With the increasing popularity of small computing devices and applications of IoT, the need for platform integrity grows both in scale and scope. In particular, the detection of successful attempts to inject a malicious software module or modify an existing one is of utmost importance. This paper promotes LiSB, a novel approach for validating software/firmware integrity and ensuring secure boot-up for resource-constrained embedded devices. LiSB is lightweight, yet very robust. A hardware primitive is used as a Root-of-Trust to support the confidentiality of generated digests and the security of the attestation protocol. Specifically, LiSB employs Physically Unclonable Functions (PUFs) to make the digest device-specific without storing any secrets in the device memory. The performance and robustness of LiSB are validated using a prototype implementation on an FPGA. The results demonstrate that LiSB outperforms recently-published and prominent commercial attestation schemes like TPM, and consumes 25 times less power than SHA-256, which serves as the core component of most existing attestation schemes. The security properties of LiSB are formally analyzed.