Expose OpenSSL constant time bignum arithmetic (pyca#4200)

* Expose BIGNUM constant time operations

This commit exposes the following functions:
BN_set_flags
BN_get_flags

BN_MONT_CTX_new
BN_MONT_CTX_set
BN_MONT_CTX_free

BN_mod_exp_mont
BN_mod_exp_mont_consttime

This commit also exposes the BN_FLG_CONSTTIME flag.

* Add myself to AUTHORS

Author: tuxxy

AUTHORS.rst

@@ -40,3 +40,4 @@ PGP key fingerprints are enclosed in parentheses.
 * Chris Wolfe <[email protected]>
 * Jeremy Lainé <[email protected]>
 * Denis Gladkikh <[email protected]>
+* John Pacific <[email protected]> (2CF6 0381 B5EF 29B7 D48C 2020 7BB9 71A0 E891 44D9)

src/_cffi_src/openssl/bignum.py

@@ -10,11 +10,17 @@
 
 TYPES = """
 typedef ... BN_CTX;
+typedef ... BN_MONT_CTX;
 typedef ... BIGNUM;
 typedef int... BN_ULONG;
 """
 
 FUNCTIONS = """
+#define BN_FLG_CONSTTIME ...
+
+void BN_set_flags(BIGNUM *, int);
+int BN_get_flags(const BIGNUM *, int);
+
 BIGNUM *BN_new(void);
 void BN_free(BIGNUM *);
 void BN_clear_free(BIGNUM *);
@@ -29,6 +35,10 @@
 BIGNUM *BN_CTX_get(BN_CTX *);
 void BN_CTX_end(BN_CTX *);
 
+BN_MONT_CTX *BN_MONT_CTX_new(void);
+int BN_MONT_CTX_set(BN_MONT_CTX *, BIGNUM *, BN_CTX *);
+void BN_MONT_CTX_free(BN_MONT_CTX *);
+
 BIGNUM *BN_copy(BIGNUM *, const BIGNUM *);
 BIGNUM *BN_dup(const BIGNUM *);
 
@@ -63,6 +73,10 @@
 int BN_exp(BIGNUM *, const BIGNUM *, const BIGNUM *, BN_CTX *);
 int BN_mod_exp(BIGNUM *, const BIGNUM *, const BIGNUM *, const BIGNUM *,
                BN_CTX *);
+int BN_mod_exp_mont(BIGNUM *, const BIGNUM *, const BIGNUM *, const BIGNUM *,
+                    BN_CTX *, BN_MONT_CTX *);
+int BN_mod_exp_mont_consttime(BIGNUM *, const BIGNUM *, const BIGNUM *,
+                              const BIGNUM *, BN_CTX *, BN_MONT_CTX *);
 int BN_gcd(BIGNUM *, const BIGNUM *, const BIGNUM *, BN_CTX *);
 BIGNUM *BN_mod_inverse(BIGNUM *, const BIGNUM *, const BIGNUM *, BN_CTX *);