Creating a Network of Trust Using X.509 Certifi cates - Implementing SSL/TLS Using Cryptography and PKI - page 235

A recent Internet-wide security analysis by Qualys Research found “22
million SSL servers with certificates that are completely invalid because
they do not match the domain name on which they reside” (see http://www
.esecurityplanet.com/features/article.php/3890171/SSL-Certificates-
In-Use-Today-Arent-All-Valid.htm ), although some of this is likely caused
by virtual hosting rather than truly invalid SSL certifi cates.

subjectPublicKeyInfo

subjectPublicKeyInfo SubjectPublicKeyInfo
Here is the heart of the certifi cate — the public key that it presents. On the client
side, when the certifi cate is received, you use the issuer, validity period, and
the subject fi eld to decide whether you trust the public key well enough to use
it to perform a key exchange. If the subject matches the host you think you're
connecting to, the certifi cate hasn't expired, and the issuer is one you trust, you
have reasonable assurance that there's no man in the middle and you can go
forward with the key exchange and, presumably, trade sensitive information
over the now-secured channel.
The defi nition for SubjectPublicKeyInfo is
SubjectPublicKeyInfo ::= SEQUENCE {
algorithm AlgorithmIdentifier,
subjectPublicKey BIT STRING }
The AlgorithmIdentifier , it should come as no surprise, includes an OID.
Two possible values of interest are shown in Table 5-4.

Table 5-4: Public-Key Algorithm OIDs

ALGORITHM IDENTIFIER OID

RSA 2A 86 48 86 F7 0D 01 01 01

Diffi e-Hellman 2A 86 48 CE 3E 02 01

NOTE Elliptic-curve Diffi e-Hellman support in X.509 certifi cates is examined
in Chapter 9.

The public key itself is defi ned here as a simple bit string. Recall from Chapter 4,
though, that you need some pretty specifi c information in a pretty specifi c for-
mat to do key exchanges, For RSA, for example, you need the modulus n and
the public exponent e . So, as it turns out, the BIT STRING here actually encodes
another ASN.1 formatted value, whose contents vary depending on the value
of the algorithm identifi er. For RSA, this is
RSAPublicKey ::= SEQUENCE {
modulus INTEGER, -- n
publicExponent INTEGER -- e -- }