### Please describe the problem.
I seem to be hitting a problem with the way git annex encrypts the credentials for S3 remotes. I am creating an S3 remote with shared encryption and embedded credentials. If I create the remote on MacOSX, enableremote fails on Debian. If I create the remote on Debian, enableremote fails on MacOSX with a similar error.  I can enable MacOSX created remotes on other MacOSX systems and Debian created remotes on other Debian systems. 
The enableremote creates a .git/annex/creds/uuid file, but the contents are garbled.

Could this be an issue with different versions of GPG on the systems? 

### What steps will reproduce the problem?
- New git annex remote on Debian.
- Clone and git annex init on MacOSX
- Create S3 remote on MacOSX with encryption=shared embedcreds=yes 
- Git annex sync
- On Debian  enable remote

Alternatively, create the S3 remote on Debian, sync and enable remote on MacOSX

### What version of git-annex are you using? On what operating system?
    MacOSX
    gpg (GnuPG/MacGPG2) 2.0.30
    libgcrypt 1.6.6
    git-annex version: 6.20170101
    build flags: Assistant Webapp Pairing Testsuite S3(multipartupload)(storageclasses) WebDAV FsEvents ConcurrentOutput TorrentParser MagicMime Feeds Quvi
    key/value backends: SHA256E SHA256 SHA512E SHA512 SHA224E SHA224 SHA384E SHA384 SHA3_256E SHA3_256 SHA3_512E SHA3_512 SHA3_224E SHA3_224 SHA3_384E SHA3_384 SKEIN256E SKEIN256 SKEIN512E SKEIN512 SHA1E SHA1 MD5E MD5 WORM URL
    remote types: git gcrypt p2p S3 bup directory rsync web bittorrent webdav tahoe glacier ddar hook external
    local repository version: 5
    supported repository versions: 3 5 6
    upgrade supported from repository versions: 0 1 2 3 4 5
    operating system: darwin x86_64

    Debian 7
    gpg (GnuPG) 1.4.12
    git-annex version: 6.20170101+gitg93d69b1-1~ndall+1
    build flags: Assistant Webapp Pairing Testsuite S3(multipartupload)(storageclasses) WebDAV Inotify DBus DesktopNotify ConcurrentOutput TorrentParser MagicMime Feeds Quvi
    key/value backends: SHA256E SHA256 SHA512E SHA512 SHA224E SHA224 SHA384E SHA384 SHA3_256E SHA3_256 SHA3_512E SHA3_512 SHA3_224E SHA3_224 SHA3_384E SHA3_384 SKEIN256E SKEIN256 SKEIN512E SKEIN512 SHA1E SHA1 MD5E MD5 WORM URL
    remote types: git gcrypt p2p S3 bup directory rsync web bittorrent webdav tahoe glacier ddar hook external
    local repository version: 5
    supported repository versions: 3 5 6
    upgrade supported from repository versions: 0 1 2 3 4 5
    operating system: linux x86_64

### Please provide any additional information below.

[[!format sh """
# Debian 7
    $ git annex enableremote S3
    enableremote S3 gpg: packet(2) with unknown version 13
      gpg error above was caused by an old git-annex bug in credentials storage. Working around it..
    git-annex: bad creds
    CallStack (from HasCallStack):
      error, called at ./Creds.hs:116:22 in main:Creds

 #   MacOSX
    git annex enableremote S34
    enableremote S34 gpg: [don't know]: invalid packet (ctb=6b)
      gpg error above was caused by an old git-annex bug in credentials storage. Working around it..
    git-annex: bad creds
    CallStack (from HasCallStack):
      error, called at ./Creds.hs:116:22 in main:Creds

"""]]

### Have you had any luck using git-annex before? (Sometimes we get tired of reading bug reports all day and a lil' positive end note does wonders)
Yes! git annex has been enormously helpful. Thanks so much for this tool.

