AVR microcontrollers (ATmega/ATtiny) use fuse lock bits to protect flash. AVR decryption unlocks lock bits to dump firmware.

AVR Lock Bits:
- LB1/LB2 set lock level.
- Lock level 3 disables both external read and write.
- Chip erase resets lock bits but also erases flash.

AVR Decryption Methods:
1. Voltage Glitching: spike Vcc during lock bit check.
2. Clock Glitching: inject clock faults.
3. High-Voltage Programming: parallel HV mode can bypass locks on old parts.
4. Decapsulation: open chip, read flash via EMMI/laser.
5. Side-Channel: power analysis attacks.

Recovered AVR firmware can be:
- Re-flashed to replacement chips
- Analyzed for bug fixes
- Used for product backup
- Used for PCB cloning and reverse engineering