nss tech note3

All About Certificate Extensions

NSS Technical Note: 3

09 May 2002
Nelson B. Bolyard

This week at least 5 different people came to me with variants of the 
same question: 

  What certificate extensions do I have to put into my cert for NSS to 
  allow it to be used for purpose <x>??

This message attempts to answer that question, and to document NSS's 
approach to validating certificates for certain purposes.

When NSS is asked to verify the validity of a certificate chain, it
verifies the validity of that cert chain for a particular purpose,
known as a SECCertUsage, as of a specific date and time.  

The list of known SECCertUsages is short:

certUsageSSLClient ...........	An SSL client authentication cert
certUsageSSLServer ...........	An ordinary SSL server cert
certUsageSSLServerWithStepUp..	An SSL server cert that allows export 
				clients to use strong crypto.
certUsageSSLCA ...............	An intermediate or root CA cert allowed 
				to issue SSL client or SSL server certs
				or other intermediate SSL CA certs.
certUsageEmailSigner .........	Used to verify S/MIME email signatures
certUsageEmailRecipient ......	Used to encrypt S/MIME emails.
certUsageObjectSigner ........	Used to verify signatures on files of 
				executable code, e.g. jar files.
certUsageStatusResponder ..... 	Used by an OCSP responder
certUsageVerifyCA ............	A CA of any kind.

Each cert has a "type" and a "key usage", each of which may contain one
or more valid values.  

Each of the above SECCertUsages translates into a required set of 
cert type and key usage for the certificate itself, and into another
set of required cert type and key usage for all the CA certs in the 
cert chain.

To determine if a cert is valid for a given cert usage, it must have the 
the cert type and key usage required for that cert usage, and all the 
CA certs in the cert chain must have the cert type and key usage required
for CA certs for that cert usage.

There are 8 Key Usages:
	CERT_SIGN
	CRL_SIGN
	DATA_ENCIPHERMENT
	DIGITAL_SIGNATURE
	GOVT_APPROVED
	KEY_AGREEMENT
	KEY_ENCIPHERMENT
	NON_REPUDIATION	

There are 9 Cert types:
	EMAIL
	EMAIL_CA
	OBJECT_SIGNING
	OBJECT_SIGNING_CA
	SSL_CA
	SSL_CLIENT
	SSL_SERVER
	STATUS_RESPONDER	
	TIME_STAMP		


For the cert being checked, the requirements are:

Cert Usage		Requried Key Usage		Required Cert Type
-------------------- 	--------------------		-----------------------
SSLClient:		DIGITAL_SIGNATURE;		SSL_CLIENT;

SSLServer:		KEY_AGREEMENT OR
			KEY_ENCIPHERMENT;		SSL_SERVER;

SSLServerWithStepUp:	GOVT_APPROVED  AND		SSL_SERVER
			KEY_AGREEMENT  or
			KEY_ENCIPHERMENT 

SSLCA: 			CERT_SIGN;			SSL_CA;

EmailSigner:		DIGITAL_SIGNATURE;		EMAIL;

EmailRecipient:		KEY_AGREEMENT OR
			KEY_ENCIPHERMENT;		EMAIL;

ObjectSigner:		DIGITAL_SIGNATURE;		OBJECT_SIGNING;

StatusResponder:	DIGITAL_SIGNATURE;		STATUS_RESPONDER;

VerifyCA		CERT_SIGN			SSL_CA OR
							EMAIL_CA OR
							OBJECT_SIGNING_CA OR
							STATUS_RESPONDER

For CA certs in the cert chain, the requirements are:

Cert Usage		Requried Key Usage	Required Cert Type
-------------------- 	--------------------	-----------------------
SSLServerWithStepUp:	GOVT_APPROVED AND 
			CERT_SIGN;		SSL_CA;

SSLClient:		CERT_SIGN;		SSL_CA;

SSLServer:	 	CERT_SIGN;		SSL_CA;

SSLCA:			CERT_SIGN;		SSL_CA;

EmailSigner:		CERT_SIGN;		EMAIL_CA or SSL_CA

EmailRecipient:		CERT_SIGN;		EMAIL_CA or SSL_CA

ObjectSigner:		CERT_SIGN;		OBJECT_SIGNING_CA;

UsageAnyCA:		CERT_SIGN;		OBJECT_SIGNING_CA OR
      						EMAIL_CA OR
          					SSL_CA;

StatusResponder: 	CERT_SIGN;		OBJECT_SIGNING_CA OR
      						EMAIL_CA OR 
          					SSL_CA;

Note:  When the required key usage is KEY_AGREEMENT OR KEY_ENCIPHERMENT,
the actual key usage required depends on the key's algorithm.  For
RSA keys, the required usage is KEY_ENCIPHERMENT. For other types of 
keys, it is KEY_AGREEMENT.


Cert Extensions:

One vital Certificate extension is the "Basic Constraints" extension.
It tells NSS whether the cert is a CA cert, or not, and affects every
other aspect of how the cert is interpreted by NSS.  The OID for this 
extension is { 2 5 29 19 }, encoded in hex as 0x55, 0x1d, 0x13.
If the extension is present and has the value TRUE, then this cert is 
taken to be a CA cert.  Otherwise it is not (except that trust flags
may override this, see discussion of trust flags farther below).

Netscape has its own openly defined Cert Type extension, which can be used
to explicitly set the Cert Type in any Cert.  The Cert Type extension has
bits in it that correspond directly to the cert types named above.  
The OID for this extension is { 2 16 840 1 113730 1 1 }
encoded in hex as  0x60, 0x86, 0x48, 0x01, 0x86, 0xf8, 0x42, 0x01, 0x01

In addition to Netscape's own Cert Type extension, NSS recognizes various
X.509 extensions.  

The X.509 key usage extension has OID { 2 5 29 0F } encoded in hex as
0x55, 0x1d, 0x0f.  If present, this extension directly determines the 
values of the 8 key usages defined above.  If absent, the cert is
assumed to be valid for all key usages.

The X.509v3 extended Key usage extension as OID { 2 5 29 37 } encoded in 
hex as 0x55, 0x1d, 0x25.  That extension contains a sequence of OIDs, each 
of which signifies one or more Cert Types, depending on the presence or 
absence of of the True Basic Constraints extension; that is, the 
interpretation of the extended Key Usage extension is controlled by 
whether the cert is a CA cert, or not.  

The following table shows the OIDs recognized in the extended key usage
extension, and how they map to cert types and key usages for CA and non-CA
certs.

extended key usage OID			non-CA cert	CA cert
-----------------------------------	--------------	----------------
SEC_OID_EXT_KEY_USAGE_EMAIL_PROTECT  	EMAIL_CA	EMAIL_CA
SEC_OID_EXT_KEY_USAGE_SERVER_AUTH	SSL_SERVER	SSL_CA
SEC_OID_EXT_KEY_USAGE_CLIENT_AUTH	SSL_CLIENT	SSL_CA
SEC_OID_EXT_KEY_USAGE_CODE_SIGN		OBJECT_SIGNING	OBJECT_SIGNING_CA
SEC_OID_EXT_KEY_USAGE_TIME_STAMP	TIME_STAMP	TIME_STAMP
SEC_OID_OCSP_RESPONDER			OCSP_RESPONDER	OCSP_RESPONDER

SEC_OID_NS_KEY_USAGE_GOVT_APPROVED	GOVT_APPROVED	GOVT_APPROVED

If the extended key usage extension is absent, the cert is assumed to have
the cert types SSL_CLIENT, SSL_SERVER and EMAIL, and if the cert is a CA
cert (as indicated by the presence of a true basic constraints extension),
the cert is also assumed to have the cert types SSL_CA, EMAIL_CA and
STATUS_RESPONDER.  If the basic constraints extension is missing, but the 
user has trusted the cert as a CA cert, the cert also gets the 
STATUS_RESPONDER cert type.  If the cert has a Fortezza type public key 
with the magic bits that signify that it is a CA, it is given cert types 
SSL_CA and EMAIL_CA.

A cert with the extended key usage extension and the Netscape cert type
extension that has the cert type SSL_CLIENT and also has an email address 
in the subject is also given the cert type EMAIL.  This allows all SSL 
client authentication certs with email addresses to also be used as email 
certs (provded they have adequate key usage).

A cert with the extended key usage extension  and the Netscape cert type
extension that as cert type SSL_CA is also always given cert type EMAIL_CA. 
This allows all SSL intermediate CAs to also be used as email intermediate CAs.

/* X.509 v3 Key Usage Extension flags */
#define KU_DIGITAL_SIGNATURE            (0x80)  /* bit 0 */
#define KU_NON_REPUDIATION              (0x40)  /* bit 1 */
#define KU_KEY_ENCIPHERMENT             (0x20)  /* bit 2 */
#define KU_DATA_ENCIPHERMENT            (0x10)  /* bit 3 */
#define KU_KEY_AGREEMENT                (0x08)  /* bit 4 */
#define KU_KEY_CERT_SIGN                (0x04)  /* bit 5 */
#define KU_CRL_SIGN                     (0x02)  /* bit 6 */

#define NS_CERT_TYPE_SSL_CLIENT         (0x80)  /* bit 0 */
#define NS_CERT_TYPE_SSL_SERVER         (0x40)  /* bit 1 */
#define NS_CERT_TYPE_EMAIL              (0x20)  /* bit 2 */
#define NS_CERT_TYPE_OBJECT_SIGNING     (0x10)  /* bit 3 */
#define NS_CERT_TYPE_RESERVED           (0x08)  /* bit 4 */
#define NS_CERT_TYPE_SSL_CA             (0x04)  /* bit 5 */
#define NS_CERT_TYPE_EMAIL_CA           (0x02)  /* bit 6 */
#define NS_CERT_TYPE_OBJECT_SIGNING_CA  (0x01)  /* bit 7 */
</x>

Document Tags and Contributors

Contributors to this page: kwilson, Sheppy
Last updated by: Sheppy,