http://xa63tpfoaqt3zru2ehxjjfbpadwj4ha6qsdvtcqtyr3b7hmt4iaq.b32.i2p/spec.html
The header begins with a type line, followed by
name=value pairs, delimited by the newline character ('\n' or 0x0A). After
the pairs are complete, a blank newline is included, followed by the line
"Size=$numBytes\n", where $numBytes is the size of the body (base10). After that comes
that many bytes making up the body of the enclosed message, followed by two
newline delimited signature lines - AuthorizationSig=$signature and
AuthenticationSig=$signature.