feat: docs, name, license

Signed-off-by: Carlos A Becker <caarlos0@gmail.com>

LICENSE

@@ -0,0 +1,21 @@
+MIT License
+
+Copyright (c) 2022 Charmbracelet, Inc
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

README.md

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-# TODO: decide name
+# Melt
 
-Backups and restores SSH private keys to/from a mnemonic set of words,
-using [bip39](https://github.com/bitcoin/bips/blob/master/bip-0039.mediawiki).
+Backup and restore a SSH private key to mnemonic word set you can memorize.
+
+## Usage
+
+You check its `backup` and `restore` subcommands:
+
+```shell
+melt backup ~/.ssh/id_ed25519
+melt restore --mnemonic "words from backup" ./recovered_id_ed25519
+```
+
+You can also pipe to and from a file directly, e.g.:
+
+```shell
+melt backup ~/.ssh/id_ed25519 > words
+melt restore ./recovered_id_ed25519 < words
+```
+
+## How it works
+
+It all comes down to the private key __seed__:
+
+> Ed25519 keys start life as a 32-byte (256-bit) uniformly random binary seed (e.g. the output of SHA256 on some random input). The seed is then hashed using SHA512, which gets you 64 bytes (512 bits), which is then split into a “left half” (the first 32 bytes) and a “right half”. The left half is massaged into a curve25519 private scalar “a” by setting and clearing a few high/low-order bits. The pubkey is generated by multiplying this secret scalar by “B” (the generator), which yields a 32-byte/256-bit group element “A”.
+> https://blog.mozilla.org/warner/2011/11/29/ed25519-keys/
+
+Knowing that, we open the key and extract its seed, and use it as __entropy__ for the [bip39][] algorithm, which states:
+
+> The mnemonic must encode entropy in a multiple of 32 bits. With more entropy security is improved but the sentence length increases. We refer to the initial entropy length as ENT. The allowed size of ENT is 128-256 bits.
+
+Doing that, we get the __mnemonic__ set of words back.
+
+To restore, we:
+
+- get the __entropy__ from the __mnemonic__
+- the __entropy__ is effectively the key __seed__, so we use it to create a SSH key pair
+- the key is effectively the same that was backup up, as the key is the same.
+
+[bip39]: https://github.com/bitcoin/bips/blob/master/bip-0039.mediawiki
+
+## Caveats
 
 At this time, only `ed25519` keys are supported.
+
+## License
+
+[MIT](https://github.com/charmbracelet/melt/raw/master/LICENSE)
+
+***
+
+Part of [Charm](https://charm.sh).
+
+<a href="https://charm.sh/"><img alt="The Charm logo" src="https://stuff.charm.sh/charm-badge-unrounded.jpg" width="400"></a>

go.mod

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-module github.com/caarlos0/keybackup
+module github.com/charmbracelet/melt
 
 go 1.17