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ctr_drbg.h File Reference

The CTR_DRBG pseudorandom generator. More...

#include "config.h"
#include "aes.h"
Include dependency graph for ctr_drbg.h:

Go to the source code of this file.

Classes

struct  mbedtls_ctr_drbg_context
 The CTR_DRBG context structure. More...
 

Macros

#define MBEDTLS_ERR_CTR_DRBG_ENTROPY_SOURCE_FAILED   -0x0034
 
#define MBEDTLS_ERR_CTR_DRBG_REQUEST_TOO_BIG   -0x0036
 
#define MBEDTLS_ERR_CTR_DRBG_INPUT_TOO_BIG   -0x0038
 
#define MBEDTLS_ERR_CTR_DRBG_FILE_IO_ERROR   -0x003A
 
#define MBEDTLS_CTR_DRBG_BLOCKSIZE   16
 
#define MBEDTLS_CTR_DRBG_KEYSIZE   32
 
#define MBEDTLS_CTR_DRBG_KEYBITS   ( MBEDTLS_CTR_DRBG_KEYSIZE * 8 )
 
#define MBEDTLS_CTR_DRBG_SEEDLEN   ( MBEDTLS_CTR_DRBG_KEYSIZE + MBEDTLS_CTR_DRBG_BLOCKSIZE )
 
#define MBEDTLS_CTR_DRBG_PR_OFF   0
 
#define MBEDTLS_CTR_DRBG_PR_ON   1
 
SECTION: Module settings

The configuration options you can set for this module are in this section. Either change them in config.h or define them using the compiler command line.

#define MBEDTLS_CTR_DRBG_ENTROPY_LEN   32
 The amount of entropy used per seed by default, in bytes. More...
 
#define MBEDTLS_CTR_DRBG_RESEED_INTERVAL   10000
 
#define MBEDTLS_CTR_DRBG_MAX_INPUT   256
 
#define MBEDTLS_CTR_DRBG_MAX_REQUEST   1024
 
#define MBEDTLS_CTR_DRBG_MAX_SEED_INPUT   384
 

Functions

void mbedtls_ctr_drbg_init (mbedtls_ctr_drbg_context *ctx)
 This function initializes the CTR_DRBG context, and prepares it for mbedtls_ctr_drbg_seed() or mbedtls_ctr_drbg_free(). More...
 
int mbedtls_ctr_drbg_seed (mbedtls_ctr_drbg_context *ctx, int(*f_entropy)(void *, unsigned char *, size_t), void *p_entropy, const unsigned char *custom, size_t len)
 This function seeds and sets up the CTR_DRBG entropy source for future reseeds. More...
 
void mbedtls_ctr_drbg_free (mbedtls_ctr_drbg_context *ctx)
 This function clears CTR_CRBG context data. More...
 
void mbedtls_ctr_drbg_set_prediction_resistance (mbedtls_ctr_drbg_context *ctx, int resistance)
 This function turns prediction resistance on or off. The default value is off. More...
 
void mbedtls_ctr_drbg_set_entropy_len (mbedtls_ctr_drbg_context *ctx, size_t len)
 This function sets the amount of entropy grabbed on each seed or reseed. More...
 
void mbedtls_ctr_drbg_set_reseed_interval (mbedtls_ctr_drbg_context *ctx, int interval)
 This function sets the reseed interval. More...
 
int mbedtls_ctr_drbg_reseed (mbedtls_ctr_drbg_context *ctx, const unsigned char *additional, size_t len)
 This function reseeds the CTR_DRBG context, that is extracts data from the entropy source. More...
 
int mbedtls_ctr_drbg_update_ret (mbedtls_ctr_drbg_context *ctx, const unsigned char *additional, size_t add_len)
 This function updates the state of the CTR_DRBG context. More...
 
void mbedtls_ctr_drbg_update (mbedtls_ctr_drbg_context *ctx, const unsigned char *additional, size_t add_len)
 This function updates the state of the CTR_DRBG context. More...
 
int mbedtls_ctr_drbg_random_with_add (void *p_rng, unsigned char *output, size_t output_len, const unsigned char *additional, size_t add_len)
 This function updates a CTR_DRBG instance with additional data and uses it to generate random data. More...
 
int mbedtls_ctr_drbg_random (void *p_rng, unsigned char *output, size_t output_len)
 This function uses CTR_DRBG to generate random data. More...
 
int mbedtls_ctr_drbg_self_test (int verbose)
 The CTR_DRBG checkup routine. More...
 
int mbedtls_ctr_drbg_seed_entropy_len (mbedtls_ctr_drbg_context *, int(*)(void *, unsigned char *, size_t), void *, const unsigned char *, size_t, size_t)
 

Detailed Description

The CTR_DRBG pseudorandom generator.

CTR_DRBG is a standardized way of building a PRNG from a block-cipher in counter mode operation, as defined in NIST SP 800-90A: Recommendation for Random Number Generation Using Deterministic Random Bit Generators.

The Mbed TLS implementation of CTR_DRBG uses AES-256 as the underlying block cipher, with a derivation function. The initial seeding grabs MBEDTLS_CTR_DRBG_ENTROPY_LEN bytes of entropy. See the documentation of mbedtls_ctr_drbg_seed() for more details.

Based on NIST SP 800-90A §10.2.1 table 3 and NIST SP 800-57 part 1 table 2, here are the security strengths achieved in typical configuration:

Note that the value of MBEDTLS_CTR_DRBG_ENTROPY_LEN defaults to:

  • 48 if the module MBEDTLS_SHA512_C is enabled and the symbol MBEDTLS_ENTROPY_FORCE_SHA256 is not enabled at compile time. This is the default configuration of the library.
  • 32 if the module MBEDTLS_SHA512_C is disabled at compile time.
  • 32 if MBEDTLS_ENTROPY_FORCE_SHA256 is enabled at compile time.

Definition in file ctr_drbg.h.

Macro Definition Documentation

◆ MBEDTLS_CTR_DRBG_BLOCKSIZE

#define MBEDTLS_CTR_DRBG_BLOCKSIZE   16

The block size used by the cipher.

Definition at line 100 of file ctr_drbg.h.

◆ MBEDTLS_CTR_DRBG_ENTROPY_LEN

#define MBEDTLS_CTR_DRBG_ENTROPY_LEN   32

The amount of entropy used per seed by default, in bytes.

This is 32 bytes because the entropy module uses SHA-256 (the SHA512 module is disabled or MBEDTLS_ENTROPY_FORCE_SHA256 is enabled).

Warning
To achieve a 256-bit security strength, you must pass a nonce to mbedtls_ctr_drbg_seed().

Definition at line 134 of file ctr_drbg.h.

◆ MBEDTLS_CTR_DRBG_KEYBITS

#define MBEDTLS_CTR_DRBG_KEYBITS   ( MBEDTLS_CTR_DRBG_KEYSIZE * 8 )

The key size for the DRBG operation, in bits.

Definition at line 102 of file ctr_drbg.h.

◆ MBEDTLS_CTR_DRBG_KEYSIZE

#define MBEDTLS_CTR_DRBG_KEYSIZE   32

The key size used by the cipher.

Definition at line 101 of file ctr_drbg.h.

◆ MBEDTLS_CTR_DRBG_MAX_INPUT

#define MBEDTLS_CTR_DRBG_MAX_INPUT   256

The maximum number of additional input Bytes.

Definition at line 144 of file ctr_drbg.h.

◆ MBEDTLS_CTR_DRBG_MAX_REQUEST

#define MBEDTLS_CTR_DRBG_MAX_REQUEST   1024

The maximum number of requested Bytes per call.

Definition at line 149 of file ctr_drbg.h.

◆ MBEDTLS_CTR_DRBG_MAX_SEED_INPUT

#define MBEDTLS_CTR_DRBG_MAX_SEED_INPUT   384

The maximum size of seed or reseed buffer in bytes.

Definition at line 154 of file ctr_drbg.h.

◆ MBEDTLS_CTR_DRBG_PR_OFF

#define MBEDTLS_CTR_DRBG_PR_OFF   0

Prediction resistance is disabled.

Definition at line 160 of file ctr_drbg.h.

◆ MBEDTLS_CTR_DRBG_PR_ON

#define MBEDTLS_CTR_DRBG_PR_ON   1

Prediction resistance is enabled.

Definition at line 162 of file ctr_drbg.h.

◆ MBEDTLS_CTR_DRBG_RESEED_INTERVAL

#define MBEDTLS_CTR_DRBG_RESEED_INTERVAL   10000

The interval before reseed is performed by default.

Definition at line 139 of file ctr_drbg.h.

◆ MBEDTLS_CTR_DRBG_SEEDLEN

#define MBEDTLS_CTR_DRBG_SEEDLEN   ( MBEDTLS_CTR_DRBG_KEYSIZE + MBEDTLS_CTR_DRBG_BLOCKSIZE )

The seed length, calculated as (counter + AES key).

Definition at line 103 of file ctr_drbg.h.

◆ MBEDTLS_ERR_CTR_DRBG_ENTROPY_SOURCE_FAILED

#define MBEDTLS_ERR_CTR_DRBG_ENTROPY_SOURCE_FAILED   -0x0034

The entropy source failed.

Definition at line 95 of file ctr_drbg.h.

◆ MBEDTLS_ERR_CTR_DRBG_FILE_IO_ERROR

#define MBEDTLS_ERR_CTR_DRBG_FILE_IO_ERROR   -0x003A

Read or write error in file.

Definition at line 98 of file ctr_drbg.h.

◆ MBEDTLS_ERR_CTR_DRBG_INPUT_TOO_BIG

#define MBEDTLS_ERR_CTR_DRBG_INPUT_TOO_BIG   -0x0038

The input (entropy + additional data) is too large.

Definition at line 97 of file ctr_drbg.h.

◆ MBEDTLS_ERR_CTR_DRBG_REQUEST_TOO_BIG

#define MBEDTLS_ERR_CTR_DRBG_REQUEST_TOO_BIG   -0x0036

The requested random buffer length is too big.

Definition at line 96 of file ctr_drbg.h.

Function Documentation

◆ mbedtls_ctr_drbg_free()

void mbedtls_ctr_drbg_free ( mbedtls_ctr_drbg_context ctx)

This function clears CTR_CRBG context data.

Parameters
ctxThe CTR_DRBG context to clear.

◆ mbedtls_ctr_drbg_init()

void mbedtls_ctr_drbg_init ( mbedtls_ctr_drbg_context ctx)

This function initializes the CTR_DRBG context, and prepares it for mbedtls_ctr_drbg_seed() or mbedtls_ctr_drbg_free().

Parameters
ctxThe CTR_DRBG context to initialize.

◆ mbedtls_ctr_drbg_random()

int mbedtls_ctr_drbg_random ( void p_rng,
unsigned char output,
size_t  output_len 
)

This function uses CTR_DRBG to generate random data.

This function automatically reseeds if the reseed counter is exceeded or prediction resistance is enabled.

Parameters
p_rngThe CTR_DRBG context. This must be a pointer to a mbedtls_ctr_drbg_context structure.
outputThe buffer to fill.
output_lenThe length of the buffer in bytes.
Returns
0 on success.
MBEDTLS_ERR_CTR_DRBG_ENTROPY_SOURCE_FAILED or MBEDTLS_ERR_CTR_DRBG_REQUEST_TOO_BIG on failure.

◆ mbedtls_ctr_drbg_random_with_add()

int mbedtls_ctr_drbg_random_with_add ( void p_rng,
unsigned char output,
size_t  output_len,
const unsigned char additional,
size_t  add_len 
)

This function updates a CTR_DRBG instance with additional data and uses it to generate random data.

This function automatically reseeds if the reseed counter is exceeded or prediction resistance is enabled.

Parameters
p_rngThe CTR_DRBG context. This must be a pointer to a mbedtls_ctr_drbg_context structure.
outputThe buffer to fill.
output_lenThe length of the buffer in bytes.
additionalAdditional data to update. Can be NULL, in which case the additional data is empty regardless of the value of add_len.
add_lenThe length of the additional data if additional is not NULL. This must be less than MBEDTLS_CTR_DRBG_MAX_INPUT and less than MBEDTLS_CTR_DRBG_MAX_SEED_INPUT - entropy_len where entropy_len is the entropy length configured for the context.
Returns
0 on success.
MBEDTLS_ERR_CTR_DRBG_ENTROPY_SOURCE_FAILED or MBEDTLS_ERR_CTR_DRBG_REQUEST_TOO_BIG on failure.

◆ mbedtls_ctr_drbg_reseed()

int mbedtls_ctr_drbg_reseed ( mbedtls_ctr_drbg_context ctx,
const unsigned char additional,
size_t  len 
)

This function reseeds the CTR_DRBG context, that is extracts data from the entropy source.

Parameters
ctxThe CTR_DRBG context.
additionalAdditional data to add to the state. Can be NULL.
lenThe length of the additional data. This must be less than MBEDTLS_CTR_DRBG_MAX_SEED_INPUT - entropy_len where entropy_len is the entropy length configured for the context.
Returns
0 on success, or MBEDTLS_ERR_CTR_DRBG_ENTROPY_SOURCE_FAILED on failure.

◆ mbedtls_ctr_drbg_seed()

int mbedtls_ctr_drbg_seed ( mbedtls_ctr_drbg_context ctx,
int(*)(void *, unsigned char *, size_t f_entropy,
void p_entropy,
const unsigned char custom,
size_t  len 
)

This function seeds and sets up the CTR_DRBG entropy source for future reseeds.

A typical choice for the f_entropy and p_entropy parameters is to use the entropy module:

The entropy length is MBEDTLS_CTR_DRBG_ENTROPY_LEN by default. You can override it by calling mbedtls_ctr_drbg_set_entropy_len().

You can provide a personalization string in addition to the entropy source, to make this instantiation as unique as possible.

Note
The seed_material value passed to the derivation function in the CTR_DRBG Instantiate Process described in NIST SP 800-90A §10.2.1.3.2 is the concatenation of the string obtained from calling f_entropy and the custom string. The origin of the nonce depends on the value of the entropy length relative to the security strength.
  • If the entropy length is at least 1.5 times the security strength then the nonce is taken from the string obtained with f_entropy.
  • If the entropy length is less than the security strength, then the nonce is taken from custom. In this case, for compliance with SP 800-90A, you must pass a unique value of custom at each invocation. See SP 800-90A §8.6.7 for more details.
Warning
When MBEDTLS_CTR_DRBG_ENTROPY_LEN is less than 48, to achieve a 256-bit security strength, you must pass a value of custom that is a nonce: this value must never be repeated in subsequent runs of the same application or on a different device.
Parameters
ctxThe CTR_DRBG context to seed. It must have been initialized with mbedtls_ctr_drbg_init(). After a successful call to mbedtls_ctr_drbg_seed(), you may not call mbedtls_ctr_drbg_seed() again on the same context unless you call mbedtls_ctr_drbg_free() and mbedtls_ctr_drbg_init() again first.
f_entropyThe entropy callback, taking as arguments the p_entropy context, the buffer to fill, and the length of the buffer. f_entropy is always called with a buffer size equal to the entropy length.
p_entropyThe entropy context to pass to f_entropy.
customThe personalization string. This can be NULL, in which case the personalization string is empty regardless of the value of len.
lenThe length of the personalization string. This must be at most MBEDTLS_CTR_DRBG_MAX_SEED_INPUT
Returns
0 on success, or MBEDTLS_ERR_CTR_DRBG_ENTROPY_SOURCE_FAILED on failure.

Referenced by schan_imp_init().

◆ mbedtls_ctr_drbg_seed_entropy_len()

int mbedtls_ctr_drbg_seed_entropy_len ( mbedtls_ctr_drbg_context ,
int(*)(void *, unsigned char *, size_t ,
void ,
const unsigned char ,
size_t  ,
size_t   
)

◆ mbedtls_ctr_drbg_self_test()

int mbedtls_ctr_drbg_self_test ( int  verbose)

The CTR_DRBG checkup routine.

Returns
0 on success, or 1 on failure.

◆ mbedtls_ctr_drbg_set_entropy_len()

void mbedtls_ctr_drbg_set_entropy_len ( mbedtls_ctr_drbg_context ctx,
size_t  len 
)

This function sets the amount of entropy grabbed on each seed or reseed.

The default value is MBEDTLS_CTR_DRBG_ENTROPY_LEN.

Note
The security strength of CTR_DRBG is bounded by the entropy length. Thus len must be at least 32 (in bytes) to achieve a 256-bit strength.
Parameters
ctxThe CTR_DRBG context.
lenThe amount of entropy to grab, in bytes. This must be at most MBEDTLS_CTR_DRBG_MAX_SEED_INPUT.

◆ mbedtls_ctr_drbg_set_prediction_resistance()

void mbedtls_ctr_drbg_set_prediction_resistance ( mbedtls_ctr_drbg_context ctx,
int  resistance 
)

This function turns prediction resistance on or off. The default value is off.

Note
If enabled, entropy is gathered at the beginning of every call to mbedtls_ctr_drbg_random_with_add() or mbedtls_ctr_drbg_random(). Only use this if your entropy source has sufficient throughput.
Parameters
ctxThe CTR_DRBG context.
resistanceMBEDTLS_CTR_DRBG_PR_ON or MBEDTLS_CTR_DRBG_PR_OFF.

◆ mbedtls_ctr_drbg_set_reseed_interval()

void mbedtls_ctr_drbg_set_reseed_interval ( mbedtls_ctr_drbg_context ctx,
int  interval 
)

This function sets the reseed interval.

The reseed interval is the number of calls to mbedtls_ctr_drbg_random() or mbedtls_ctr_drbg_random_with_add() after which the entropy function is called again.

The default value is MBEDTLS_CTR_DRBG_RESEED_INTERVAL.

Parameters
ctxThe CTR_DRBG context.
intervalThe reseed interval.

◆ mbedtls_ctr_drbg_update()

void mbedtls_ctr_drbg_update ( mbedtls_ctr_drbg_context ctx,
const unsigned char additional,
size_t  add_len 
)

This function updates the state of the CTR_DRBG context.

Warning
This function cannot report errors. You should use mbedtls_ctr_drbg_update_ret() instead.
Note
If add_len is greater than MBEDTLS_CTR_DRBG_MAX_SEED_INPUT, only the first MBEDTLS_CTR_DRBG_MAX_SEED_INPUT Bytes are used. The remaining Bytes are silently discarded.
Parameters
ctxThe CTR_DRBG context.
additionalThe data to update the state with. This must not be NULL unless add_len is 0.
add_lenLength of additional data. This must be at most MBEDTLS_CTR_DRBG_MAX_SEED_INPUT.

◆ mbedtls_ctr_drbg_update_ret()

int mbedtls_ctr_drbg_update_ret ( mbedtls_ctr_drbg_context ctx,
const unsigned char additional,
size_t  add_len 
)

This function updates the state of the CTR_DRBG context.

Parameters
ctxThe CTR_DRBG context.
additionalThe data to update the state with. This must not be NULL unless add_len is 0.
add_lenLength of additional in bytes. This must be at most MBEDTLS_CTR_DRBG_MAX_SEED_INPUT.
Returns
0 on success.
MBEDTLS_ERR_CTR_DRBG_INPUT_TOO_BIG if add_len is more than MBEDTLS_CTR_DRBG_MAX_SEED_INPUT.
An error from the underlying AES cipher on failure.