cran-r debian readme used to include security flaw

I had to bust the cache on one of my Docker images and when I rebuilt it I
noticed something rather concerning from the `apt-get install` step:

gpg: requesting key E084DAB9 from hkp server ha.pool.sks-keyservers.net
gpg: key E084DAB9: public key "Totally Legit Signing Key <
mallory at example.org>" imported
gpg: key E084DAB9: public key "Michael Rutter <marutter at gmail.com>" imported
gpg: Total number processed: 2
gpg:               imported: 2  (RSA: 2)

The "Totally Legit Signing Key" didn't look so totally legit to me. It
turns out it's because I followed the installation instructions from cran-r
from way back (
https://web.archive.org/web/20170702124141/https://cran.r-project.org/bin/linux/ubuntu/README.html
) and had this in my Dockerfile:

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys E084DAB9

Which was only a 32 bit GPG key. Someone recently got around to exploiting
this:
https://seclists.org/oss-sec/2018/q3/174

You can see in his description that  `Totally Legit Signing Key <mallory ()
example org>` is listed as proof of the exploit.

Now if you go to:
https://cran.r-project.org/bin/linux/ubuntu/README.html

now the line has since been changed to use a 64 bit GPG key:
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys
0x51716619e084dab9

which actually makes this exploit much harder. This is great, except that
if I hadn't been watching the output as closely I never would have known
that the code I copied from the official R installation instructions were
insecure.

The author of the exploit report actually commented on our pull request to
change this:
https://github.com/AlexsLemonade/refinebio/pull/590#issuecomment-419234831

and had this to say:

Duplicating 64-bit key IDs is not trivial, but it could be still within

budget of a large criminal organization.

Please consider using full fingerprint instead, i.e.:

E298A3A825C0D65DFD57CBB651716619E084DAB9



In conclusion, this email is intended to be a warning to anyone who has
that line sitting in a script, dockerfile, or worse a blog post. I would
also like to raise the question about whether or not anything more can be
done to alert others. Finally I would recommend updating Cran's readme to
suggest using the full fingerprint instead of the 64-bit key like so:

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys
E298A3A825C0D65DFD57CBB651716619E084DAB9

Thank you,
- Kurt

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