PCR
also: Platform Configuration Register
PCR (Platform Configuration Register) is a memory location in a Trusted Platform Module (TPM) that stores cryptographic measurements of system components, used to verify that a system hasn't been tampered with during boot.
PCR stands for Platform Configuration Register and is a core security feature of Trusted Platform Module (TPM) chips found on modern computers. Each PCR is a 20 or 32-byte register (depending on TPM version) that holds a cryptographic hash of system firmware, bootloader, kernel, and other early-boot components.
During system startup, the BIOS/UEFI measures each component before execution and extends the corresponding PCR by hashing its current value together with the new measurement. This creates a chain of trust—any unauthorized modification to boot components will produce a different PCR value, immediately revealing tampering.
PCRs are commonly used by dm-verity, IMA (Integrity Measurement Architecture), and disk encryption tools like cryptsetup to automatically unlock secrets only when the system's measurements match expected "golden" PCR values. For example, a sealed encryption key might only be released if PCR 0 (firmware) through PCR 7 (secure boot state) match known-good hashes.