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sphinx: a password Store that Perfectly Hides from Itself (No Xaggeration)

libsphinx is a cryptographic password storage as described in https://eprint.iacr.org/2015/1099

and as presented by the Levchin Prize winner 2018 Hugo Krawczyk on Real World Crypto https://www.youtube.com/watch?v=px8hiyf81iM

What is this thing?

It allows you to have only a few (at least one) passwords that you need to remember, while at the same time provides unique 40 (ASCII) character long very random passwords (256 bit entropy). Your master password is encrypted (blinded) and sent to the password storage server which (without decrypting) combines your encrypted password with a big random number and sends this (still encrypted) back to you, where you can decrypt it (it's a kind of end-to-end encryption of passwords) and use the resulting unique, strong and very random password to register/login to various services. The resulting strong passwords make offline password cracking attempts infeasible. If say you use this with google and their password database is leaked your password will still be safe.

How is this different from my password storage which stores the passwords in an encrypted database? Most importantly using an encrypted database is not "end-to-end" encrypted. Your master password is used to decrypt the database read out the password and send it back to you. This means whoever has your database can try to crack your master password on it, or can capture your master password while you type or send it over the network. Then all your passwords are compromised. If some attacker compromises your traditional password store it's mostly game over for you. Using sphinx the attacker controlling your password store learns nothing about your master nor your individual passwords. Also even if your strong password leaks, it's unique and cannot be used to login to other sites or services.

Installing

Install libsodium and libsodium-dev using your operating system provided package management.

Building everything should (hopefully) be quite simple afterwards:

cd src
make

Library

libsphinx builds a library, which you can use to build your own password manager either in C/C++ or any other language that can bind to this library.

The Sphinx API

The Library exposes the following 3 functions for the FK-PTR protocol (the password storage):

void sphinx_challenge(const uint8_t *pwd, const size_t p_len, uint8_t *bfac, uint8_t *chal);

int sphinx_respond(const uint8_t *chal, const uint8_t *secret, uint8_t *resp);

int sphinx_finish(const uint8_t *pwd, const size_t p_len,
                  const uint8_t *bfac, const uint8_t *resp,
                  uint8_t *rwd);

Standalone Binaries

libsphinx comes with very simple binaries implementing the sphinx protocol, so you can build your own password storage even from shell scripts. Each step in the SPHINX protocol is handled by one binary:

step 1 - challenge

The following creates a challenge for a device:

echo -n "shitty master password" | ./challenge >c 2>b

The master password is passed in through standard input.

The challenge is sent to standard output.

A blinding factor is stored in a tempfile, the name of this file is output to stderr. This tempfile is needed in the last step again.

step 2 - device responds

Pass the challenge from step 1 on standard input like:

./respond secret <c >r0

The response is sent to standard output.

step 3 - derive password

To derive a (currently hex) password, pass the response from step 2 on standard input and the filename of the tempfile and the challenge from step 1 like:

fname=$(cat b) ./derive $fname c <r0 >pwd0

The derived password is sent to standard output and currently is a 32 byte binary string. Please note that currently this only outputs the unblinded H(pwd)^k, for the full protocol this should be hashed again with the password prepended.

step 4 - transform into ASCII password

The output from step 3 is a 32 byte binary string, most passwords have some limitations to accept only printable - ASCII - chars. bin2pass.py is a python script in the pwdsphinx python module which takes a binary input on standard input and transforms it into an ASCII password. It can have max two parameters the classes of characters allowed ([u]pper-, [l]ower-case letters, [d]igits and [s]ymbols) and the size of the password. The following examples should make this clear:

Full ASCII, max size:

./bin2pass.py <pwd0

no symbols, max size:

./bin2pass.py uld <pwd0

no symbols, 8 chars:

./bin2pass.py uld 8 <pwd0

only digits, 4 chars:

./bin2pass.py d 4 <pwd0

only letters, 16 chars:

./bin2pass.py ul 16 <pwd0

Credits

This project was funded through the NGI0 PET Fund, a fund established by NLnet with financial support from the European Commission's Next Generation Internet programme, under the aegis of DG Communications Networks, Content and Technology under grant agreement No 825310.