# The Odyssey of Entropy: Cryptography
## 3 Entropy and Encryption
### Modeling, Design and Implementation
#### 3.1.1 Security Models Related to Entropy
It is well known that the only real-world implementation of perfect secrecy (in its pure notion) is one-time pad (OTP), wherein the key is at least as long as the plaintext and needs to be updated with each new plaintext. However, some variants of perfect secrecy have received research focus. In the following, we review some well-studied variants of perfect secrecy as well as some related research works. * **Perfect Forward Secrecy:** Perfect forward secrecy depends on frequent changes in the encryption/decryption key (e.g., with each call or each message in a conversation, or each web page reload) in order to prevent the cryptosystem from being broken if a key is compromised. Several researchers have proposed encryption systems providing perfect forward secrecy to be used in different communication systems. For example, two email protocols with perfect forward secrecy were proposed by [PERSON] et al. [46]. However, some flaws in the reasoning presented by [PERSON] et al. [46] were reported by [PERSON] [47], and two new robust email protocols with guaranteed perfect forward secrecy were introduced by [PERSON] and [PERSON] [48]. Later on, a method for cryptanalysis of the protocols proposed by [PERSON] and [PERSON] [48] was presented by [PERSON] and [PERSON] [49]. In recent years, perfect forward secrecy has been considered in several other areas. For example, a lightweight transport layer security protocol with perfect secrecy was proposed by [PERSON] et al. [50]. As another example, perfect secrecy is guaranteed to be provided by a high-performance key agreement protocol proposed by [PERSON] et al. [51]. * **One-Time Pad (OTP):** OTP is the only perfectly secure cryptographic scheme used in real-world applications. Although some researchers believe that OTP is more of a key safeguarding scheme than a cryptosystem [52], a vast number of research works have considered OTP as (part of) the security solution in a broad spectrum of applications. The feasibility of perfectly secure cryptography using imperfect random sources was studied by [PERSON] and [PERSON] [53]. [PERSON] et al. [54] proposed an OTP cryptosystem in which the receiver does not need the OTP to decrypt the ciphertext, while the OTP fully affects the plaintext from the adversaries point of view. The application of OTP in scenarios where the receiver may not be trustworthy was studied by [PERSON] and [PERSON] [55]. An OTP-based cryptosystem was proposed by [PERSON] and [PERSON] [56] for BANs (body area networks), wherein messages are short, and large volumes of NVM (non-volatile memory) are available. This cryptosystem stores pre-calculated OTPs in the NVM for future use. Moreover, OTPs have been used in a spectrum of environments, such as multi-user one-hop wireless networks [57], IMDs (implantable medical devices) [58], UAVs (ynmanned aerial vehicles) [59], medical images [60], mobile instant messaging [61], coded networks [62] and credit cards [63]. OTP has been also used in quantum computing, especially in QKD (quantum key distribution) [64; 65; 66; 67; 68; 69; 70].