Question

I am using RSA algorithm for shared key encryption/decryption.I am using open ssl libraries and c language.RSA_size() returns 256bytes.the key(data to enrypt) size is more than 256bytes but less than 500 bytes.The RSA keysize used for encryption is 1024.keypair is generated using openssl library.
If data to encrypt is less than or equals to 245 bytes the encryption works fine(because of padding).Otherwise i have to break the data(not sure it is ok as iam trying to exchange shared key)Is there any other way to increase RSA_size.my code doesn’t call RAND_seed()(not sure what argment to pass) before calling rsa_public_encrypt. Any help is appreciated.

Answer 1

You can either break your data into chunks, or use the “seal”/”open” OpenSSL functions.

Essentially what they do is generate a random set of keys, encrypt the keys using the public key and then use those keys to encrypt whatever you want. That way you’re not limited to specific sizes and decryption is not too slow (you only decrypt the random keys using RSA, the rest is done using symmetric encryption so it’s a lot faster).

I suggest you have a look at EVP_SealInit and EVP_OpenInit.