pirserver/protocol_client

The tor process will launch the pirclient process, and hold pipes to the
pirclient's stdin, stdout, and stderr.  All communication between the
processes is over those pipes.  Anything the pirclient outputs over the
stderr pipe should be logged by the tor process.  If the pirclient finds
its stdin has reached EOF, it should terminate.

The general format of request messages (tor -> pirclient) is:

request_id: 8 bytes
request_type: 1 byte
body_len: 4 bytes (big-endian unsigned int)
body: (body_len) bytes

The general format of response messages (pirclient -> tor) is:

request_id: 8 bytes (must match the request_id being responded to)
response_type: 1 byte
body_len: 4 bytes (big-endian unsigned int)
body: (body_len) bytes

There are two request types:

PIRCLIENT_REQUEST_CREATE (0x41)
Request the creation of a private query
    The body_len must be at least 32.  The first 32 bytes of the body
    are the plaintext query, and the remaining bytes are the current
    pirserver params for the server we're going to send this query to.

PIRCLIENT_REQUEST_EXTRACT (0x42)
Extract the plaintext data from a PIR response
    The body_len must be at least 8.  The first 8 bytes of the body
    are the Query ID returned in the PIRCLIENT_RESPONSE_CREATE message,
    and the remaining bytes are the PIR response.  Any given Query ID
    must be used in at most one PIRCLIENT_REQUEST_EXTRACT message (and
    it should be exactly one).

There are two response types:

PIRCLIENT_RESPONSE_CREATE (0xBF)
Reply with a PIR query
    The body begins with an 8-byte Query ID (which the tor process
    should hold on to, to match it to the corresponding PIR response in
    the PIRCLIENT_REQUEST_EXTRACT message).  The remainder of the body
    is the PIR query to send to the server.  A body_len of 0 indicates
    an error.

PIRCLIENT_RESPONSE_EXTRACT (0xBE)
Reply with the plaintext data extracted from a PIR response
    The body is the plaintext of the response received from the PIR
    server.  A body_len of 0 indicates an error.