Bob Meckle writes in:

I have recently come across a situation where it would be greatly beneficial to build a script to check revocation dates on certificates issued using a certain template, and send an email to our certificate staff letting them know which certificates will expire within the next 6 weeks. I am wondering if you guys have any tricks up your sleeve in regards to this situation?

Thanks for writing in Bob. This is actually quite simple on Windows. One of my favorite features of PowerShell is that dir (an alias for Get-ChildItem) can be used on pretty much any hierarchy, including the certificate store. After we have a list of the certificates we simply filter by the expiration date. Here is how we can find expiring certificates:

PS C:\> Get-ChildItem -Recurse cert: | Where-Object { $_.NotAfter -le (Get-Date).AddDays(42) -and $_.NotAfter -ge (Get-Date) }

We start off by getting a recursive list of the certificates. Then the results are piped into the Where-Object cmdlet to filter for the certificates that expire between today and six weeks (42 days) from now (inclusive). Additional filtering can be added by simply modifying the Where-Object filter, per Bob's original request. We can shorten the command using aliases and shortened parameter names, as well as store the output to be emailed.

PS C:\> $tomail = ls -r cert: | ? { $_.NotAfter -le (Get-Date).AddDays(42) -and $_.NotAfter -ge (Get-Date) }

Emailing is quite simple too. PowerShell version 4 adds a new cmdlet, Send-MailMessage. We can send the certificate information like this:

PS C:\> Send-MailMessage -To me@blah.com -From cert@blah.com -Subject "Expiring Certs" -Body $tomail

Pretty darn simple if you ask me. Hal, what do you have up your sleeve?

Tim, you're getting soft with all these cool PowerShell features. Why when I was your age... Oh, nevermind! Kids these days! Hmph!

If I'm feeling curmudgeonly, it's because this is far from a cake-walk in Linux. Obviously, I can't check a Windows certificate store remotely from a Linux machine. So I thought I'd focus on checking remote web site certificates (which could be Windows, Linux, or anything else) from my Linux command line.

The trick for checking a certificate is fairly well documented on the web:

$ echo | openssl s_client -connect www.google.com:443 2>/dev/null | 
     openssl x509 -noout -dates
notBefore=Jun  4 08:58:29 2014 GMT
notAfter=Sep  2 00:00:00 2014 GMT

We use the OpenSSL built-in "s_client" to connect to the target web server and dump the certificate information. The "2>/dev/null" drops some extraneous standard error logging. The leading "echo" piped into the standard input makes sure that we close the connection right after receiving the certificate info. Otherwise our command line will hang and never return to the shell prompt. We then use OpenSSL again to output the information we want from the downloaded certificate. Here we're just requesting the "-dates"-- "-noout" stops the openssl command from displaying the certificate itself.

However, there is much more information you can parse out. Here's a useful report that displays the certificate issuer, certificate name, and fingerprint in addition to the dates:

$ echo | openssl s_client -connect www.google.com:443 2>/dev/null | 
     openssl x509 -noout -issuer -subject -fingerprint -dates
issuer= /C=US/O=Google Inc/CN=Google Internet Authority G2
subject= /C=US/ST=California/L=Mountain View/O=Google Inc/CN=www.google.com
SHA1 Fingerprint=F4:E1:5F:EB:F1:9F:37:1A:29:DA:3A:74:BF:05:C5:AD:0A:FB:B1:95
notBefore=Jun  4 08:58:29 2014 GMT
notAfter=Sep  2 00:00:00 2014 GMT

If you want a complete dump, just use "... | openssl x509 -text".

But let's see if we can answer Bob's question, at least for a single web server. Of course, that's going to involve some date arithmetic, and the shell is clumsy at that. First I need to pull off just the "notAfter" date from my output:

$ echo | openssl s_client -connect www.google.com:443 2>/dev/null | 
     openssl x509 -noout -dates | tail -1 | cut -f2 -d=
Sep  2 00:00:00 2014 GMT

You can convert a time stamp string like this into a "Unix epoch" date with the GNU date command:

$ date +%s -d 'Sep  2 00:00:00 2014 GMT'
1409616000

To do it all in one command, I just use "$(...) for command output substitution:

$ date +%s -d "$(echo | openssl s_client -connect www.google.com:443 2>/dev/null | 
                     openssl x509 -noout -dates | tail -1 | cut -f2 -d=)"
1409616000

I can calculate the difference between this epoch date and the current epoch date ("date +%s") with some shell arithmetic and more command output substitution:

$ echo $(( $(date +%s -d "$(echo | openssl s_client -connect www.google.com:443 2>/dev/null | 
                                openssl x509 -noout -dates | tail -1 | cut -f2 -d=)") 
            - $(date +%s) ))
5452606

Phew! This is already getting pretty long-winded, but now I can check my result against Bob's 6 week threshold (that's 42 days or 3628800 seconds):

$ [[ $(( $(date +%s -d "$(echo | openssl s_client -connect www.google.com:443 2>/dev/null | 
                              openssl x509 -noout -dates | tail -1 | cut -f2 -d=)") - $(date +%s) )) 
    -gt 3628800 ]] && echo GOOD || echo EXPIRING
GOOD

Clearly this is straying all too close to the borders of Scriptistan, but if you wanted to check several web sites, you could add an outer loop:

$ for d in google.com sans.org facebook.com twitter.com; do 
    echo -ne $d\\t; 
    [[ $(( $(date +%s -d "$(echo | openssl s_client -connect www.$d:443 2>/dev/null | 
                            openssl x509 -noout -dates | tail -1 | cut -f2 -d=)") - $(date +%s) ))
       -gt 3628800 ]] && echo GOOD || echo EXPIRING; 
done
google.com	GOOD
sans.org	GOOD
facebook.com	GOOD
twitter.com	GOOD

See? It's all good, Bob!