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-rw-r--r--doc/crypto/hmac.pod18
1 files changed, 11 insertions, 7 deletions
diff --git a/doc/crypto/hmac.pod b/doc/crypto/hmac.pod
index 58a57f47bb4f..2c8f20a20188 100644
--- a/doc/crypto/hmac.pod
+++ b/doc/crypto/hmac.pod
@@ -60,13 +60,17 @@ function B<evp_md> and the key B<key> which is B<key_len> bytes
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>).