I'm running ffmpeg v2.8.1 from Homebrew, and trying to convert [Audible](https://www.audible.com) files but there's some trick I"m missing.

This command produces this error output.

```
ffmpeg -activation_bytes 1CEB00DA -i Volume1.aax -vn -c:a copy output.mp4

[mov,mp4,m4a,3gp,3g2,mj2 @ 0x7fc9a4010600] [aax] file checksum == d72f1f04e3c73d0bc68e742db1bc69b58dc3a500
[mov,mp4,m4a,3gp,3g2,mj2 @ 0x7fc9a4010600] [aax] mismatch in checksums!
[mov,mp4,m4a,3gp,3g2,mj2 @ 0x7fc9a4010600] error reading header
Volume1.aax: Invalid data found when processing input

```

I know the file is ok, because it plays in iTunes. I does have the file image/cover art embedded in it as well,

From the ffmpeg docs, I read
[Audible AAX files are encrypted M4B files](http://ffmpeg.org/ffmpeg-all.html#Audible-AAX), and they can be decrypted by specifying a 4 byte activation secret.

I'm presuming - probably incorrectly - that the one given is ok. If not, how does one find that secret.