nss tech note4

Pulling certificate extension information out of SSL certificates

NSS Technical Note: 4

Note: This document contains code snippets that focus on essential aspects of the task and often do not illustrate all the cleanup that needs to be done. Also, this document does not attempt to be an exhaustive survey of all possible ways to do a certain task; it merely tries to show a certain way.

Include these files

#include "ssl.h"
#include "cert.h"

Get the handle of the cert associated with an SSL connection

CERTCertificate* cert = SSL_PeerCertificate(PRFileDesc *fd);
If SSL client, this will get you the server's cert handle;
If SSL server, this will get you the client's cert handle IF client auth is enabled

CERTCertificate* cert = SSL_LocalCertificate(PRFileDesc *fd);
If SSL client, this will get you the client cert's handle, IF client auth happened
If SSL server, this will get you the server's cert handle

Don't forget to clean up the cert handle when you're done with it

void CERT_DestroyCertificate(CERTCertificate *cert);

Some info is readily available

cert->subjectName (char*)
cert->issuerName (char*)
cert->emailAddr (char*)
OR char *CERT_GetCertificateEmailAddress(CERTCertificate *cert);
cert->keyUsage (unsigned int)

To break the issuer and subject names into components

Pass &(cert->issuer) or &(cert->subject) to the following functions

char *CERT_GetCommonName(CERTName *name);
char *CERT_GetCertEmailAddress(CERTName *name);
char *CERT_GetCountryName(CERTName *name);
char *CERT_GetLocalityName(CERTName *name);
char *CERT_GetStateName(CERTName *name);
char *CERT_GetOrgName(CERTName *name);
char *CERT_GetOrgUnitName(CERTName *name);
char *CERT_GetDomainComponentName(CERTName *name);
char *CERT_GetCertUid(CERTName *name);

Example code to illustrate access to the info is given below.

Background on cert extensions

An extension has the following attributes

Object Id (OID) : A unique OID represents an algorithm, a mechanism, a piece of information, etc. Examples: X500 RSA Encryption, Certificate Basic Constraints, PKCS#7 Digested Data, etc.
There is a long list of pre-defined OIDs, and new ones can be added dynamically by an application.
The OID data structure contains an array of identifier bytes (each byte is a "level" in a hierarchical namespace), a text description, and some other things.
Critical : indicates whether the extension is critical
Value : The value of the extension

Looping through all extensions

CERTCertExtension** extensions =cert->extensions;
if (extensions)
{
    while (*extensions)
    {
        SECItem *ext_oid = &(*extensions)->id;
        SECItem *ext_critical = &(*extensions)->critical;
        SECItem *ext_value = &(*extensions)->value;

        /* id attribute of the extension */
        SECOidData *oiddata = SECOID_FindOID(ext_oid);
        if (oiddata == NULL)
        {
            /* OID not found */
            /* SECItem ext_oid has type (SECItemType), data (unsigned char *) and len (unsigned int) fields
               - the application interprets these */
            .......
        }
        else
        {
            char *name = oiddata->desc; /* name of the extension */
            .......
        }

        /* critical attribute of the extension */
        if (ext_critical->len > 0)
       {
            if (ext_critical->data[0])
                /* the extension is critical */
            else
                /* the extension is not critical */
       }

        /* value attribute of the extension */
        /* SECItem ext_value has type (SECItemType), data (unsigned char *) and len (unsigned int) fields
            - the application interprets these */
        SECOidTag oidtag = SECOID_FindOIDTag(ext_oid);
       switch (oidtag)
       {
            case a_tag_that_app_recognizes:
                .....

            case .....
                ......
       }

        extensions++;
    }
}

An example custom cert extension

struct _myCertExtData
{
    SECItem version;
    SECItem streetaddress;
    SECItem phonenum;
    SECItem rfc822name;
    SECItem id;
    SECItem maxusers;
};

typedef struct _myCertExtData myCertExtData;

/* template used for decoding the extension */
const SEC_ASN1Template myCertExtTemplate[] = {
    { SEC_ASN1_SEQUENCE, 0, NULL, sizeof( myCertExtData ) },
    { SEC_ASN1_INTEGER, offsetof(myCertExtData, version) },
    { SEC_ASN1_OCTET_STRING, offsetof( myCertExtData, streetaddress ) },
    { SEC_ASN1_OCTET_STRING, offsetof( myCertExtData, phonenum ) },
    { SEC_ASN1_OCTET_STRING, offsetof( myCertExtData, rfc822name ) },
    { SEC_ASN1_OCTET_STRING, offsetof( myCertExtData, id ) },
    { SEC_ASN1_INTEGER, offsetof(myCertExtData, maxusers ) },
    { 0 }
};

/* OID for my cert extension - replace 0xff with appropriate values*/
static const unsigned char myoid[] = { 0xff, 0xff, 0xff, 0xff, .... };
static const SECItem myoidItem = { (SECItemType) 0, (unsigned char *)myoid, sizeof(myoid) };

SECItem myextvalue;
myCertExtData data;
SECStatus rv = CERT_FindCertExtensionByOID(cert, &myoidItem, &myextvalue);
if (rv == SECSuccess)
{
    SEC_ASN1DecoderContext * context = SEC_ASN1DecoderStart(NULL, &data, myCertExtTemplate);
    rv = SEC_ASN1DecoderUpdate( context, (const char *)(myextvalue.data), myextvalue.len);
    if (rv == SECSuccess)
    {
        /* Now you can extract info from SECItem fields of your extension data structure */
        /* See "Misc helper functions" below */
        .......

        /* free the SECItem fields */
        SECITEM_FreeItem(&data.version, PR_FALSE);
        SECITEM_FreeItem(&data.streetaddress, PR_FALSE);
        ......
        SECITEM_FreeItem(&data.maxusers, PR_FALSE);
    }
}

Some miscellaneous helper functions

Compare two SECItems (e.g., two OIDs)
PRBool SECITEM_ItemsAreEqual(const SECItem *a, const SECItem *b);
Interpreting a SECItem value as an integer
If SECItem *item->len <=4, then int value = DER_GetInteger(item);
Interpreting a SECItem value as a string
Use string copy functions to copy item->len bytes from item->data and null terminate explicitly

Some higher level extension functions

Get a specific extension from the list of extensions, given the extension tag
SECStatus CERT_FindCertExtension (CERTCertificate *cert, int tag, SECItem *value);
Get a specific extension from the ISSUER's cert
SECStatus CERT_FindIssuerCertExtension (CERTCertificate *cert, int tag, SECItem *value);
Get the value of an extension with the given OID
SECStatus CERT_FindCertExtensionByOID (CERTCertificate *cert, SECItem *oid, SECItem *value);
Get the decoded value of the "Basic Constraints" extension
SECStatus CERT_FindBasicConstraintExten (CERTCertificate *cert, CERTBasicConstraints *value);
Get value of the keyUsage extension. This uses PR_Alloc to allocate buffer for the decoded value, The caller should free up the storage allocated in value->data.
SECStatus CERT_FindKeyUsageExtension (CERTCertificate *cert, SECItem *value);
Get decoded value of the subjectKeyID extension. This uses PR_Alloc to allocate buffer for the decoded value, The caller should free up the storage allocated in value->data.
SECStatus CERT_FindSubjectKeyIDExten (CERTCertificate *cert, SECItem *retItem);

For more information

Browse through the NSS source code online at https://lxr.mozilla.org/mozilla/source/security/nss/ and https://lxr.mozilla.org/security/
documentation on some cert funcs https://www.mozilla.org/projects/security/pki/nss/ref/ssl/sslcrt.html