12. Privacy

Target

Unlock the contract.

Weakness

Similarly to the Level 8 -Vault, the contract's security relies on the value of a variable defined as private. This variable is actually publicly visible.

Solidity Concepts

The layout of storage data in slots and how to read data from storage with getStorageAt were covered in Level 8 -Vault.

The slots are 32 bytes long. 1 byte = 8 bits = 2 nibbles = 2 hexadecimal digits.
In practice, when using e.g getStorageAt we get string hashes of length 64 + 2 ('0x') = 66.

Hack

1. Analyse storage layout:

   slot	variable
   0	bool (1 bit long)
   1	ID (256 bits long)
   2	awkwardness (16 bytes) - denomination (8 bytes) - flattening (8 bytes)
   3	data[0] (32 bytes long)
   4	data[1] (32 bytes long)
   5	data[2] (32 bytes long)

   The _key variable is slot 5.

2. Take the first 16 bytes = take the first 2 ('0x') + 2 * 16 = 34 characters of the bytestring.

Takeaways

• Same as for Level 8 -Vault:
  • All storage is publicly visible, even private variables
  • Don't store passwords or secret data on chain without hashing them first
• Storage optimization
  • Use memory instead of storage if persisting data in state is not necessary
  • Order variables in such way that slots occupdation is maximized.