Potential Under-Constrained Problem within `AgeLimit` Template

[Koukyosyumei]

Hi! I noticed that the AgeLimit template is under-constrained due to the lack of necessary checks and constraints. Below are the specific issues and suggested fixes:

1. Constraint Creation with <-- Operator

The <-- operator does not create a constraint, allowing a malicious prover to replace the right-hand side with arbitrary values without detection by the verifier.

For example:

22    out <-- greaterThan.out;
23    out === 1;

This can be bypassed by replacing out <-- greaterThan.out with out <-- 1. To prevent this, the code should be updated to enforce constraints explicitly:

out <== greaterThan.out;
out === 1;

2. Input Range Checking for GreaterThan(N)

The GreaterThan(N) component assumes that inputs are constrained to fit within N bits. However, without range checks, excessively large inputs can cause incorrect outputs due to overflow.

For instance, consider:

ageLimit = 21888242871839275222246405745257275088548364400416034343698204186575808495592
age = 8

Here, GreaterThan(8) produces 1, which is incorrect.

Suggested Fix:

Use Num2Bits from CircomLib to constrain the inputs' bit lengths before passing them to GreaterThan(N):

component ageBits = Num2Bits(8);
ageBits.in <== age;

component ageLimitBits = Num2Bits(8);
ageLimitBits.in <== ageLimit;

This ensures that the inputs are correctly constrained to 8 bits, avoiding overflow issues.