1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
|
# msr
*Small public key verification tool*
Inspired by OpenBSD's [signify](http://cvsweb.openbsd.org/cgi-bin/cvsweb/src/usr.bin/signify/signify.c) and [minisign](https://jedisct1.github.io/minisign/) I decided to spend some time learning [libsodium](https://github.com/jedisct1/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.
## Usage
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
## Specifications
At the behest of libsodium, we use [ed25519](http://ed25519.cr.yp.to/) for all things signing, `scryptsalsa208sha256` for the KDF, and [BLAKE2](https://blake2.net/) 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.
### Secret Key Files
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 bytes`
* `kdf_opsl` and `kdf_meml` are the operations and memory limits for the KDF (defined in libsodium as `crypto_pwhash_scryptsalsa208sha256_{OPS,MEM}LIMIT_SENSITIVE`)
* `checksum = BLAKE2( <sig_alg> || <key id> || <secret key)`
* `encrypted key = <kdf output> ^ (<key id> || <secret key> || <checksum>)`
### Public Key Files
untrusted comment: <1024 bytes, arbitrarily changeable>
base64( <signature algorithm> || <key id> || <public key> )
### Signatures
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> )
### Dependencies
`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.
### License
GPL3+
### Author's notes
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!
|
