Lenticrypt: a Provably Plausibly Deniable Cryptosystem

Back in 2009, I wrote about a thought experiment on how to subvert copyright law via plausible deniability. A couple years ago I expanded on that thought experiment by proposing a seedling idea on how to accomplish it via cryptography. Since then, I’ve slowly been developing that seedling into a functioning proof-of-concept, which has culminated in the creation of the Lenticrypt project:

Lenticrypt can generate a single ciphertext file such that different plaintexts are generated depending on which key is used for decryption:

$ python lenticrypt.py -e key1 plaintext1 -e key2 plaintext2 -o output.enc

$ python lenticrypt.py -d key1 output.enc | diff - plaintext1 -s
Files - and plaintext1 are identical

$ python lenticrypt.py -d key2 output.enc | diff - plaintext2 -s
Files - and plaintext2 are identical

Unlike alternative plausibly deniable cryptosystems like the recently discontinued TrueCrypt—whose ciphertext size grows in proportion to the number of plaintexts (i.e., hidden volumes) it encrypts—Lenticrypt’s ciphertext size is proportional to the largest plaintext it encrypts. This is because Lenticrypt shares bytes in the cyphertext between each of the plaintexts it encrypts; they are not stored in separate regions of the ciphertext. Therefore, there is no straightforward way to estimate the number of plaintexts that are “hidden” inside a single ciphertext.

In fact, Lenticrypt has the theoretical property that, under reasonable assumptions, there is always a near 100% probability that there exists an key in the public domain that will decrypt a given ciphertext to any desired plaintext, even if that key is not known. Therefore, even if an incriminating plaintext is revealed, the author of the ciphertext can plausibly deny having created it because there is a non-zero probability that the plaintext was legitimately decrypted by random chance.

More technical details on the cryptosystem as well as additional use-cases are described in Issue 0x04 of The International Journal of PoC||GTFO.

Note that Issue 0x04 of PoC||GTFO is a polyglot: Among other things, you can also treat the .pdf file as if it were a .zip. If you extract it, there are some neat Leticrypt-related Easter eggs inside the feelies.