  We study the universal coding under side-channel attacks posed and
investigated by Oohama and Santoso (2022). They proposed a theoretical security
model for Shannon cipher system under side-channel attacks, where the adversary
is not only allowed to collect ciphertexts by eavesdropping the public
communication channel, but is also allowed to collect the physical information
leaked by the devices where the cipher system is implemented on such as running
time, power consumption, electromagnetic radiation, etc. For any distributions
of the plain text, any noisy channels through which the adversary observe the
corrupted version of the key, and any measurement device used for collecting
the physical information, we can derive an achievable rate region for
reliability and security such that if we compress the ciphertext with rate
within the achievable rate region, then: (1) anyone with secret key will be
able to decrypt and decode the ciphertext correctly, but (2) any adversary who
obtains the ciphertext and also the side physical information will not be able
to obtain any information about the hidden source as long as the leaked
physical information is encoded with a rate within the rate region.
