CMS streaming filter BIO
returns a streaming filter BIO chain based on
cms. The output of the filter is written to
out. Any data written to the chain is automatically
translated to a BER format CMS structure of the appropriate type.
The chain returned by this function behaves like a standard filter BIO. It supports non blocking I/O. Content is processed and streamed on the fly and not all held in memory at once: so it is possible to encode very large structures. After all content has been written through the chain, BIO_flush(3) must be called to finalise the structure.
CMS_STREAM flag must be included in
the corresponding flags parameter of the
cms creation function.
If an application wishes to write additional data to out, BIOs should be removed from the chain using BIO_pop(3) and freed with BIO_free(3) until out is reached. If no additional data needs to be written, BIO_free_all(3) can be called to free up the whole chain.
Any content written through the filter is used verbatim: no canonical translation is performed.
It is possible to chain multiple BIOs to, for example, create a triple wrapped signed, enveloped, signed structure. In this case it is the application's responsibility to set the inner content type of any outer CMS_ContentInfo structures.
Large numbers of small writes through the chain should be avoided as this will produce an output consisting of lots of OCTET STRING structures. Prepending a BIO_f_buffer(3) buffering BIO will prevent this.
BIO_new_CMS() returns a
BIO chain when successful or
NULL if an error occurred. The error can be obtained
BIO_new(3), CMS_ContentInfo_new(3), CMS_encrypt(3), CMS_sign(3)
BIO_new_CMS() first appeared in OpenSSL
1.0.0 and has been available since OpenBSD 6.7.
There is currently no corresponding inverse BIO which can decode a CMS structure on the fly.