Add initial fuzz test

fuzz/fuzz_roundtrip.py

@@ -0,0 +1,42 @@
+import sys
+import atheris
+
+with atheris.instrument_imports():
+    import rsa
+
+@atheris.instrument_func
+def TestOneInput(input_bytes):
+    fdp = atheris.FuzzedDataProvider(input_bytes)
+    key = fdp.ConsumeIntInRange(16, 9999)
+    message = fdp.ConsumeBytes(atheris.ALL_REMAINING)
+
+    try:
+        pub, priv = rsa.newkeys(key)
+    except ValueError:
+        # newskeys raises a ValueError in the event of a legit error. The fuzzer
+        # will often generate input that triggers such errors, and we can simply
+        # ignore them, although the fuzzer should continue running and thus we
+        # return.
+        return
+
+    try:
+        encrypted = rsa.encrypt(message, pub)
+    except OverflowError:
+        # encrypt calls into _pad_for_encryption which raises an overflow error.
+        # Similar to above, the fuzzer will generate inputs that trigger this
+        # exception and in this event we simply want the fuzzer to continue.
+        # As such, we return so the fuzzer can continue with its next iteration.
+        return
+
+    decrypted = rsa.decrypt(encrypted, priv)
+    assert(decrypted == message)
+
+
+def main():
+    atheris.instrument_all()
+    atheris.Setup(sys.argv, TestOneInput, enable_python_coverage=True)
+    atheris.Fuzz()
+
+
+if __name__ == "__main__":
+    main()