Tag: OCSP

One minute… what is a digital certificate?

This blog post covers

Why use certificates?

Certificates can be used as an authentication mechanism. It allows identity information to be sent over a network, and the remote end to validate the information before using it. For this to work, we need a “Certificate Authority” who can validate the certificate. When the remote end gets the “certified certificate” it checks the certification and validity and if successful can use this. Checking can involve sending an electronic request to the Certificate Authority asking if the certificate is still valid.

What is a certificate?

A certificate has several components (conceptually I think of a certificate as an envelope with several bits of paper within it).

  • A public key. See One minute explanation of public keys and private keys.
  • Information about the owner, such as name and address or organization
  • Meta information – such as it can be used for document signing (but not certificate signing), and validity dates.
  • Signing information.

Information about the owner

A certificate has information about the “owner”. Common attributes include

  • Common Name CN=Colin Paice
  • Organisation O=My Org
  • Organisational Unit OU=TEST
  • Country C=GB

Meta information.

The certificate contains information on how it should be used. For example

  • This certificate can be used as a server’s certificate – but not as a client certificate.
  • This certificate can be used for signature but not encryption.
  • This certificate can be used to sign other certificate (see below).

A server (or client) could decide to ignore some of these attributes. A proper server will honour the information, and if the client’s certificate does not have the “can be used by a client” attribute set, it will reject the certificate.

How do you trust a certificate?

If you have been sent a certificate – how do you tell if it is genuine? This was one of the problems in the early days of secure communications.

This problem was solved by having an organisation we both trust, and having this organisation “sign” a certificate.

Imagine there is a UK Certificate Authority (UKCA). To get your certificate approved…

  • you take your certificate, your passport, and proof of address to the Certificate Authority (UKCA).
  • A clerk in the CA, checks that the name in your certificate matches your passport, and the address in the certificate matches the proof of address your provided.
  • The clerk can check “the permissions” within your certificate, such as this certificate can/cannot be used as a certificate authority, or can/cannot be used as a server. The CA may not care!
  • The CA take an electronic copy of your certificate and perform a checksum calculation on the contents. If the certificate is changed in any way, the checksum calculation will be a different number.
  • The CA then encrypts the checksum calculation, prints it out, and staples the encrypted value and a copy of the CA’s public certificate to the back of your certificate.
  • The certificate has now been signed by the CA.

You send me a copy of your certificate and attachement created by the CA.

When I get a copy of your certificate (and attachment),

  • I check the CA’s public certificate that you sent me is the same as the copy I already have.
  • I do the same check sum calculation as the CA authority did.
  • I use the CA’s public certificate to decrypt the CA’s encrypted checksum.
  • The two checksum values should match!

If the checksum values match, then I can trust that the information in the certificate is the same as the information you showed to the CA when getting you certificate “signed”. Of course if you provided a fake passport, all I know is that what I see is what the CA saw.

Intermediate CAs and certificate chains

Of course it costs you money to get your certificate signed by a CA. To reduce this cost you can set up your own enterprise Certificate Authority. You create a certificate called CN=MYORGCA, and set the attribute set to allow it to sign other certificates. You take this the the UKCA and get it signed.

When Joe wants his personal certificate signed, he come to you,

  • you check Joe’s corporate ID badge has the same name as in Joe’s certificate
  • you check any other data fits within your enterprise standards, such as expiry date.
  • you use your enterprise CA certificate(CN=MYORGCA) to sign Joe’ certificate.
  • Joe get a package with
    • Joe’s certificate,
    • the checksum from the enterprise signing, and the enterprise public certificate.
    • a copy of the UKCA’s certificate and encrypted checksum for your enterprise’s CA.

When I get Joe’s certificate and attachments.

  • I can see if I have a copy of your enterprise public certificate.
  • No I do not, but I do have a copy of the UKCA public certificate,
  • I validate the your enterprise’s public certificate, using the UKCA certificate
  • If your enterprises public certificate validates successfully, I can then use it to validate Joe’s certificate.

This means I can validate certificates sent to me, as long as I have at least one of the certificates in the chain.

You could take this further and have a departmental CA authority.

Is the certificate still valid?

If someone leaves your organisation you want to ensure that any certificates issued to that person cannot be used.

You can have an expiry date in the certificate, so within a year or two the certificate will expire. This is not good enough, and you want to make the invalidate the certificate as soon as possible. This can be done through an Online Certificate Status Protocol (OCSP) attribute in the certificate. Basically this says go and ask the URL if this certificate if it is still valid.
How do you know who to ask? A certificate has url information within in it Authority Information Access: OCSP – URI:http://ColinsCert.Checker.com/ . The back end serving the URL is typically an LDAP server.

If you think about this for a few minutes this may not seem a good idea. For every secure connection handshake, you have to issue a query to a remote server somewhere to see if the certificate is still valid. This will seriously affect performance. This was solved by having a “OCSP response valid time”, sent as part of the response from the OCSP server. This known as stapling (I picture this as stapling a valid-until ticket to a certificate). The OCSP server can say “This certificate is valid, and assume it is valid for n hours”. The first time the client uses the certificate it may take longer because of the OCSP checks. For successive requests , the client can then use the certificate without doing any OCSP checks – until the time period has expired.

Do I trust the sender?

A bad actor could have copied the certificate from a valid server, and presented it to the client. The client checks it, and it looks ok, but in reality it came from a bad server.

You can include Subject Alternate Name (or SAN) in the certificate. This is the URL or IP address of the server. When the certificate is sent from the server, the client checks that the IP address of the server is in the list of SANs in the certificate. If it does not match then the certificate is rejected. Example SANs DNS:www.example.com, IP:10.1.1.2

If the DNS:name is used, this will also require a connection to a DNS to get back the IP address, which will add time to TLS hand shake to the server.

You should use SAN for the server, but not for the client. A Server will have a fixed, or limited choice of values. A client can get a random IP address and so is unlikely to match a value in the certificate.

Are my digital certificates still valid and are they slowing down my channel start?

Digital certificates are great. They allow program to program communication so each end can get information to identify the other end, and the programs can then communicate securely, with encryption, or just checking the payload has not changed.

A certificate is basically a file with two parts (or two files)  – a public certificate and a private key. You can publicize the public part to any one who wants it (which is why is is called the public part). Anyone with the private key can use it to say they are you. (If you can get access to the private key, then you can impersonate the identity)

There are times when you want to say this certificate is not longer valid. For example when I worked at IBM, I had a certificate on my laptop to access the IBM mail servers.

  • If my laptop was stolen, IBM would need to revoke the certificate.
  • When I retired from IBM, IBM revoked my certificate to prevent me from trying to access my IBM mail using my old certificate from my personal laptop.

Managing these certificate is difficult. There could be billions of certificates in use today.

Your server should validate every connection request to ensure the certificate sent from the client is still valid.

A client should validate the certificate sent by the server to ensure that it is connecting to a valid server.

In the early days of certificates, there was a big list of revoked certificates – the Certificate Revocation List(CRL). If a certificate is on the list then it has been revoked. You tend to have an LDAP server within your firewall which contains these lists of revoked certificates.

This was a step in the right direction, but it is difficult to keep these lists up to date, when you consider how many certificates are in use today, and how many organizations generate certificates. How often do I need to refresh my list? If the CRL server was to refresh it every day, it may be up to one day out of date, and report “this certificate is ok” – when it had been revoked.

These days there is a technique called Online Certificate Status Protocol (OCSP). Basically this says go and ask the site which issued this certificate if it is still valid. This is a good idea – and they say the simple ideas are the best.
How do you know who to ask? A certificate can have url information within in it Authority Information Access: OCSP – URI:http://ColinsCert.Checker.com/, or you can specify URL information in the queue manager configuration for those certificates without the Authority Information Access(AIA) information.

Often the URL in the certificate is outside of your organization, and outside of your firewall. To access the OCSP site you may need to have an SSL Proxy server which has access through the firewall.

You can configure MQ to use a (one) OCSP server for those certificates not using AIA information.  If your organization is a multinational company, you may be working with other companies who use different Certificate Authorities.   If you have certificates from more than one CA, you will not be able use MQ to check all of them to see if they are still valid.  You may want to set up an offline job which runs periodically and checks the validity of the certificates.

Starting the MQ channel can be slow

When an TLS/SSL MQ channel is started, you can use OCSP or CRL to check that a certificate is valid. This means sending a request to a remote server and waiting for a response.   The LDAP server for CRL requests is likely to be within your domain,  as your organization manages it. The OCSP  server could be outside of your control, and in the external network.  If this server is slow, or the access to the server is slow, the channel will be slow to start.  For many customers the network time within a site is under 1 millisecond, between two sites could be 50 ms. Going to an external site the time could be seconds – and dependent on the traffic to the external site.
This time may be acceptable when starting the channel, first thing in the morning, but restarting a channel during a busy period can cause a spike in traffic because no messages flow while the channel is starting. For example

ChannelwithRestart

No messages will flow while the channel is starting, and this delay will add to the round trip time of the messages.

How do I check to see if I have a problem

This is tough. MQ does not provide any information. I used MQ internal trace when debugging problems, but you cannot run with trace enabled during normal running.

There are two parts to the validation request. The time to get to and from the server, and the time to process your request once it has got to the server.

You can use TCP Ping to get to the server (or to the proxy server).  If you are using a proxy server you cannot “ping” through the proxy server.

Openssl provides a many functions for creating and managing certificates.

You can use the command

time openssl s_client -connect server:port
or
time openssl s_client -connect -proxy host:port server:port

The “time” is a linux command which tells your the duration in milliseconds of the command following it.

The openssl s_client is a powerful ssl client program. The -connect… tries connecting to server:port. You can specify -proxy host:port to use the proxy.

The server at the remote end may not recognize the request – but you will get the response time of the request.

Running this on my laptop I got

time openssl s_client -connect 127.0.0.1:8888
CONNECTED(00000005)
140566846001600:error:1408F10B:SSL routines:ssl3_get_record:wrong version number:../ssl/record/ssl3_record.c:332:
—
no peer certificate available
—
No client certificate CA names sent
—
SSL handshake has read 5 bytes and written 233 bytes

Verification: OK
…
real 0m0.012s

The request took 0.012 seconds.

My openssl OCSP server reported

OCSP Response Data:
OCSP Response Status: malformedrequest (0x1)

You can use openssl ocsp …. to send a certificate validation request to the OCSP server and check the validity of the system – but you would have to first extract the certificates from the iKeyman keystore.

How do I test this?

I’ve created the instructions I used:

More information about OCSP.

There is a good article in the IBM Knowledge Centre here .

The article says
To check the revocation status of a digital certificate using OCSP, IBM MQ determines which OCSP responder to contact in one of two ways:

  1. Using a URL specified in an authentication information object or specified by a client application.
  2. Using the AuthorityInfoAccess (AIA) certificate extension in the certificate to be checked.

Configure a QM for OSCP checking for certificates without AIA information.

You can configure a queue manager to do OCSP checking, for those certificates without AIA information within the certificate.

There is a queue manager attribute SSLCRLNL ( SSL Certificate RevocationList Name List) which points to a name list.   This name list has a list of AUTHINFO object names.
The name list can have up to one AUTHINFO object for OCSP checking and up to 10 AUTHINFO objects for CRL checking.

You define an AUTHINFO object to define the URL of an OCSP server.

Define AUTHINFO(MYOSCP) AUTHTYPE(OCSP) OCSPURL(‘HTTP://MyOSCP.server’)

Create a name list, and add the AUTHINFO to it.

Use alter qmgr  SSLCRLNL(name) and refresh security type(SSL)
You need to change the qm.ini file SSL stanza of the queue manager configuration file see here.

OCSPAuthentication=REQUIRED|WARN|OPTIONAL
OCSPCheckExtensions= YES|NO
SSLHTTPProxyName= string

If you have a fire wall around your network, you can use SSLHTTPProxyName to get through your fire wall.

There is some good information here.

Configure client OSCP checking for certificates without AIA information.

You need the CCDT created by a queue manager rather than a JSON CCDT.
When you configure a queue manager with the AUTHINFO objects and the queue manager SSLCRLNL attributes, the information is copied to the CCDT.

This CCDT is in the usual location, for example the /prefix/qmgrs/QUEUEMANAGERNAME/@ipcc directory.

You can use a CCDT from one queue manager, when accessing other queue managers.

You need to make the CCDT file available to the client machines, for example email or FTP,  or use URL access to the CCDT.

You also should configure the mqclient.ini file see here.

How do I check to see if my certificates have AIA information.

You can use the iKeyman GUI to display details about the certificate, or a command line like

/opt/mqm/bin/runmqckm -cert -details -db key.kdb -pw password -label CLIENT

This gives output like

Label: CLIENT
Key Size: 2048
Version: X509 V3
Serial Number: 01
Issued by: ….

Extensions:
– AuthorityInfoAccess: ObjectId: 1.3.6.1.5.5.7.1.1 Criticality=false
AuthorityInfoAccess [
[accessMethod: ocsp
accessLocation: URIName: http://ocspserver.my.host/
]]

Can I turn this OCSP checking off ?

For example if you think you have a problem with OCSP server response time.

In the mqclient.ini you can set

  • ClientRevocationChecks = DISABLED No attempt is made to load certificate revocation configuration from the CCDT and no certificate revocation checking is done
  • OCSPCheckExtensions = NO   This says ignore the URL in the AIA information within a certificate.

See SSL stanza of the client configuration file.

In the qm.ini you can set

  • OCSPCheckExtensions=NO  This says ignore the URL in the AIA information within a certificate.
  • alter qmgr SSLCRLNL(‘ ‘) and refresh security type(SSL)

See SSL stanza of the queue manager configuration file and Revoked certificates and OCSP.

How do I tell if I have a problem with OCSP?

There are no events or messages which tell you the response time of requests.

You may get message AMQ9716

AMQ9716
Remote SSL certificate revocation status check failed for channel …
Explanation
IBM MQ failed to determine the revocation status of the remote SSL certificate for one of the following reasons:
(a) The channel was unable to contact any of the CRL servers or OCSP responders for the certificate.
(b) None of the OCSP responders contacted knows the revocation status of the certificate.
(c) An OCSP response was received, but the digital signature of the response could not be verified.
You can change the queue manager configuration to not produce these messages, by setting ClientRevocationChecks = OPTIONAL
From this message you cannot tell if the request got to the server.
The easiest way may be to ask the network people to take a packet trace to the URL(s) and review the time of the requests and the responses.

Using the AuthorityInfoAccess (AIA) certificate extension in the certificate.

You can create certificates containing the URL needed to validate the certificate. Most of the IBM MQ documentation assumes you have already have a certificate with this information in it.
You can use openssl to create a certificate with AIA information, and import it into the iKeyman keystore.   See here.

You cannot use IBM GSKIT program iKeyman to generate this data because it does not support it. You can use iKeyman to display the information once it is inside the keystore.

Timing the validation request

Openssl has a command to validate a certificate for example

openssl ocsp -CAfile cacert.pem -issuer cacert.pem -cert servercert.pem -url http://OCSP.server.com:port -resp_text

You can use the linux time command, for example

time openssl ocsp -CAfile cacert.pem -issuer cacert.pem -cert servercert.pem -url http://OCSP.server.com:port -resp_text

you get

Response verify OK
servercert.pem: good
real 0m0.040s

The time taken to go to a OCSP server on the same machine is 40 milliseconds. The time for a ping to 127.0.0.1 was also 40 ms.

Thanks…

Thanks to Morag of MQGEM, and Gwydion at IBM for helping me get my head round this topic.