# Tamper and Clone-Resistant Authentication Scheme for Medical Image Systems
## 3 The SUC Concept and Its Realization as an Alternative to the PUF  
This section is a slightly-modified version of the same section of our earlier publications [31; 32] on the SUC design technique. It aims to make the paper self-contained, more understandable, and to provide the reader with further information on SUC-creation process. Figure 3 illustrates a possible SUC-creation process in a modern System-on-Chip (SoC) Field Programmable Gate Arrays (FPGA). The required SoC FPGA device should fulfil the following requirements:  
* A non-volatile/flash-based FPGA-fabric. * Self-reconfigurable FPGA device. * FPGA with an internal true random number generator (TRNG) meeting the requirements of a NIST standard. In such an FPGA, the SUC-creation process may proceed by a program (software package) called GENIE as follows:  
* A cipher-class \(\{E_{1},\ E_{2}\ \ldots\ E_{\sigma}\}\) with large cardinality (\(\sigma\ \rightarrow\ \infty\)) is generated. * A single-event process with the help of the internal TRNG leads to a one-time random choosing of a cipher \(E_{j}\) from the generated class \(\{E_{1},\ E_{2}\ \ldots\ E_{\sigma}\}\). * Lastly, all the dashed symbols (entities) are completely eliminated, irreversibly abolished, and fully removed from the chip in Figure 3. What remains inside the chip is just an irreversible, unrepeatable, and unpredictable cipher module \(E_{j}\) as unknown cipher-choice even to the designer himself. It should be noted again that an emerging VLSI device with a self-reconfiguring capability is fundamentally required to realize usable unknown structures so-called SUC as \"an electronic mutation\" [33]. The concept of unknown ciphers is considered as a new security paradigm. The unknown cipher is basically designed and proposed to provide a clone-resistant identity in an authentication scheme [8]. Therefore, the unknown cipher does not violate [PERSON]'s principle for a cryptosystem as long as the unknown cipher does not deal with protecting the communications between at least two parties, which requires the cipher design to be public knowledge and commonly known to all parties except the cipher-key (based on [PERSON]'s principle). It should be also pointed out that the SUC-security paradigm cannot be classified under \"security by obscurity\", where the cipher is designed to be exclusively known to the designer/manufacturer, and then kept obscure. Furthermore, if the cipher designer is not able to precisely predict and determine the generated cipher, then the cipher is considered as not known/unknown. Here, we assume that \"unclonability\" is only possible and attained if unknown structures are generated. Unknown Ciphers as Clone-Resistant Modules  
To construct a clone-resistant watermarking approach, it is required that each medical device embeds its unique SUC as an unclonable or clone-resistant identity. Generating a hardwired function SUC is based on the following key idea: \"The only secret which can be kept unrevealed is the one which nobody knows\" [34]. Figure 4 shows the SUC-creation phase processed in a secure environment as follows [32]:  
1. A software package \"GENIE\" as an SUC creator is shortly injected by a trusted authority (TA) into a SoC FPGA. 2. The GENIE generates/chooses a cipher with the help of an internal unpredictable bit stream from the internal TRNG. 3. The GENIE is irreversibly eliminated and completely removed from the SoC FPGA. What remains inside the SoC FPGA is an unchangeable, non-repeatable, and unremovable cipher (SUC) which no one knows. Figure 3: Possible secret unknown cipher (SUC) generation scenario. Adapted from [32]. SUC enrollment phase [32]:  
4. TA randomly chooses a set of plaintexts \(\{x_{1}\),... \(x_{T}\}\) out of the 2\({}^{n}\) possible inputs, where the SUC-input size is \(n\) bits input-size. 5. TA stimulates the SoC with the set of plaintexts \(\{x_{1}\),... \(x_{T}\}\) to get the corresponding ciphertexts \(\{y_{1}\),... \(y_{T}\}\) using its SUC. 6.