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Catch AES failure in mbedtls_ctr_drbg_random

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The functions mbedtls_ctr_drbg_random() and
mbedtls_ctr_drbg_random_with_add() could return 0 if an AES function
failed. This could only happen with alternative AES
implementations (the built-in implementation of the AES functions
involved never fail), typically due to a failure in a hardware
accelerator.

Bug reported and fix proposed by Johan Uppman Bruce and Christoffer
Lauri, Sectra.
This commit is contained in:
Gilles Peskine 2019-11-28 09:45:32 +01:00
parent f0486052b7
commit 86dd9501ba
2 changed files with 5 additions and 1 deletions
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4
ChangeLog
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@ -20,6 +20,10 @@ Security
timings on the comparison in the key generation enabled the attacker to timings on the comparison in the key generation enabled the attacker to
learn leading bits of the ephemeral key used during ECDSA signatures and to learn leading bits of the ephemeral key used during ECDSA signatures and to
recover the private key. Reported by Jeremy Dubeuf. recover the private key. Reported by Jeremy Dubeuf.
* Catch failure of AES functions in mbedtls_ctr_drbg_random(). Uncaught
failures could happen with alternative implementations of AES. Bug
reported and fix proposed by Johan Uppman Bruce and Christoffer Lauri,
Sectra.
Bugfix Bugfix
* Remove redundant line for getting the bitlen of a bignum, since the variable * Remove redundant line for getting the bitlen of a bignum, since the variable

2
library/ctr_drbg.c
View File

@ -517,7 +517,7 @@ int mbedtls_ctr_drbg_random_with_add( void *p_rng,
exit: exit:
mbedtls_platform_zeroize( add_input, sizeof( add_input ) ); mbedtls_platform_zeroize( add_input, sizeof( add_input ) );
mbedtls_platform_zeroize( tmp, sizeof( tmp ) ); mbedtls_platform_zeroize( tmp, sizeof( tmp ) );
return( 0 ); return( ret );
} }
int mbedtls_ctr_drbg_random( void *p_rng, unsigned char *output, size_t output_len ) int mbedtls_ctr_drbg_random( void *p_rng, unsigned char *output, size_t output_len )