fix: apply security audit results

In particular, this PR consider FPS-14/15/16/17/18/19/20.  Changes are applied for FPS-15/16/20.

README.md

@@ -17,7 +17,7 @@ There are currently four different crates:
 
 
 - [**Filecoin Proofs (`filecoin-proofs`)**](./filecoin-proofs)
-  A wrapper around `storage-proofs`, providing an FFI-exported API callable from C (and in practice called by [go-filecoin](https://github.com/filecoin-project/go-filecoin') via cgo). Filecoin-specific values of setup parameters are included here, and circuit parameters generated by Filecoin’s (future) trusted setup will also live here.
+  A wrapper around `storage-proofs`, providing an FFI-exported API callable from C (and in practice called by [go-filecoin](https://github.com/filecoin-project/go-filecoin) via cgo). Filecoin-specific values of setup parameters are included here, and circuit parameters generated by Filecoin’s (future) trusted setup will also live here.
 
 
     ![FPS crate dependencies](/img/fps-dependencies.png?raw=true)

storage-proofs/core/src/crypto/feistel.rs

@@ -222,7 +222,7 @@ mod tests {
                 // Since every element in the set is reversibly mapped to another element also in the set,
                 // this is indeed a permutation.
                 assert_eq!(i, v, "failed to permute");
-                assert!(p <= *n, "output number is too big");
+                assert!(p < *n, "output number is too big");
             }
         }
     }

storage-proofs/core/src/sector.rs

@@ -40,8 +40,8 @@ impl fmt::Display for SectorId {
 }
 
 impl SectorId {
-    pub fn as_fr_safe(self) -> [u8; 31] {
-        let mut buf: [u8; 31] = [0; 31];
+    pub fn as_fr_safe(self) -> [u8; 32] {
+        let mut buf: [u8; 32] = [0; 32];
         byteorder::LittleEndian::write_u64(&mut buf[0..8], self.0);
         buf
     }