Skip to main content

OCSP certificate validation - packet analysis

Online Certificate Status Protocol (OCSP) is an IETF standard defined in RFC 2560 (http://www.ietf.org/rfc/rfc2560.txt). OCSP is used for real-time certificate status checking. OCSP uses HTTP as its transport mechanism. Transaction consists of an HTTP query using an HTTP POST verb and an HTTP 200 response. 

1. OCSP request (MIME type: ocsp-request):

HTTP header:

POST /ocsp HTTP/1.1
Cache-Control: no-cache
Connection: Keep-Alive
Pragma: no-cache
Content-Type: application/ocsp-request
Accept: */*
User-Agent: Microsoft-CryptoAPI/6.2
Content-Length: 86
Host: srv3.kp.local

The request contains hash of the issuer's name and public key along with the serial number of the certificate to be validated:



2. OCSP response (MIME type: ocsp-response): 

HTTP header:

HTTP/1.1 200 OK

Cache-Control: max-age=580
Content-Length: 1256
Content-Type: application/ocsp-response
Expires: Mon, 22 Apr 2013 10:34:16 GMT
Last-Modified
: Mon, 22 Apr 2013 08:59:16 GMT
ETag: "407c5206fbf20cfa69f6110435a82fd4"
Server: Microsoft-IIS/8.0
Date: Mon, 22 Apr 2013 09:59:35 GMT


OCSP response contains the revocation status 


To prevent spoofing attacks, the response is signed by the responder. In order to validate the signature, certificate containing public key of the responder is returned. This could lead to a problem whereby OCSP signing certificate revocation would be checked leading to a "verification loop". According to the RFC's 2560 section 4.2.2.2.1 there are three ways of overcoming this issue. Microsoft CA implementation uses special extension "id-pkix-ocsp-nochek". This extention tells the requester not to validate status of the OCSP signing certificate. The risk of the certificate being misused is mitigated by using very short certificate validity periods. 


OCSP signing certificate:



Corresponding OCSP responder signing configuration:


Labels:

Comments

Post a Comment

Popular posts from this blog

x.509 Certificates - Critical vs non-critical extensions

Extensions are used to associate additional information with the user or the key.  Each certificate extension has three attributes - extnID, critical, extnValue extnID - Extension ID - an OID that specifies the format and definitions of the extension critical - Critical flag - Boolean value extnValue - Extension value  Criticality flag specifies whether the information in an extension is important. If an application doesn't recognize the extension marked as critical, the certificate cannot be accepted. If an extension is not marked as critical (critical value False) it can be ignored by an application. In Windows, critical extensions are marked with a yellow exclamation mark,  View certificate extensions using OpenSSL: # openssl x509 -inform pem -in cert.pem -text -noout (output abbreviated)         X509v3 extensions:             X509v3 Key Usage: critical                 Digital Signature, Key Encipherment             X509v3 Subject Key Identifier
Post a Comment
Read more

DNS response and error types

In this post we explore common DNS response codes. We will cover the following responses: NOERROR SERVFAIL NXDOMAIN NODATA REFUSED Throughout article we’ll refer to the following RFCs: RFC 1034 - DOMAIN NAMES - CONCEPTS AND FACILITIES RFC 2308 - Negative Caching of DNS Queries (DNS NCACHE) RFC 2136 - Dynamic Updates in the Domain Name System (DNS UPDATE) RFC 8914 - Extended DNS Errors Response Codes - RCODEs The DNS RCODES are best defined in RFC2316 .  They signify what type of response was sent by the server. “RCODE   Response code - this four bit field is undefined in requests and set in responses.”   The table below shows the summary of the currently defined RCODEs. Mnemonic Val Description NOERROR 0 No error condition.
Post a Comment
Read more

Count number of lines - 'findstr'

How do I count number of lines in a command output? findstr /r/n "^" | find /c ":" Above commands will display number of lines output by whatever command (well, nearly whatever) you specify in the front.  For example:  C:\>ping localhost | findstr /r/n "^" | find /c ":" FINDSTR: // ignored 12 This comes handy if you want to find out how many OUs you have in Active Directory: dsquery ou  -limit 0 | findstr /r/n "^" | find /c ":" How many user accounts there are: dsquery user -limit 0 | findstr /r/n "^" | find /c ":" Computers: dsquery computer -limit | findstr /r/n "^" | find /c ":"
Post a Comment
Read more