ima: Document and replace keys and adapt scripts for EC keys

For shorted file signatures use EC keys rather than RSA keys.
Document the debug keys and their purpose.
Adapt the scripts for creating these types of keys to now
create EC keys.

Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
Signed-off-by: Armin Kuster <akuster808@gmail.com>

meta-integrity/scripts/ima-gen-local-ca.sh

@@ -18,7 +18,6 @@ GENKEY=ima-local-ca.genkey
 
 cat << __EOF__ >$GENKEY
 [ req ]
-default_bits = 2048
 distinguished_name = req_distinguished_name
 prompt = no
 string_mask = utf8only
@@ -33,10 +32,11 @@ emailAddress = john.doe@example.com
 basicConstraints=CA:TRUE
 subjectKeyIdentifier=hash
 authorityKeyIdentifier=keyid:always,issuer
-# keyUsage = cRLSign, keyCertSign
+keyUsage = cRLSign, keyCertSign
 __EOF__
 
-openssl req -new -x509 -utf8 -sha1 -days 3650 -batch -config $GENKEY \
+openssl req -new -x509 -utf8 -sha256 -days 36500 -batch -config $GENKEY \
+        -newkey ec -pkeyopt ec_paramgen_curve:prime256v1 \
         -outform DER -out ima-local-ca.x509 -keyout ima-local-ca.priv
 
 openssl x509 -inform DER -in ima-local-ca.x509 -out ima-local-ca.pem