Calulates the keyidentifier using a SHA1 hash over the BIT STRING
from SubjectPublicKeyInfo as defined in RFC2459.
Digest digest = new SHA1Digest();
byte[] resBuf = new byte[digest.getDigestSize()];
byte[] bytes = spki.getPublicKeyData().getBytes();
digest.update(bytes, 0, bytes.length);
digest.doFinal(resBuf, 0);
this.keyidentifier=resBuf;