From 82048ad9e3e4a96a38c8fa6a529798f40e33acb1 Mon Sep 17 00:00:00 2001
From: Nguyễn Gia Phong <mcsinyx@disroot.org>
Date: Sat, 14 Nov 2020 11:10:22 +0700
Subject: [usth/ICT3.4] Secure information

---
 usth/ICT3.4/powmod.scm | 9 +++++++++
 1 file changed, 9 insertions(+)
 create mode 100644 usth/ICT3.4/powmod.scm

(limited to 'usth/ICT3.4/powmod.scm')

diff --git a/usth/ICT3.4/powmod.scm b/usth/ICT3.4/powmod.scm
new file mode 100644
index 0000000..b21e995
--- /dev/null
+++ b/usth/ICT3.4/powmod.scm
@@ -0,0 +1,9 @@
+(define (square x) (* x x))
+(display
+  (let powmod ((x (read)) (e (read)) (m (read)))
+    ; let's ignore negative e here
+    (cond ((= e 0) 1)
+          ((= e 1) (remainder x m))
+          ((even? e) (remainder (square (powmod x (/ e 2) m)) m))
+          (else (remainder (* (square (powmod x (quotient e 2) m)) x) m)))))
+(newline)
-- 
cgit 1.4.1