c_rehash scans directories and calculates a hash value of each .pem
file in the specified directory list and creates symbolic links
for each file, where the name of the link is the hash value.
This utility is useful as many programs that use OpenSSL require
directories to be set up like this in order to find certificates.
If any directories are named on the command line, then those are
processed in turn. If not, then the SSL_CERT_DIR environment variable
is consulted; this shold be a colon-separated list of directories,
like the Unix PATH variable.
If that is not set then the default directory (installation-specific
but often /usr/local/ssl/certs) is processed.
In order for a directory to be processed, the user must have write
permissions on that directory, otherwise it will be skipped.
The links created are of the form HHHHHHHH.D, where each H
is a hexadecimal character and D is a single decimal digit.
When processing a directory, c_rehash will first remove all links
that have a name in that syntax. If you have links in that format
used for other purposes, they will be removed.
Hashes for CRLs look similar except the letter r appears after
the period, like this: HHHHHHHH.rD.
Multiple objects may have the same hash; they will be indicated by
incrementing the D value. Duplicates are found by comparing the
full SHA-1 fingerprint. A warning will be displayed if a duplicate
A warning will also be displayed if there are .pem files that
cannot be parsed as either a certificate or a CRL.
The program uses the openssl program to compute the hashes and
fingerprints. If not found in the users PATH, then set the
OPENSSL environment variable to the full pathname.
Any program can be used, it will be invoked as follows for either
a certificate or CRL:
$OPENSSL x509 -hash -fingerprint -noout -in FFFFFF
$OPENSSL crl -hash -fingerprint -noout -in FFFFFF
where FFFFFF is the filename. It must output the hash of the
file on the first line, and the fingerprint on the second,
optionally prefixed with some text and an equals sign.