diff --git a/index.md b/index.md
index b015d17..d4a96ac 100644
--- a/index.md
+++ b/index.md
@@ -7,11 +7,15 @@
 - ["Probabilistic Proof Systems"](http://people.cs.georgetown.edu/jthaler/COSC544.html) course notes
 - Vitalik Buterin's introduction to SNARKs, part [1](https://medium.com/@VitalikButerin/quadratic-arithmetic-programs-from-zero-to-hero-f6d558cea649), [2](https://medium.com/@VitalikButerin/exploring-elliptic-curve-pairings-c73c1864e627), and [3](https://medium.com/@VitalikButerin/zk-snarks-under-the-hood-b33151a013f6); and STARKs, part [1](https://vitalik.ca/general/2017/11/09/starks_part_1.html), [2](https://vitalik.ca/general/2017/11/22/starks_part_2.html), and [3](https://vitalik.ca/general/2018/07/21/starks_part_3.html).
 
-## History of Zero-knowledge proofs
+## History of Proof Systems
 
 - Invention of zero-knowledge
   - [Zero-Knowledge Proofs [GMR85]](http://groups.csail.mit.edu/cis/crypto/classes/6.876/papers/gmr-ZK.pdf)
   - [Non-Interactive ZK [BFM88]](https://dl.acm.org/citation.cfm?id=62222)
+- Witness-Indistinguishable proof systems
+  - [Definition of witness-indistinguishability [FS90]](https://dl.acm.org/citation.cfm?id=100272)
+  - [Zaps and Their Applications [DN00]](http://www.wisdom.weizmann.ac.il/~/naor/PAPERS/zap.pdf)
+  - Non-interactive zaps [GOS06,Raf15,FO18]
 - Important landmarks for zk-SNARKs
   - [Succinct ZK[K92]](http://people.csail.mit.edu/vinodv/6892-Fall2013/efficientargs.pdf)
   - [Succinct Non-Interactive ZK [M94]](https://projecteuclid.org/download/pdf_1/euclid.lnl/1235415908)
@@ -48,6 +52,11 @@
 - Hybrid NIZK ([ACM18])
 - Aurora ([BCRSVW18])
 
+[GOS06]: http://www0.cs.ucl.ac.uk/staff/J.Groth/NIZKJournal.pdf
+[Raf15]: https://ia.cr/2015/050
+[GO18]: https://ia.cr/2018/228
+[FS90]: https://dl.acm.org/citation.cfm?id=100272
+[DN00]: http://www.wisdom.weizmann.ac.il/~/naor/PAPERS/zap.pdf
 [GGPR13]: https://eprint.iacr.org/2012/215
 [PGHR13]: https://eprint.iacr.org/2013/279.pdf
 [BCGTV13]: https://eprint.iacr.org/2013/507