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I'm trying to understand the file format of the secring.gpg file. I couldn't really find any official documentation of the file format so I decided to create a new key-pair and use the S2K (string-to-key) function simple S2K, which basically means that the entered passphrase is only hashed once with the specified message digest algorithm. So I created the new key-pair using the command:

gpg --gen-key --s2k-digest-algo MD5 --s2k-mode 0

I was able to verify that the key was created with the right specs by using:

gpg --list-packets secring.pgp

The partial output is:

:secret key packet:
    version 4, algo 1, created 1397496587, expires 0
    skey[0]: [1024 bits]
    skey[1]: [17 bits]
    simple S2K, algo: 3, SHA1 protection, hash: 1
    protect IV:  06 8a d3 02 e9 df 49 8a
    encrypted stuff follows

The 'hash: 1' should mean that MD5 is used and simple S2K means hashed once, no salt. You can find this information in the RFC 4880 Section 3.7.

I used the passphrase 'test' which should result in the MD5 hash '098f...b4f6'. The problem is I am not able to find this hash in the secring.gpg file. I used a HexViewer and was able to find the string FE 03 00 01, which means (following the RFC):

FE = initiate the algo specs

03 = symmetric cipher (CAST5)

00 = simple S2K

01 = MD5

Which also confirms that the right spec is used. Something that made me curious is that I found this string twice (for the master and the sub-key, I think). But still there should be somewhere my hashed passphrase.

Am I not understand something? Or am I looking in the wrong place?

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