lib: add AESGenCTRFunc

1 files changed, 1 insertions, 23 deletions

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)