*croc* allows any two computers to directly and securely transfer files and folders. When sending a file, *croc* generates a random code phrase which must be shared with the recipient so they can receive the file. The code phrase encrypts all data and metadata and also serves to authorize the connection between the two computers in a intermediary relay. The relay connects the TCP ports between the two computers and does not store any information (and all information passing through it is encrypted).
I hear you asking, *Why another open-source peer-to-peer file transfer utilities?* [There](https://github.com/cowbell/sharedrop) [are](https://github.com/webtorrent/instant.io) [great](https://github.com/kern/filepizza) [tools](https://github.com/warner/magic-wormhole) [that](https://github.com/zerotier/toss) [already](https://github.com/ipfs/go-ipfs) [do](https://github.com/zerotier/toss) [this](https://github.com/nils-werner/zget). But, after review, [I found it was useful to make another](https://schollz.github.io/sending-a-file/). Namely, *croc* has no dependencies (just [download a binary and run](https://github.com/schollz/croc/releases/latest)), it works on any operating system, and its blazingly fast because it does parallel transfer over multiple TCP ports.
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, `wss://croc3.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). You can also define your own ports with `-tcp` and `-port`. Relays can also be customized to which elliptic curve they will use (default is siec).
*croc* is similar to [magic-wormhole](https://github.com/warner/magic-wormhole#design) in spirit and design. Like *magic-wormhole*, *croc* generates a code phrase for you to share with your friend which allows secure end-to-end transferring of files and folders through a intermediary relay that connects the TCP ports between the two computers. Also like *magic-wormhole*, security is enabled by performing password-authenticated key exchange (PAKE) with the weak code phrase to generate a session key on both machines without passing any private information between the two. The session key is then verified and used to encrypt the content and meta-data with AES-256. If at any point the PAKE fails, an error will be reported and the file will not be transferred. More details on the PAKE transfer can be found at [github.com/schollz/pake](https://github.com/schollz/pake) and below in the [protocol](#protocol).
The intermediary relay uses websockets to have bidirectional communication with potential senders and recipients. Only one sender and one recipient is allowed in a channel at once. The relay helps to deliver the PAKE information, and once both parties agree, it will staple the connections between the sender and recipient and pipe all incoming TCP data from the sender to the recipient.
When sending a file with *croc*, a local relay is initiated which will try to discover local peers for connection. If a recipient is found locally, then the local connections are used and the public relay is never used. This way, *croc* can be used without a public internet connection (i.e. to transfer files over LAN).
This is an outline of the protocol used here. The basic PAKE protocol is from [Dan Boneh and Victor Shoup's crypto book](https://crypto.stanford.edu/%7Edabo/cryptobook/BonehShoup_0_4.pdf) (pg 789, "PAKE2 protocol).
1.**Sender** requests new channel and receives empty channel from **Relay**, or obtains the channel they request (or an error if it is already occupied).
2.**Sender** generates *u* using PAKE from secret *pw*.
3.**Sender** sends *u* to **Relay** and the type of curve being used. Returns error if channel is already occupied by sender, otherwise it uses it.
4.**Sender** communicates channel + secret *pw* to **Recipient** (human interaction).
5.**Recipient** connects to channel and receives UUID.
6.**Recipient** requests *u* from **Relay** using the channel. Returns error if it doesn't exist yet.
7.**Recipient** generates *v*, session key *k_B*, and hashed session key *H(k_B)* using PAKE from secret *pw*.
8.**Recipient** sends *v*, *H(H(k_B))* to **Relay**.
9.**Sender** requests *v*, *H(H(k_B))* from **Relay**.
10.**Sender** uses *v* to generate its session key *k_A* and *H(k_A)*, and checks *H(H(k_A))*==*H(H(k_B))*. **Sender** aborts here if it is incorrect.
11.**Sender** gives the **Relay** authentication *H(k_A)*.
12.**Recipient** requests *H(k_A)* from relay and checks against its own. If it doesn't match, then bail.
13.**Sender** connects to **Relay** tcp ports and identifies itself using channel+UUID.
14.**Sender** encrypts data with *k*.
15.**Recipient** connects to **Relay** tcp ports and identifies itself using channel+UUID.
16.**Relay** realizes it has both recipient and sender for the same channel so it staples their connections. Sets *stapled* to `true`.
17.**Sender** asks **Relay** whether connections are stapled.
18.**Sender** sends data over TCP.
19.**Recipient** closes relay when finished. Anyone participating in the channel can close the relay at any time. Any of the routes except the first ones will return errors if stuff doesn't exist.
I am awed by all the [great contributions](#acknowledgements) made! 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/).
