CodingMediumcod-g996

Subject CryptographyLevel Mid–Senior~15 minCommon in Security interviewsIndustries Software development

Question

Implement base64url decoding (the inverse of JWT-style encoding). Given a base64url string `s` that has had its '=' padding stripped, restore the padding, decode it, and return the original UTF-8 string. The input uses the URL-safe alphabet ('-' and '_'). An empty input decodes to an empty string. Assume the input is well-formed.

Implement

base64url_decode(s: str) → str

Examples

in["aGVsbG8"]out"hello"

What a strong answer looks like

State your approach and its time/space complexity out loud before you optimize. Handle the edge cases (empty input, duplicates, overflow), and say why you chose this over the brute force. Green tests are the floor, not the grade.

Learn the concepts

Vibe coding: describe the solution in plain language (or narrate it) and the coach grades your approach. Generating runnable code from your description is coming next.

Run or narrate your approach, then ask the coach.