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