File dhm.h

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.

Warning

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.

Warning

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.

Defines

MBEDTLS_ERR_DHM_BAD_INPUT_DATA: Bad input parameters.

MBEDTLS_ERR_DHM_READ_PARAMS_FAILED: Reading of the DHM parameters failed.

MBEDTLS_ERR_DHM_MAKE_PARAMS_FAILED: Making of the DHM parameters failed.

MBEDTLS_ERR_DHM_READ_PUBLIC_FAILED: Reading of the public values failed.

MBEDTLS_ERR_DHM_MAKE_PUBLIC_FAILED: Making of the public value failed.

MBEDTLS_ERR_DHM_CALC_SECRET_FAILED: Calculation of the DHM secret failed.

MBEDTLS_ERR_DHM_INVALID_FORMAT: The ASN.1 data is not formatted correctly.

MBEDTLS_ERR_DHM_ALLOC_FAILED: Allocation of memory failed.

MBEDTLS_ERR_DHM_FILE_IO_ERROR: Read or write of file failed.

MBEDTLS_ERR_DHM_SET_GROUP_FAILED: Setting the modulus and generator failed.

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).

MBEDTLS_DHM_RFC3526_MODP_2048_G_BIN

MBEDTLS_DHM_RFC3526_MODP_3072_P_BIN

MBEDTLS_DHM_RFC3526_MODP_3072_G_BIN

MBEDTLS_DHM_RFC3526_MODP_4096_P_BIN

MBEDTLS_DHM_RFC3526_MODP_4096_G_BIN

MBEDTLS_DHM_RFC7919_FFDHE2048_P_BIN

MBEDTLS_DHM_RFC7919_FFDHE2048_G_BIN

MBEDTLS_DHM_RFC7919_FFDHE3072_P_BIN

MBEDTLS_DHM_RFC7919_FFDHE3072_G_BIN

MBEDTLS_DHM_RFC7919_FFDHE4096_P_BIN

MBEDTLS_DHM_RFC7919_FFDHE4096_G_BIN

MBEDTLS_DHM_RFC7919_FFDHE6144_P_BIN

MBEDTLS_DHM_RFC7919_FFDHE6144_G_BIN

MBEDTLS_DHM_RFC7919_FFDHE8192_P_BIN

MBEDTLS_DHM_RFC7919_FFDHE8192_G_BIN

Enums

enum mbedtls_dhm_parameter

Which parameter to access in mbedtls_dhm_get_value().

Values:

enumerator MBEDTLS_DHM_PARAM_P: The prime modulus.

enumerator MBEDTLS_DHM_PARAM_G: The generator.

enumerator MBEDTLS_DHM_PARAM_X: Our secret value.

enumerator MBEDTLS_DHM_PARAM_GX: Our public key = G^X mod P.

enumerator MBEDTLS_DHM_PARAM_GY: The public key of the peer = G^Y mod P.

enumerator MBEDTLS_DHM_PARAM_K: The shared secret = G^(XY) mod P.

Functions

void mbedtls_dhm_init(mbedtls_dhm_context *ctx)

This function initializes the DHM context.

Parameters: ctx – The DHM context to initialize.

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.

Returns

An MBEDTLS_ERR_DHM_XXX error code on failure.

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.

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().

Note

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.

Returns

An MBEDTLS_ERR_DHM_XXX error code 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.

Returns

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.

Returns

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 (*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.

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.

Returns

An MBEDTLS_ERR_DHM_XXX error code on failure.

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.

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.

Returns

An MBEDTLS_ERR_DHM_XXX error code on failure.

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.

Returns

MBEDTLS_ERR_DHM_BAD_INPUT_DATA if param is invalid.

Returns

An MBEDTLS_ERR_MPI_XXX error code if the copy fails.

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.

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.

Returns

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.

Returns

An MBEDTLS_ERR_DHM_XXX or MBEDTLS_ERR_PEM_XXX error code on failure.

int mbedtls_dhm_self_test(int verbose)

The DMH checkup routine.

Returns: 0 on success.
Returns: 1 on failure.