In some encryption systems it is necessary to preserve the format and length of the encrypted data. This kind of encryption is called FPE (Format Preserving Encryption) or DPE (Datatype Preserving Encryption). For example, in the case of PCS (Physical Coding Sublayer) at Gigabit Ethernet optical communications, the 8b10b symbol flow can be encrypted at line rate thanks to a symmetric encryption scheme that must preserve the format and coding properties of 8b10b symbols. In this case, since a high speed data flow needs to be encrypted preserving its format, the usage of an FPE stream cipher could be advantageous. For this purpose, a new keystream generator scheme is proposed in this work. It is based on a secure block cipher working in CTR (Counter) mode whose output is subjected to a modulo operation. Moreover, a security analysis for this mode is carried out, deriving an expression for the IND-CPA (Indistinguishability under Chosen Plaintext Attack) advantage of any adversary, concluding that this mode can be considered secure in the same way as traditional modes are. Furthermore, the proposed structure has been implemented over an FPGA (Field Programmable Gate Array) device and adapted to a Gigabit Ethernet application.

In this work a new self-synchronized encryption method for 10 Gigabit optical links is proposed and developed. Necessary modifications to introduce this kind of encryption in physical layers based on 64b/66b encoding, such as 10 GBase-R, have been considered. The proposed scheme encrypts directly the 64b/66b blocks by using a symmetric stream cipher based on an FPE (Format Preserving Encryption) block cipher operating in PSCFB (Pipelined Statistical Cipher Feedback) mode. One of the main novelties in this paper is the security analysis done for this mode. For the first time, an expression for the IND-CPA (Indistinguishability under Chosen-Plaintext Attack) advantage of any adversary over this scheme has been derived. Moreover, it has been concluded that this mode can be considered secure in the same way of traditional modes are. In addition, the overall system has been simulated and implemented in an FPGA (Field Programmable Gate Array). An encrypted optical link has been tested with Ethernet data frames, concluding that it is possible to cipher traffic at this level, getting maximum throughput and hiding traffic pattern from passive eavesdroppers.

A new technique for improving the security of a chaotic cryptosystem has been proposed. An encryption system based on a Skew Tent Map using this technique has been implemented on a Virtex 7 FPGA. The results show that, using this method, the randomness of the sequences can be considerably improved by using a small number of extra resources.

In this work a new physical layer encryption method for Ethernet 1000Base-X standard is proposed. The proposed encryption scheme uses a chaotic stream cipher to encrypt the 8b10b symbols flow at PCS (Physical Coding Sublayer) level. The complete system has been implemented in an FPGA (Field Programmable Gate Array). Experimental results are analyzed, concluding that it is possible to cipher traffic at this level and hide the complete Ethernet traffic pattern from any malicious observer. Moreover, no overhead is introduced during encryption and no throughput losses are produced.