Contributed by Hassan Salmani; University of Connecticut
After each 128'hFFFF_FFFF_FFFF_FFFF_FFFF_FFFF_FFFF_FFFF encryptions, the Trojan gets activated and leaks the secret key from a cryptographic chip running the AES algorithm through a covert channel. The channel adapts the concepts from spread spectrum communications (also known as Code-Division Multiple Access (CDMA)) to distribute the leakage of single bits over many clock cycles. The Trojan employs this method by using a pseudo-random number generator (PRNG) to create a CDMA code sequence, the PRNG initialized to the input plaintext. The code sequence is then used to XOR modulate the secret information bits. The modulated sequence is forwarded to a leakage circuit (LC) to set up a covert CDMA channel in the power side-channel. The LC is realized by connecting eight identical flip-flop elements to the single output of the XOR gate to mimic a large capacitance .