Implement oblivious index operation (facebookresearch#284)

Summary:
Pull Request resolved: facebookresearch#284

This is needed for the concat function in AsciiString when accessing a character in the AsciiString while the index is secret.

Overloaded the [ ] operator so the index could be a secret Int object. Iterate through the AsciiString and assign the return value to be the current character if we are at the secret index. Return a signed secret Int object.

Differential Revision: https://www.internalfb.com/diff/D37892641?entry_point=27

fbshipit-source-id: 0b81990773e5635c2d6c33a945c96410ae863f5a

example/edit_distance/AsciiString.h

@@ -149,6 +149,17 @@ class AsciiString : public scheduler::SchedulerKeeper<schedulerId> {
     return data_[i];
   }
 
+  frontend::Int<true, 8, true, schedulerId, usingBatch> operator[](
+      frontend::Int<false, maxWidth, true, schedulerId, usingBatch> i) {
+    frontend::Int<true, 8, true, schedulerId, usingBatch> ret(0, 0);
+    for (size_t j = 0; j < maxWidth; j++) {
+      frontend::Int<false, maxWidth, false, schedulerId, usingBatch> indexInInt(
+          j);
+      ret = ret.mux(indexInInt == i, data_[j]);
+    }
+    return ret;
+  }
+
   AsciiString<maxWidth, isSecret, schedulerId, usingBatch> toUpperCase() const;
   AsciiString<maxWidth, isSecret, schedulerId, usingBatch> toLowerCase() const;
 

example/edit_distance/test/AsciiStringTest.cpp

@@ -311,4 +311,30 @@ TEST(AsciiStringTest, testPrivateSizeBatch) {
   }
 }
 
+TEST(AsciiStringTest, testObliviousIndex) {
+  scheduler::SchedulerKeeper<0>::setScheduler(
+      std::make_unique<scheduler::PlaintextScheduler>(
+          scheduler::WireKeeper::createWithUnorderedMap()));
+  using SecAsciiString = AsciiString<15, true, 0>;
+  using PubAsciiString = AsciiString<15, false, 0>;
+
+  std::string s1 = "Foo";
+  std::string s2 = "Testing";
+
+  PubAsciiString v1(s1);
+  SecAsciiString v2(s2, 1);
+
+  for (size_t i = 0; i < 15; i++) {
+    frontend::Int<false, 15, true, 0> privateIndex(i, 0);
+    if (i < s1.size())
+      EXPECT_EQ((int64_t)s1[i], v1[privateIndex].openToParty(0).getValue());
+    else
+      EXPECT_EQ(0, v1[privateIndex].openToParty(0).getValue());
+    if (i < s2.size())
+      EXPECT_EQ((int64_t)s2[i], v2[privateIndex].openToParty(1).getValue());
+    else
+      EXPECT_EQ(0, v2[privateIndex].openToParty(1).getValue());
+  }
+}
+
 } // namespace fbpcf::edit_distance