tdo

commit 3e942aad1e42e2e9380a7203b6187a827477b1fa
parent 5449f48e21138128d9936541f3d5dbf2eeba3b76
Author: david cochran <about.trout@gmail.com>
Date:   Tue,  4 Nov 2025 06:32:35 -0800

add layer5 solution

Diffstat:
A
cmd/layer5/decode.go
 | 
86
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

1 file changed, 86 insertions(+), 0 deletions(-)
diff --git a/cmd/layer5/decode.go b/cmd/layer5/decode.go
@@ -0,0 +1,86 @@
+package main
+
+import (
+	"bytes"
+	"crypto/aes"
+	"crypto/cipher"
+	"encoding/binary"
+	"errors"
+	"log"
+	"os"
+
+	"github.com/abtrout/tdo"
+)
+
+func main() {
+	bs, err := tdo.DecodePipedInput()
+	if err != nil {
+		log.Fatalf("Failed to read layer input: %v", err)
+	}
+
+	// Split input based on instructions.
+	kek := bs[0:32]
+	keyIV, keyCT := bs[32:40], bs[40:80]
+	ctIV, ct := bs[80:96], bs[96:]
+	// Decrypt the key with kek and keyIV.
+	block, err := aes.NewCipher(kek)
+	if err != nil {
+		log.Fatalf("Failed to make NewCipher: %v", err)
+	}
+	key, err := KeyUnwrap(block, keyIV, keyCT)
+	if err != nil {
+		log.Fatalf("Failed to unwrap key: %v", err)
+	}
+	// Decrypt the ct with key and iv.
+	block, err = aes.NewCipher(key)
+	if err != nil {
+		log.Fatalf("Failed to make NewCipher: %v", err)
+	}
+	out := make([]byte, len(ct))
+	stream := cipher.NewCTR(block, ctIV)
+	stream.XORKeyStream(out, ct)
+
+	if _, err := os.Stdout.Write(out); err != nil {
+		log.Fatalf("Failed to write decoded output: %v", err)
+	}
+}
+
+// Key Unwrap is not implemented in the standard library.
+// https://datatracker.ietf.org/doc/html/rfc3394#section-2.2.2
+func KeyUnwrap(block cipher.Block, iv, ct []byte) ([]byte, error) {
+	// setup
+	a := make([]byte, 8)
+	copy(a, ct[0:8])
+	n := (len(ct) / 8) - 1 // n+1 blocks
+	r := make([][]byte, n)
+	for i := range r {
+		r[i] = ct[(i+1)*8 : (i+2)*8]
+	}
+	// unwrap
+	ctr := make([]byte, 8)
+	for j := 5; j >= 0; j-- {
+		for i := len(r) - 1; i >= 0; i-- {
+			binary.BigEndian.PutUint64(ctr, uint64(n*j+i+1))
+			// a = a ^ ctr
+			for i := range a {
+				a[i] ^= ctr[i]
+			}
+			// b = Decrypt(a || r[i])
+			b := append(a, r[i]...)
+			block.Decrypt(b, b)
+			// reset for next round
+			copy(a, b[0:8])
+			copy(r[i], b[8:16])
+		}
+	}
+	// verify
+	if !bytes.Equal(a, iv) {
+		return nil, errors.New("iv mismatch")
+	}
+	// output
+	var res []byte
+	for _, b := range r {
+		res = append(res, b...)
+	}
+	return res, nil
+}