challenge-su
Copyright 2016 Joshua Juran
This is an implementation of su that authenticates non-root users by
issuing a cryptographic challenge, which must be answered by an Ed25519
signature of the challenge using the correct secret key. It's intended
for use with Android devices.
Unlike the typical Android su, this one does not solicit confirmation
by the user through a Superuser app. Instead, it issues a cryptographic
challenge on stdout, to which it expects a response on stdin.
The challenge is a sequence of 32 mostly-random bytes, in hexadecimal
form. The response is an Ed25519 signature of the challenge. For the
benefit of adb shell users, the actual message to be signed consists
of the 64 hexadecimal digits (using lowercase letters) followed by LF --
65 bytes in total. The trailing newline is included for convenience
with utilities like echo and cat.
The message must be signed with the Ed25519 secret key corresponding to
the public key installed on the device. The resulting 64-byte signature
must then be encoded in hexadecimal (either case) and entered on stdin
(presumably via Paste), followed by a newline (LF). If the signature
verifies, then su sets the gid and uid to root and execs the shell.
The intended use case is a user running adb shell from a trusted host
where the secret key is stored, seeking to invoke a root shell for the
purpose of performing privileged maintenance tasks.
DISCLAIMER: I'm not a cryptographer or a security expert. Don't assume this program achieves any particular standard of security or correctness. Read the code and do your own analysis.
This program is free software: you can redistribute it and/or modify it under the terms of the GNU Affero General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.
This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Affero General Public License for more details.
You should have received a copy of the GNU Affero General Public License along with this program. If not, see http://www.gnu.org/licenses/.