=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_verify_recover(3)|EVP_PKEY_verify_recover(3)>, =head1 HISTORY These functions were first added to OpenSSL 1.0.0. =cut