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-rw-r--r--challenge/c18.go47
1 files changed, 47 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