Plans for adding cipher mode to file headers

[Will Morrison <camocrazed@gmail.com>]

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To make the changes to store the cipher mode in the file header, we
are proposing the following.

1. Change ECRYPTFS_SUPPORTED_FILE_VERSION to 4. This should prevent
old versions of eCryptfs from trying to read new style headers.

2. Add a new cipher mode field in the appropriate packets of version 4
file headers. (I believe these are tag 1 and tag 3, for asymmetric and
symmetric keys). Since there is no equivalent to this field in the
OpenPGP RFCs, we will be creating a new list of constants similar to
the ones in ecryptfs.h for the mode type.

3. When reading a file header and initializing a crypt_stat, if the
version number is 4 or greater, read the mode out of the header,
otherwise, default to CBC.

4. When writing out headers, refer to the file_version field in the
crypt_stat to determine what to write out. If it's 4 or greater,
include the mode field.

This should result in the new version 4 header being written for all
new files. Old files would still be read and written with the version
3 headers and default to using CBC mode. Older versions of eCryptfs
should refuse to open files with version 4 headers.

Does this make sense? If not, what are we missing?

Thanks,
- -Will
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