While previous work on lens identification by chromatic aberration succeeded in distinguishing lenses of different
model, the CA patterns obtained were not stable enough to support distinguishing different copies of the same
lens. This paper discusses on how to eliminate two major hurdles in the way of obtaining a stable lens CA pattern.
The first hurdle was overcome by using a white noise pattern as shooting target to supplant the conventional
but misalignment-prone checkerboard pattern. The second hurdle was removed by the introduction of the lens
focal distance, which had not received the attention it deserves. Consequently, we were able to obtain a stable
enough CA pattern distinguishing different copies of the same lens. Finally, with a complete view of the lens CA
pattern feature space, it is possible to fulfil lens identification among a large lens database.
An emerging form of steganographic communication uses ciphertext to replace the output of a random or strong
pseudo-random number generator. PRNG-driven media, for example computer animated backdrops in video-conferencing
channels, can then be used as a covert channel, if the PRNG bits that generated a piece of content
can be estimated by the recipient.
However, all bits sent over such a channel must be computationally indistinguishable from i.i.d. coin flips. Ciphertext
messages and even key exchange datagrams are easily shaped to match this distribution; however, when
placing these messages into a continous stream of PRNG bits, the sender is unable to provide synchronization
markers, metadata, or error correction to ensure the message's location and proper decoding.
In this paper we explore methods for message transmission and steganographic key exchange in such a "coin
flip" channel. We establish that key exchange is generally not possible in this channel if an adversary possesses
even a modest noise budget. If the warden is not vigilant in adding noise, however, communication is very simple.