SCA hardening for mod. field inversion in EC_GROUP
This commit adds a dedicated function in `EC_METHOD` to access a modular field inversion implementation suitable for the specifics of the implemented curve, featuring SCA countermeasures. The new pointer is defined as: `int (*field_inv)(const EC_GROUP*, BIGNUM *r, const BIGNUM *a, BN_CTX*)` and computes the multiplicative inverse of `a` in the underlying field, storing the result in `r`. Three implementations are included, each including specific SCA countermeasures: - `ec_GFp_simple_field_inv()`, featuring SCA hardening through blinding. - `ec_GFp_mont_field_inv()`, featuring SCA hardening through Fermat's Little Theorem (FLT) inversion. - `ec_GF2m_simple_field_inv()`, that uses `BN_GF2m_mod_inv()` which already features SCA hardening through blinding. From a security point of view, this also helps addressing a leakage previously affecting conversions from projective to affine coordinates. This commit also adds a new error reason code (i.e., `EC_R_CANNOT_INVERT`) to improve consistency between the three implementations as all of them could fail for the same reason but through different code paths resulting in inconsistent error stack states. Co-authored-by:Nicola Tuveri <nic.tuv@gmail.com> (cherry picked from commit e0033efc ) Reviewed-by:
Matt Caswell <matt@openssl.org> Reviewed-by:
Nicola Tuveri <nic.tuv@gmail.com> (Merged from https://github.com/openssl/openssl/pull/8263)
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