From aeac109e5013a6e3aeb2d7f9a7149207a05b369e Mon Sep 17 00:00:00 2001
From: makejian <makejian@xiaomi.com>
Date: Sun, 25 Jun 2023 20:35:21 +0800
Subject: [PATCH] crypto/bn:Add exponentiation algorithm in bignum

add exponentiation algorithm: pow_mod_faster
Signed-off-by: makejian <makejian@xiaomi.com>
---
 crypto/bn.c         | 32 ++++++++++++++++++++++++++++++++
 include/crypto/bn.h |  5 +++++
 2 files changed, 37 insertions(+)

diff --git a/crypto/bn.c b/crypto/bn.c
index 3e3aead74d..e21368bab9 100644
--- a/crypto/bn.c
+++ b/crypto/bn.c
@@ -762,3 +762,35 @@ void bignum_assign(FAR struct bn *dst, FAR struct bn *src)
       dst->array[i] = src->array[i];
     }
 }
+
+void pow_mod_faster(FAR struct bn *a, FAR struct bn *b,
+                    FAR struct bn *n, FAR struct bn *res)
+{
+  struct bn tmpa;
+  struct bn tmpb;
+  struct bn tmp;
+  bignum_assign(&tmpa, a);
+  bignum_assign(&tmpb, b);
+
+  bignum_from_int(res, 1); /* r = 1 */
+
+  while (1)
+    {
+      if (tmpb.array[0] & 1)            /* if (b % 2) */
+        {
+          bignum_mul(res, &tmpa, &tmp); /*   r = r * a % m */
+          bignum_mod(&tmp, n, res);
+        }
+
+      bignum_rshift(&tmpb, &tmp, 1); /* b /= 2 */
+      bignum_assign(&tmpb, &tmp);
+
+      if (bignum_is_zero(&tmpb))
+        {
+          break;
+        }
+
+      bignum_mul(&tmpa, &tmpa, &tmp);
+      bignum_mod(&tmp, n, &tmpa);
+    }
+}
diff --git a/include/crypto/bn.h b/include/crypto/bn.h
index 722c318efe..4c0e69a41f 100644
--- a/include/crypto/bn.h
+++ b/include/crypto/bn.h
@@ -204,4 +204,9 @@ void bignum_isqrt(FAR struct bn *a, FAR struct bn *b);
 
 void bignum_assign(FAR struct bn *dst, FAR struct bn *src);
 
+/* CRK_EXP_MOD algorithm */
+
+void pow_mod_faster(FAR struct bn *a, FAR struct bn *b,
+                    FAR struct bn *n, FAR struct bn *res);
+
 #endif /* __INCLUDE_CRYPTO_BIGNUM_H */