while converting some certificates from keystore to openssl/pem I noticed for the first time that there are "Bag Attributes" prepended to the certs.
The look like this:
Bag Attributes
friendlyName: CN=PositiveSSL CA,O=Comodo CA Limited,L=Salford,ST=Greater Manchester,C=GB
subject=/C=GB/ST=Greater Manchester/L=Salford/O=Comodo CA Limited/CN=PositiveSSL CA
issuer=/C=US/ST=UT/L=Salt Lake City/O=The USERTRUST Network/OU=http://www.usertrust.com/CN=UTN-USERFirst-Hardware
Do they serve any function?
I noticed that I like them because they make my chain-files (a concatenation of certificates) more clear. Sadly the ca certs I download don't have them.
So how do I generate them?
To be exact, you apparently mean converting (or just reading) with the openssl pkcs12 (import) utility a PKCS#12 file, which can be supported by Java as a keystore but was not the default (update) until Java9 in 2017. PKCS#12 was designed and normally is used for a privatekey and the cert(s) (usually multiple) for that key, although the format is flexible enough to allow lone cert(s). OpenSSL commandline pkcs12 -export requires a privatekey, although it will add "extra" certs, and a program calling the API can apparently do no privatekey. In my experience, Java didn't support lone cert(s) in PKCS#12 before version 8, and in my 8 and 9 has two attributes: pkcs9.friendlyName and 2.16.840.1.113894.746875.1.1 which is apparently an Oracle-defined trustedKeyUsage. Most lone certs are not stored, or downloaded, as PKCS#12.
PKCS#12 is defined in terms of several (slightly different) "bag" structures that contain various things, primarily privatekeys and certs with optional attributes attached that are unsurprisingly called "bag attributes"; your case (apparently) has only cert(s). These attributes follow the now-conventional structure of an arbitrary number of pairs of OID plus value depending on the OID. Note in your display only friendlyName is a bag attribute, indicated because it is indented under the heading.
The subject= and issuer= lines are fields from the cert itself which the openssl pkcs12 (import) utility extracts and prints for convenience. If that is sufficient, you can display them for any cert with the x509 utility; in particular if you want to have them before the PEM-encoded cert "blob" in the way pkcs12 output does, use openssl x509 -in infile -subject -issuer -out outfile. This does one cert, so if you have a chain in a PEM file you need to split it apart and do each cert separately, and possibly combine again afterwards; for example something like
# split into files cert_1, cert_2, etc.
$ awk <chain.pem -va="openssl x509 -subject -issuer >cert_"
'/^-----BEGIN/{b=a (++n);x=1}x{print|b}/^-----END/{close(b);x=0}'
# output entire "bag" to stdout (with blank lines between certs)
$ awk <chain.pem -va="openssl x509 -subject -issuer" \
'/^-----BEGIN/{b=a;x=1}x{print|b}/^-----END/{close(b);x=0;print""}'
As a comparison, openssl s_client -showcerts does something very similar: it outputs subject and issuer with each cert blob from the received chain, labelling them with a level number, "s:" and "i:".
