Use padding with intro requests

   PK_ID is the hash of Bob's public key.  RP is NUL-terminated.

   The data is encrypted to Bob's PK as follows: Suppose Bob's PK is L octets

   long.  If the data to be encrypted is shorter than L, then it is encrypted

   directly (with no padding).  If the data is at least as long as L, then a

   randomly generated 16-byte symmetric key is prepended to the data, after

   which the first L-16 bytes of the data are encrypted with Bob's PK; and the

   rest of the data is encrypted with the symmetric key.

   [There's very little here is that is recognizable by Bob to let him

    know if he decrypted it correctly. Perhaps we should add OAEP padding

    to the RSA -- there is space -RD]

   long.  If the data to be encrypted is shorter than L-42, then it is

   encrypted directly (with OAEP padding).  If the data is at least as long

   as L-42, then a randomly generated 16-byte symmetric key is prepended to

   the data, after which the first L-16-42 bytes of the data are encrypted with

   Bob's PK; and the rest of the data is encrypted with the symmetric key.

1.9. Introduction: From the Introduction Point to Bob's OP