"BEGIN RSA PUBLIC KEY" is PKCS#1, which can only contain RSA keys. "BEGIN PUBLIC KEY" is PKCS#8, which can contain a variety of formats.
- To convert from PKCS#8 to PKCS#1:
openssl rsa -pubin -in pub8.pem -RSAPublicKey_out -out pub1.pem
- To convert from PKCS#1 to PKCS#8: