EVP_KDF_HKDF.7ssl man page

EVP_KDF_HKDF — The HKDF EVP_KDF implementation

Description

Support for computing the HKDF KDF through the EVP_KDF API.

The EVP_KDF_HKDF algorithm implements the HKDF key derivation function. HKDF follows the “extract-then-expand” paradigm, where the KDF logically consists of two modules. The first stage takes the input keying material and “extracts” from it a fixed-length pseudorandom key K. The second stage “expands” the key K into several additional pseudorandom keys (the output of the KDF).

Numeric identity

EVP_KDF_HKDF 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_SALT
EVP_KDF_CTRL_SET_MD
EVP_KDF_CTRL_SET_KEY

These controls work as described in “CONTROLS” in EVP_KDF_CTX(3).

EVP_KDF_CTRL_RESET_HKDF_INFO

This control does not expect any arguments.

Resets the context info buffer to zero length.

EVP_KDF_CTRL_ADD_HKDF_INFO

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

Sets the info value to the first infolen bytes of the buffer info.  If a value is already set, the contents of the buffer are appended to the existing value.

The total length of the context info buffer cannot exceed 1024 bytes; this should be more than enough for any normal use of HKDF.

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.

EVP_KDF_CTRL_SET_HKDF_MODE

This control expects one argument: int mode

Sets the mode for the HKDF operation. There are three modes that are currently defined:

EVP_KDF_HKDF_MODE_EXTRACT_AND_EXPAND

This is the default mode.  Calling EVP_KDF_derive(3) on an EVP_KDF_CTX set up for HKDF will perform an extract followed by an expand operation in one go. The derived key returned will be the result after the expand operation. The intermediate fixed-length pseudorandom key K is not returned.

In this mode the digest, key, salt and info values must be set before a key is derived otherwise an error will occur.

EVP_KDF_HKDF_MODE_EXTRACT_ONLY

In this mode calling EVP_KDF_derive(3) will just perform the extract operation. The value returned will be the intermediate fixed-length pseudorandom key K.  The keylen parameter must match the size of K, which can be looked up by calling EVP_KDF_size() after setting the mode and digest.

The digest, key and salt values must be set before a key is derived otherwise an error will occur.

EVP_KDF_HKDF_MODE_EXPAND_ONLY

In this mode calling EVP_KDF_derive(3) will just perform the expand operation. The input key should be set to the intermediate fixed-length pseudorandom key K returned from a previous extract operation.

The digest, key and info values must be set before a key is derived otherwise an error will occur.

EVP_KDF_ctrl_str() type string: “mode”

The value string is expected to be one of: “EXTRACT_AND_EXPAND”, “EXTRACT_ONLY” or “EXPAND_ONLY”.

Notes

A context for HKDF can be obtained by calling:

 EVP_KDF_CTX *kctx = EVP_KDF_CTX_new_id(EVP_KDF_HKDF);

The output length of an HKDF expand operation is specified via the keylen parameter to the EVP_KDF_derive(3) function.  When using EVP_KDF_HKDF_MODE_EXTRACT_ONLY the keylen parameter must equal the size of the intermediate fixed-length pseudorandom key otherwise an error will occur. For that mode, the fixed output size can be looked up by calling EVP_KDF_size() after setting the mode and digest on the EVP_KDF_CTX.

Example

This example derives 10 bytes using SHA-256 with the secret key “secret”, salt value “salt” and info value “label”:

 EVP_KDF_CTX *kctx;
 unsigned char out[10];

 kctx = EVP_KDF_CTX_new_id(EVP_KDF_HKDF);

 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_SALT, "salt", (size_t)4) <= 0) {
     error("EVP_KDF_CTRL_SET_SALT");
 }
 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_ADD_HKDF_INFO, "label", (size_t)5) <= 0) {
     error("EVP_KDF_CTRL_ADD_HKDF_INFO");
 }
 if (EVP_KDF_derive(kctx, out, sizeof(out)) <= 0) {
     error("EVP_KDF_derive");
 }

 EVP_KDF_CTX_free(kctx);

Conforming to

RFC 5869

See Also

EVP_KDF_CTX, EVP_KDF_CTX_new_id(3), EVP_KDF_CTX_free(3), EVP_KDF_ctrl(3), EVP_KDF_size(3), EVP_KDF_derive(3), “CONTROLS” in EVP_KDF_CTX(3)

Info

2019-07-25 1.1.1c OpenSSL