Add instructions to README

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "age-plugin-yubikey"
-description = "[BETA] YubiKey plugin for age."
+description = "[BETA] YubiKey plugin for age clients"
 version = "0.0.0"
 authors = ["Jack Grigg <thestr4d@gmail.com>"]
 repository = "https://github.com/str4d/age-plugin-yubikey"

README.md

@@ -1,13 +1,105 @@
-# YubiKey plugin for age
+# YubiKey plugin for age clients
 
-Work in progress.
+`age-plugin-yubikey` is a plugin for [age](https://age-encryption.org/v1) clients
+like [`age`](https://age-encryption.org) and [`rage`](https://str4d.xyz/rage),
+which enables files to be encrypted to age identities stored on YubiKeys.
 
-Do not use this yet with a YubiKey you care about.
+This plugin is in **BETA**; we strongly recommend using this with a new YubiKey,
+or one that you do not care about.
 
-Not even at a 0.1 release yet.
+## Installation
 
-Do not assume we will keep support for anything this currently may do to your YubiKey!
+On Windows, Linux, and macOS, you can use the
-We have a UX goal in mind, and will make breaking changes until we get there.
+[pre-built binaries](https://github.com/str4d/age-plugin-yubikey/releases).
+
+If your system has Rust 1.51+ installed (either via `rustup` or a system
+package), you can build directly from source:
+
+```
+cargo install age-plugin-yubikey
+```
+
+Help from new packagers is very welcome.
+
+## Configuration
+
+There are two ways to configure a YubiKey as an `age` identity. You can run the
+plugin binary directly to use a simple text interface, which will create an age
+identity file:
+
+```
+$ age-plugin-yubikey
+```
+
+Or you can use command-line flags to programmatically generate an identity and
+print it to standard output:
+
+```
+$ age-plugin-yubikey --generate \
+    [--serial SERIAL] \
+    [--slot SLOT] \
+    [--name NAME] \
+    [--pin-policy PIN-POLICY] \
+    [--touch-policy TOUCH-POLICY]
+```
+
+Once an identity has been created, you can regenerate it later:
+
+```
+$ age-plugin-yubikey --identity [--serial SERIAL] --slot SLOT
+```
+
+## Usage
+
+The age recipients contained in all connected YubiKeys can be printed on
+standard output:
+
+```
+$ age-plugin-yubikey --list
+```
+
+To encrypt files to these YubiKey recipients, ensure that `age-plugin-yubikey`
+is accessible in your `PATH`, and then use the recipients with an age client as
+normal (e.g. `rage -r age1yubikey1...`).
+
+The output of the `--list` command can also be used directly to encrypt files to
+all recipients (e.g. `age -R filename.txt`).
+
+To decrypt files encrypted to a YubiKey identity, pass the identity file to the
+age client as normal (e.g. `rage -d -i yubikey-identity.txt`).
+
+## Advanced topics
+
+### Agent support
+
+`age-plugin-yubikey` does not provide or interact with an agent for decryption.
+As age plugin binaries have short lifetimes (they only run while the age client
+is running), this means that YubiKey identities configured with a PIN policy of
+`once` will actually prompt for the PIN on every decryption.
+
+A decryption agent will most likely be implemented as a separate age plugin that
+interacts with [`yubikey-agent`](https://github.com/FiloSottile/yubikey-agent),
+enabling YubiKeys to be used simultaneously with age and SSH.
+
+### Manual setup and technical details
+
+`age-plugin-yubikey` only officially supports YubiKeys set up either via the
+text interface or the `--generate` flag.
+
+In practice, any PIV token with an ECDSA P-256 key and certificate in one of the
+20 "retired" slots should work. You can list all age-compatible keys with:
+
+```
+$ age-plugin-yubikey --list-all
+```
+
+`age-plugin-yubikey` implements several automatic security management features:
+
+- If it detects that the default PIN is being used, it will prompt the user to
+  change the PIN. The PUK is then set to the same value as the PIN.
+- If it detects that the default management key is being used, it generates a
+  random management key and stores it in PIN-protected metadata.
+  `age-plugin-yubikey` does not support custom management keys.
 
 ## License