A BIO is an I/O stream abstraction; essentially OpenSSL's answer to the C library's
FILE *. OpenSSL comes with a number of useful BIO types predefined, or you can create your own.
BIOs come in two flavors: source/sink, or filter. BIOs can be chained together. Each chain always has exactly one source/sink, but can have any number (zero or more) of filters.
Reading from a BIO can be done with BIO_read and
Writing to a BIO can be done with