What is the certificate enrollment process?

The general procedure to issue certificates in a Public Key Infrastructure is more or less the following:

1. the client generates a key pair, private and public

2. the client generates a CSR (Certificate Signing Request) including attributes like Common Name and the Public Key. Signs it with the private key and sends it to the server

3. The server builds the X509 Certificate with the CSR data, signs it with the CA private key and returns the X509 to client

4. the client stores the private key and the certificate in a KeyStore

What CA generate?

The x509 certificate

What is P12 file

A file in PKCS#12 format (.pfx or .p12) containing a key store

what is .cer file contain

The public part of the certificate (not private key) in DER or PEM format

EDITED - CSR generation on Android

Gradle dependencies

compile 'com.madgag.spongycastle:core:1.51.0.0'
compile 'com.madgag.spongycastle:pkix:1.51.0.0'

Generate KeyPair and CSR

//Generate KeyPair
KeyPairGenerator keyGen = KeyPairGenerator.getInstance("RSA");
keyGen.initialize(KEY_SIZE, new SecureRandom());
KeyPair keyPair = keyGen.generateKeyPair();

//Generate CSR in PKCS#10 format encoded in DER
PKCS10CertificationRequest csr = CsrHelper.generateCSR(keyPair, commonname);
byte  CSRder[] = csr.getEncoded();

Utilities

public class CsrHelper {

  private final static String DEFAULT_SIGNATURE_ALGORITHM = "SHA256withRSA";
  private final static String CN_PATTERN = "CN=%s, O=Aralink, OU=OrgUnit";

  private static class JCESigner implements ContentSigner {

        private static Map<String, AlgorithmIdentifier> ALGOS = new HashMap<String, AlgorithmIdentifier>();

        static {
            ALGOS.put("SHA256withRSA".toLowerCase(), new AlgorithmIdentifier(
                    new ASN1ObjectIdentifier("1.2.840.113549.1.1.11")));
            ALGOS.put("SHA1withRSA".toLowerCase(), new AlgorithmIdentifier(
                    new ASN1ObjectIdentifier("1.2.840.113549.1.1.5")));

        }

        private String mAlgo;
        private Signature signature;
        private ByteArrayOutputStream outputStream;

        public JCESigner(PrivateKey privateKey, String sigAlgo) {
            //Utils.throwIfNull(privateKey, sigAlgo);
            mAlgo = sigAlgo.toLowerCase();
            try {
                this.outputStream = new ByteArrayOutputStream();
                this.signature = Signature.getInstance(sigAlgo);
                this.signature.initSign(privateKey);
            } catch (GeneralSecurityException gse) {
                throw new IllegalArgumentException(gse.getMessage());
            }
        }

        @Override
        public AlgorithmIdentifier getAlgorithmIdentifier() {
            AlgorithmIdentifier id = ALGOS.get(mAlgo);
            if (id == null) {
                throw new IllegalArgumentException("Does not support algo: " +
                        mAlgo);
            }
            return id;
        }

        @Override
        public OutputStream getOutputStream() {
            return outputStream;
        }

        @Override
        public byte[] getSignature() {
            try {
                signature.update(outputStream.toByteArray());
                return signature.sign();
            } catch (GeneralSecurityException gse) {
                gse.printStackTrace();
                return null;
            }
        }
    }

//Create the certificate signing request (CSR) from private and public keys
public static PKCS10CertificationRequest generateCSR(KeyPair keyPair, String cn) throws IOException,
            OperatorCreationException {
        String principal = String.format(CN_PATTERN, cn);

        ContentSigner signer = new JCESigner (keyPair.getPrivate(),DEFAULT_SIGNATURE_ALGORITHM);

        PKCS10CertificationRequestBuilder csrBuilder = new JcaPKCS10CertificationRequestBuilder(
                new X500Name(principal), keyPair.getPublic());
        ExtensionsGenerator extensionsGenerator = new ExtensionsGenerator();
        extensionsGenerator.addExtension(Extension.basicConstraints, true, new BasicConstraints(
                true));
        csrBuilder.addAttribute(PKCSObjectIdentifiers.pkcs_9_at_extensionRequest,
                extensionsGenerator.generate());
        PKCS10CertificationRequest csr = csrBuilder.build(signer);

        return csr;
    }
}