keyfunc/README.md

# 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> [-- sftp options...]`

Adds the `pub` line to `~/.ssh/authorized_keys` on the host. No command is run
on the host: the file is fetched, changed here, and written back with the
system `sftp` client in batch mode.

The first connection fetches `~/.ssh/authorized_keys`. The file reads as empty
only when `sftp` reported that file as not being there — the one line naming
that path. The same wording anywhere else in the session does not count: `ssh`
writes `No such file or directory` about an `-i` it cannot find, on a session
that then authenticates through the agent. When `sftp` failed for any other
reason — the file is there and cannot be read, the connection did not come up —
the tool prints what `sftp` said and exits with status 1 without writing
anything, rather than put a file back holding the new key alone. What `sftp`
cannot tell apart is a missing file and one in a directory it cannot enter, so a
`~/.ssh` whose mode shuts the user out reads as a host with no file; the second
connection sets that mode to `0700` and writes, as on a host that has none. If
an identical line is already in the file, the tool prints
`already present` and connects no further. Otherwise the line is added (after a
newline, if the file did not end with one) and a second connection:

- creates `~/.ssh` and sets it to mode `0700`;
- uploads the new file as `~/.ssh/authorized_keys.keyfunc-<random>` and sets it
  to mode `0600`;
- renames that file over `~/.ssh/authorized_keys`.

The tool then prints `added`. So a run that adds a line connects twice. The
rename is the step that either happens or does not: the file on the host is
never half-written. `sftp` does it in one step against servers that offer
OpenSSH's POSIX rename extension, as OpenSSH's own server does; a server
without it may refuse to rename onto a file that is already there.

If a step fails, the tool prints what `sftp` said, removes nothing, and exits
with status 1. It names the uploaded file only when the step that failed was
the upload or one after it, which is where a file of that name can be on the
host; a failure before the upload names none. Everything `sftp`
writes goes to standard error, so the tool's own standard output is only
`added` or `already present`.

Anything after `--` is passed to `sftp` unchanged, which is where the port goes
(`-P 2222`, not `-p`). How the connection authenticates is up to the user's
normal `ssh` setup, except that batch mode does not prompt: a key or an agent
has to do it, not a typed password.

### `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
```