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### Revision(s):

Revision #2 to TR18-061 | 10th April 2018 15:50

#### Indistinguishability by adaptive procedures with advice, and lower bounds on hardness amplification proofs

Revision #2
Authors: Aryeh Grinberg, Ronen Shaltiel, Emanuele Viola
Accepted on: 10th April 2018 15:50
Keywords:

Abstract:

We study how well can $q$-query decision trees distinguish between the
following two distributions: (i) $R=(R_1,\ldots,R_N)$ that are i.i.d.
variables, (ii) $X=(R|R \in A)$ where $A$ is an event s.t. $\Pr[R \in A] \ge 2^{-a}$. We prove two lemmas:

- Forbidden-set lemma: There exists $B \subseteq [N]$ of
size $poly(a,q,\frac{1}{\eta})$ such that $q$-query trees that do not
query variables in $B$ cannot distinguish $X$ from $R$ with
advantage $\eta$.

- Fixed-set lemma: There exists $B \subseteq [N]$ of size
$poly(a,q,\frac{1}{\eta})$ and $v\in \B^B$ such that $q$-query trees
do not distinguish $(X|X_B=v)$ from $(R|R_B=v)$ with advantage
$\eta$.

The first can be seen as an extension of past work by Edmonds,
Impagliazzo, Rudich and Sgall (Computational Complexity 2001), Raz
(SICOMP 1998), and Shaltiel and Viola (SICOMP 2010) to adaptive
decision trees. We can also derive it from recent, independent work by
Meir and Wigderson (ECCC 2017) who use different techniques.

We use the second, fixed-set lemma to prove lower bounds on black-box
proofs for hardness amplification that amplify hardness from $\delta$ to
$1/2-\epsilon$. Specifically:

- Reductions must make $q=\Omega(\log(1/\delta)/\epsilon^2)$
queries, implying a size loss factor'' of $q$. We also prove the
lower bound $q=\Omega(\log(1/\delta)/\epsilon)$ for error-less''
hardness amplification proofs, and for direct-product lemmas. These
bounds are tight.

- Reductions can be used to compute Majority on
$\Omega(1/\epsilon)$ bits, implying that black box proofs cannot amplify
hardness of functions that are hard against constant depth circuits
(unless they are allowed to use Majority gates).

Both items extend to pseudorandom-generator constructions.

These results prove conjectures in Viola's Ph.D.~Thesis (2006), and
improve on three incomparable previous works (Shaltiel and Viola,
SICOMP 2010; Gutfreund and Rothblum, RANDOM 2008; Artemenko and
Shaltiel, Computational Complexity 2014).

Changes to previous version:

Remark 1.5.

Revision #1 to TR18-061 | 8th April 2018 15:46

#### Indistinguishability by adaptive procedures with advice, and lower bounds on hardness amplification proofs

Revision #1
Authors: Aryeh Grinberg, Ronen Shaltiel, Emanuele Viola
Accepted on: 8th April 2018 15:46
Keywords:

Abstract:

We study how well can $q$-query decision trees distinguish between the
following two distributions: (i) $R=(R_1,\ldots,R_N)$ that are i.i.d.
variables, (ii) $X=(R|R \in A)$ where $A$ is an event s.t. $\Pr[R \in A] \ge 2^{-a}$. We prove two lemmas:

- Forbidden-set lemma: There exists $B \subseteq [N]$ of
size $poly(a,q,\frac{1}{\eta})$ such that $q$-query trees that do not
query variables in $B$ cannot distinguish $X$ from $R$ with
advantage $\eta$.

- Fixed-set lemma: There exists $B \subseteq [N]$ of size
$poly(a,q,\frac{1}{\eta})$ and $v\in \B^B$ such that $q$-query trees
do not distinguish $(X|X_B=v)$ from $(R|R_B=v)$ with advantage
$\eta$.

The first can be seen as an extension of past work by Edmonds,
Impagliazzo, Rudich and Sgall (Computational Complexity 2001), Raz
(SICOMP 1998), and Shaltiel and Viola (SICOMP 2010) to adaptive
decision trees. We can also derive it from recent, independent work by
Meir and Wigderson (ECCC 2017) who use different techniques.

We use the second, fixed-set lemma to prove lower bounds on black-box
proofs for hardness amplification that amplify hardness from $\delta$ to
$1/2-\epsilon$. Specifically:

- Reductions must make $q=\Omega(\log(1/\delta)/\epsilon^2)$
queries, implying a size loss factor'' of $q$. We also prove the
lower bound $q=\Omega(\log(1/\delta)/\epsilon)$ for error-less''
hardness amplification proofs, and for direct-product lemmas. These
bounds are tight.

- Reductions can be used to compute Majority on
$\Omega(1/\epsilon)$ bits, implying that black box proofs cannot amplify
hardness of functions that are hard against constant depth circuits
(unless they are allowed to use Majority gates).

Both items extend to pseudorandom-generator constructions.

These results prove conjectures in Viola's Ph.D.~Thesis (2006), and
improve on three incomparable previous works (Shaltiel and Viola,
SICOMP 2010; Gutfreund and Rothblum, RANDOM 2008; Artemenko and
Shaltiel, Computational Complexity 2014).

Changes to previous version:

Only change is the addition of an acknowledgement.

### Paper:

TR18-061 | 6th April 2018 16:59

#### Indistinguishability by adaptive procedures with advice, and lower bounds on hardness amplification proofs

TR18-061
Authors: Aryeh Grinberg, Ronen Shaltiel, Emanuele Viola
Publication: 6th April 2018 16:59
Keywords:

Abstract:

We study how well can $q$-query decision trees distinguish between the
following two distributions: (i) $R=(R_1,\ldots,R_N)$ that are i.i.d.
variables, (ii) $X=(R|R \in A)$ where $A$ is an event s.t. $\Pr[R \in A] \ge 2^{-a}$. We prove two lemmas:

- Forbidden-set lemma: There exists $B \subseteq [N]$ of
size $poly(a,q,\frac{1}{\eta})$ such that $q$-query trees that do not
query variables in $B$ cannot distinguish $X$ from $R$ with
advantage $\eta$.

- Fixed-set lemma: There exists $B \subseteq [N]$ of size
$poly(a,q,\frac{1}{\eta})$ and $v\in \B^B$ such that $q$-query trees
do not distinguish $(X|X_B=v)$ from $(R|R_B=v)$ with advantage
$\eta$.

The first can be seen as an extension of past work by Edmonds,
Impagliazzo, Rudich and Sgall (Computational Complexity 2001), Raz
(SICOMP 1998), and Shaltiel and Viola (SICOMP 2010) to adaptive
decision trees. We can also derive it from recent, independent work by
Meir and Wigderson (ECCC 2017) who use different techniques.

We use the second, fixed-set lemma to prove lower bounds on black-box
proofs for hardness amplification that amplify hardness from $\delta$ to
$1/2-\epsilon$. Specifically:

- Reductions must make $q=\Omega(\log(1/\delta)/\epsilon^2)$
queries, implying a size loss factor'' of $q$. We also prove the
lower bound $q=\Omega(\log(1/\delta)/\epsilon)$ for error-less''
hardness amplification proofs, and for direct-product lemmas. These
bounds are tight.

- Reductions can be used to compute Majority on
$\Omega(1/\epsilon)$ bits, implying that black box proofs cannot amplify
hardness of functions that are hard against constant depth circuits
(unless they are allowed to use Majority gates).

Both items extend to pseudorandom-generator constructions.

These results prove conjectures in Viola's Ph.D.~Thesis (2006), and
improve on three incomparable previous works (Shaltiel and Viola,
SICOMP 2010; Gutfreund and Rothblum, RANDOM 2008; Artemenko and
Shaltiel, Computational Complexity 2014).

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