Generating a PagarMe CardHash manually using Java / Android

pagarme-cardhash-java-android.java

import android.util.Base64;
import java.io.IOException;
import java.net.URLEncoder;
import java.security.InvalidKeyException;
import java.security.KeyFactory;
import java.security.NoSuchAlgorithmException;
import java.security.PublicKey;
import java.security.spec.InvalidKeySpecException;
import java.security.spec.X509EncodedKeySpec;
import javax.crypto.BadPaddingException;
import javax.crypto.Cipher;
import javax.crypto.IllegalBlockSizeException;
import javax.crypto.NoSuchPaddingException;

//...

Cipher cipher;
KeyFactory keyFactory;

try {
    cipher = Cipher.getInstance("RSA/None/PKCS1PADDING");
    keyFactory = KeyFactory.getInstance("RSA");
} catch (NoSuchAlgorithmException | NoSuchPaddingException cardHashGenerationImpossible) {

    //DEVICE (IF ANDROID) OR JAVA ENVIRONMENT UNABLE TO GENERATE CARD HASHES
    //AS IT WON'T BE ABLE TO PROPERLY ENCRYPT USER CARD INFORMATION.

    //AS PER https://docs.pagar.me/api/?shell#gerando-card_hash-manualmente,
    //PAGARME REQUIRES PUBLIC CRYPTOGRAPHY USING RSA W/ PKCS1Padding.

}

//...

//USING RETROFIT TO GET https://api.pagar.me/1/transactions/card_hash_key.

Call<JsonObject> fetchPagarMeCardHashPublicKey = AppApi.getInstance()
    .getPagarMeCardHashPublicKey(Constants.PAGAR_ME_ENCRYPTION_KEY);
fetchPagarMeCardHashPublicKey.enqueue(new Callback<JsonObject>() {

    @Override
    public void onResponse(Call<JsonObject> call, Response<JsonObject> response) {
        if (response.isSuccessful() && response.body() != null) {
            try {
                JsonObject responseBody = response.body();
                int cardHashId = responseBody.get("id").getAsInt();

                String rawPublicKey = responseBody.get("public_key").getAsString();

                //PAGARME PUBLIC KEYS COME IN PEM FORMAT, SO WE NEED TO CONVERT'EM
                //TO DER FORMAT SO THAT THEY CAN PLAY NICE WITH X509EncodedKeySpec.

                rawPublicKey = rawPublicKey
                    .replaceAll("\\s*-----BEGIN PUBLIC KEY-----\\s*", "")
                    .replaceAll("\\s*-----END PUBLIC KEY-----\\s*", "")
                    .replaceAll("\\s", "");
                byte[] decodedPublicKey = Base64.decode(rawPublicKey.getBytes("UTF-8"), 0);

                //SUCCESSFULLY CONVERTED THE PUBLIC KEY TO DER FORMAT.

                X509EncodedKeySpec publicKeySpec = new X509EncodedKeySpec(decodedPublicKey);
                PublicKey publicKey = keyFactory.generatePublic(publicKeySpec);
                cipher.init(Cipher.ENCRYPT_MODE, publicKey);

                //NOW FORMATTING CARD INFORMATION IN THE FORMAT EXPECTED BY PAGARME.

                String cardInformation = String.format(Locale.US,
                    "card_number=%s&card_holder_name=%s&card_expiration_date=%s"
                    + "&card_cvv=%s", creditCardNumber.replace(" ", "").trim(),
                    URLEncoder.encode(profileCreditCardName.getText().trim(), "UTF-8").replace("+", "%20"),
                    creditCardGoodThru.replace("/", "").trim(), profileCreditCardCvc.getText().trim());

                //THEN WE USE OUR CIPHER TO ENCRYPT AND THEN BASE64-ENCODE THE CARD INFORMATION,
                //AND FINALLY, APPEND THE ASSIGNED CARD ID AS PREFIX OF THE RESULT TO COMPOSE THE CARD HASH.

                byte[] encryptedCardInformation = cipher.doFinal(cardInformation.getBytes("UTF-8"));
                String cardHash = String.format(Locale.US, "%d_%s", cardHashId,
                    (Base64.encodeToString(encryptedCardInformation, Base64.DEFAULT)))
                        .replaceAll("\\s", "");
                
                //NOW YOU MIGHT WANT TO SEND THE CARD HASH TO YOUR BACKEND SERVER.
                        
            } catch (InvalidKeySpecException | InvalidKeyException invalidKey) {
                //PROBLEMS WITH THE PUBLIC KEY RECEIVED FROM PAGARME.
            } catch (BadPaddingException | IllegalBlockSizeException | IOException  encryptionException) {
                //I/O ERRORS HAPPENED WHILE TRYING TO ENCRYPT CARD INFORMATION.
            }
        } else {
            //PAGARME PROBABLY DIDN'T LIKE THE PAGAR_ME_ENCRYPTION_KEY YOU USED IN THE GET REQUEST...
        }
    }

    @Override
    public void onFailure(Call<JsonObject> call, Throwable throwable) {
        //COULD NOT RECEIVE PUBLIC KEY FROM PAGARME.
    }

});