KEYCTL_DH_COMPUTE

Section: Linux Key Management Calls (3)
Updated: 07 Apr 2016
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NAME

keyctl_dh_compute - Compute a Diffie-Hellman shared secret or public key
keyctl_dh_compute_kdf - Derive key from a Diffie-Hellman shared secret

SYNOPSIS

#include <keyutils.h>

long keyctl_dh_compute(key_serial_t private, key_serial_t prime,
key_serial_t base, char *buffer, size_t buflen);

long keyctl_dh_compute_alloc(key_serial_t private,
key_serial_t prime, key_serial_t base, void **_buffer);

long keyctl_dh_compute_kdf(key_serial_t private, key_serial_t prime,
key_serial_t base, char *hashname, char *otherinfo,
size_t otherinfolen, char *buffer, size_t buflen);

DESCRIPTION

keyctl_dh_compute() computes a Diffie-Hellman public key or shared secret. That computation is:

: base ^ private ( mod prime )

When base is a key containing the shared generator value, the remote public key is computed. When base is a key containing the remote public key, the shared secret is computed.

base, private, and prime must all refer to user-type keys containing the parameters for the computation. Each of these keys must grant the caller read permission in order for them to be used.

buffer and buflen specify the buffer into which the computed result will be placed. buflen may be zero, in which case the buffer is not used and the minimum buffer length is fetched.

keyctl_dh_compute_alloc() is similar to keyctl_dh_compute() except that it allocates a buffer big enough to hold the payload data and places the data in it. If successful, a pointer to the buffer is placed in *_buffer. The caller must free the buffer.

keyctl_dh_compute_kdf() derives a key from a Diffie-Hellman shared secret according to the protocol specified in SP800-56A. The Diffie-Hellman computation is based on the same primitives as discussed for keyctl_dh_compute().

To implement the protocol of SP800-56A base is a key containing the remote public key to compute the Diffie-Hellman shared secret. That shared secret is post-processed with a key derivation function.

The hashname specifies the Linux kernel crypto API name for a hash that shall be used for the key derivation function, such as sha256. The hashname must be a NULL terminated string. See /proc/crypto for available hashes on the system.

Following the specification of SP800-56A section 5.8.1.2 the otherinfo parameter may be provided. The format of the OtherInfo field is defined by the caller. The caller may also specify NULL as a valid argument when no OtherInfo data shall be processed. The length of the otherinfo parameter is specified with otherinfolen and is restricted to a maximum length by the kernel.

The KDF returns the requested number of bytes specified with the genlen or the buflen parameter depending on the invoked function.

buffer and buflen specify the buffer into which the computed result will be placed.

RETURN VALUE

On success keyctl_dh_compute() returns the amount of data placed into the buffer when buflen is non-zero. When buflen is zero, the minimum buffer length to hold the data is returned.

On success keyctl_dh_compute_alloc() returns the amount of data in the buffer.

On error, both functions set errno to an appropriate code and return the value -1.

ERRORS

ENOKEY: One of the keys specified is invalid or not readable.
EINVAL: The buffer pointer is invalid or buflen is too small.
EOPNOTSUPP: One of the keys was not a valid user key.
EMSGSIZE: When using keyctl_dh_compute_kdf(), the size of either otherinfolen or buflen is too big.

LINKING

This is a library function that can be found in libkeyutils. When linking, -lkeyutils should be specified to the linker.