*croc* is a tool that allows any two computers to simply and securely transfer files and folders. There are many tools that can do this but AFAIK *croc* is the only tool that is easily installed and used on any platform, *and* has secure peer-to-peer transferring, *and* has the capability to resume broken transfers.
*croc* securely transfers data using *code phrases* - a combination of three random words (mnemonicoded 4 bytes). The code phrase is shared between the sender and the recipient for password authenticated key exchange ([PAKE](https://github.com/schollz/pake)), a cryptographic method to use a shared weak key (the "code phrase") to generate a strong key for secure end-to-end encryption. By default, a code phrase can only be used once between two parties so an attacker would have a chance of less than 1 in *4 billion* to guess the code phrase correctly to steal the data. An attacker with the wrong code phrase will fail the PAKE and the sender will be notified without any data transfering. Only two people with the right code phrase will be able to computers transfer encrypted data through a relay.
The actual data transfer is accomplished using a relay, either using raw TCP sockets or websockets. If both computers are on the LAN network then *croc* will use a local relay, otherwise a public relay is used. All the data going through the relay is encrypted using the PAKE-generated session key, so the relay can't spy on information passing through it. The data is transferred in blocks, where each block is compressed and encrypted, and the recipient keeps track of blocks received so that it can resume the transfer if interrupted.
*croc* differs from a utility like *scp* because it doesn't require any two computers to have enabled port-forwarding. Instead, *croc* will uses a relay - a temporary server setup locally (if both computers are on lan) or publicly (default is at croc4.schollz.com). Any two computers can connect to the relay, and after securing their channel with PAKE, they can transfer encrypted metadata and data through the relay. The relay works by first having the computers communicate the PAKE protocol via websockets, and then exchanging encrypted metadata, and then stapling the TCP connections directly so that they can transfer directly.
My motivation to write *croc*, as stupid as it sounds, is because I wanted to create a program that made it easy to send a 3GB+ PBS documentary to my friend in a different country. My friend has a Windows computer and is not comfortable using a terminal. So I wanted to write a program that, while secure, is simple to receive a file. *croc* accomplishes this, and now I find myself using it almost everyday at work. To receive a file you can just download the executable and double click on it. The name is inspired by the [fable of the frog and the crocodile](https://web.archive.org/web/20180926035731/http://allaboutfrogs.org/stories/crocodile.html).
The first example shows the basic transfer of some file or folder from computer 1 to computer 2. _These two gifs should run in sync if you force-reload (Ctl+F5)_
The second example shows how you can restart a broken transfer. Here, computer 2 presses Ctl+C during a transfer to abruptly break the connection, and then resumes by having computer 1 re-send the file. _These two gifs should run in sync if you force-reload (Ctl+F5)_
In version 4, there is now a GUI available for [Windows](https://github.com/schollz/croc/releases/latest) and [Linux](https://github.com/schollz/croc/releases/latest):
Or, you can [install Go](https://golang.org/dl/) and build from source with `go get github.com/schollz/croc`. Since *croc* uses [Go modules](https://golang.org/doc/go1.11#modules) it requires Go version 1.11+.
to receive (you'll be prompted to enter the code phrase). Note, by default, you don't need any arguments for receiving, instead you will be prompted to enter the code phrase. This makes it possible for you to just double click the executable to run (nice for those of us that aren't computer wizards).
*croc* automatically will attempt to start a local connection on your LAN to transfer the file much faster. It uses [peer discovery](https://github.com/schollz/peerdiscovery), basically broadcasting a message on the local subnet to see if another *croc* user wants to receive the file. *croc* will utilize the first incoming connection from either the local network or the public relay and follow through with PAKE.
You can change this behavior by forcing *croc* to use only local connections (`--local`) or force to use the public relay only (`--no-local`):
In this case *croc* will automatically use the stdin data and send and assign a filename like "croc-stdin-123456789". To receive to stdout at you can always just use the `-yes` and `-stdout` flags which will automatically approve the transfer and pipe it out to stdout.
The relay is needed to staple the parallel incoming and outgoing connections. The relay temporarily stores connection information and the encrypted meta information. The default uses a public relay at, `croc4.schollz.com`. You can also run your own relay, it is very easy, just run:
Make sure to open up TCP ports (see `croc relay --help` for which ports to open). Relays can also be customized to which elliptic curve they will use (default is siec).
*croc* has been through many iterations, and I am awed by all the great contributions! If you feel like contributing, in any way, by all means you can send an Issue, a PR, ask a question, or tweet me ([@yakczar](http://ctt.ec/Rq054)).
- ...[@warner](https://github.com/warner) for the [idea](https://github.com/warner/magic-wormhole).
- ...[@tscholl2](https://github.com/tscholl2) for the [encryption gists](https://gist.github.com/tscholl2/dc7dc15dc132ea70a98e8542fefffa28).
- ...[@skorokithakis](https://github.com/skorokithakis) for [code on proxying two connections](https://www.stavros.io/posts/proxying-two-connections-go/).
