EVP_KDF_SS

Section: OpenSSL (7)
Updated: 2020-07-28
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NAME

EVP_KDF_SS - The Single Step / One Step EVP_KDF implementation

DESCRIPTION

The EVP_KDF_SS algorithm implements the Single Step key derivation function (SSKDF). SSKDF derives a key using input such as a shared secret key (that was generated during the execution of a key establishment scheme) and fixedinfo. SSKDF is also informally referred to as 'Concat KDF'.

Auxilary function

The implementation uses a selectable auxiliary function H, which can be in the backported version only a:

H(x) = hash(x, digest=md)

Numeric identity

EVP_KDF_SS is the numeric identity for this implementation; it can be used with the EVP_KDF_CTX_new_id() function.

Supported controls

The supported controls are:

EVP_KDF_CTRL_SET_MD

This control works as described in ``CONTROLS'' in EVP_KDF_CTX(3).

EVP_KDF_CTRL_SET_KEY

This control expects two arguments: "unsigned char *secret", "size_t secretlen"

The shared secret used for key derivation. This control sets the secret.

EVP_KDF_ctrl_str() takes two type strings for this control:

"secret": The value string is used as is.
"hexsecret": The value string is expected to be a hexadecimal number, which will be decoded before being passed on as the control value.

EVP_KDF_CTRL_SET_SSKDF_INFO

This control expects two arguments: "unsigned char *info", "size_t infolen"

An optional value for fixedinfo, also known as otherinfo. This control sets the fixedinfo.

EVP_KDF_ctrl_str() takes two type strings for this control:

"info": The value string is used as is.
"hexinfo": The value string is expected to be a hexadecimal number, which will be decoded before being passed on as the control value.

NOTES

A context for SSKDF can be obtained by calling:

EVP_KDF_CTX *kctx = EVP_KDF_CTX_new_id(EVP_KDF_SS);

The output length of an SSKDF is specified via the "keylen" parameter to the EVP_KDF_derive(3) function.

EXAMPLE

This example derives 10 bytes using H(x) = SHA-256, with the secret key ``secret'' and fixedinfo value ``label'':

  EVP_KDF_CTX *kctx;
  unsigned char out[10];

  kctx = EVP_KDF_CTX_new_id(EVP_KDF_SS);

  if (EVP_KDF_ctrl(kctx, EVP_KDF_CTRL_SET_MD, EVP_sha256()) <= 0) {
      error("EVP_KDF_CTRL_SET_MD");
  }
  if (EVP_KDF_ctrl(kctx, EVP_KDF_CTRL_SET_KEY, "secret", (size_t)6) <= 0) {
      error("EVP_KDF_CTRL_SET_KEY");
  }
  if (EVP_KDF_ctrl(kctx, EVP_KDF_CTRL_SET_SSKDF_INFO, "label", (size_t)5) <= 0) {
      error("EVP_KDF_CTRL_SET_SSKDF_INFO");
  }
  if (EVP_KDF_derive(kctx, out, sizeof(out)) <= 0) {
      error("EVP_KDF_derive");
  }

  EVP_KDF_CTX_free(kctx);

CONFORMING TO

NIST SP800-56Cr1.

HISTORY

This functionality was added to OpenSSL 3.0.0.

COPYRIGHT

Licensed under the Apache License 2.0 (the ``License''). You may not use this file except in compliance with the License. You can obtain a copy in the file LICENSE in the source distribution or at <https://www.openssl.org/source/license.html>.