= sgn - pseudonymous digital signatures

sgn allows you to mark files such that others can verify the same
person wrote them.

*** THIS IS ALPHA SOFTWARE, NOT YET PROPERLY PEER-REVIEWED.  USE AT OWN RISK. ***

== Requirements

* Ruby 1.8.
* OpenSSL 0.9.8 or newer, openssl(1).
* Support for /dev/fd/.

== Usage

Generate a key with `sgn-genkey`:

    % sgn-genkey
    Generating key...
    read EC key
    writing EC key
    Wrote public key /Users/azurediamond/.sgn.pub.
    Do you want to encrypt the private key [recommended] (y/n)? y
    read EC key
    writing EC key
    Enter PEM pass phrase: hunter2
    Verifying - Enter PEM pass phrase: hunter2
    Wrote private key /Users/azurediamond/.sgn.pem.
    Created key with fingerprint 358b3f83e291a784ac4fa141225815da728c0e80.

Sign a file with `sgn`:

    % sgn FILE >FILE.signed
    Enter pass phrase for /Users/azurediamond/.sgn.pem: hunter2
    % tail -2 FILE.signed
    SK: MC4wEAYHKoZIzj0CAQYFK4EEAB8DGgADx8YQzTwcnJeizBVIiAr70e+/eamw7exP
    SH: MDQCGG8SE8+uf0y8c4uOMi0dJBIf41taxJYg8QIYLAnAy9FyP382fcTFI3ztsGmWCMvaJHf9

Copy it, modify it:
    % cp FILE.signed FILE.changed
    % ed FILE.changed
    ,s/private/public/g
    w
    q

Verify the files with `sgn-check`:

    % sgn-check FILE*
    file not signed: FILE
    358b3f83e291a784ac4fa141225815da728c0e80 FILE.changed FAILED
    358b3f83e291a784ac4fa141225815da728c0e80 FILE.signed OK

As you can see, the signed files contain your fingerprints, and the
modification has been discovered.  Note that "OK" only means the
content check passed; only the fingerprint identifies the signer!
Thus

    f28c6a572a78961cdd91244a41e4e423a4c2334f FILE.changed OK

would mean the file was not signed by the signer of FILE.signed (or
with a different key).

== How it works

sgn uses modern elliptic public cryptography to sign files using ECDSA
and includes the public key in all signed files to allow
offline-checking.  Currently, a 192-bit SECG key is used for signing
(which is stronger than an equivalent 1024-bit RSA key), and SHA1 for
hashing (I'd like to use a better hash, but that's all OpenSSL has for
ECDSA.)  Due to the compact size of key and hash, both easily fit into
an 80 character line and don't clutter the things you want to sign.

NOTA BENE: You must not lose or leak your private key.  There is no
revocation mechanism.  Encrypt it if you intend to sign critical
information!

== Miscellaneous

It is your own problem to authorize your fingerprints.  There is no PKI.

You can place SH:/SK: markers at places you prefer (e.g. inside
comments) to store the signature there.  Avoid SH:/SK: strings elsewhere.

Make a key for each identity you have.

sgn.el provides a few Emacs macros to sign and check buffers.

== Copyright

Written by 1b5c1836ae72386d5812a0eb63135abeacb38c76, see LICENSE.