Search code examples
javaencryptioncryptographyaesrsa

RSA maximum message length with specific padding

I'm trying to implement some encryption in my Java application. The encryption should work as follows:

  1. Generate random n bits AES key
  2. Encrypt AES key using RSA public key
  3. Send encrypted AES key to server
  4. Encrypt data with AES key + send to server

I'm having trouble deciding what padding algorithm to use. I read OAEPWithSHA-1AndMGF1Padding is a good choice, but what is the maximum message length I can encrypt with this?

I need to know the exact maximum bytes my AES key can be to be encrypted using RSA with OAEPWithSHA-1AndMGF1Padding.

The length of the RSA key makes difference too. I will probably go for 2048 bits.

TL;DR: How many bytes can I encrypt using a 2048 bits RSA key with OAEPWithSHA-1AndMGF1Padding?

Solution

  • What you describe is known as Hybrid Encryption and is a common way to get the speed of symmetric-key crypto systems combined with the benefits of the two keys of RSA.

    As for the specifics:

    A 2048 bit RSA key allows for 256 bytes of which the OAEP padding takes 42 bytes, leaving around 214 bytes for encrypted data. An AES-256 key is 256 bits (32 bytes) long, so there is plenty of space for it.