In the article referred to by Agnes,   Farmer, Sprout and Witzel argue that the Indus script is not a language-based writing system. 

Parpola, Asko, 2008. Is the Indus script indeed not a writing system? Pp. 111-131 in: Airavati: Felicitation volume in honour of Iravatham Mahadevan. Chennai: Varalaaru.com. 

I think the Indus script is partially decipherable, but with methodology that differs completely from that of Yajnadeva: 

I have proposed (at least to some extent) verifiable Proto-Dravidian interpretations for some two dozen signs in my book Deciphering the Indus Script (Cambridge UP 1994), 

summarized in chapter 21 (pp. 266-291) of my book The Roots of Hinduism (OUP 2015).  Some further interpretations based on the same methodology 

and basic premises have been presented in other publications, one forthcoming. But it is very difficult to progress further, and  with the presently available source material 

a complete decipherment of the Indus script is impossible. 

With best wishes, Asko