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Title "BIO_f_md 3" For nroff, turn off justification. Always turn off hyphenation; it makes
way too many mistakes in technical documents.
BIO_f_md, BIO_set_md, BIO_get_md, BIO_get_md_ctx - message digest BIO filter Header "SYNOPSIS" .Vb 2 #include <openssl/bio.h> #include <openssl/evp.h> \& BIO_METHOD * BIO_f_md(void); int BIO_set_md(BIO *b,EVP_MD *md); int BIO_get_md(BIO *b,EVP_MD **mdp); int BIO_get_md_ctx(BIO *b,EVP_MD_CTX **mdcp); .Ve Header "DESCRIPTION" \fIBIO_f_md() returns the message digest \s-1BIO\s0 method. This is a filter \s-1BIO\s0 that digests any data passed through it, it is a \s-1BIO\s0 wrapper for the digest routines EVP_DigestInit(), EVP_DigestUpdate() and EVP_DigestFinal().

Any data written or read through a digest \s-1BIO\s0 using BIO_read() and \fIBIO_write() is digested.

\fIBIO_gets(), if its size parameter is large enough finishes the digest calculation and returns the digest value. BIO_puts() is not supported.

\fIBIO_reset() reinitialises a digest \s-1BIO.\s0

\fIBIO_set_md() sets the message digest of \s-1BIO \s0b to md: this must be called to initialize a digest \s-1BIO\s0 before any data is passed through it. It is a BIO_ctrl() macro.

\fIBIO_get_md() places the a pointer to the digest BIOs digest method in mdp, it is a BIO_ctrl() macro.

\fIBIO_get_md_ctx() returns the digest BIOs context into mdcp.

Header "NOTES" The context returned by BIO_get_md_ctx() can be used in calls to EVP_DigestFinal() and also the signature routines EVP_SignFinal() and EVP_VerifyFinal().

The context returned by BIO_get_md_ctx() is an internal context structure. Changes made to this context will affect the digest \s-1BIO\s0 itself and the context pointer will become invalid when the digest \s-1BIO\s0 is freed.

After the digest has been retrieved from a digest \s-1BIO\s0 it must be reinitialized by calling BIO_reset(), or BIO_set_md() before any more data is passed through it.

If an application needs to call BIO_gets() or BIO_puts() through a chain containing digest BIOs then this can be done by prepending a buffering \s-1BIO.\s0

Header "RETURN VALUES" \fIBIO_f_md() returns the digest \s-1BIO\s0 method.

\fIBIO_set_md(), BIO_get_md() and BIO_md_ctx() return 1 for success and 0 for failure.

Header "EXAMPLES" The following example creates a \s-1BIO\s0 chain containing an \s-1SHA1\s0 and \s-1MD5\s0 digest \s-1BIO\s0 and passes the string \*(L"Hello World\*(R" through it. Error checking has been omitted for clarity.

.Vb 10 BIO *bio, *mdtmp; char message[] = "Hello World"; bio = BIO_new(BIO_s_null()); mdtmp = BIO_new(BIO_f_md()); BIO_set_md(mdtmp, EVP_sha1()); /* For BIO_push() we want to append the sink BIO and keep a note of * the start of the chain. */ bio = BIO_push(mdtmp, bio); mdtmp = BIO_new(BIO_f_md()); BIO_set_md(mdtmp, EVP_md5()); bio = BIO_push(mdtmp, bio); /* Note: mdtmp can now be discarded */ BIO_write(bio, message, strlen(message)); .Ve

The next example digests data by reading through a chain instead:

.Vb 10 BIO *bio, *mdtmp; char buf[1024]; int rdlen; bio = BIO_new_file(file, "rb"); mdtmp = BIO_new(BIO_f_md()); BIO_set_md(mdtmp, EVP_sha1()); bio = BIO_push(mdtmp, bio); mdtmp = BIO_new(BIO_f_md()); BIO_set_md(mdtmp, EVP_md5()); bio = BIO_push(mdtmp, bio); do { rdlen = BIO_read(bio, buf, sizeof(buf)); /* Might want to do something with the data here */ } while(rdlen > 0); .Ve

This next example retrieves the message digests from a \s-1BIO\s0 chain and outputs them. This could be used with the examples above.

.Vb 10 BIO *mdtmp; unsigned char mdbuf[EVP_MAX_MD_SIZE]; int mdlen; int i; mdtmp = bio; /* Assume bio has previously been set up */ do { EVP_MD *md; mdtmp = BIO_find_type(mdtmp, BIO_TYPE_MD); if(!mdtmp) break; BIO_get_md(mdtmp, &md); printf("%s digest", OBJ_nid2sn(EVP_MD_type(md))); mdlen = BIO_gets(mdtmp, mdbuf, EVP_MAX_MD_SIZE); for(i = 0; i < mdlen; i++) printf(":%02X", mdbuf[i]); printf("\en"); mdtmp = BIO_next(mdtmp); } while(mdtmp); \& BIO_free_all(bio); .Ve

Header "BUGS" The lack of support for BIO_puts() and the non standard behaviour of \fIBIO_gets() could be regarded as anomalous. It could be argued that BIO_gets() and BIO_puts() should be passed to the next \s-1BIO\s0 in the chain and digest the data passed through and that digests should be retrieved using a separate BIO_ctrl() call. Header "SEE ALSO" \s-1TBA\s0