dhm.h File Reference

This file contains Diffie-Hellman-Merkle (DHM) key exchange definitions and functions. More...

#include "mbedtls/private_access.h"
#include "mbedtls/build_info.h"
#include "mbedtls/bignum.h"
#include "dhm_alt.h"

Include dependency graph for dhm.h:

This graph shows which files directly or indirectly include this file:

Go to the source code of this file.

Macros
#define	MBEDTLS_ERR_DHM_BAD_INPUT_DATA   -0x3080
#define	MBEDTLS_ERR_DHM_READ_PARAMS_FAILED   -0x3100
#define	MBEDTLS_ERR_DHM_MAKE_PARAMS_FAILED   -0x3180
#define	MBEDTLS_ERR_DHM_READ_PUBLIC_FAILED   -0x3200
#define	MBEDTLS_ERR_DHM_MAKE_PUBLIC_FAILED   -0x3280
#define	MBEDTLS_ERR_DHM_CALC_SECRET_FAILED   -0x3300
#define	MBEDTLS_ERR_DHM_INVALID_FORMAT   -0x3380
#define	MBEDTLS_ERR_DHM_ALLOC_FAILED   -0x3400
#define	MBEDTLS_ERR_DHM_FILE_IO_ERROR   -0x3480
#define	MBEDTLS_ERR_DHM_SET_GROUP_FAILED   -0x3580
#define	MBEDTLS_DHM_RFC3526_MODP_2048_P_BIN
#define	MBEDTLS_DHM_RFC3526_MODP_2048_G_BIN   { 0x02 }
#define	MBEDTLS_DHM_RFC3526_MODP_3072_P_BIN
#define	MBEDTLS_DHM_RFC3526_MODP_3072_G_BIN   { 0x02 }
#define	MBEDTLS_DHM_RFC3526_MODP_4096_P_BIN
#define	MBEDTLS_DHM_RFC3526_MODP_4096_G_BIN   { 0x02 }
#define	MBEDTLS_DHM_RFC7919_FFDHE2048_P_BIN
#define	MBEDTLS_DHM_RFC7919_FFDHE2048_G_BIN   { 0x02 }
#define	MBEDTLS_DHM_RFC7919_FFDHE3072_P_BIN
#define	MBEDTLS_DHM_RFC7919_FFDHE3072_G_BIN   { 0x02 }
#define	MBEDTLS_DHM_RFC7919_FFDHE4096_P_BIN
#define	MBEDTLS_DHM_RFC7919_FFDHE4096_G_BIN   { 0x02 }
#define	MBEDTLS_DHM_RFC7919_FFDHE6144_P_BIN
#define	MBEDTLS_DHM_RFC7919_FFDHE6144_G_BIN   { 0x02 }
#define	MBEDTLS_DHM_RFC7919_FFDHE8192_P_BIN
#define	MBEDTLS_DHM_RFC7919_FFDHE8192_G_BIN   { 0x02 }

Enumerations
enum	mbedtls_dhm_parameter {   MBEDTLS_DHM_PARAM_P, MBEDTLS_DHM_PARAM_G, MBEDTLS_DHM_PARAM_X, MBEDTLS_DHM_PARAM_GX,   MBEDTLS_DHM_PARAM_GY, MBEDTLS_DHM_PARAM_K }

Functions
void	mbedtls_dhm_init (mbedtls_dhm_context *ctx)
	This function initializes the DHM context. More...
int	mbedtls_dhm_read_params (mbedtls_dhm_context *ctx, unsigned char **p, const unsigned char *end)
	This function parses the DHM parameters in a TLS ServerKeyExchange handshake message (DHM modulus, generator, and public key). More...
int	mbedtls_dhm_make_params (mbedtls_dhm_context *ctx, int x_size, unsigned char *output, size_t *olen, int(*f_rng)(void *, unsigned char *, size_t), void *p_rng)
	This function generates a DHM key pair and exports its public part together with the DHM parameters in the format used in a TLS ServerKeyExchange handshake message. More...
int	mbedtls_dhm_set_group (mbedtls_dhm_context *ctx, const mbedtls_mpi *P, const mbedtls_mpi *G)
	This function sets the prime modulus and generator. More...
int	mbedtls_dhm_read_public (mbedtls_dhm_context *ctx, const unsigned char *input, size_t ilen)
	This function imports the raw public value of the peer. More...
int	mbedtls_dhm_make_public (mbedtls_dhm_context *ctx, int x_size, unsigned char *output, size_t olen, int(*f_rng)(void *, unsigned char *, size_t), void *p_rng)
	This function creates a DHM key pair and exports the raw public key in big-endian format. More...
int	mbedtls_dhm_calc_secret (mbedtls_dhm_context *ctx, unsigned char *output, size_t output_size, size_t *olen, int(*f_rng)(void *, unsigned char *, size_t), void *p_rng)
	This function derives and exports the shared secret (G^Y)^X mod P. More...
size_t	mbedtls_dhm_get_bitlen (const mbedtls_dhm_context *ctx)
	This function returns the size of the prime modulus in bits. More...
size_t	mbedtls_dhm_get_len (const mbedtls_dhm_context *ctx)
	This function returns the size of the prime modulus in bytes. More...
int	mbedtls_dhm_get_value (const mbedtls_dhm_context *ctx, mbedtls_dhm_parameter param, mbedtls_mpi *dest)
	This function copies a parameter of a DHM key. More...
void	mbedtls_dhm_free (mbedtls_dhm_context *ctx)
	This function frees and clears the components of a DHM context. More...
int	mbedtls_dhm_parse_dhm (mbedtls_dhm_context *dhm, const unsigned char *dhmin, size_t dhminlen)
	This function parses DHM parameters in PEM or DER format. More...
int	mbedtls_dhm_parse_dhmfile (mbedtls_dhm_context *dhm, const char *path)
	This function loads and parses DHM parameters from a file. More...
int	mbedtls_dhm_self_test (int verbose)
	The DMH checkup routine. More...

Detailed Description

This file contains Diffie-Hellman-Merkle (DHM) key exchange definitions and functions.

Diffie-Hellman-Merkle (DHM) key exchange is defined in RFC-2631: Diffie-Hellman Key Agreement Method and Public-Key Cryptography Standards (PKCS) #3: Diffie Hellman Key Agreement Standard.

RFC-3526: More Modular Exponential (MODP) Diffie-Hellman groups for Internet Key Exchange (IKE) defines a number of standardized Diffie-Hellman groups for IKE.

RFC-5114: Additional Diffie-Hellman Groups for Use with IETF Standards defines a number of standardized Diffie-Hellman groups that can be used.

Warning

The security of the DHM key exchange relies on the proper choice of prime modulus - optimally, it should be a safe prime. The usage of non-safe primes both decreases the difficulty of the underlying discrete logarithm problem and can lead to small subgroup attacks leaking private exponent bits when invalid public keys are used and not detected. This is especially relevant if the same DHM parameters are reused for multiple key exchanges as in static DHM, while the criticality of small-subgroup attacks is lower for ephemeral DHM.

For performance reasons, the code does neither perform primality nor safe primality tests, nor the expensive checks for invalid subgroups. Moreover, even if these were performed, non-standardized primes cannot be trusted because of the possibility of backdoors that can't be effectively checked for.

Diffie-Hellman-Merkle is therefore a security risk when not using standardized primes generated using a trustworthy ("nothing up my sleeve") method, such as the RFC 3526 / 7919 primes. In the TLS protocol, DH parameters need to be negotiated, so using the default primes systematically is not always an option. If possible, use Elliptic Curve Diffie-Hellman (ECDH), which has better performance, and for which the TLS protocol mandates the use of standard parameters.

Definition in file dhm.h.

Macro Definition Documentation

#define MBEDTLS_DHM_RFC3526_MODP_2048_G_BIN   { 0x02 }

Definition at line 476 of file dhm.h.

#define MBEDTLS_DHM_RFC3526_MODP_2048_P_BIN

RFC 3526, RFC 5114 and RFC 7919 standardize a number of Diffie-Hellman groups, some of which are included here for use within the SSL/TLS module and the user's convenience when configuring the Diffie-Hellman parameters by hand through mbedtls_ssl_conf_dh_param.

The following lists the source of the above groups in the standards:

• RFC 5114 section 2.2: 2048-bit MODP Group with 224-bit Prime Order Subgroup
• RFC 3526 section 3: 2048-bit MODP Group
• RFC 3526 section 4: 3072-bit MODP Group
• RFC 3526 section 5: 4096-bit MODP Group
• RFC 7919 section A.1: ffdhe2048
• RFC 7919 section A.2: ffdhe3072
• RFC 7919 section A.3: ffdhe4096
• RFC 7919 section A.4: ffdhe6144
• RFC 7919 section A.5: ffdhe8192

The constants with suffix "_p" denote the chosen prime moduli, while the constants with suffix "_g" denote the chosen generator of the associated prime field.

The constants further suffixed with "_bin" are provided in binary format, while all other constants represent null-terminated strings holding the hexadecimal presentation of the respective numbers.

The primes from RFC 3526 and RFC 7919 have been generating by the following trust-worthy procedure:

• Fix N in { 2048, 3072, 4096, 6144, 8192 } and consider the N-bit number the first and last 64 bits are all 1, and the remaining N - 128 bits of which are 0x7ff...ff.
• Add the smallest multiple of the first N - 129 bits of the binary expansion of pi (for RFC 5236) or e (for RFC 7919) to this intermediate bit-string such that the resulting integer is a safe-prime.
• The result is the respective RFC 3526 / 7919 prime, and the corresponding generator is always chosen to be 2 (which is a square for these prime, hence the corresponding subgroup has order (p-1)/2 and avoids leaking a bit in the private exponent).

Definition at line 442 of file dhm.h.

#define MBEDTLS_DHM_RFC3526_MODP_3072_G_BIN   { 0x02 }

Definition at line 528 of file dhm.h.

#define MBEDTLS_DHM_RFC3526_MODP_3072_P_BIN

Definition at line 478 of file dhm.h.

#define MBEDTLS_DHM_RFC3526_MODP_4096_G_BIN   { 0x02 }

Definition at line 596 of file dhm.h.

#define MBEDTLS_DHM_RFC3526_MODP_4096_P_BIN

Definition at line 530 of file dhm.h.

#define MBEDTLS_DHM_RFC7919_FFDHE2048_G_BIN   { 0x02 }

Definition at line 632 of file dhm.h.

#define MBEDTLS_DHM_RFC7919_FFDHE2048_P_BIN

Definition at line 598 of file dhm.h.

#define MBEDTLS_DHM_RFC7919_FFDHE3072_G_BIN   { 0x02 }

Definition at line 684 of file dhm.h.

#define MBEDTLS_DHM_RFC7919_FFDHE3072_P_BIN

Definition at line 634 of file dhm.h.

#define MBEDTLS_DHM_RFC7919_FFDHE4096_G_BIN   { 0x02 }

Definition at line 752 of file dhm.h.

#define MBEDTLS_DHM_RFC7919_FFDHE4096_P_BIN

Definition at line 686 of file dhm.h.

#define MBEDTLS_DHM_RFC7919_FFDHE6144_G_BIN   { 0x02 }

Definition at line 852 of file dhm.h.

#define MBEDTLS_DHM_RFC7919_FFDHE6144_P_BIN

Definition at line 754 of file dhm.h.

#define MBEDTLS_DHM_RFC7919_FFDHE8192_G_BIN   { 0x02 }

Definition at line 984 of file dhm.h.

#define MBEDTLS_DHM_RFC7919_FFDHE8192_P_BIN

Definition at line 854 of file dhm.h.

#define MBEDTLS_ERR_DHM_ALLOC_FAILED   -0x3400

Allocation of memory failed.

Definition at line 88 of file dhm.h.

#define MBEDTLS_ERR_DHM_BAD_INPUT_DATA   -0x3080

Bad input parameters.

Definition at line 74 of file dhm.h.

#define MBEDTLS_ERR_DHM_CALC_SECRET_FAILED   -0x3300

Calculation of the DHM secret failed.

Definition at line 84 of file dhm.h.

#define MBEDTLS_ERR_DHM_FILE_IO_ERROR   -0x3480

Read or write of file failed.

Definition at line 90 of file dhm.h.

#define MBEDTLS_ERR_DHM_INVALID_FORMAT   -0x3380

The ASN.1 data is not formatted correctly.

Definition at line 86 of file dhm.h.

#define MBEDTLS_ERR_DHM_MAKE_PARAMS_FAILED   -0x3180

Making of the DHM parameters failed.

Definition at line 78 of file dhm.h.

#define MBEDTLS_ERR_DHM_MAKE_PUBLIC_FAILED   -0x3280

Making of the public value failed.

Definition at line 82 of file dhm.h.

#define MBEDTLS_ERR_DHM_READ_PARAMS_FAILED   -0x3100

Reading of the DHM parameters failed.

Definition at line 76 of file dhm.h.

#define MBEDTLS_ERR_DHM_READ_PUBLIC_FAILED   -0x3200

Reading of the public values failed.

Definition at line 80 of file dhm.h.

#define MBEDTLS_ERR_DHM_SET_GROUP_FAILED   -0x3580

Setting the modulus and generator failed.

Definition at line 92 of file dhm.h.

Enumeration Type Documentation

enum mbedtls_dhm_parameter

Which parameter to access in mbedtls_dhm_get_value().

Enumerator
MBEDTLS_DHM_PARAM_P	The prime modulus.
MBEDTLS_DHM_PARAM_G	The generator.
MBEDTLS_DHM_PARAM_X	Our secret value.
MBEDTLS_DHM_PARAM_GX	Our public key = G^X mod P.
MBEDTLS_DHM_PARAM_GY	The public key of the peer = G^Y mod P.
MBEDTLS_DHM_PARAM_K	The shared secret = G^(XY) mod P.

Definition at line 95 of file dhm.h.

Function Documentation

int mbedtls_dhm_calc_secret	(	mbedtls_dhm_context *	ctx,
		unsigned char *	output,
		size_t	output_size,
		size_t *	olen,
		int(*)(void *, unsigned char *, size_t)	f_rng,
		void *	p_rng
	)

This function derives and exports the shared secret (G^Y)^X mod P.

Note

If f_rng is not NULL, it is used to blind the input as a countermeasure against timing attacks. Blinding is used only if our private key X is re-used, and not used otherwise. We recommend always passing a non-NULL f_rng argument.

Parameters

ctx	The DHM context to use. This must be initialized and have its own private key generated and the peer's public key imported.
output	The buffer to write the generated shared key to. This must be a writable buffer of size output_size Bytes.
output_size	The size of the destination buffer. This must be at least the size of ctx->len (the size of P).
olen	On exit, holds the actual number of Bytes written.
f_rng	The RNG function. Must not be NULL. Used for blinding.
p_rng	The RNG context to be passed to f_rng. This may be NULL if f_rng doesn't need a context parameter.

Returns

0 on success.

An MBEDTLS_ERR_DHM_XXX error code on failure.

void mbedtls_dhm_free

(

mbedtls_dhm_context *

ctx

)

This function frees and clears the components of a DHM context.

Parameters

ctx	The DHM context to free and clear. This may be NULL, in which case this function is a no-op. If it is not NULL, it must point to an initialized DHM context.

size_t mbedtls_dhm_get_bitlen

(

const mbedtls_dhm_context *

ctx

)

This function returns the size of the prime modulus in bits.

Parameters

ctx	The DHM context to query.

Returns

The size of the prime modulus in bits, i.e. the number n such that 2^(n-1) <= P < 2^n.

size_t mbedtls_dhm_get_len

(

const mbedtls_dhm_context *

ctx

)

This function returns the size of the prime modulus in bytes.

Parameters

ctx	The DHM context to query.

Returns

The size of the prime modulus in bytes, i.e. the number n such that 2^(8*(n-1)) <= P < 2^(8*n).

int mbedtls_dhm_get_value	(	const mbedtls_dhm_context *	ctx,
		mbedtls_dhm_parameter	param,
		mbedtls_mpi *	dest
	)

This function copies a parameter of a DHM key.

Parameters

ctx	The DHM context to query.
param	The parameter to copy.
dest	The MPI object to copy the value into. It must be initialized.

Returns

0 on success.

MBEDTLS_ERR_DHM_BAD_INPUT_DATA if field is invalid.

An MBEDTLS_ERR_MPI_XXX error code if the copy fails.

void mbedtls_dhm_init

(

mbedtls_dhm_context *

ctx

)

This function initializes the DHM context.

Parameters

ctx	The DHM context to initialize.

int mbedtls_dhm_make_params	(	mbedtls_dhm_context *	ctx,
		int	x_size,
		unsigned char *	output,
		size_t *	olen,
		int(*)(void *, unsigned char *, size_t)	f_rng,
		void *	p_rng
	)

This function generates a DHM key pair and exports its public part together with the DHM parameters in the format used in a TLS ServerKeyExchange handshake message.

Note

This function assumes that the DHM parameters ctx->P and ctx->G have already been properly set. For that, use mbedtls_dhm_set_group() below in conjunction with mbedtls_mpi_read_binary() and mbedtls_mpi_read_string().

In a TLS handshake, this is the how the server generates and exports its DHM key material.

Parameters

ctx	The DHM context to use. This must be initialized and have the DHM parameters set. It may or may not already have imported the peer's public key.
x_size	The private key size in Bytes.
olen	The address at which to store the number of Bytes written on success. This must not be NULL.
output	The destination buffer. This must be a writable buffer of sufficient size to hold the reduced binary presentation of the modulus, the generator and the public key, each wrapped with a 2-byte length field. It is the responsibility of the caller to ensure that enough space is available. Refer to mbedtls_mpi_size() to computing the byte-size of an MPI.
f_rng	The RNG function. Must not be NULL.
p_rng	The RNG context to be passed to f_rng. This may be NULL if f_rng doesn't need a context parameter.

Returns

0 on success.

An MBEDTLS_ERR_DHM_XXX error code on failure.

int mbedtls_dhm_make_public	(	mbedtls_dhm_context *	ctx,
		int	x_size,
		unsigned char *	output,
		size_t	olen,
		int(*)(void *, unsigned char *, size_t)	f_rng,
		void *	p_rng
	)

This function creates a DHM key pair and exports the raw public key in big-endian format.

Note

The destination buffer is always fully written so as to contain a big-endian representation of G^X mod P. If it is larger than ctx->len, it is padded accordingly with zero-bytes at the beginning.

Parameters

ctx	The DHM context to use. This must be initialized and have the DHM parameters set. It may or may not already have imported the peer's public key.
x_size	The private key size in Bytes.
output	The destination buffer. This must be a writable buffer of size olen Bytes.
olen	The length of the destination buffer. This must be at least equal to ctx->len (the size of P).
f_rng	The RNG function. This must not be NULL.
p_rng	The RNG context to be passed to f_rng. This may be NULL if f_rng doesn't need a context argument.

Returns

0 on success.

An MBEDTLS_ERR_DHM_XXX error code on failure.

int mbedtls_dhm_parse_dhm	(	mbedtls_dhm_context *	dhm,
		const unsigned char *	dhmin,
		size_t	dhminlen
	)

This function parses DHM parameters in PEM or DER format.

Parameters

dhm	The DHM context to import the DHM parameters into. This must be initialized.
dhmin	The input buffer. This must be a readable buffer of length dhminlen Bytes.
dhminlen	The size of the input buffer dhmin, including the terminating NULL Byte for PEM data.

Returns

0 on success.

An MBEDTLS_ERR_DHM_XXX or MBEDTLS_ERR_PEM_XXX error code on failure.

int mbedtls_dhm_parse_dhmfile	(	mbedtls_dhm_context *	dhm,
		const char *	path
	)

This function loads and parses DHM parameters from a file.

Parameters

dhm	The DHM context to load the parameters to. This must be initialized.
path	The filename to read the DHM parameters from. This must not be NULL.

Returns

0 on success.

An MBEDTLS_ERR_DHM_XXX or MBEDTLS_ERR_PEM_XXX error code on failure.

int mbedtls_dhm_read_params	(	mbedtls_dhm_context *	ctx,
		unsigned char **	p,
		const unsigned char *	end
	)

This function parses the DHM parameters in a TLS ServerKeyExchange handshake message (DHM modulus, generator, and public key).

Note

In a TLS handshake, this is the how the client sets up its DHM context from the server's public DHM key material.

Parameters

ctx	The DHM context to use. This must be initialized.
p	On input, *p must be the start of the input buffer. On output, *p is updated to point to the end of the data that has been read. On success, this is the first byte past the end of the ServerKeyExchange parameters. On error, this is the point at which an error has been detected, which is usually not useful except to debug failures.
end	The end of the input buffer.

Returns

0 on success.

An MBEDTLS_ERR_DHM_XXX error code on failure.

int mbedtls_dhm_read_public	(	mbedtls_dhm_context *	ctx,
		const unsigned char *	input,
		size_t	ilen
	)

This function imports the raw public value of the peer.

Note

In a TLS handshake, this is the how the server imports the Client's public DHM key.

Parameters

ctx	The DHM context to use. This must be initialized and have its DHM parameters set, e.g. via mbedtls_dhm_set_group(). It may or may not already have generated its own private key.
input	The input buffer containing the G^Y value of the peer. This must be a readable buffer of size ilen Bytes.
ilen	The size of the input buffer input in Bytes.

Returns

0 on success.

An MBEDTLS_ERR_DHM_XXX error code on failure.

int mbedtls_dhm_self_test

(

int

verbose

)

The DMH checkup routine.

Returns

0 on success.

1 on failure.

int mbedtls_dhm_set_group	(	mbedtls_dhm_context *	ctx,
		const mbedtls_mpi *	P,
		const mbedtls_mpi *	G
	)

This function sets the prime modulus and generator.

Note

This function can be used to set ctx->P, ctx->G in preparation for mbedtls_dhm_make_params().

Parameters

ctx	The DHM context to configure. This must be initialized.
P	The MPI holding the DHM prime modulus. This must be an initialized MPI.
G	The MPI holding the DHM generator. This must be an initialized MPI.

Returns

0 if successful.

An MBEDTLS_ERR_DHM_XXX error code on failure.