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Asked: 2026-06-05T08:21:51+00:00

I want that encrypted data have the same length as my original text, because

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I want that encrypted data have the same length as my original text, because I must respect the constraint of length. I am using BouncyCastle.

Here’s my code to generate the keys pair:

KeyPairGenerator keyGen = KeyPairGenerator.getInstance("RSA");
keyGen.initialize(512);
KeyPair keypair = keyGen.genKeyPair();
PublicKey pub = keypair.getPublic();
byte[] pubs = pub.getEncoded();

And here’s my code to encrypt:

Cipher cipher = Cipher.getInstance("RSA/ECB/PKCS1PADDING");
PublicKey pk = KeyFactory.getInstance("RSA").generatePublic(new X509EncodedKeySpec(pubs));
cipher.init(Cipher.ENCRYPT_MODE, pk);
byte[] cipherBytes = cipher.doFinal(plainArray);

The encoded data is very large, what can I do to make as small as original data ?

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1 Answer

  1. Editorial Team

    Nothing, at least not regarding the RSA. RSA requires a certain amount of padding to be secure, and as the data will be seen as random data by any compression method, you cannot compress it either.

    Of course, you should not directly encrypt data directly using RSA, you should encrypt a random session/data key instead. But even then, you will have the encrypted session key as overhead.

    You can remove some bits for the asymmetric encrypted data/key by using Elliptic Curve Cryptography (the output of which is double the key size minimum, but key sizes are much smaller to achieve the same level of security). EC cryptography is not for the meek though, it has lots of complexity.

    512 bit RSA is considered insecure by the way. Use 1024 as minimum or keep to the ECRYPT II or NIST recommendations listed at http://www.keylength.com/ .

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