bip85keys: rename from bip85ssh and group commands by key type

The tool will cover more key types than SSH, so the directory, module
and binary become bip85keys and the commands are grouped by type:
"bip85keys ssh pub|priv|install|ssh" now, "bip85keys age pub|priv"
planned. The README states the age derivation (agehd's path in
sneak/secret) and that adding a type is one package plus one
subcommand. The SSH application id and everything else stay as before.

Model: fable-5-1

bip85keys/README.md

@@ -0,0 +1,140 @@
+# bip85keys
+
+`bip85keys` turns a BIP-39 mnemonic (or a BIP-32 `xprv`) into key pairs that can
+be recreated from that secret at any time. The same secret, 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 secret is turned into a key like this:
+
+1. mnemonic -> BIP-39 seed (empty passphrase), or the `xprv` is parsed directly;
+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'/592366788'/<app>'/<n>'
+```
+
+- `83696968` is the fixed BIP-85 purpose.
+- `592366788` is the vendor id, `sha256("berlin.sneak") & 0x7fffffff`, the same
+  one `agehd` uses.
+- `app` is the application id of the key type; each type has its own.
+- `n` is the key index: flag `--index` / `-n`, default `0`.
+
+## Giving it the secret
+
+The secret is never taken as a command-line argument. It is looked for in this
+order; the first one found wins:
+
+1. `--mnemonic-file <path>` or `--xprv-file <path>`: the file holds the secret
+   on one line.
+2. Environment variable `BIP85KEYS_MNEMONIC` or `BIP85KEYS_XPRV`.
+3. A prompt on the terminal with echo turned off. The prompt accepts either a
+   mnemonic or an `xprv`; a value starting with `xprv` is treated as an `xprv`.
+
+If none of these is available and standard input is not a terminal, the tool
+refuses and exits with status 1.
+
+Every command takes `--index` / `-n` and the secret flags above, and has
+`--help`. `bip85keys --version` prints the version set at build time.
+
+## SSH keys: `bip85keys ssh`
+
+Only ed25519 keys are produced. The application id is `1822331379`,
+`sha256("bip85ssh") & 0x7fffffff`, so the path is
+`m/83696968'/592366788'/1822331379'/<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 AAAAC3NzaC1lZDI1NTE5AAAAIA1esgfi4OeaywgKh0o5r/8lMOUlUD/N+YoAiC8SNEML
+index 1: ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIB26T7hdDuUF6wfTQ7NpIpyeTgGha4NlhQjaAhap5dqs
+```
+
+### `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 ssh <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 derive exactly what
+`sneak/secret` derives with its `agehd` package: application id `733482323`
+(`sha256("secret") & 0x7fffffff`), so the path is
+`m/83696968'/592366788'/733482323'/<n>'`; the 32 bytes from step 4 are clamped
+as X25519 requires and encoded as an `AGE-SECRET-KEY-1...` identity. An age key
+from this tool for a given mnemonic and index will equal the one `secret`
+derives.
+
+## 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 ssh`,
+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
+bip85keys ssh priv -n 3 > ~/.ssh/id_bip85_3
+bip85keys ssh install -n 3 user@example.com
+bip85keys ssh ssh -n 3 user@example.com uptime
+```