secure_hash v2

Attachmentscrypto - 48810 solves

It has come to our attention that the released version of our SecureHash class was in fact not completely secure. We're deeply sorry about that, and after whipping our sole developer an appropriate number of times we now proudly present SecureHash v2. It features twice as much security as before, and is now certified to be absolutely free of bugs.

Update: the binary was compiled with g++ and libstdc++, 64bit


1 hour



  1. Calculate the bucket number of
  2. Search 1000 credentials has same bucket number and register them.
  3. Login with

The task has two operation, register and login. It store the credential using:

sha512(sha512(name) + password)

As far as I know, no collision attack is available on this kind of digest method. But the server has some interesting code when it search for hash:

auth_result lookup_keyvalue(const std::string& name, const std::string& password) {
    std::string digest = sha512sum(name, password);
    size_t bucket = values.bucket(digest);

    auto it = values.begin(bucket), end = values.end(bucket);
    size_t iterations = 0;
    size_t MAX_ITERATIONS = 1000;

    while (it != end) {
        if (*it++ == digest)
            return AUTH_SUCCESS; // 1

        // Avoid DoS attacks by fixing upper time limit.
        if (iterations++ >= MAX_ITERATIONS)
            return AUTH_TIMEOUT; // 2

    return AUTH_FAILURE; // 0
            if (table.lookup_keyvalue(name, password)) {
                // login successfully, check root and print flag

We can login when it reach the limit of iterations, Search for 1000 credentials with same bucket number as root, pwd, register them and login with root, pwd to get the flag. Here's the code.