3des encryption and decryption explained in detail

3DES, also known as 3DESede or TripleDES, is a cryptographic scheme that applies three rounds of DES encryption and can be reversed. In 1975, IBM developed and released the Data Encryption Standard (DES) algorithm. However, due to its relatively short key length, DES was vulnerable to brute-force attacks. To enhance security, the 3DES algorithm was introduced by applying the DES encryption process three times on each data block. Despite being based on a public algorithm, 3DES ensures data security through the use of a unique secret key. Some may wonder if 3DES is still secure today. So far, no successful attack has been reported against 3DES, and breaking it would certainly cause a major shock in the information security community. This makes 3DES a reliable choice for many applications, especially where backward compatibility with DES is required. **Brief Analysis of the 3DES Encryption Algorithm** 3DES is not a new encryption algorithm but rather a mode of operation of the original DES algorithm. It is one of the most commonly used symmetric encryption algorithms and offers greater security than standard DES. The encryption and decryption processes involve applying the DES algorithm three times, using different keys in a specific order. Suppose EK() and DK() represent the encryption and decryption functions of DES, P represents the plaintext, and C represents the ciphertext. The encryption and decryption formulas are as follows: - **Encryption**: C = EK3(DK2(EK1(P))) — this means encrypting, then decrypting, then encrypting again. - **Decryption**: P = DK1(EK2(DK3(C))) — this means decrypting, then encrypting, then decrypting again. In 3DES, three keys (K1, K2, and K3) are used. Normally, the key is double-length, meaning K1 corresponds to the left 8 bytes, K2 to the right 8 bytes, and K3 to the left 8 bytes again. Each key is 8 bytes long, resulting in a total of 16 or 24 bytes depending on whether the keys are repeated or distinct. Since DES operates on 8-byte blocks, when implementing 3DES, data must be divided into such blocks. If the last block is smaller than 8 bytes, padding is applied. Common padding methods include: - **Zeros Padding**: Filling the remaining bytes with zeros. - **PKCS5 Padding**: Adding bytes that indicate the number of padding bytes. For example, if you have 9 bytes of data: - **First block**: F0 F1 F2 F3 F4 F5 F6 F7 - **Second block**: F8 00 00 00 00 00 00 00 (for Zeros Padding) - **Second block**: F8 07 07 07 07 07 07 07 (for PKCS5 Padding) The actual implementation of DES is quite complex, so developers often rely on built-in APIs from platforms like Android and iOS. Here’s a simplified version of how 3DES can be implemented in both environments. **Android Code Example:** ```java public byte[] triDesEncrypt(byte[] desKey, byte[] desData, int flag) { byte[] keyFirst8 = new byte[8]; byte[] keySecond8 = new byte[8]; if (desKey.length > 8) { for (int i = 0; i < 8; i++) { keyFirst8[i] = desKey[i]; } } else { return null; } if (desKey.length > 16) { for (int i = 0; i < desKey.length - 8; i++) { keySecond8[i] = desKey[i + 8]; } } else { for (int i = 0; i < 8; i++) { keySecond8[i] = desKey[i + 8]; } } byte[] tmpKey = new byte[8]; byte[] tmpData = new byte[8]; System.arraycopy(keyFirst8, 0, tmpKey, 0, 8); System.arraycopy(desData, 0, tmpData, 0, 8); int mode = flag; byte[] result = unitDes(tmpKey, tmpData, mode); System.arraycopy(keySecond8, 0, tmpKey, 0, 8); System.arraycopy(result, 0, tmpData, 0, 8); mode = (mode == 1) ? 0 : 1; result = unitDes(tmpKey, tmpData, mode); System.arraycopy(keyFirst8, 0, tmpKey, 0, 8); System.arraycopy(result, 0, tmpData, 0, 8); mode = (mode == 1) ? 0 : 1; result = unitDes(tmpKey, tmpData, mode); return result; } ``` **iOS Code Example (Swift):** ```swift func encrypt(with src: Data, key1: Data, key2: Data, key3: Data) -> Data? { guard src.count > 0, key1.count > 0, key2.count > 0, key3.count > 0 else { return nil } let key = NSMutableData() key.append(key1) key.append(key2) key.append(key3) let initVec = "01234567" as NSString let keyPtr = key.bytes let ivPtr = initVec.utf8String var buffer = [UInt8](repeating: 0, count: src.count + kCCBlockSize3DES) var movedBytes = 0 let status = CCCrypt( CCOperation(kCCEncrypt), CCAlgorithm(kCCAlgorithm3DES), CCOptions(kCCOptionPKCS7Padding | kCCOptionECBMode), keyPtr, kCCKeySize3DES, ivPtr, src.bytes, src.count, &buffer, buffer.count, &movedBytes ) return status == kCCSuccess ? Data(bytes: buffer, count: movedBytes) : nil } func decrypt(with src: Data, key1: Data, key2: Data, key3: Data) -> Data? { guard src.count > 0, key1.count > 0, key2.count > 0, key3.count > 0 else { return nil } let key = NSMutableData() key.append(key1) key.append(key2) key.append(key3) let initVec = "01234567" as NSString let keyPtr = key.bytes let ivPtr = initVec.utf8String var buffer = [UInt8](repeating: 0, count: src.count + kCCBlockSize3DES) var movedBytes = 0 let status = CCCrypt( CCOperation(kCCDecrypt), CCAlgorithm(kCCAlgorithm3DES), CCOptions(kCCOptionPKCS7Padding | kCCOptionECBMode), keyPtr, kCCKeySize3DES, ivPtr, src.bytes, src.count, &buffer, buffer.count, &movedBytes ) return status == kCCSuccess ? Data(bytes: buffer, count: movedBytes) : nil } ``` These code examples demonstrate how 3DES can be implemented on mobile platforms, ensuring secure data transmission and storage. While 3DES is gradually being replaced by more modern algorithms like AES, it remains an important part of legacy systems and continues to be widely used in certain industries.

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