feat(cyber_security): add password_strength_checker CLI

Author: Saiprashanth Pulisetti

cyber_security/password_strength_checker.py

@@ -0,0 +1,136 @@
+#!/usr/bin/env python3
+"""Password strength checker (entropy-based) with actionable feedback.
+
+Usage:
+  python -m cyber_security.password_strength_checker "P@ssw0rd!"
+  python -m cyber_security.password_strength_checker --help
+"""
+from __future__ import annotations
+
+import argparse
+import math
+import re
+import sys
+from dataclasses import dataclass
+
+
+@dataclass
+class StrengthResult:
+    bits_of_entropy: float
+    estimated_search_space: int
+    category: str
+    feedback: list[str]
+
+
+CHARSETS = {
+    "lower": "abcdefghijklmnopqrstuvwxyz",
+    "upper": "ABCDEFGHIJKLMNOPQRSTUVWXYZ",
+    "digits": "0123456789",
+    "symbols": "!@#$%^&*()-_=+[{]}\\|;:'\",<.>/?`~",
+    "space": " ",
+}
+
+
+def estimate_entropy(password: str) -> StrengthResult:
+    if not password:
+        return StrengthResult(0.0, 1, "empty", ["Password is empty."])
+
+    used = 0
+    charspace = 0
+
+    def uses(charset: str) -> bool:
+        return any(c in charset for c in password)
+
+    if uses(CHARSETS["lower"]):
+        charspace += 26
+        used += 1
+    if uses(CHARSETS["upper"]):
+        charspace += 26
+        used += 1
+    if uses(CHARSETS["digits"]):
+        charspace += 10
+        used += 1
+    if uses(CHARSETS["symbols"]):
+        charspace += len(CHARSETS["symbols"])
+        used += 1
+    if uses(CHARSETS["space"]):
+        charspace += 1
+        used += 1
+
+    # Fallback if classifier failed for some unusual unicode input
+    if charspace == 0:
+        unique_chars = len(set(password))
+        charspace = max(unique_chars, 1)
+
+    length = len(password)
+    # Shannon-style estimate: log2(charspace^length) = length * log2(charspace)
+    entropy = length * math.log2(max(charspace, 1))
+
+    # Heuristics reduce entropy for common patterns
+    penalties = 0.0
+    common_subs = [("password", 10), ("1234", 8), ("qwerty", 8)]
+    lower_pw = password.lower()
+    for patt, pen in common_subs:
+        if patt in lower_pw:
+            penalties += pen
+    if re.search(r"^(?:[A-Za-z]+\d+|\d+[A-Za-z]+)$", password):
+        penalties += 5
+    entropy = max(0.0, entropy - penalties)
+
+    # Category mapping
+    if entropy < 28:
+        category = "very weak"
+    elif entropy < 36:
+        category = "weak"
+    elif entropy < 60:
+        category = "moderate"
+    elif entropy < 80:
+        category = "strong"
+    else:
+        category = "very strong"
+
+    feedback: list[str] = []
+    if length < 12:
+        feedback.append("Use at least 12-16 characters.")
+    if used < 3:
+        feedback.append("Mix upper/lowercase, digits, and symbols.")
+    if re.search(r"(.)\\1{2,}", password):
+        feedback.append("Avoid repeated characters.")
+    if re.search(r"(\d){4,}", password):
+        feedback.append("Avoid long digit sequences.")
+
+    estimated_space = int(2 ** entropy) if entropy < 60 else int(10 ** (entropy / math.log2(10)))
+    return StrengthResult(entropy, estimated_space, category, feedback)
+
+
+def parse_args(argv: list[str]) -> argparse.Namespace:
+    parser = argparse.ArgumentParser(description="Estimate password strength and provide feedback.")
+    parser.add_argument("password", nargs="?", help="Password string to evaluate. If omitted, read from stdin.")
+    parser.add_argument("-q", "--quiet", action="store_true", help="Print only the entropy and category.")
+    return parser.parse_args(argv)
+
+
+def main(argv: list[str] | None = None) -> int:
+    args = parse_args(sys.argv[1:] if argv is None else argv)
+    pwd = args.password
+    if pwd is None:
+        pwd = sys.stdin.read().rstrip("\n")
+
+    result = estimate_entropy(pwd)
+
+    if args.quiet:
+        print(f"{result.bits_of_entropy:.2f} bits\t{result.category}")
+        return 0
+
+    print(f"Entropy: {result.bits_of_entropy:.2f} bits")
+    print(f"Estimated search space: ~{result.estimated_search_space}")
+    print(f"Category: {result.category}")
+    if result.feedback:
+        print("Feedback:")
+        for item in result.feedback:
+            print(f" - {item}")
+    return 0
+
+
+if __name__ == "__main__":  # pragma: no cover
+    raise SystemExit(main())