hacks/bip85keys/README.md

# bip85keys

`bip85keys` 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 key type: `bip85keys ssh ...` for ed25519 SSH keys (in
this PR) and `bip85keys age ...` for age identities (planned, see below).

## 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 `BIP85KEYS_MNEMONIC_COMMAND`: the same, as a shell
   command held in the environment.
3. Environment variable `BIP85KEYS_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`.
`bip85keys --version` prints the version set at build time.

## SSH keys: `bip85keys 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
```

### `bip85keys ssh pub`

Prints one `authorized_keys` line to standard output:

```
ssh-ed25519 <key> <comment>
```

The comment defaults to `bip85keys/ssh/<n>`; change it with `--comment`.

### `bip85keys 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`.

### `bip85keys 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.

### `bip85keys 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: `bip85keys age` (planned)

Not in this PR, which delivers the SSH type first; it is the next type to add.
`bip85keys age pub` and `bip85keys age priv` will use application number
`657169`, path `m/83696968'/657169'/<n>'`; the 32 bytes from step 4 are clamped
as X25519 requires and encoded as an `AGE-SECRET-KEY-1...` 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.

## 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 ./bip85keys
make check    # fmt-check, lint (golangci-lint) and tests
```

Examples:

```
bip85keys ssh pub -n 3 --mnemonic-command 'secret get foo'
bip85keys ssh priv -n 3 > ~/.ssh/id_bip85_3
bip85keys ssh install -n 3 user@example.com
bip85keys ssh to -n 3 user@example.com uptime
```