- NAME
- SYNOPSIS
- DESCRIPTION
- NOTES
- RETURN VALUES
- EXAMPLE
- SEE ALSO
- HISTORY

# NAME

EVP_PKEY_derive_init, EVP_PKEY_derive_set_peer, EVP_PKEY_derive - derive public key algorithm shared secret.

# 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);

# DESCRIPTION

The `EVP_PKEY_derive_init()` function initializes a public key algorithm context using key 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 ctx. If key is NULL then the maximum size of the output buffer is written to the keylen parameter. If key is not NULL then before the call the keylen parameter should contain the length of the key buffer, if the call is successful the shared secret is written to key and the amount of data written to keylen.

# 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.

# 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.

# 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 */

# SEE ALSO

EVP_PKEY_CTX_new, EVP_PKEY_encrypt, EVP_PKEY_decrypt, EVP_PKEY_sign, EVP_PKEY_verify, EVP_PKEY_verify_recover,

# HISTORY

These functions were first added to OpenSSL 1.0.0.