Array value does not change after update

[sebw42]

Hello,
I noticed that when using arrays, the value of an element is sometimes not updated right away.
Here is an example:

arr = sint.Array(1)
arr.assign_all(42)

# change arr[0] to 0
new_value = sint(0) * sint(0)
arr.assign(new_value, cint(0))

# use arr[0] in a calculation. Should result in 0
times_one = arr[0] * sint(1)

# prints `0 ; 42`
print_ln('%s ; %s', arr[0].reveal(), times_one.reveal())

Assigning new_value to arr[0] only takes effect after calculating the value of times_one.
During the calculation, arr[0] seems to still be 42.

After a bit of trial and error, I found out that the bug disappears in the following cases:

• if no multiplication is involved when calculating new_value or times_one
• if not both operands of the two multiplications are secret
• if we use a python integer instead of a cint in arr.assign

Thanks for looking into this!

[mkskeller]

This is because the compiler cannot recognise that the cint(0) is the same as 0. You can either introduce a break_point() after the assign call or surround the commands with memory protection:

program.protect_memory(True)
...
program.protect_memory(False)

[sebw42]

Thank you for the quick response!
The same thing can also happen if the index is a python integer:

arr = sint.Array(1)
arr.assign(sint(1) * sint(1), 0)
arr.assign(sint(0), 0)
_ignored = sint(1) * sint(1)
print_ln('%s', arr[0].reveal()) # prints 1

Again, the wrong output does not occur if there is no secret multiplication before/after the assignment of sint(0) to arr[0].
Introducing a break_point() after one of the assigns also fixes it.
protect_memory or compiling with -M does not solve the issue here however.

Does that mean I have to add a break_point() after every assign to make sure the value is updated in time?

Thanks!

[mkskeller]

Thank you for raising this. This is indeed a bug, and 8a4b029 should fix it.