Good day,
I don't fully understand ocsp certificate verification. In order
to better understand it, I want to do it manually. I can already
do that with certificates.
$ openssl
s_client -connect openssl.org:443 -showcerts
# I save the server.crt and intermediate.crt
$ openssl verify -no-CApath -partial_chain -trusted
intermediate.crt server.crt
server.crt: OK
Manually:
# Get the ASN id's of the TBS and Signature
$ asn=`openssl
asn1parse -i -in server.crt |egrep -e '(^ .*: SEQUENCE|: BIT
STRING)'`
$ asn_tbs=`echo
"$asn" | head -1 | awk -F: '{print $1}' | sed 's/ //g'`
$ asn_sig=`echo
"$asn" | tail -1 | awk -F: '{print $1}' | sed 's/ //g'`
# Get tbs
openssl asn1parse -in server.crt -strparse ${asn_tbs} -out
server.tbs > /dev/null
# Hash tbs
$ cat
server.tbs | openssl sha256 -binary > server.tbs.sha256
# Get
signature (ignore 'header too long' error)
$ openssl
asn1parse -in server.crt -strparse ${asn_sig} -out server.sig
> /dev/null
# Get public key of intermediate
$ openssl
x509 -in intermediate.crt -noout -pubkey > intermediate.pub
# Recover
(decrypt) the signature
$ openssl
rsautl -inkey intermediate.pub -pubin -in server.sig -out
server.sig.recovered
# Verify.
Ignore the first line of server.sig.recovered, this is the hash
algoritm designator
$ od
-An -tx1 -w19 server.sig.recovered
30 31 30 0d 06 09 60 86 48 01 65 03 04 02 01 05 00 04 20
87 36 67 06 ba d7 10 18 72 d3 f6 58 00 a9 34 78 bc 82 bf
57 37 20 ab 82 04 fb 04 78 38 e2 d3 a2
$ od
-An -tx1 -w19 server.tbs.sha256
87 36 67 06 ba d7 10 18 72 d3 f6 58 00 a9 34 78 bc 82 bf
57 37 20 ab 82 04 fb 04 78 38 e2 d3 a2
Yay. Now how do I do that with OCSP ?
# Get OCSP
$ ocsp=`openssl
x509 -noout -ocsp_uri -in server.crt`
# Verify
$ ocsp_response=`openssl ocsp -noverify -no_nonce -respout
ocsp.resp -reqout ocsp.req -issuer intermediate.crt -cert
server.crt -text -url $ocsp`
$ echo "$ocsp_response" | grep server.crt
server.crt: good
Manually:
# Get the signature. Can't find how to do this with asn1parse
$ for byte in `echo "$ocsp_response" | grep -A40 " Signature
Algorithm" | grep -B40 "server.crt" | egrep -ve '(Signature
Algorithm|server.crt)' | sed -e 's/ //g' -e 's/:/ /g'`; do
echo -ne "\x$byte"
done > ocsp.resp.sig
# Recover (decrypt) the
signature
$ openssl rsautl -inkey intermediate.pub -pubin -in
ocsp.resp.sig -out ocsp.resp.sig.recovered
# Print the
decrypted signature (looks good, first line is hash algorithm
designator, length looks ok, no errors)
$ od -An -tx1 -w19 ocsp.resp.sig.recovered
30 31 30 0d 06 09 60 86 48 01 65 03 04 02 01 05 00 04 20
b4 83 f2 c3 4a 6c 1b 4e df 66 b4 d5 31 0b 58 c3 60 3c e9
20 0f 4f b0 df 61 88 2f c0 e0 25 66 a8
But.. How to
extract the tbs data from the response, so I can sha256 that
and compare ?
