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RSA Decryption: check private value after decryption #7167

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Jan 29, 2024
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RSA Decryption: check private value after decryption #7167

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11 changes: 11 additions & 0 deletions wolfcrypt/src/rsa.c
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Original file line number Diff line number Diff line change
Expand Up @@ -2723,6 +2723,17 @@ static int RsaFunctionSync(const byte* in, word32 inLen, byte* out,
if (mp_to_unsigned_bin_len_ct(tmp, out, (int)*outLen) != MP_OKAY)
ret = MP_TO_E;
}
#ifdef WOLFSSL_RSA_CHECK_D_ON_DECRYPT
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Is this something that should be on be default and optionally turned off rather than off be default and optionally turned on?

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Would it make sense to enable with WOLFSSL_CHECK_SIG_FAULTS?

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WOLFSSL_CHECK_SIG_FAULTS is currently for signature and ECC.
This code is only for RSA decryption and not signing.
If there was a better generic 'check data after op' define, I would consider enabling it as part of that.

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@SparkiDev should this be on by default and have a different macro name to disable it?

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There are very limited circumstances that would require the checking of d.
It is unnecessary work that we should avoided if possible.

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Let's merge as-is then document clearly those reasons

if ((ret == 0) && (type == RSA_PRIVATE_DECRYPT)) {
mp_sub(&key->n, &key->p, tmp);
mp_sub(tmp, &key->q, tmp);
mp_add_d(tmp, 1, tmp);
mp_mulmod(&key->d, &key->e, tmp, tmp);
if (!mp_isone(tmp)) {
ret = MP_EXPTMOD_E;
}
}
#endif
#else
(void)type;
(void)key;
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