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Parse RSA parameters DP, DQ and QP from PKCS1 private keys #352

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merged 4 commits into from
Jan 31, 2020
Merged

Parse RSA parameters DP, DQ and QP from PKCS1 private keys #352

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80cc811
Parse RSA parameters DP, DQ and QP from PKCS1 private keys
jack-fortanix Jan 22, 2020
8c2631b
Address review comments
jack-fortanix Jan 23, 2020
6023975
Avoid memory leak when RSA-CRT is not enabled in build
jack-fortanix Jan 27, 2020
2e9eef4
Final review comments
jack-fortanix Jan 28, 2020
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40 changes: 37 additions & 3 deletions library/pkparse.c
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Original file line number Diff line number Diff line change
Expand Up @@ -769,14 +769,48 @@ static int pk_parse_key_pkcs1_der( mbedtls_rsa_context *rsa,
goto cleanup;
p += len;

/* Complete the RSA private key */
if( ( ret = mbedtls_rsa_complete( rsa ) ) != 0 )
#if !defined(MBEDTLS_RSA_NO_CRT)
/*
* The RSA CRT parameters DP, DQ and QP are nominally redundant, in
* that they can be easily recomputed from D, P and Q. However by
* parsing them from the PKCS1 structure it is possible to avoid
* recalculating them which both reduces the overhead of loading
* RSA private keys into memory and also avoids side channels which
* can arise when computing those values, since all of D, P, and Q
* are secret. See https://eprint.iacr.org/2020/055 for a
* description of one such attack.
*/

/* Import DP */
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Could we please add a comment here as a reminder for the future to ensure that nobody "optimises" this code out?

Basically explaining that although we could compute these values, we choose not to because this way it is faster and reduces the attack surface exposed to side channel attacks.

if( ( ret = mbedtls_asn1_get_tag( &p, end, &len,
MBEDTLS_ASN1_INTEGER ) ) != 0 ||
( ret = mbedtls_mpi_read_binary( &rsa->DP, p, len ) ) != 0 )
goto cleanup;
p += len;
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/* Import DQ */
if( ( ret = mbedtls_asn1_get_tag( &p, end, &len,
MBEDTLS_ASN1_INTEGER ) ) != 0 ||
( ret = mbedtls_mpi_read_binary( &rsa->DQ, p, len ) ) != 0 )
goto cleanup;
p += len;

/* Check optional parameters */
/* Import QP */
if( ( ret = mbedtls_asn1_get_tag( &p, end, &len,
MBEDTLS_ASN1_INTEGER ) ) != 0 ||
( ret = mbedtls_mpi_read_binary( &rsa->QP, p, len ) ) != 0 )
goto cleanup;
p += len;
#else
/* Verify existance of the CRT params */
if( ( ret = mbedtls_asn1_get_mpi( &p, end, &T ) ) != 0 ||
( ret = mbedtls_asn1_get_mpi( &p, end, &T ) ) != 0 ||
( ret = mbedtls_asn1_get_mpi( &p, end, &T ) ) != 0 )
goto cleanup;
#endif

/* Complete the RSA private key */
if( ( ret = mbedtls_rsa_complete( rsa ) ) != 0 )
goto cleanup;

if( p != end )
Expand Down
11 changes: 10 additions & 1 deletion library/rsa.c
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Original file line number Diff line number Diff line change
Expand Up @@ -250,6 +250,9 @@ int mbedtls_rsa_complete( mbedtls_rsa_context *ctx )
{
int ret = 0;
int have_N, have_P, have_Q, have_D, have_E;
#if !defined(MBEDTLS_RSA_NO_CRT)
int have_DP, have_DQ, have_QP;
#endif
int n_missing, pq_missing, d_missing, is_pub, is_priv;

RSA_VALIDATE_RET( ctx != NULL );
Expand All @@ -260,6 +263,12 @@ int mbedtls_rsa_complete( mbedtls_rsa_context *ctx )
have_D = ( mbedtls_mpi_cmp_int( &ctx->D, 0 ) != 0 );
have_E = ( mbedtls_mpi_cmp_int( &ctx->E, 0 ) != 0 );

#if !defined(MBEDTLS_RSA_NO_CRT)
have_DP = ( mbedtls_mpi_cmp_int( &ctx->DP, 0 ) != 0 );
have_DQ = ( mbedtls_mpi_cmp_int( &ctx->DQ, 0 ) != 0 );
have_QP = ( mbedtls_mpi_cmp_int( &ctx->QP, 0 ) != 0 );
#endif

/*
* Check whether provided parameters are enough
* to deduce all others. The following incomplete
Expand Down Expand Up @@ -325,7 +334,7 @@ int mbedtls_rsa_complete( mbedtls_rsa_context *ctx )
*/

#if !defined(MBEDTLS_RSA_NO_CRT)
if( is_priv )
if( is_priv && !(have_DP && have_DQ && have_QP))
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{
ret = mbedtls_rsa_deduce_crt( &ctx->P, &ctx->Q, &ctx->D,
&ctx->DP, &ctx->DQ, &ctx->QP );
Expand Down