www.gryphel.com/c/minivmac/extras/sigcheck - feedback

SigCheck *Alpha*


Download

sigcheck-1.1.0.zip (111K) a zipped hfs disk image and checksum file. The disk image can be mounted with Mini vMac. Includes source code.

SigCheck is a tool for checking digital signatures. It is intended to eventually replace PSgCheck, a similar tool that uses an incompatible format. (Almost all the digital signatures found throughout this website are still in that format.)

Screenshot

To use SigCheck, launch the application, and in the editing window that appears, paste in a signed message. For example:

--------- GRY SIGNED TEXT ---------

Twas brillig, and the slithy toves
did gyre and gimble in the wabe:
All mimsy were the borogoves,
and the mome raths outgrabe.

------- BEGIN GRY SIGNATURE -------
Gry/AXuKqWsF8Rh5/Bb045yIowANlvp/V/ymKoDa55Wb7dW/xGfPmca8oLw+Rv9d
cSQpcL+XpzESnWqpK/uNg7lTIe4wdOzo1/s6bHU0u27J+A5EngkYQiAMHeETq6Nx
DM8viQbkNx2UZrDxyOLCGXW1N4txMktyjC3DYzRMH/oKBVHxgP6B2j8sipEgp3Zf
-------- END GRY SIGNATURE --------

Copy all of the indented text above, which includes the message body, the signature, and headers and footers. Then paste it into SigCheck, which requires getting the clipboard into the emulated Macintosh, if SigCheck is run inside Mini vMac. You can get text into the emulated Macintosh using the “Host Paste” command in the Edit Menu of SigCheck. It is like the “Paste” command, except that it uses the clipboard of the real computer instead of the clipboard of the emulated computer. The keyboard shortcut is Command-Option-V. (The “Host Paste” command has similar effect to using ClipIn and then the normal “Paste” command.)

Then click on the status bar, at the bottom of the window below the editing area. (Choosing the ‘Go’ command from the File menu, of the emulated Macintosh, will also work. The keyboard shortcut is Command-G.)

The editing area is cleared. Next paste in the public key for the signed message. For this example:

----- BEGIN GRY PUBLIC KEY -----
Gry+PKAIAAA/AXuKqWsF8Rh5Ie4/vORBnTxYf8FPpRl/n6hpgDgfJ5MAOsAEyePE
nSfjmL3I9emNTuW/iCIHlCl/WkaWyMQZ+NDb1ZsnSLyOKwOycVkR3JwJbUamM4wy
jUuoWV6jzuhqWZobpGPMSN4B3ivHXtcNVm5SVmnAL13T4FCLnx+TxaYRAQABwA54
------ END GRY PUBLIC KEY ------

Before continuing, it would be a good idea to set Mini vMac to All Out speed. Then, on a modern computer, SigCheck should take less than a second to run. At 1x speed, or on a real Macintosh Plus, it takes much longer. The code of SigCheck is intended to be simple to understand and maintain, as opposed to fast.

Now, click on the status bar again. If all is well, the status bar should say “Good Signature.” Otherwise you should get an alert with some error message.

Besides the signed messages on this website, anyone can create their own signed message using the SigWrite tool. Public and secret key pairs for these tools can be created with MakeKeys.

If SigCheck says the signature is good, that is strong evidence that the message was signed by the owner of the public key (the person who has the corresponding secret key). But you should be aware of a number of weaknesses:

First, the key might not belong to who you think it does. If someone has hacked the Gryphel Project website, or is intercepting all traffic between you and the website, then they can replace the public keys displayed on this website as well. So you shouldn’t just get the key from the website whenever you need it, you should save your own copy. That still doesn’t protect you when you first get the key. One possible protection is to find other copies of the key on the web and compare them.

Second, the key might have been stolen. Once anyone else knows the secret key, it is pretty much useless. Securing information on a computer is a difficult problem. Actually, it is impossible to prove that a computer is completely secure. A computer not connected to the internet is much more likely to be secure, but that is usually impractical.

Third, the key might have been broken. The security of a key depends on the difficulty of factoring a large number into two primes. In 2009, a 768 bit key was broken. As of this writing (2018), no one has publicly broken a 1024 bit key, however there have been predictions that it would be possible around now. It is quite possible that some large government organization now has that capability. Even if that is so, for most uses a 1024 bit key is probably still safe for signing. The capability would most likely be quietly used for decrypting. Forging signatures would make that capability publicly known, and so less useful.

Fourth, as SigCheck descends from the old MacPGP software, it is using the old md5 checksum algorithm. Currently md5 is generally considered hopelessly weakened, and unsuitable for any purpose. However, as far as I know, there is still no known publicly known practical “Preimage” attack, which is what is most important for signing. That is, if I create a file that has a certain md5 checksum, there is no practical way known yet for someone else to construct a different file with the same checksum. But it is possible to create two files with the same md5 checksum. So you have to be careful about signing a statement that you have looked at a file created by someone else, with a given md5 checksum, and it is good. Because they might have another file with the same checksum that isn’t good. And you have to be careful about signing a statement that contains a significant amount of text from someone else, especially if your text that precedes their text is predictable. Which seems to be what happened to Microsoft with the “Flame” malware.

SigCheck is in part descended from MacPGP source code, which, as far as I can tell, allows derived works for noncommercial use.

Here is the md5 checksum for the download, signed with Gryphel Key 1:

-----BEGIN PGP SIGNED MESSAGE-----

46757c0a10d20e1d52927bdc6f2325f5 sigcheck-1.1.0.zip

-----BEGIN PGP SIGNATURE-----
Version: 2.6.3ia
Charset: noconv

iQCVAwUBWq5ULNiugru7WJg5AQGaxAQAi3mh1iTDbZS05Ccsr1WTY303Eyhyw02A
zJWanln9Wz1p+9Rh2LlP0WQTtU8yifP1BwfvNuf8O1WAniep9pNTIDKaynWCnN5R
ZnYQpXMjrL956sIR2EKbmU62DCA34H8omSyxm4MGi5XOorF9uTkcVBOCtSA8RNpn
MiKxq3l7bzA=
=cNYZ
-----END PGP SIGNATURE-----

See the Compiling page for instructions on compiling SigCheck from the source code.

:

If you find SigCheck useful, please consider helping the Gryphel Project, of which it is a part.

gryphel logo, 1K
www.gryphel.com/c/minivmac/extras/sigcheck - feedback
copyright (c) 2018 C. Pratt - last update 3/18/2018