From 51cce8132d7bd21f51613a1a78ea6652ffb222f4 Mon Sep 17 00:00:00 2001
From: rsiddharth <s@ricketyspace.net>
Date: Wed, 3 Mar 2021 20:14:16 -0500
Subject: lib: add AESGenCTRFunc

---
 challenge/c18.go | 24 +-----------------------
 1 file changed, 1 insertion(+), 23 deletions(-)

(limited to 'challenge')

diff --git a/challenge/c18.go b/challenge/c18.go
index 00d1146..a84776e 100644
--- a/challenge/c18.go
+++ b/challenge/c18.go
@@ -11,31 +11,9 @@ import (
 
 // Cryptopals #18 - Implement CTR, the stream cipher mode
 func C18() {
-	// Generates counter function for AES CTR mode.
-	genCTRFunc := func(nonce uint64) func() []byte {
-		ctr := uint64(0) // Counter
-		ff := uint64(0xFF)
-		cf := func() []byte {
-			cb := make([]byte, 16) // counter block
-			var i, j uint
-			// Put nonce in the first 8 bytes in cb in little endian format
-			for i = 0; i < 8; i++ {
-				n := nonce & (ff << (i * 8)) // Reset all except the i^th byte of the nonce
-				cb[i] = byte(n >> (i * 8))   // Retrieve i^th byte of the nonce
-			}
-			// Put counter in the next 8 bytes in cb in little endian format
-			for i, j = 8, 0; i < 16; i, j = i+1, j+1 {
-				n := ctr & (ff << (j * 8)) // Reset all except the j^th byte of the counter
-				cb[i] = byte(n >> (j * 8)) // Retrieve j^th byte of the counter
-			}
-			ctr += 1 // Increment counter by 1
-			return cb
-		}
-		return cf
-	}
 	cipher := lib.Base64ToBytes("L77na/nrFsKvynd6HzOoG7GHTLXsTVu9qvY/2syLXzhPweyyMTJULu/6/kXX0KSvoOLSFQ==")
 	key := lib.StrToBytes("YELLOW SUBMARINE")
-	ctrFunc := genCTRFunc(0)
+	ctrFunc := lib.AESGenCTRFunc(0)
 	plain, err := lib.AESDecryptCTR(cipher, key, ctrFunc)
 	if err != nil {
 		fmt.Printf("decryption failed: %v", err)
-- 
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