This is more or less (but mostly *less*) a Golang port of [@warner's](https://github.com/warner) [*magic-wormhole*](https://github.com/warner/magic-wormhole) which allows you to directly transfer files between computers. I wrote this because I wanted to send my friend Jessie a file using *magic-wormhole*. However, when I told Jessie how to install the dependencies for *magic-wormhole* she made this face: :sob:. So, nominally, *croc* does the same thing (file transfer directly between computers) without dependencies so you can just double-click on your computer, even if you use Windows.
[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, no we don't, because after review, [I found it was useful to make a new one](https://schollz.github.io/sending-a-file/).
Note, by default, you don't need any arguments for receiving! This makes it possible for you to just double click the executable to run (nice for those of us that aren't computer wizards).
Or, if you are like my good friend Jessie and "*just can't even*" with programming, [download the latest release for your system](https://github.com/schollz/croc/releases/latest).
*croc* relies on a TCP relay 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, `cowyo.com`, which has no guarantees except that I guarantee to turn if off as soon as it gets abused ([click here to check the current status of the public relay](https://stats.uptimerobot.com/lOwJYIgRm)).
Now, when you use *croc* to send and receive you should add `-server your-server.com` to use your relay server.
_Note:_ If you are behind a firewall, make sure to open up TCP ports 27001-27009.
# How does it work?
*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 transfering of files. The similarities may diverge from here.
The code phrase is 16 random bits that are [menemonic encoded](http://web.archive.org/web/20101031205747/http://www.tothink.com/mnemonic/) plus a prepended integer to specify number of threads. This code phrase is hashed using sha256 and sent to the relay which maps that key to that connection. When the relay finds a matching key for both the receiver and the sender (i.e. they both have the same code phrase), then the sender transmits the encrypted metadata to the receiver through the relay. Then the receiver decrypts and reviews the metadata (file name, size), and chooses whether to consent to the transfer.
After the receiver consents to the transfer, the sender transmits encrypted data through the relay. The relay setups up [Go channels](https://golang.org/doc/effective_go.html?h=chan#channels) for each connection which pipes all the data incoming from that sender's connection out to the receiver's connection. After the transmission the channels are destroyed and all the connection and meta data information is wiped from the relay server. The encrypted file data never is stored on the server.
Encryption uses PBKDF2 (see [RFC2898](http://www.ietf.org/rfc/rfc2898.txt)) where the code phrase shared between the sender and receiver is used as the passphrase. For each of the two encrypted data blocks (metadata stored on relay server, and file data transmitted), a random 8-byte salt is used and a IV is generated according to [NIST Recommendation for Block ciphers, Section 8.2](http://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication800-38d.pdf).
**Decryption**
On the receiver's computer, each piece of received encrypted data is written to a separate file. These files are concatenated and then decrypted. The hash of the decrypted file is then checked against the hash transmitted from the sender (part of the meta data block).
# License
MIT
# Acknowledgements
Thanks...
- ...[@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/).
