DESCRIPTIONA BIO is an I/O abstraction, it hides many of the underlying
I/O details from an application. If an application uses a BIO for
its I/O it can transparently handle SSL connections, unencrypted
network connections and file I/O.
There are two type of BIO, a source/sink BIO and a filter
BIO.
As its name implies a source/sink BIO is a source and/or sink
of data, examples include a socket BIO and a file BIO.
A filter BIO takes data from one BIO and passes it through
to another, or the application. The data may be left unmodified
(for example a message digest BIO) or translated (for example an
encryption BIO). The effect of a filter BIO may change according
to the I/O operation it is performing: for example an encryption
BIO will encrypt data if it is being written to and decrypt data
if it is being read from.
BIOs can be joined together to form a chain (a single BIO
is a chain with one component). A chain normally consist of one
source/sink BIO and one or more filter BIOs. Data read from or written
to the first BIO then traverses the chain to the end (normally a
source/sink BIO).
HP Open Source Security for OpenVMS Volume 2:... 
CRYPTO Application Programming Interface (API)... 
