EVP_MD_CTX_clear_flags
EVP_MD_CTX_clear_flags(ctx, EVP_MD_CTX_FLAG_CLEANED | EVP_MD_CTX_FLAG_FINALISED);
EVP_MD_CTX_clear_flags(out, EVP_MD_CTX_FLAG_KEEP_PKEY_CTX);
EVP_MD_CTX_clear_flags(out, EVP_MD_CTX_FLAG_KEEP_PKEY_CTX);
EVP_MD_CTX_clear_flags(ctx, EVP_MD_CTX_FLAG_KEEP_PKEY_CTX);
EVP_MD_CTX_clear_flags(ctx, EVP_MD_CTX_FLAG_FINALISED);
void EVP_MD_CTX_clear_flags(EVP_MD_CTX *ctx, int flags);