Silas Richelson

Haakon Larsen, Tushant Mittal, Silas Richelson et al.

Let $f: T\to \{ 0,1 \}$ be a Boolean function on the Boolean half-slice, $T$, \ie elements of $\{0,1\}^n$ with Hamming weight $n/2$. We show that if $f(x)+f(y)=f(x+y)$ holds with probability $\frac{1+δ}{2}$ over a uniform pair $(x,y)$ such that $x,y,x+y\in T$, then $f$ agrees with some linear function on at least $\frac{1+δ}{2}-o(1)$ fraction of the points in $T$. More generally, we show that if $f$ passes the natural $k$-query BLR test with probability $\frac{1+δ}{2}$ for any $k\geq3$, then it must agree with some affine function at $\frac{1+δ^{\frac{1}{k-2}}}{2}-o(1)$ fraction of the points in $T$. The only other known linearity test for the slice in the low soundness regime (i.e., when $δ$ can be arbitrarily small) was given by Kalai, Lifshitz, Minzer, and Ziegler [FOCS'24]. Our result improves upon this result in two significant ways: firstly, it works for $k=3$ queries, instead of requiring $k\geq4$; secondly, our result is sharper, e.g., when $k=4$, we are able to conclude an agreement of $\frac{1+\sqrtδ}{2}-o(1)$ instead of $\frac{1+c\sqrtδ}{2}$ for $c\approx.0035$. In particular, our result matches (up to the $o(1)$ term) the conclusion one obtains over the full hypercube via the classical BLR analysis. Our main technical contribution is a new dense model theorem using bounds on Krawtchouk polynomials. Using these Krawtchouk polynomial bounds, we also obtain a simple $k$-query test ($k\geq 5$) that avoids any use of the dense model machinery. This simplified test naturally extends to the slice over the $q$-ary hypercube, giving the first such result over larger alphabets.

Thibaut Horel, Sunoo Park, Silas Richelson et al.

We study secure and undetectable communication in a world where governments can read all encrypted communications of citizens. We consider a world where the only permitted communication method is via a government-mandated encryption scheme, using government-mandated keys. Citizens caught trying to communicate otherwise (e.g., by encrypting strings which do not appear to be natural language plaintexts) will be arrested. The one guarantee we suppose is that the government-mandated encryption scheme is semantically secure against outsiders: a perhaps advantageous feature to secure communication against foreign entities. But what good is semantic security against an adversary that has the power to decrypt? Even in this pessimistic scenario, we show citizens can communicate securely and undetectably. Informally, there is a protocol between Alice and Bob where they exchange ciphertexts that look innocuous even to someone who knows the secret keys and thus sees the corresponding plaintexts. And yet, in the end, Alice will have transmitted her secret message to Bob. Our security definition requires indistinguishability between unmodified use of the mandated encryption scheme, and conversations using the mandated encryption scheme in a modified way for subliminal communication. Our topics may be thought to fall broadly within the realm of steganography: the science of hiding secret communication in innocent-looking messages, or cover objects. However, we deal with the non-standard setting of adversarial cover object distributions (i.e., a stronger-than-usual adversary). We leverage that our cover objects are ciphertexts of a secure encryption scheme to bypass impossibility results which we show for broader classes of steganographic schemes. We give several constructions of subliminal communication schemes based on any key exchange protocol with random messages (e.g., Diffie-Hellman).