2 files changed, 89 insertions, 0 deletions
diff --git a/challenge/c27.go b/challenge/c27.go
new file mode 100644
index 0000000..7b5554e
--- /dev/null
+++ b/challenge/c27.go
@@ -0,0 +1,87 @@
+// Copyright © 2021 rsiddharth <s@ricketyspace.net>
+// SPDX-License-Identifier: ISC
+
+package challenge
+
+import (
+	"fmt"
+
+	"ricketyspace.net/cryptopals/lib"
+)
+
+func C27() {
+	// Generate random key.
+	key, err := lib.RandomBytes(16)
+	if err != nil {
+		fmt.Printf("Error: %v", err)
+		return
+	}
+
+	// Make IV same as `key`.
+	iv := key
+
+	// Encrypt `plain` with AES-CBC
+	encrypt := func(plain []byte) []byte {
+		return lib.AESEncryptCBC(plain, key, iv)
+	}
+
+	// Same as lib.AESDecryptCBC but ignores padding error and
+	// checks if plain text has high ASCII values.
+	//
+	// Always returns full plain text; even when there is an
+	// error.
+	decrypt := func(cipher []byte) ([]byte, error) {
+		iter := len(cipher) / 16
+
+		lc := iv
+		output := make([]byte, 0)
+		for i := 0; i < iter; i++ {
+			s := (i * 16)
+			e := (i * 16) + 16
+			c := cipher[s:e]
+			output = append(output, lib.FixedXORBytes(
+				lib.AESInvCipher(c, key), lc)...)
+
+			lc = c
+		}
+
+		// Undo padding
+		plain, err := lib.Pkcs7PaddingUndo(output)
+		if err != nil {
+			// If padding undo fails, just use `output`.
+			plain = output
+		}
+
+		// Check if `plain` high ASCII
+		for _, p := range plain {
+			if p >= 128 {
+				return plain, lib.CPError{"Has high ASCII values"}
+			}
+		}
+
+		return plain, nil
+	}
+
+	// Encrypt atleast 3 blocks of plain text.
+	c := encrypt(lib.StrToBytes(
+		"As soon as you are born they make you feel small"))
+
+	// Modify cipher.
+	copy(c[32:48], c[0:16])              // C_3 <- C_1
+	copy(c[16:32], lib.FillBytes(0, 16)) // C_2 <- 0
+
+	// Try decrypting.
+	p, err := decrypt(c)
+	if err != nil {
+		// Has high ASCII values; recover key.
+		k := lib.FixedXORBytes(p[0:16], p[32:48])
+		if lib.BytesEqual(k, key) {
+			fmt.Printf("Recovered key: %v\n", k)
+			return
+		}
+	}
+	fmt.Printf("Could not recover key\n")
+}
+
+// Output:
+// Recovered key: [62 228 155 158 92 189 217 142 58 118 162 140 165 41 152 237]
diff --git a/cryptopals.go b/cryptopals.go
index 79df27d..312d5aa 100644
--- a/cryptopals.go
+++ b/cryptopals.go
@@ -71,5 +71,7 @@ func main() {
 		challenge.C25()
 	case 26:
 		challenge.C26()
+	case 27:
+		challenge.C27()
 	}
 }