Venue: ICLR 2023 Conference

Reviews:

final_score: rating: 6: marginally above the acceptance threshold
review_content: summary_of_the_paper: In this paper, the authors formalize the relationship between perfect security and couplings of distributions, and propose the first instance of a steganography method that has the benefits of non-trivial efficiency and perfect security guarantees. Experiments with GPT-2 and WaveRNN demonstrate the effectiveness of the proposed method. Also, the code is provided for reproducibility. 
strength_and_weaknesses: The paper is well-written. The authors start by formulizing and explaining the relationship between perfect security and couplings of distributions through two results. Then, the authors accordingly introduce the first instance of steganography algorithm with non-trivial efficiency and perfect security guarantees.

Since steganography is not my primary research area, the theoretical part of this paper is beyond my knowledge. So, I cannot comment its theoretical contribution reaches the standard of ICLR. But it seems to me that the proposed method is well-motivated, based on two findings that the perfect steganographic security is, in fact, equivalent to a coupling problem and achieving maximal transmission efficiency among perfect security procedures is equivalent to a minimum entropy coupling problem. So, I rate this paper as borderline accept for now and will refer to the comments of other reviewers for my final rating.

clarity,_quality,_novelty_and_reproducibility: - The paper is well-written.
- The proposed method seems novel, i.e., the first instance of steganography algorithm with non-trivial efficiency and perfect security guarantees.
- Code is provided for reproducibility.
summary_of_the_review: Steganography is not my primary research area. It seems to me that the paper has a solid theoretical support.
review_title: null

final_score: rating: 1: strong reject
review_content: summary_of_the_paper: In this paper, the authors show that a steganography procedure is perfectly secure under Cachin (1998)’s information theoretic-model of steganography if and only if it is induced by a coupling. Moreover, the authors show that, among perfectly secure procedures, a procedure is maximally efficient if and only if it is induced by a minimum entropy coupling.
strength_and_weaknesses: Strength::
Security is an important topic in computing and should be pursued.

Weakness::
The paper can be improved based on the following points.
1) The title of the paper is not appropriate as it is misleading and should be changed. In security field, there is no such thing as "perfectly secure" mechanism. Every security measures has its own Pros and Cons depending on circumstances. The title should be revised.
2) In section 1, it is mentioned, "A procedure is said to be perfectly secure if it guarantees a divergence of zero." This is only true in theoretical sense and not in practical. So, this should be made clear in the statement with appropriate citations.
3) The motivation in the section 1 is not strong enough. Provide a motivational case study to engage the readers and to prove the importance of such research.
4) Section 2 (Background) is a bit misleading in written format as it mostly consist of problem formulation rather than background research. Please rename the title and provide background research on other existing related methods.
5) In section 3 & 4, please define each variables used/mentioned for readability proposes. Not everyone who would be reading this paper would be a field expert in computer security.
6) Section 5 (experimental evaluation) is weak. Comparative study is lacking and should be included to show the efficacy of the claims being made. Pleas provide that.
7) In a conference paper, it might be better to segregate future works into a separate section along with discussions. And also have a separate conclusion section.
8) The paper seems a bit out of scope for this conference and might be better tuned towards computer security related conferences.
clarity,_quality,_novelty_and_reproducibility: Lacks clarity and novelty is questionable. Please note the weaknesses section in the review.
summary_of_the_review: 1) The title of the paper is not appropriate as it is misleading and should be changed. In security field, there is no such thing as "perfectly secure" mechanism. Every security measures has its own Pros and Cons depending on circumstances. The title should be revised.
2) In section 1, it is mentioned, "A procedure is said to be perfectly secure if it guarantees a divergence of zero." This is only true in theoretical sense and not in practical. So, this should be made clear in the statement with appropriate citations.
3) The motivation in the section 1 is not strong enough. Provide a motivational case study to engage the readers and to prove the importance of such research.
4) Section 2 (Background) is a bit misleading in written format as it mostly consist of problem formulation rather than background research. Please rename the title and provide background research on other existing related methods.
5) In section 3 & 4, please define each variables used/mentioned for readability proposes. Not everyone who would be reading this paper would be a field expert in computer security.
6) Section 5 (experimental evaluation) is weak. Comparative study is lacking and should be included to show the efficacy of the claims being made. Pleas provide that.
7) In a conference paper, it might be better to segregate future works into a separate section along with discussions. And also have a separate conclusion section.
8) The paper seems a bit out of scope for this conference and might be better tuned towards computer security related conferences.
review_title: null

final_score: rating: 8: accept, good paper
review_content: summary_of_the_paper: This paper presents theoretical proofs of perfectly secure (zero KL divergence) steganography, particularly that it depends on being induced by a coupling (necessary and sufficient condition) and that for efficiency it will also depend on a minimum entropy coupling (that benefit from very recent advances in the approximation of minimum entropy coupling iteratively). The paper presents experiments showing practicality of the assumptions in a (unrealistic) noise-free channel. 
strength_and_weaknesses: 
Strengths:
1) The paper provides thorough theoretical derivations for this increasingly interesting and important topic
2) The paper is very clear and methodologically correct
3) The fact that a theoretical constraint is also efficient is a very interesting result of this paper

Weaknesses:
1) The fact that the paper assumed the Cachin (1998) model with transmission being made by a noise-free (perfect) channel is, as pointed out by the authors, a drawback that limits the importance and applicability of these results. 
2) Nowadays, important applications of steganography are not limited to 1D signals and thus application on 2D images are of extreme importance, opening relevant applications. I suggest the authors briefly discuss this in the paper, particularly the applicability of the paper results to images.

clarity,_quality,_novelty_and_reproducibility: The paper is very clear with a high overall quality.
The results are quite novel and reproducibility does not seem to be an issues.

summary_of_the_review: The paper is a theoretical paper with thorough derivations and very clear discussions. The result impact is somehow limited to the assumptions of a noise-free channel, but still important.
review_title: null

final_score: rating: 6: marginally above the acceptance threshold
review_content: summary_of_the_paper: This work proposes to view the task of steganography as a minimum entropy coupling problem. The authors show that (1) coupling algorithms yield steganography procedures with perfect security; (2) the approach induced by minimum entropy coupling maximizes information throughput; (3) apply the gained insights to steganography with deep generative model covertext distributions. 
strength_and_weaknesses: ## Strengths
(+) To the best of my knowledge, this is the first work approaching the task of steganography as a minimum entropy coupling problem.
(+) To the best of my knowledge, this is the first steganography algorithm to achieve perfect security guarantees (in the context of Cachin (1998)’s information theoretic model of steganography)  
(+) The presented results based on the Kullback-Leibler divergences are convincing (close to zero)   
(+) The approach is well motivated by the existing theory.  

## Weaknesses
(-) This work relies on the assumption that “the exact distribution of covertext is assumed to be known”. However, in practice, this might not be the case, since the model of language might not be perfectly known. Hence, for most practical scenarios this approach might not provide perfectly secure steganography.  
(-) While the results in Figure 2 are close to zero, they are not exactly zero. Does this mean that perfect security is not obtained as claimed?  
(-) The authors evaluated the security of their approach empirically, however, it is not clear if this method can withstand modern steganalysis methods [A, B, C, D], or human evaluation.   

[A] Modern Text Hiding, Text Steganalysis, and Applications: A Comparative Analysis; Entropy. 2019  
[B] Linguistic Steganalysis With Graph Neural Networks; IEEE Signal Processing Letters. 2021  
[C] A fast and efficient text steganalysis method; IEEE Signal Processing Letters. 2019  
[D] Exploiting language model for efficient linguistic steganalysis; ICASSP 2022  

clarity,_quality,_novelty_and_reproducibility: ## Clarity
This work is technically sound to the best of my judgment.

## Quality 
This work is clearly written and easy to follow.

## Novelty
This work holds novelty for a subgroup of the ML community. 

## Reproducibility
This work is reproducible to the best of my judgment. 

summary_of_the_review: I believe that this work holds some novelty for a subgroup in the ML community. While the assumptions made in this work heavily rely on the assumptions made by Cachin might not hold in practical scenarios, the authors provide a “secure” steganography approach from a theoretical standpoint. The authors also showed that their approach is effective in some practical scenarios.
review_title: null

Meta-review: metareview:_summary,_strengths_and_weaknesses: In this work, the authors show that a steganography procedure has "perfect" security under Cachin's information theoretic-model of steganography if and only if it is induced by a coupling. They also show that among perfectly secure procedures, a procedure is maximally efficient if and only if it is induced by a minimum entropy coupling. The authors also develop steganography algorithms that achieve perfect security guarantees with non-trivial efficiency and are highly scalable. Empirical validation is provided by comparing a minimum entropy coupling-based approach to three modern baselines using GPT-2 and WaveRNN as communication channels. The results suggest that it may be natural to view information-theoretic steganography through the lens of minimum entropy coupling. Most reviewers thought the paper is well-written, the method is novel, and liked the fact that this is the first instance of steganography algorithm with non-trivial efficiency and perfect security guarantees. The reviewers raised concerns about using the Cachin model, restriction to 1D, perfectly secure definition and other issues. In my opinion the authors provided an adequate response to the reviewers. I recommend acceptance.






justification_for_why_not_higher_score: The paper makes interesting contributions but does have some flaws so does not merit an oral presentation based on the scores and my reading of the paper
justification_for_why_not_lower_score: I am choosing to disregard one reviewer which clearly seemed to have made quite a few mistakes this puts the average score at 6.7 making it a non-borderline accept paper.
Decision: Accept: poster