The use of embedded secret information such as keys for cryptographic applications, unique identifiers for authentication, and activation of on-chip features is becoming increasingly employed in ICs. The generation of secret bit strings using PUFs offers distinct cost and security advantages over conventional approaches of storing keys in RAM, ROM, or programmable fuses. For example, PUFs can be used to efficiently implement challenge–response authentication without the need to pre-program or load pre-assigned keys. As the PUF characteristics are not directly revealed by the challenge-response mechanism, such an IC is highly resistant to attacks. Below are some of the current and emerging solutions employing PUF technology.