7 Novel Vulnerabilities in shamir.py — Static Analysis Report

## 7 Novel Vulnerabilities in `shamir.py` — Static Analysis Report

Hello @bitaps-com team,

I have conducted a thorough static and runtime analysis of the `shamir_secret_sharing.py` implementation in the PyBTC library as part of the ZeroNights X bug bounty program.
---

### Bug 1 — `assert` in `_gf256_div` silently removed in production (`python -O`)

**Severity:** High
**Location:** `_gf256_div()`, line 55

The line `assert a == _gf256_mul(r, b)` is the only verification that GF(256) division produced a correct result. Python removes all `assert` statements when executed with the `-O` (optimize) flag, which is standard practice in many production deployments. Any arithmetic error therefore silently propagates incorrect GF elements into every share for the affected byte, with no exception raised.

```python
# Current — silently gone with python -O
assert a == _gf256_mul(r, b)

# Fix
if _gf256_mul(r, b) != a:
    raise ArithmeticError("GF256 division verification failed")
```

---

### Bug 2 — `restore_secret({})` crashes with `TypeError` instead of a clean error

**Severity:** High
**Location:** `restore_secret()`, end of validation block

Passing an empty dict leaves `share_length = None`. The subsequent `for i in range(share_length)` raises `TypeError: 'NoneType' object cannot be interpreted as an integer` — an internal crash rather than a meaningful error message. Confirmed by execution.

```python
# Fix — add at entry
if not shares:
    raise ValueError("No shares provided")
```

---

### Bug 3 — Empty secret `b""` splits silently but `restore_secret` always fails

**Severity:** Medium
**Location:** `split_secret()` line 114 vs `restore_secret()` line 141

`split_secret(2, 3, b"")` succeeds without error (the byte loop runs 0 iterations), producing shares of length 0. `restore_secret` then raises `"Invalid shares"` because `share_length == 0` is explicitly rejected. The API is asymmetric: a user who accidentally splits an empty secret believes they hold a valid backup when recovery is structurally impossible.

```python
# Fix in split_secret
if len(secret) == 0:
    raise ValueError("Secret must not be empty")
```

---

### Bug 4 — Duplicate share indices cause unhandled `ZeroDivisionError` inside interpolation

**Severity:** Medium
**Location:** `restore_secret()` → `_interpolation()`

`restore_secret` never validates that all share indices are distinct. If two shares carry the same index, the Lagrange denominator `GF_sub(x_j, x_m) = x_j XOR x_m = 0`, which triggers `ZeroDivisionError` deep inside `_gf256_div` with no descriptive context. A caller assembling shares from user input can trivially produce this condition.

```python
# Fix
if len(set(shares.keys())) != len(shares):
    raise ValueError("Share indices must be unique")
```

---

### Bug 5 — `_gf256_pow` is O(b) loop; should be O(1) via precomputed tables

**Severity:** Medium / DoS
**Location:** `_gf256_pow()` line 28, called from `_fn()`

`_gf256_pow(a, b)` multiplies in a loop `b-1` times. For threshold=200 and a 32-byte secret this causes approximately **636,800 unnecessary multiplications** that the already-present exp/log tables make entirely redundant. This is a DoS surface on resource-constrained devices and is inconsistent with every other arithmetic primitive in the file.

```python
# Fix — O(1)
def _gf256_pow(a, b):
    if b == 0: return 1
    if a == 0: return 0
    return EXP_TABLE[(LOG_TABLE[a] * b) % 255]
```

---

### Bug 6 — `restore_secret(x=0)` parameter is publicly exposed — silent wrong output

**Severity:** Low
**Location:** `restore_secret()` function signature

`x=0` is the Lagrange evaluation target and must always be 0 to recover `f(0) = secret`. Exposing it as a public keyword argument allows a caller to accidentally pass `x=1` and silently receive a share value instead of the secret, with no error raised.

```python
# Fix — remove the parameter
def restore_secret(shares):   # x=0 is an internal detail
```

---

### Bug 7 — `import time` is a dead import

**Severity:** Info
**Location:** Line 2

`time` is imported but referenced nowhere in the file. In a cryptographic library this is worth flagging: it suggests timing-based code (possibly entropy seeding) was removed incompletely, and it is worth confirming no timing-dependent security property was inadvertently removed with it.

---

## Verification

All bugs above were identified through static code analysis and confirmed via local execution. No live shares, mnemonics, or wallet keys were used in this research.

---

## Bounty 

If any of these findings qualify for a bounty reward, please send to:

**Bitcoin address:** `bc1qw40vafh6cl57hxu8696arnsnqcmvv06qt9hhj2`

Thank you 

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7 Novel Vulnerabilities in shamir.py — Static Analysis Report #79