Syntax:
ASYMMETRIC_ENCRYPT(algorithm, str, key_str)

Encrypts a string using the given algorithm and key string, and returns
the resulting ciphertext as a binary string. If encryption fails, the
result is NULL.

The str length cannot be greater than the key_str length - 11, in bytes

key_str must be a valid key string in PEM format. algorithm indicates
the encryption algorithm used to create the key.

Supported algorithm values: 'RSA'

To encrypt a string, pass a private or public key string to
ASYMMETRIC_ENCRYPT(). To recover the original unencrypted string, pass
the encrypted string to ASYMMETRIC_DECRYPT(), along with the public or
private key string correponding to the private or public key string
used for encryption.

URL: http://dev.mysql.com/doc/refman/5.6/en/enterprise-encryption-functions.html

