A.P. Lawrence

2005/04/28 PEM files (openssl)

Ascii versions of certificate files are usually stored in .pem files. You might find them under your httpd directory:

# ls -l /etc/httpd/conf/*pem
-r--------  1 root root 3289 Nov 18 07:26 /etc/httpd/conf/httpd.pem

If you look inside such a file, you'll see something like this:

# head /etc/httpd/conf/*pem
-----BEGIN RSA PRIVATE KEY-----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Funny thing is, "file" doesn't know anything special about this:

# file /etc/httpd/conf/*pem
/etc/httpd/conf/httpd.pem: ASCII text

And in fact, they shouldn't be called pem files at all. From https://digitalid.soltrus.com/globalServer/INSTALL/techDocs.html

The ApacheSSL documentation, and the docs for the SSLeay toolkit,
refer to certificates and certificate requests as "PEM" files. They
are not. ApacheSSL, like all SSL secure servers, uses the (standard)
X.509 certificate format. X.509 certificates are binary files,
which are difficult to send around by mail. So SSLeay stores them
in BASE64 encoded format, between '-----BEGIN-----' and '-----END-----'
lines. BASE64 encoding was defined as part of the (old) Privacy
Enhanced Mail (PEM) specification, which is why the documentation
calls them "PEM format" files.

The original PEM description can be found at http://www.networksorcery.com/enp/data/pem.htm

Actually, the X.509 format is apparently "Abstract Syntax Notation" or ASN.1. (http://www.cs.indiana.edu/~chiuk/security/glossary/):

    ASN.1 is OSI's notation for for specifying abstract types and values.

    ASN.1 does not specify how these objects are encoded into bits.
    This is specified by a set of encoding rules. Two are in common
    use: the Basic Encoding Rules (BER), and the Distinguished
    Encoding Rules (DER). BER specifies more than one way to encode
    some values, while using DER results in a unique encoding for
    each ASN.1 value.

    The X.509 certificate specification is specified with ASN.1.

    Kerberos 5 uses ASN.1 and DER to encode its protocol messages. 

That makes it all perfectly clear, doesn't it?

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