Symmetric and Hashing

XOR Decrypt Round Trip

Use the same toy keystream to recover the original nibbles. This is toy arithmetic only, not deployable security.

cryptography This lesson shows why both sides must know the same symmetric key stream.

Example

Use the same toy keystream to recover the original nibbles.

highlighted = computed this step

Step 1 — Set up

Set up the exact toy cryptography values.

cipher nibbles, keystream((1, 2, 12), (6, 7, 8))\begin{array}{c|c}\text{cipher nibbles, keystream}&\text{((1, 2, 12), (6, 7, 8))}\end{array}

Step 2 — Decrypt byte 1

Compute the highlighted cryptography value.

index, cipher, key, plain(1, 1, 6, 7)\begin{array}{c|c}\text{index, cipher, key, plain}&\hlmath{\text{(1, 1, 6, 7)}}\end{array}

Step 3 — Decrypt byte 2

Compute the highlighted cryptography value.

index, cipher, key, plain(2, 2, 7, 5)\begin{array}{c|c}\text{index, cipher, key, plain}&\hlmath{\text{(2, 2, 7, 5)}}\end{array}

Step 4 — Decrypt byte 3

Compute the highlighted cryptography value.

index, cipher, key, plain(3, 12, 8, 4)\begin{array}{c|c}\text{index, cipher, key, plain}&\hlmath{\text{(3, 12, 8, 4)}}\end{array}

Final Step — Round trip

Compute the highlighted cryptography value.

decrypted vs originalmatch\begin{array}{c|c}\text{decrypted vs original}&\hlmath{\text{match}}\end{array}