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Use large Hamming distance for signature validity

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If signature_is_good is 0 (invalid) of 1 (valid), then it's all too easy for
an active physical attacker to turn invalid into valid by flipping a single
bit in RAM, on the bus or in a CPU register.

Use a special value to represent "valid" that can't easily be reached by
flipping a few bits.
This commit is contained in:
Manuel Pégourié-Gonnard 2019-11-08 11:14:09 +01:00
parent d1e55dfce6
commit f66657ac44
1 changed files with 8 additions and 3 deletions
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11
library/x509_crt.c
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@ -2884,6 +2884,10 @@ static int x509_crt_check_parent( const mbedtls_x509_crt_sig_info *sig_info,
return( 0 ); return( 0 );
} }
/* This value is different enough from 0 that it's hard for an active physical
* attacker to reach it just by flipping a few bits. */
#define X509_SIGNATURE_IS_GOOD 0x7f5a5a5a
/* /*
* Find a suitable parent for child in candidates, or return NULL. * Find a suitable parent for child in candidates, or return NULL.
* *
@ -2915,7 +2919,8 @@ static int x509_crt_check_parent( const mbedtls_x509_crt_sig_info *sig_info,
* - [in] child: certificate for which we're looking for a parent * - [in] child: certificate for which we're looking for a parent
* - [in] candidates: chained list of potential parents * - [in] candidates: chained list of potential parents
* - [out] r_parent: parent found (or NULL) * - [out] r_parent: parent found (or NULL)
* - [out] r_signature_is_good: 1 if child signature by parent is valid, or 0 * - [out] r_signature_is_good: set to X509_SIGNATURE_IS_GOOD if
* child signature by parent is valid, or to 0
* - [in] top: 1 if candidates consists of trusted roots, ie we're at the top * - [in] top: 1 if candidates consists of trusted roots, ie we're at the top
* of the chain, 0 otherwise * of the chain, 0 otherwise
* - [in] path_cnt: number of intermediates seen so far * - [in] path_cnt: number of intermediates seen so far
@ -3040,7 +3045,7 @@ check_signature:
{ {
mbedtls_platform_enforce_volatile_reads(); mbedtls_platform_enforce_volatile_reads();
if( ret_fi == 0 ) if( ret_fi == 0 )
signature_is_good = 1; signature_is_good = X509_SIGNATURE_IS_GOOD;
} }
if( top && ! signature_is_good ) if( top && ! signature_is_good )
@ -3522,7 +3527,7 @@ find_parent:
} }
/* signature was checked while searching parent */ /* signature was checked while searching parent */
if( ! signature_is_good ) if( signature_is_good != X509_SIGNATURE_IS_GOOD )
*flags |= MBEDTLS_X509_BADCERT_NOT_TRUSTED; *flags |= MBEDTLS_X509_BADCERT_NOT_TRUSTED;
{ {