Gets the signature algorithm OID string from the CRL.
An OID is represented by a set of positive whole number separated
by ".", that means,<br>
<positive whole number>.<positive whole number>.<...>
For example, the string "1.2.840.10040.4.3" identifies the SHA-1
with DSA signature algorithm defined in
<a href="http://www.ietf.org/rfc/rfc3279.txt">RFC 3279: Algorithms and
Identifiers for the Internet X.509 Public Key Infrastructure Certificate
and CRL Profile</a>.
Gets the signature algorithm OID string from the CRL. An OID is represented by a set of positive whole number separated by ".", that means,<br> <positive whole number>.<positive whole number>.<...> For example, the string "1.2.840.10040.4.3" identifies the SHA-1 with DSA signature algorithm defined in <a href="http://www.ietf.org/rfc/rfc3279.txt">RFC 3279: Algorithms and Identifiers for the Internet X.509 Public Key Infrastructure Certificate and CRL Profile</a>.
@return the signature algorithm oid string.