Example:

Your block contains: TX_001, TX_002, TX_003, TX_007, TX_010, TX_011

2
Extract the Last Digit from Each TX ID

For each transaction ID, take only the last digit (the ones place).

Example:

TX_001 → last digit is 1
TX_002 → last digit is 2
TX_003 → last digit is 3
TX_007 → last digit is 7
TX_010 → last digit is 0
TX_011 → last digit is 1
3
Add All the Last Digits Together

Sum all the extracted digits to get your block hash.

Example:

1 + 2 + 3 + 7 + 0 + 1 = ?
Step by step:
1 + 2 = 3
3 + 3 = 6
6 + 7 = 13
13 + 0 = 13
13 + 1 = 14
Block Hash = 14
Why This Works: Just like a real hash, our simplified hash changes when you select different transactions. If you swap TX_001 for TX_005, your hash would change from 14 to 18 (replacing 1 with 5). This demonstrates how hashes uniquely identify block contents!

Quick Reference Table

TX ID Last Digit TX ID Last Digit
TX_0011 TX_0111
TX_0022 TX_0122
TX_0033 TX_0133
TX_0044 TX_0144
TX_0055 TX_0155
TX_0066 TX_0166
TX_0077 TX_0177
TX_0088 TX_0188
TX_0099 TX_0199
TX_0100 TX_0200

Practice Exercise

Calculate the hash for this block:

Selected transactions: TX_004, TX_006, TX_008, TX_009, TX_015

Your work:

Click to reveal answer
TX_004 → 4
TX_006 → 6
TX_008 → 8
TX_009 → 9
TX_015 → 5
4 + 6 + 8 + 9 + 5 = 32

Common Mistakes to Avoid

  • Don't include transactions you didn't select in your block
  • Don't forget to extract the last digit (use 7 from TX_007, not 007)
  • Remember TX_010 and TX_020 have last digit 0 (not 1 or 2)
  • Double-check your addition – use a calculator if needed!

Connection to Real Blockchains

Real blockchain hashes (like SHA-256) are much more complex, producing a 64-character hexadecimal string like: 0000000000000000000abc...

But the principle is the same: the hash is a unique fingerprint that:

  • Changes completely if any transaction is modified
  • Can be quickly verified by others
  • Links blocks together in a chain (each block includes the previous block's hash)

Our simplified version teaches these core concepts without requiring cryptographic calculations!

© Joerg Osterrieder 2025-2026. All rights reserved.