# How Much Data Can You Encrypt with RSA Keys?

When someone first begins to consider using encryption to protect business data, they discover that there are two general types: symmetric (AES) and asymmetric (RSA). At first glance, which one you would choose can be confusing.One of the differences between the two is speed. Symmetric encryption is much faster than asymmetric. The exact difference is implementation dependent, but may be on the order of 100 to 1000 times faster.

It is widely known that AES encrypts a 16-byte block of data at a time. However, how much data can be encrypted at one time with an RSA key is usually only discussed in vague terms such as “use RSA to encrypt session keys.” This raises the question of how much data can be encrypted by an RSA key in a single operation.

The typical encryption scenario is to encrypt with a public key and decrypt with the private key. OpenSSL provides the RSA_public_encrypt and RSA_private_decrypt functions to implement this.

The first parameter to the RSA_public_encrypt function is flen. This is an integer that indicates the number of bytes to encrypt. Its maximum value depends on the padding mode. For OAEP padding, recommended for all new applications, it must be less than the size of the key modulus – 41 (all in bytes).

To get the size of the modulus of an RSA key call the function RSA_size.

The modulus size is the key size in bits / 8. Thus a 1024-bit RSA key using OAEP padding can encrypt up to (1024/8) – 42 = 128 – 42 = 86 bytes.

A 2048-bit key can encrypt up to (2048/8) – 42 = 256 – 42 = 214 bytes.

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