Saturday, April 4, 2015

Expecting more - gpg and expect

Hi everyone!

I recently needed to ssh to a machine, with a hard to remember password, and was unable to use ssh keys.

I had seen lots of examples for connecting though ssh that looked something like this...

#!/usr/bin/expect -f
set timeout 60
spawn ssh

expect "Password: "
send "Password\r"

send "\r" # This is successful. I am able to login successfully to the first machine

# at this point, carry on scripting the first ssh session:
send "ssh\r"
expect ...

Can you see something terribly wrong there?  The password is stored in plain text in the file! Even setting restrictive file access permissions, that's not a very secure way of handling passwords.

I had read about gpg for storing/extracting secrets when I was learning puppet with the puppet 3 cookbook.

The steps to use gpg are as follows.
#generate gpg key
temp@old-laptop:~$ gpg --gen-key
gpg (GnuPG) 1.4.16; Copyright (C) 2013 Free Software Foundation, Inc.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

gpg: directory `/home/temp/.gnupg' created
gpg: new configuration file `/home/temp/.gnupg/gpg.conf' created
gpg: WARNING: options in `/home/temp/.gnupg/gpg.conf' are not yet active during this run
gpg: keyring `/home/temp/.gnupg/secring.gpg' created
gpg: keyring `/home/temp/.gnupg/pubring.gpg' created
Please select what kind of key you want:
   (1) RSA and RSA (default)
   (2) DSA and Elgamal
   (3) DSA (sign only)
   (4) RSA (sign only)
Your selection? 1
RSA keys may be between 1024 and 4096 bits long.
What keysize do you want? (2048)
Requested keysize is 2048 bits
Please specify how long the key should be valid.
         0 = key does not expire
      <n>  = key expires in n days
      <n>w = key expires in n weeks
      <n>m = key expires in n months
      <n>y = key expires in n years
Key is valid for? (0)
Key does not expire at all
Is this correct? (y/N) y

You need a user ID to identify your key; the software constructs the user ID
from the Real Name, Comment and Email Address in this form:
    "Heinrich Heine (Der Dichter) <>"

Real name: John Doe
Email address:
Comment: example
You selected this USER-ID:
    "John Doe (example) <>"

Change (N)ame, (C)omment, (E)mail or (O)kay/(Q)uit? o
You need a Passphrase to protect your secret key. 
gpg: /home/temp/.gnupg/trustdb.gpg: trustdb created
gpg: key CFE3FAA2 marked as ultimately trusted
public and secret key created and signed.

gpg: checking the trustdb
gpg: 3 marginal(s) needed, 1 complete(s) needed, PGP trust model
gpg: depth: 0  valid:   1  signed:   0  trust: 0-, 0q, 0n, 0m, 0f, 1u
pub   2048R/CFE3FAA2 2015-04-04
      Key fingerprint = 5851 BF85 33A2 01E5 895F  0BD2 34B0 F74F CFE3 FAA2
uid                  John Doe (example) <>
sub   2048R/D7422C09 2015-04-04

#make file containing secret - please do it with an editor!
temp@old-laptop:~$ echo "1234" > ~/passfile

#encrypt file
temp@old-laptop:~$ gpg -e -r ~/passfile

#remove plaintext file
temp@old-laptop:~$ ls
examples.desktop  passfile  passfile.gpg
temp@old-laptop:~$ rm passfile

#decrypt file
temp@old-laptop:~$ gpg -d ~/passfile.gpg

You need a passphrase to unlock the secret key for
user: "John Doe (example) <>"
2048-bit RSA key, ID D7422C09, created 2015-04-04 (main key ID CFE3FAA2)

can't connect to `/run/user/1000/keyring-WFzMA0/gpg': Permission denied
gpg: can't connect to `/run/user/1000/keyring-WFzMA0/gpg': connect failed
gpg: encrypted with 2048-bit RSA key, ID D7422C09, created 2015-04-04
      "John Doe (example) <>"

But, how do I tie that into an expect script for remotely logging in?

I initially found the exec statement for expect.
set password [exec gpg -d passfile.gpg]
It just didn't work properly for me.  GPG would prompt me for my passphrase, but it would output some notification text and the key to my display, and while it would be stored in the variable, it confused expect and didn't properly spawn my ssh connection. 

If I faked out getting the value directly, it would function as expected
set password [exec echo 1234]
Hrm... so what was I doing wrong? If I ran the same command and grabbed the output and redirected it to a file, it only contained my password.

gpg -d passfile.gpg > passfile.out
cat passfile.out

After reading about 50 different stack overflow posts that mostly said 'Why don't you just use ssh keys' or just had examples in plain text, I FINALLY found a clue!

This poor guy got some bad advice, where they aren't using the exec statement (he's just assigning the command string to the password variable), but he had the part that I missed.... the --quiet and --no-tty arguments!

once I added those to my gpg command, everything finally worked!

Here is my final example script -
 set pass [exec gpg --decrypt --quiet --no-tty passfile.gpg]

spawn ssh temp@localhost
expect "password:"
send "$pass\r"