Why Does My Java AES/CBC Decryption Produce Corrupted Plaintext?

Decrypted Ciphertext Contains Malformed Initial Bytes in Java AES/CBC

In the provided code, after decryption using AES/CBC with PKCS5 padding, the initial portion of the plaintext appears corrupted.

Root Cause

The issue stems from neglecting to convert the encrypted and decrypted bytes to strings. In the decryption loop, the cipher's output is directly written to the output stream. As a result, the first bytes of the plaintext, which contain the padding information, are incorrectly interpreted as part of the message.

Solution

To solve this problem, convert the ciphertext and plaintext to strings using appropriate character encodings. This ensures that the padding is correctly handled and the plaintext is displayed accurately.

Improved Code Example

// Before encryption, encode the plaintext as a string
byte[] plaintextBytes = "Hello there. How are you? Have a nice day.".getBytes(StandardCharsets.UTF_8);
// ... encrypt and decrypt as before ...

// Convert the decrypted bytes to a string
String decryptedText = new String(decryptedBytes, StandardCharsets.UTF_8);
System.out.println("Decrypted text: " + decryptedText);

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