2 files changed, 49 insertions, 0 deletions
diff --git a/challenge/c18.go b/challenge/c18.go
new file mode 100644
index 0000000..00d1146
--- /dev/null
+++ b/challenge/c18.go
@@ -0,0 +1,47 @@
+// Copyright © 2021 rsiddharth <s@ricketyspace.net>
+// SPDX-License-Identifier: ISC
+
+package challenge
+
+import (
+	"fmt"
+
+	"ricketyspace.net/cryptopals/lib"
+)
+
+// 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)
+	plain, err := lib.AESDecryptCTR(cipher, key, ctrFunc)
+	if err != nil {
+		fmt.Printf("decryption failed: %v", err)
+	}
+	fmt.Printf("%v\n", lib.BytesToStr(plain))
+}
+
+// Output:
+// Yo, VIP Let's kick it Ice, Ice, baby Ice, Ice, baby
diff --git a/cryptopals.go b/cryptopals.go
index bfff39c..222923d 100644
--- a/cryptopals.go
+++ b/cryptopals.go
@@ -53,5 +53,7 @@ func main() {
 		challenge.C16()
 	case 17:
 		challenge.C17()
+	case 18:
+		challenge.C18()
 	}
 }