Small public key verification tool
Inspired by OpenBSD's signify and minisign I decided to spend some time learning libsodium and re-implement minisign for myself.
Currently, this programme is a superset of minisign (and so partially signify) but in the future signatures and or keys may become incompatible -- it seemed in bad taste to do that immediately.
Compile it and run msr --help and you'll find
-G, --generate[=FILE] Generate a new key pair, storing in
`FILE.{pub,key}' (FILE defaults to msr)
-S, --sign-detached=FILE Sign FILE by generating a separate signature
-T, --sign-text=FILE Sign FILE by appending a signature
-V, --verify-detached=FILE Verify the detached signature on FILE
-X, --verify-text=FILE Verify the inline text signature in FILE
-f, --signature-file=FILE Use FILE as the signature file for detached
signing and verification purposes
-p, --pubkey-file=FILE Use the public key in FILE
--pubkey-string=STR Use the public key encoded in STR
-s, --seckey-file=FILE Use the secret key in FILE
--seckey-string=STR Use the secret key encoded in STR
--password-file=FILE Load secret key passphrase from FILE
--comment-pubkey=STR Use STR for the default untrusted comment in the
generated public key file
--comment-seckey=STR Use STR for the default untrusted comment in the
generated secret key file
-t, --comment-trusted=STR Use STR for the trusted comment when making a
detached signature
-u, --comment-untrusted=STR Use STR for the default untrusted comment when
making a detached signature
-q, --quiet Produce no output
-?, --help Give this help list
--usage Give a short usage message
--version Print program version
At the behest of libsodium, we use ed25519 for all things signing, scryptsalsa208sha256 for the KDF, and BLAKE2 for computing hashes otherwise. Thus, for what follows we have
sig_alg = Ed
chk_alg = B2
kdf_alg = Sc
Finally, to minimise key collisions and provide a convenient necessary match criterion, each key is assigned a key_id which is eight random bytes.
untrusted comment: <1024 bytes, arbitrarily changeable>
base64( <sig_alg> || <kdf_alg> || <chk_alg> || <kdf_salt> || <kdf_opsl> || <kdf_meml> || <encrypted key> )
where
kdf_salt = 32 random byteskdf_opslandkdf_memlare the operations and memory limits for the KDF (defined in libsodium ascrypto_pwhash_scryptsalsa208sha256_{OPS,MEM}LIMIT_SENSITIVE)checksum = BLAKE2( <sig_alg> || <key id> || <secret key)encrypted key = <kdf output> ^ (<key id> || <secret key> || <checksum>)
untrusted comment: <1024 bytes, arbitrarily changeable>
base64( <signature algorithm> || <key id> || <public key> )
Detached signatures have the format
untrusted comment: <1024 bytes, arbitrarily changeable>
base64( <signature algorithm> || <key id> || <signature> )
trusted comment: <1024 bytes, fixed at signing>
base64( <global signature> )
where
signature = ed25519( <file data> )global signature = ed25519( <signature> || <trusted comment> )
whereas inline signatures are simply of the form
<file contents>
--- BEGIN SIGNATURE ---
base64( <signature algorithm> || <key id> || <signature> )
libsodium, argp and a compiler/stdlib that will understand -D_GNU_SOURCE (for non-modifying basename)
I doubt it's that portable, but it should probably work on most unix-y systems.
GPL3+
The idea that you can squeeze public key verification into just a few bytes (sub ~100 for everything concerned) and have it still be "128 bit strong" is really amazing. Moreover, I felt that this would be a good learning exercise -- I haven't really done much library interfacing in C (this project entailed using both libsodium and argp), and I haven't ever concluded a mid-sized C project before. I have no doubt the code is crufty and poorly designed, but I had fun and it was an interesting paradigm shift from my usual language of choice.
All input welcome and desired!
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