Document hash functions.

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diff --git a/doc/crypto/hmac.pod b/doc/crypto/hmac.pod
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+=pod
+
+=head1 NAME
+
+HMAC, HMAC_Init, HMAC_Update, HMAC_Final - HMAC message authentication code
+
+=head1 SYNOPSIS
+
+ #include <openssl/hmac.h>
+
+ unsigned char *HMAC(const EVP_MD *evp_md, const void *key,
+               int key_len, const unsigned char *d, int n,
+               unsigned char *md, unsigned int *md_len);
+
+ void HMAC_Init(HMAC_CTX *ctx, const void *key, int key_len,
+               const EVP_MD *md);
+ void HMAC_Update(HMAC_CTX *ctx, const unsigned char *data, int len);
+ void HMAC_Final(HMAC_CTX *ctx, unsigned char *md, unsigned int *len);
+
+ void HMAC_cleanup(HMAC_CTX *ctx);
+
+=head1 DESCRIPTION
+
+HMAC is a MAC (message authentication code), i.e. a keyed hash
+function used for message authentication, which is based on a hash
+function.
+
+HMAC() computes the message authentication code of the B<n> bytes at
+B<d> using the hash function B<evp_md> and the key B<key> which is
+B<key_len> bytes long.
+
+It places the result in B<md> (which must have space for the output of
+the hash function, which is no more than B<EVP_MAX_MD_SIZE> bytes).
+If B<md> is NULL, the digest is placed in a static array.  The size of
+the output is placed in B<md_len>, unless it is B<NULL>.
+
+B<evp_md> can be EVP_sha1(), EVP_ripemd160() etc.
+B<key> and B<evp_md> may be B<NULL> if a key and hash function have
+been set in a previous call to HMAC_Init() for that B<HMAC_CTX>.
+
+HMAC_cleanup() erases the key and other data from the B<HMAC_CTX>.
+
+The following functions may be used if the message is not completely
+stored in memory:
+
+HMAC_Init() initializes a B<HMAC_CTX> structure to use the hash
+function B<evp_md> and the key B<key> which is B<key_len> bytes long.
+
+HMAC_Update() can be called repeatedly with chunks of the message to
+be authenticated (B<len> bytes at B<data>).
+
+HMAC_Final() places the message authentication code in B<md>, which
+must have space for the hash function output.
+
+=head1 RETURN VALUES
+
+HMAC() returns a pointer to the message authentication code.
+
+HMAC_Init(), HMAC_Update(), HMAC_Final() and HMAC_cleanup() do not
+return values.
+
+=head1 CONFORMING TO
+
+RFC 2104
+
+=head1 SEE ALSO
+
+L<sha(3)|sha(3)>, L<evp(3)|evp(3)>
+
+=head1 HISTORY
+
+HMAC(), HMAC_Init(), HMAC_Update(), HMAC_Final() and HMAC_cleanup()
+are available since SSLeay 0.9.0.
+
+=cut