PKCS7_DATAINIT(3) Library Functions Manual PKCS7_DATAINIT(3)

PKCS7_dataInit — construct a BIO chain for adding or retrieving content

library “libcrypto”
#include <openssl/pkcs7.h>

BIO *
PKCS7_dataInit(PKCS7 *p7, BIO *indata);

() constructs a BIO chain in preparation for putting data into or retrieving data out of p7. Depending on the contentType of p7, the created chain starts with:

for SignedData:
one or more BIO_f_md(3) message digest filters
for EnvelopedData:
one BIO_f_cipher(3) encryption filter
for SignedAndEnvelopedData:
one or more BIO_f_md(3) message digest filters followed by one BIO_f_cipher(3) encryption filter
for DigestedData:
one BIO_f_md(3) message digest filter
for arbitrary data:
no filter BIO

One additional BIO is appended to the end of the chain, depending on the first condition that holds in the following list:

indata
if the indata argument is not NULL. This only makes sense while verifying a detached signature, in which case indata is expected to supply the content associated with the detached signature.
BIO_s_null(3)
if the contentType of p7 is SignedData and it is configured to contain a detached signature. This only makes sense while creating the detached signature.
BIO_new_mem_buf(3)
when reading from a SignedData or DigestedData object. () attaches the end of the chain to the nested content of p7.
BIO_s_mem(3)
otherwise. This is the most common case while writing data to p7. PKCS7_dataFinal(3) can later be used to transfer the data from the memory BIO into p7.

Before calling PKCS7_dataInit() in order to add content, PKCS7_new(3), PKCS7_set_type(3), and PKCS7_content_new(3) are typically required to create p7, to choose its desired type, and to allocate the nested ContentInfo structure. Alternatively, for SignedData, PKCS7_sign(3) can be used with the PKCS7_PARTIAL or PKCS7_STREAM flags or for EnvelopedData, PKCS7_encrypt(3) with the PKCS7_STREAM flag.

After calling (), the desired data can be written into the returned BIO, BIO_flush(3) can be called on it, PKCS7_dataFinal(3) can be used to transfer the processed data from the returned memory BIO to the p7 structure, and the chain can finally be destroyed with BIO_free_all(3).

While () does support the EnvelopedData and SignedAndEnvelopedData types, using it for these types is awkward and error prone except when using PKCS7_encrypt(3) for the setup because PKCS7_content_new(3) does not support these two types. So in addition to creating p7 itself and setting its type, the nested ContentInfo structure also needs to be constructed with PKCS7_new(3) and PKCS7_set_type(3) and manually inserted into the correct field of the respective sub-structure of p7.

PKCS7_dataInit() can also be called on a fully populated object of type SignedData or DigestedData. After that, BIO_read(3) can be used to retrieve data from it. In this use case, do not call PKCS7_dataFinal(3); simply proceed directly to BIO_free_all(3) after reading the data.

PKCS7_dataInit() returns a BIO chain on success or NULL on failure. It fails if p7 is NULL, if the content field of p7 is empty, if the contentType of p7 is unsupported, if a cipher is required but none is configured, or if any required operation fails, for example due to lack of memory or for various other reasons.

BIO_new(3), BIO_read(3), PKCS7_content_new(3), PKCS7_dataFinal(3), PKCS7_encrypt(3), PKCS7_final(3), PKCS7_new(3), PKCS7_set_type(3), PKCS7_sign(3)

PKCS7_dataInit() first appeared in SSLeay 0.8.1 and has been available since OpenBSD 2.4.

This function does not support EncryptedData.

If p7 is a fully populated structure containing EnvelopedData, SignedAndEnvelopedData, or arbitrary data, PKCS7_dataInit() returns a BIO chain that ultimately reads from an empty memory BIO, so reading from it will instantly return an end-of-file indication rather than reading the actual data contained in p7.

June 8, 2025 Debian