Getting a Illegalblocksizeexception: Data Must Not Be Longer Than 256 Bytes When Using Rsa

getting a IllegalBlockSizeException: Data must not be longer than 256 bytes when using rsa

The RSA algorithm can only encrypt data that has a maximum byte length
of the RSA key length in bits divided with eight minus eleven padding
bytes, i.e. number of maximum bytes = key length in bits / 8 - 11.

So basicly you divide the key length with 8 -11(if you have padding). For example if you have a 2048bit key you can encrypt 2048/8 = 256 bytes (- 11 bytes if you have padding). So, either use a larger key or you encrypt the data with a symmetric key, and encrypt that key with rsa (which is the recommended approach).

That will require you to:

1. generate a symmetric key
2. Encrypt the data with the symmetric key
3. Encrypt the symmetric key with rsa
4. send the encrypted key and the data
5. Decrypt the encrypted symmetric key with rsa
6. decrypt the data with the symmetric key
7. done :)

Java RSA decryption javax.crypto.IllegalBlockSizeException: Data must not be longer than 256 bytes

As Topaco mentioned (all credits to him) when decrypting, the ciphertext must be Base64 decoded and not UTF8 encoded:

In my case, it was UTF8 encoded.

Base64.getDecoder().decode("OixtTJRXe2nDRWDBqSs9m4wN[...]17/MKpw==") worked.

RSA decryption error - IllegalBlockSizeException: Data must not be longer than 128 bytes

Your signature string contains 256 characters, however this is hexadecimal and really represents 128 bytes.

Before you verify the signature, you must convert it back to a byte array. This is not achieved through someString.getBytes() but rather via DatatypeConverter.parseHexBinary(someString) (or any other method you prefer from Google).

Also, I would strongly recommend you use the Signature class rather than the Cipher class when signing messages. Currently your code can only handle messages that are smaller than 128 bytes in length (smaller, in fact, due to padding). Instead, you should be hashing the message prior to signing (e.g. using the SHA256withRSA mechanism).

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