RSA encryption length calculation formula

The length of data that can be encrypted using RSA is determined primarily by the size of the key you're using. You appear to be using OAEP, so the maximum length is:

keyLength - 2 - 2 * hashLength

WherekeyLengthIs the length of the RSA modulus in bytes. You're using a 1024 bit key so:

keyLength = 1024 / 8 = 128

And since you're using OAEP with SHA-1

hashLength = 20

So the maximum you can encrypt is:

128 - 2 - 2 * 20 = 86

Which is exactly what your example shows.

To Encrypt more data, youCocouldUse a larger RSA key, but RSA really shouldn't be used for encrypting large amounts of data, so is usually used as part of a hybrid cryptosystem where RSA is instead used to encrypt a randomly generated key for a random Ric algorithm like AES, and then the specified Ric algorithm is used to encrypt the actual data so avoiding the length limits associated with RSA.

RSA encryption length calculation formula