keyfunc/README.md

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+# keyfunc
+
+`keyfunc` turns a BIP-39 mnemonic into key pairs that can be recreated from
+that mnemonic at any time. The same mnemonic, key type and index always give the
+same key.
+
+It uses the BIP-85 entropy deriver from `git.eeqj.de/sneak/secret/pkg/bip85` and
+takes the same steps as that repository's `agehd` package.
+
+Commands are grouped by what is derived: `keyfunc ssh ...` for ed25519 SSH
+keys, `keyfunc age ...` for age identities and for encrypting and decrypting
+with them, and `keyfunc mnemonic ...` for child mnemonics derived from the
+main one.
+
+## Derivation
+
+The mnemonic is turned into a key like this:
+
+1. mnemonic -> BIP-39 seed (empty passphrase);
+2. seed -> BIP-32 master key;
+3. master key -> 64 bytes of BIP-85 entropy at the path below;
+4. entropy -> BIP-85 DRNG (SHAKE256); read 32 bytes;
+5. those 32 bytes become the key in the way the key type needs.
+
+The path is:
+
+```
+m/83696968'/<app>'/<n>'
+```
+
+- `83696968` is the fixed BIP-85 purpose.
+- `app` is the application number of the key type. There is no vendor id: the
+  path is meant as a standard any implementation can follow, not something tied
+  to one tool. Each key type's number is spelled the way BIP-85 spells its own
+  RSA application (`828365` is the ASCII codes of `R`, `S`, `A` written out):
+  SSH is `838372` (`S` `S` `H`), age is `657169` (`A` `G` `E`).
+- `n` is the key index: flag `--index` / `-n`, default `0`.
+
+## Giving it the mnemonic
+
+The mnemonic itself is never a command-line argument. It is looked for in this
+order; the first one found wins:
+
+1. `--mnemonic-command <command>`: a shell command, run with `sh -c`, whose
+   standard output is the mnemonic. Example: `--mnemonic-command 'secret get
+   foo'`. Whitespace around the output is dropped. If the command exits with a
+   non-zero status, the tool prints its standard error and exits with status 1.
+2. Environment variable `KEYFUNC_MNEMONIC_COMMAND`: the same, as a shell
+   command held in the environment.
+3. Environment variable `KEYFUNC_MNEMONIC`: the mnemonic itself.
+4. A prompt on the terminal with echo turned off.
+
+If none of these is available and standard input is not a terminal, the tool
+refuses and exits with status 1. A mnemonic that fails the BIP-39 checksum is
+refused with a message saying so.
+
+Every command takes `--index` / `-n` and `--mnemonic-command`, and has `--help`.
+`keyfunc --version` prints the version set at build time.
+
+## SSH keys: `keyfunc ssh`
+
+Only ed25519 keys are produced. The application number is `838372`, so the
+path is `m/83696968'/838372'/<n>'`. The 32 bytes from step 4 are the ed25519
+seed.
+
+Test vector, mnemonic
+`abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon about`:
+
+```
+index 0: ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIJZOtOczrc/7CQytcuFwt7s4r8KjkZWkwjLZWBaFKD+7
+index 1: ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIOEWY8+/gmHYVC4u0Y0I4FKs+eVUulTPHfk9VtXw1tMF
+```
+
+### `keyfunc ssh pub`
+
+Prints one `authorized_keys` line to standard output:
+
+```
+ssh-ed25519 <key> <comment>
+```
+
+The comment defaults to `keyfunc/ssh/<n>`; change it with `--comment`.
+
+### `keyfunc ssh priv`
+
+Prints the unencrypted private key in OpenSSH format (the
+`-----BEGIN OPENSSH PRIVATE KEY-----` block that `ssh` reads) to standard output
+and nothing else, so it can be redirected into a file. The key's comment is the
+same as for `pub`.
+
+### `keyfunc ssh install <[user@]host> [-- ssh options...]`
+
+Runs the system `ssh` to the host and, on the host:
+
+- creates `~/.ssh` with mode `0700` if it is missing;
+- creates `~/.ssh/authorized_keys` with mode `0600` if it is missing;
+- appends the `pub` line only if an identical line is not already there.
+
+It then prints `added` or `already present`. How this `ssh` connection
+authenticates is up to the user's normal `ssh` setup (existing keys, agent,
+password). Anything after `--` is passed to `ssh` unchanged.
+
+### `keyfunc ssh to <host> [ssh arguments...]`
+
+Derives the key, serves it from an SSH agent that runs inside the tool on a unix
+socket in a new private `0700` temporary directory, then runs the system `ssh`
+with `-o IdentityAgent=<that socket>` followed by the host and all remaining
+arguments unchanged. The tool exits with `ssh`'s exit status and removes the
+socket and directory on the way out. The private key is never written to disk.
+
+## age identities: `keyfunc age`
+
+The application number is `657169`, path `m/83696968'/657169'/<n>'`. The 32
+bytes from step 4 are clamped as X25519 requires and become an age identity,
+the same steps `sneak/secret` takes in its `agehd` package. `secret` derives at
+a vendor-specific path today; for its keys to equal this tool's it moves to
+this path, which is a change in `secret`, not here.
+
+### `keyfunc age pub`
+
+Prints the recipient, the `age1...` public key, on one line.
+
+### `keyfunc age priv`
+
+Prints the identity, the `AGE-SECRET-KEY-1...` line, and nothing else.
+
+### `keyfunc age encrypt [-n N] [--to <recipient>...] [-o <file>] [<file>]`
+
+Encrypts the file (or standard input) with age. The recipients are the derived
+identity's own recipient, plus any given with `--to`, so the same mnemonic can
+always decrypt what it encrypted. Output goes to `-o` or standard output;
+`--armor` writes the text form. Nothing is written except the output.
+
+### `keyfunc age decrypt [-n N] [-o <file>] [<file>]`
+
+Decrypts the file (or standard input) with the derived identity. Output goes to
+`-o` or standard output. If the identity is not one of the recipients, the tool
+says so and exits with status 1.
+
+## Derived mnemonics: `keyfunc mnemonic`
+
+### `keyfunc mnemonic [-n N] [--words 12|18|24]`
+
+Prints a child mnemonic derived from the main one, using BIP-85's own mnemonic
+application (number `39`, English, path
+`m/83696968'/39'/0'/<words>'/<n>'`, entropy taken as the specification says,
+not through step 4). Default 12 words. A child mnemonic is a full mnemonic in
+its own right: it can seed another `keyfunc`, another wallet, or `secret`, and
+it never has to be written down, since it can be derived again.
+
+## Adding a key type
+
+Adding a key type is one package under `internal/` that turns the 32 derived
+bytes into that type's key, plus one cobra subcommand under `internal/cli/` that
+groups its commands.
+
+## Errors
+
+Errors go to standard error and the exit status is 1, except for `ssh to`,
+which passes through `ssh`'s own exit status.
+
+## Building and running
+
+```
+make build    # produces ./keyfunc
+make check    # fmt-check, lint (golangci-lint) and tests
+```
+
+Examples:
+
+```
+keyfunc ssh pub -n 3 --mnemonic-command 'secret get foo'
+keyfunc ssh priv -n 3 > ~/.ssh/id_bip85_3
+keyfunc ssh install -n 3 user@example.com
+keyfunc ssh to -n 3 user@example.com uptime
+keyfunc age pub -n 0
+keyfunc age encrypt -n 0 --armor -o notes.age notes.txt
+keyfunc age decrypt -n 0 notes.age
+keyfunc mnemonic -n 1 --words 24
+```