DES Decryption Calculator
DES is a legacy cipher. This educational tool helps inspect classic Feistel decryption behavior, byte flow, padding removal, and mode handling.
Example Data Table
| Scenario | Mode | Ciphertext | Key | IV | Expected Plaintext |
|---|---|---|---|---|---|
| FIPS demonstration block | ECB | 85E813540F0AB405 | 133457799BBCDFF1 | 0000000000000000 | 0123456789ABCDEF |
| Binary training sample | CBC | 6C1F0E209EBB7A11A4D00B24D7D7A1C8 | 133457799BBCDFF1 | 1234567890ABCDEF | Example varies with padding policy |
| Readable text check | CFB | Q1dFUlRZMTIzNDU2 | QUJDREVGR0g= | MTIzNDU2Nzg= | Review visible characters after decryption |
The first row is the classic DES reference example. Nonprintable plaintext is best reviewed in hex output mode.
Formula Used
DES is a 16-round Feistel network working on 64-bit blocks. The tool applies the initial permutation, runs 16 rounds with reversed round keys, swaps halves, then applies the final permutation.
| Round split | Li = Ri-1 |
|---|---|
| Round merge | Ri = Li-1 XOR F(Ri-1, Ki) |
| Feistel function | F(R, K) = P(S(E(R) XOR K)) |
| Decryption rule | Use the same structure with the 16 subkeys in reverse order. |
ECB and CBC require ciphertext lengths that are exact multiples of eight bytes. CFB and OFB can process shorter trailing segments.
How to Use This Calculator
- Paste the ciphertext in hex, base64, or raw text form.
- Enter the DES key and choose the correct input encoding.
- Select the working mode: ECB, CBC, CFB, or OFB.
- Provide an IV for CBC, CFB, or OFB operations.
- Choose padding removal and your preferred output display format.
- Pick an exact-length policy or allow auto pad and truncate.
- Press Decrypt Now to generate the result section above the form.
- Review the visible text, raw hex, base64, round subkeys, and byte chart.
- Use the CSV and PDF buttons to export your report.
Frequently Asked Questions
1. Why does the tool ask for an IV?
CBC, CFB, and OFB modes depend on an initialization vector. ECB does not use one, so the IV field is ignored there.
2. Why is my decrypted output unreadable?
The key, mode, IV, or padding may be wrong. Many DES examples also decrypt to binary bytes rather than readable text, so inspect hex output.
3. What does the length policy change?
DES needs eight-byte keys and eight-byte IVs. Exact mode enforces that. Auto mode pads short values with null bytes or truncates longer ones.
4. When should I remove PKCS#7 padding?
Use PKCS#7 only when the original encrypted plaintext was padded that way. Removing the wrong padding scheme can damage valid plaintext bytes.
5. Does this tool support Triple DES?
No. This page is designed for classic single-key DES so learners can inspect the original 16-round block process more clearly.
6. Why are round keys shown in reverse?
DES decryption uses the same Feistel structure as encryption, but the sixteen round subkeys are applied in the opposite order.
7. Is DES secure for modern projects?
No. DES is considered obsolete because its key size is too small. Use modern ciphers such as AES for real protection.
8. What does the Plotly graph show?
The chart plots decimal values for the first decrypted bytes. It helps visualize byte distribution and spot repeating patterns or nonprintable outputs.