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?? 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 */