EVP_PKEY_derive() docs.

diff --git a/doc/crypto/EVP_PKEY_derive.pod b/doc/crypto/EVP_PKEY_derive.pod
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+=pod
+
+=head1 NAME
+
+EVP_PKEY_derive_init, EVP_PKEY_derive_set_peer, EVP_PKEY_derive - derive public key algorithm shared secret.
+
+=head1 SYNOPSIS
+
+ #include <openssl/evp.h>
+
+ int EVP_PKEY_derive_init(EVP_PKEY_CTX *ctx);
+ int EVP_PKEY_derive_set_peer(EVP_PKEY_CTX *ctx, EVP_PKEY *peer);
+ int EVP_PKEY_derive(EVP_PKEY_CTX *ctx, unsigned char *key, size_t *keylen);
+
+=head1 DESCRIPTION
+
+The EVP_PKEY_derive_init() function initializes a public key algorithm
+context using key B<pkey> for shared secret derivation.
+
+The EVP_PKEY_derive_set_peer() function sets the peer key: this will normally
+be a public key.
+
+The EVP_PKEY_derive() derives a shared secret using B<ctx>.
+If B<key> is B<NULL> then the maximum size of the output buffer is written to
+the B<keylen> parameter. If B<key> is not B<NULL> then before the call the
+B<keylen> parameter should contain the length of the B<key> buffer, if the call
+is successful the shared secret is written to B<key> and the amount of data
+written to B<keylen>.
+
+=head1 NOTES
+
+After the call to EVP_PKEY_derive_init() algorithm specific control
+operations can be performed to set any appropriate parameters for the
+operation.
+
+The function EVP_PKEY_derive() can be called more than once on the same
+context if several operations are performed using the same parameters.
+
+=head1 RETURN VALUES
+
+EVP_PKEY_derive_init() and EVP_PKEY_derive() return 1 for success and 0
+or a negative value for failure. In particular a return value of -2
+indicates the operation is not supported by the public key algorithm.
+
+=head1 EXAMPLE
+
+Derive shared secret (for example DH or EC keys):
+
+ #include <openssl/evp.h>
+ #include <openssl/rsa.h>
+
+ EVP_PKEY_CTX *ctx;
+ unsigned char *skey;
+ size_t skeylen;
+ EVP_PKEY *pkey, *peerkey;
+ /* NB: assumes pkey, peerkey have been already set up */
+
+ ctx = EVP_PKEY_CTX_new(pkey);
+ if (!ctx)
+       /* Error occurred */
+ if (EVP_PKEY_derive_init(ctx) <= 0)
+       /* Error */
+ if (EVP_PKEY_derive_set_peer(ctx, peerkey) <= 0)
+       /* Error */
+
+ /* Determine buffer length */
+ if (EVP_PKEY_derive(ctx, NULL, &skeylen) <= 0)
+       /* Error */
+
+ skey = OPENSSL_malloc(skeylen);
+
+ if (!skey)
+       /* malloc failure */
+ 
+ if (EVP_PKEY_derive(ctx, skey, &skeylen) <= 0)
+       /* Error */
+
+ /* Shared secret is skey bytes written to buffer skey */
+
+=head1 SEE ALSO
+
+L<EVP_PKEY_CTX_new(3)|EVP_PKEY_CTX_new(3)>,
+L<EVP_PKEY_encrypt(3)|EVP_PKEY_encrypt(3)>,
+L<EVP_PKEY_decrypt(3)|EVP_PKEY_decrypt(3)>,
+L<EVP_PKEY_sign(3)|EVP_PKEY_sign(3)>,
+L<EVP_PKEY_verify(3)|EVP_PKEY_verify(3)>,
+L<EVP_PKEY_verifyrecover(3)|EVP_PKEY_verifyrecover(3)>,
+
+=head1 HISTORY
+
+These functions were first added to OpenSSL 0.9.9.
+
+=cut