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  1. #include <stdio.h>
  2. #include <stdlib.h>
  3. #include <string.h>
  4.  
  5. /* ------------------------------
  6.    Simple Hash Function (Pedagogical)
  7.    NOT cryptographically secure
  8. -------------------------------- */
  9. unsigned int simple_hash(const char *data)
  10. {
  11.     unsigned int hash = 0;
  12.     while (*data)
  13.     {
  14.         hash = hash * 31 + *data;
  15.         data++;
  16.     }
  17.     return hash;
  18. }
  19.  
  20. /* ------------------------------
  21.    Block Structure with Hash Pointer
  22. -------------------------------- */
  23. typedef struct Block
  24. {
  25.     char data[100];
  26.     unsigned int previous_hash;  // hash pointer part
  27.     struct Block *previous;      // normal pointer part
  28. } Block;
  29.  
  30. /* ------------------------------
  31.    Create New Block
  32. -------------------------------- */
  33. Block *create_block(const char *data, Block *previous_block)
  34. {
  35.     Block *new_block = (Block *)malloc(sizeof(Block));
  36.     strcpy(new_block->data, data);
  37.     new_block->previous = previous_block;
  38.  
  39.     if (previous_block == NULL)
  40.         new_block->previous_hash = 0;  // Genesis block
  41.     else
  42.         new_block->previous_hash = simple_hash(previous_block->data);
  43.  
  44.     return new_block;
  45. }
  46.  
  47. /* ------------------------------
  48.    Verify Integrity of Chain
  49. -------------------------------- */
  50. int verify_chain(Block *current)
  51. {
  52.     while (current->previous != NULL)
  53.     {
  54.         unsigned int recalculated_hash =
  55.             simple_hash(current->previous->data);
  56.  
  57.         if (recalculated_hash != current->previous_hash)
  58.             return 0; // Tampering detected
  59.  
  60.         current = current->previous;
  61.     }
  62.     return 1; // Chain is valid
  63. }
  64.  
  65. /* ------------------------------
  66.    Print Blockchain
  67. -------------------------------- */
  68. void print_chain(Block *current)
  69. {
  70.     while (current != NULL)
  71.     {
  72.         printf("Block Data: %s\n", current->data);
  73.         printf("Previous Hash: %u\n", current->previous_hash);
  74.         printf("-------------------------\n");
  75.  
  76.         current = current->previous;
  77.     }
  78. }
  79.  
  80. /* ------------------------------
  81.    Main Demonstration
  82. -------------------------------- */
  83. int main()
  84. {
  85.     Block *genesis = create_block("Genesis Block", NULL);
  86.     Block *block2 = create_block("Second Block", genesis);
  87.     Block *block3 = create_block("Third Block", block2);
  88.  
  89.     printf("=== Original Chain ===\n");
  90.     print_chain(block3);
  91.  
  92.     if (verify_chain(block3))
  93.         printf("Chain is VALID\n");
  94.     else
  95.         printf("Chain is INVALID\n");
  96.  
  97.     printf("\n--- Tampering with Block 2 ---\n");
  98.     strcpy(block2->data, "Hacked Data");
  99.  
  100.     if (verify_chain(block3))
  101.         printf("Chain is VALID\n");
  102.     else
  103.         printf("Chain is INVALID (Tampering Detected)\n");
  104.  
  105.     return 0;
  106. }
  107.  
Success #stdin #stdout 0.01s 5288KB
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stdin
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Standard input is empty
stdout
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=== Original Chain ===
Block Data: Third Block
Previous Hash: 2248563169
-------------------------
Block Data: Second Block
Previous Hash: 3198240725
-------------------------
Block Data: Genesis Block
Previous Hash: 0
-------------------------
Chain is VALID

--- Tampering with Block 2 ---
Chain is INVALID (Tampering Detected)
https://ideone.com/KTUTFv
language:
C (gcc 8.3)
created:
14 hours ago
visibility:
public

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