Clarify digest change in HMAC_Init_ex()

long. It is deprecated and only included for backward compatibility

with OpenSSL 0.9.6b.

HMAC_Init_ex() initializes or reuses a B<HMAC_CTX> structure to use

the function B<evp_md> and key B<key>. Either can be NULL, in which

case the existing one will be reused. HMAC_CTX_init() must have been

called before the first use of an B<HMAC_CTX> in this

function. B<N.B. HMAC_Init() had this undocumented behaviour in

previous versions of OpenSSL - failure to switch to HMAC_Init_ex() in

programs that expect it will cause them to stop working>.

HMAC_Init_ex() initializes or reuses a B<HMAC_CTX> structure to use the hash

function B<evp_md> and key B<key>. If both are NULL (or B<evp_md> is the same

as the previous digest used by B<ctx> and B<key> is NULL) the existing key is

reused. B<ctx> must have been created with HMAC_CTX_new() before the first use

of an B<HMAC_CTX> in this function. B<N.B. HMAC_Init() had this undocumented

behaviour in previous versions of OpenSSL - failure to switch to HMAC_Init_ex()

in programs that expect it will cause them to stop working>.

B<NB: if HMAC_Init_ex() is called with B<key> NULL and B<evp_md> is not the

same as the previous digest used by B<ctx> then an error is returned

because reuse of an existing key with a different digest is not supported.>

HMAC_Update() can be called repeatedly with chunks of the message to

be authenticated (B<len> bytes at B<data>).