.\" $OpenBSD: EVP_PKEY_derive.3,v 1.11 2024/07/21 08:25:33 tb Exp $ .\" full merge up to: OpenSSL 48e5119a Jan 19 10:49:22 2018 +0100 .\" .\" This file is a derived work. .\" The changes are covered by the following Copyright and license: .\" .\" Copyright (c) 2023 Ingo Schwarze .\" .\" Permission to use, copy, modify, and distribute this software for any .\" purpose with or without fee is hereby granted, provided that the above .\" copyright notice and this permission notice appear in all copies. .\" .\" THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES .\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF .\" MERCHANTABILITY AND FITNESS. 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IN NO EVENT SHALL THE OpenSSL PROJECT OR .\" ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, .\" SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; .\" LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, .\" STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) .\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED .\" OF THE POSSIBILITY OF SUCH DAMAGE. .\" .Dd $Mdocdate: July 21 2024 $ .Dt EVP_PKEY_DERIVE 3 .Os .Sh NAME .Nm EVP_PKEY_derive_init , .Nm EVP_PKEY_derive_set_peer , .Nm EVP_PKEY_CTX_get0_peerkey , .Nm EVP_PKEY_derive .Nd derive public key algorithm shared secret .Sh SYNOPSIS .In openssl/evp.h .Ft int .Fo EVP_PKEY_derive_init .Fa "EVP_PKEY_CTX *ctx" .Fc .Ft int .Fo EVP_PKEY_derive_set_peer .Fa "EVP_PKEY_CTX *ctx" .Fa "EVP_PKEY *peerkey" .Fc .Ft EVP_PKEY * .Fo EVP_PKEY_CTX_get0_peerkey .Fa "EVP_PKEY_CTX *ctx" .Fc .Ft int .Fo EVP_PKEY_derive .Fa "EVP_PKEY_CTX *ctx" .Fa "unsigned char *key" .Fa "size_t *keylen" .Fc .Sh DESCRIPTION .Fn EVP_PKEY_derive_init initializes the public key algorithm context .Fa ctx for shared secret derivation using the .Vt EVP_PKEY object already stored in .Fa ctx . The library provides built-in support for keys with an .Xr EVP_PKEY_base_id 3 of .Dv EVP_PKEY_DH , .Dv EVP_PKEY_EC , .Dv EVP_PKEY_HKDF , and .Dv EVP_PKEY_X25519 . .Pp After the call to .Fn EVP_PKEY_derive_init , algorithm specific control operations can optionally be performed to set any appropriate parameters for the operation. .Pp .Fn EVP_PKEY_derive_set_peer configures the .Fa ctx , which already needs to be initialized with .Fn EVP_PKEY_derive_init , .Xr EVP_PKEY_encrypt_init 3 , or .Xr EVP_PKEY_decrypt_init 3 , to use the .Fa peerkey , which is normally a public key. In case of success, the reference count of the .Fa peerkey is incremented by one. Consequently, the caller needs to call .Xr EVP_PKEY_free 3 on the .Fa peerkey when the caller no longer needs it, even if it is still in use by .Fa ctx . .Pp .Fn EVP_PKEY_derive derives a shared secret using .Fa ctx . If .Fa key is .Dv NULL , then the maximum size of the output buffer is written to the .Fa keylen parameter. If .Fa key is not .Dv NULL then before the call the .Fa keylen parameter should contain the length of the .Fa key buffer. If the call is successful, the shared secret is written to .Fa key and the amount of data written to .Fa keylen . .Pp The function .Fn EVP_PKEY_derive can be called more than once on the same context if several operations are performed using the same parameters. .Sh RETURN VALUES .Fn EVP_PKEY_derive_init , .Fn EVP_PKEY_derive_set_peer , and .Fn 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. .Pp For .Fn EVP_PKEY_derive_set_peer , a return value of \-1 can for example occur if .Fa ctx is not properly initialized, does not contain an .Vt EVP_PKEY that can be retrieved with .Xr EVP_PKEY_CTX_get0_pkey 3 , the .Xr EVP_PKEY_id 3 of both keys mismatch, or .Xr EVP_PKEY_cmp_parameters 3 reports mismatching key parameters. .Pp .Fn EVP_PKEY_derive fails with a return value of \-1 for example if .Fa ctx has not been successfully initialized with .Fn EVP_PKEY_derive_init . .Pp .Fn EVP_PKEY_CTX_get0_peerkey returns an internal pointer to the .Fa peerkey used by .Fa ctx without incrementing its reference count. .Sh EXAMPLES Derive shared secret (for example DH or EC keys): .Bd -literal -offset indent #include #include EVP_PKEY_CTX *ctx; unsigned char *skey; size_t skeylen; EVP_PKEY *pkey, *peerkey; /* Assumes that pkey and peerkey have already been set up. */ ctx = EVP_PKEY_CTX_new(pkey, NULL); 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 = malloc(skeylen); if (!skey) /* malloc failure */ if (EVP_PKEY_derive(ctx, skey, &skeylen) <= 0) /* Error */ /* Shared secret is skey bytes written to buffer skey */ .Ed .Sh SEE ALSO .Xr EVP_PKEY_CTX_new 3 , .Xr EVP_PKEY_decrypt 3 , .Xr EVP_PKEY_encrypt 3 , .Xr EVP_PKEY_meth_set_derive 3 , .Xr EVP_PKEY_sign 3 , .Xr EVP_PKEY_verify 3 , .Xr EVP_PKEY_verify_recover 3 , .Xr X25519 3 .Sh HISTORY .Fn EVP_PKEY_derive_init , .Fn EVP_PKEY_derive_set_peer , .Fn EVP_PKEY_CTX_get0_peerkey , and .Fn EVP_PKEY_derive first appeared in OpenSSL 1.0.0 and have been available since .Ox 4.9 .