# 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'/'/' ``` - `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 `: 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'/'`. 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 ``` The comment defaults to `keyfunc/ssh/`; 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 [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=` 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'/'`. 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 ...] [-o ] []` 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 ] []` 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'/'/'`, 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 ```