Checking SSL Certificates

Verify whether a certificate matches a private key and has valid timestamp

The certificate is the signed public key (and some meta data); to verify whether a certificate matches the private key, one has to extract the public key of both and compare them.

Both the rsa and the x509 subcommand of openssl have a -modulus option to extract the modulus; they can also extract the complete public key, but the option has a different name for each of the subcommands:

1
2

openssl x509 -in "$CRT" -noout -pubkey
openssl rsa -in "$KEY" -pubout 2>/dev/null

The timestamps are in the text output of the certificate in the fields “Not Before:” and “Not After :”:

1
2

openssl x509 -in "$CRT" -text -noout | \
awk -- 'BEGIN {FS=": ";b="";e=""}  /^ *Not Before *: / { b = $2 } /^ *Not After *: / { e = $2 } END { print b; print e }

You can convert those timestamps to “seconds since 1970-01-01 00:00:00 UTC” width date +%s, making it easy to compare them to the current date (in the same format).

All in one script (you have to set KEY and CRT before; you probably may want to check whether the files exist too):

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21

A=$(openssl x509 -in "$CRT" -noout -pubkey)
B=$(openssl rsa -in "$KEY" -pubout 2>/dev/null)
if [ "$A" != "$B" ]; then
    echo "cert doesn't match private key" >&2
    printf "$CRT pubkey:\n%s\n$KEY pubkey:\n%s\n" "$A" "$B" >&2
else
    DATES=$(openssl x509 -in "$CRT" -text -noout |
        awk -- 'BEGIN {FS=": ";b="";e=""}  /^ *Not Before *: / { b = $2 } /^ *Not After *: / { e = $2 } END { print b; print e }')
    begin=$(echo "$DATES" | head -n1)
    end=$(echo "$DATES" | tail -n1)
    ubegin=$(date -d "$begin" +%s)
    uend=$(date -d "$end" +%s)
    unow=$(date +%s)
    if [ $unow -lt $ubegin -o $uend -lt $unow ]; then
        echo "Invalid timestamps for $CRT, was valid for:" >&2
        echo "$DATES" >&2
    else
        echo "Valid timestamps for $CRT (which matches privkey):"
        echo "$DATES"
    fi
fi