Date: Tue, 09 Apr 2013 10:41:19 +0200
From: Yann Droneaud <ydroneaud@...eya.com>
To: <discussions@...sword-hashing.net>
Subject: Is this really safe ...

Hi,

Is it really safe to use GitHub to store a secret key ring ?

Someone point me to this:

https://github.com/skeeto/dotfiles#readme

     You may have noticed, yes, I have my private PGP key in here! 
Dangerous?!
     Maybe, it's an experiment. It's got a strong passphrase on it and 
I've
     pumped up the key strengthening settings in GPG, like so,

     gpg --s2k-cipher-algo AES256 --s2k-digest-algo SHA512 --s2k-mode 3 
\
         --s2k-count 10000000 --edit-key <key id>

     Then run passwd in the key editor. That's over 10 million rounds of
     SHA-512 which takes a half-second to compute on my laptop. 
Brute-forcing
     my passphrase should be completely impractical. I invite anyone to 
prove
     me wrong

The file is available here:

https://github.com/skeeto/dotfiles/blob/master/gnupg/secring.gpg

Fingerprints:

66D1 2CF3 F6D0 5FD4 BA2A  8F69 6133 82C5 48B2 B841
424F 2EDC 295A FEC7 1547  53AA 0D65 65A4 A540 67E2


Armored key 48B2B841 and fed to http://www.pgpdump.net/cgi-bin/pgpdump, 
you may read:

PGPdump Results

Old: Secret Key Packet(tag 5)(966 bytes)
     Ver 4 - new
     Public key creation time - Sat Feb 26 15:35:47 UTC 2011
     Pub alg - RSA Encrypt or Sign(pub 1)
     RSA n(2048 bits) - ...
     RSA e(17 bits) - ...
     Sym alg - AES with 256-bit key(sym 9)
     Iterated and salted string-to-key(s2k 3):
         Hash alg - SHA512(hash 10)
         Salt - f7 5a 1b fe 8c 73 5a 9f
         Count - 10485760(coded count 212)
     IV - a6 61 4a 95 44 1e 7e 90 88 c3 01 70 8d 56 2e 11
     Encrypted RSA d
     Encrypted RSA p
     Encrypted RSA q
     Encrypted RSA u
     Encrypted SHA1 hash

Additionally, there's a subkey:

Old: Secret Subkey Packet(tag 7)(966 bytes)
     Ver 4 - new
     Public key creation time - Sat Feb 26 15:35:47 UTC 2011
     Pub alg - RSA Encrypt or Sign(pub 1)
     RSA n(2048 bits) - ...
     RSA e(17 bits) - ...
     Sym alg - AES with 256-bit key(sym 9)
     Iterated and salted string-to-key(s2k 3):
         Hash alg - SHA512(hash 10)
         Salt - f7 5a 1b fe 8c 73 5a 9f
         Count - 10485760(coded count 212)
     IV - e5 22 39 0c 29 73 ce fe 5b be 8d 2f 6c f5 d5 42
     Encrypted RSA d
     Encrypted RSA p
     Encrypted RSA q
     Encrypted RSA u
     Encrypted SHA1 hash

And it uses the same salt.

Using gpg --dump secring.gpg and looking for key A54067E2

:secret key packet:
         version 4, algo 1, created 1320238566, expires 0
         skey[0]: [2048 bits]
         skey[1]: [17 bits]
         iter+salt S2K, algo: 9, SHA1 protection, hash: 10, salt: 
bce919fc9921bc5d
         protect count: 212
         protect IV:  af 2d cf d2 6e dc 35 87 8c a4 27 af cc c7 25 70

:secret sub key packet:
         version 4, algo 1, created 1320238566, expires 0
         skey[0]: [2048 bits]
         skey[1]: [17 bits]
         iter+salt S2K, algo: 9, SHA1 protection, hash: 10, salt: 
bce919fc9921bc5d
         protect count: 212
         protect IV:  1e 72 83 b6 c6 c6 7a 50 53 81 b8 c6 9f 24 c0 50


According RFC 4880 http://tools.ietf.org/html/rfc4880#section-3.7.1.3
"Count" is not the iteration count, but the count of byte to be hashed:

    Iterated-Salted S2K hashes the passphrase and salt data multiple
    times.  The total number of octets to be hashed is specified in the
    encoded count in the S2K specifier.  Note that the resulting count
    value is an octet count of how many octets will be hashed, not an
    iteration count.

So if I understand, the iteration count will be equal to "Count / Hash 
output size",
eg 163840 rounds.

I haven't read any implementation of the Iterated-Salted S2K algorithm, 
but it seems, well,
worst than PBKDF. I tought PGP would use such algorithm to protect 
secret keys.


Regards.

-- 
Yann Droneaud
OPTEYA

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