我的加密逻辑出了什么问题？

Question

I thought I had my encryption classes sorted but a problem has arisen.

My client/server model is simple. The client connects to the server, sends any messages in the message queue and then disconnects.

The first message that is sent is absolutely fine, but any messages sent after that it appears that the first block (first 16 characters of the string) is wrong. The strange thing is that all the other blocks appear to be fine, which is very strange as I am CBC.

Encryption settings:

encryptionAlgorithm = "AES";
encryptionBitCount = 256;
encryptionMessageLength = 176;
hashingAlgorithm = "PBEWithSHA256And256BitAES-CBC-BC";
hashingCount = //some number;
cipherTransformation = "AES/CBC/PKCS7Padding";
salt = //some bytes;

My logic for client works as following:

• Init cipher via: cipher.init(Cipher.ENCRYPT_MODE, cipherInitKey);
• Connect to server

for each message is message queue

• Generate and write IV to socket via: stream.write(cipher.getParameters().getParameterSpec(IvParameterSpec.class).getIV());
• Generate a write ciphertext to socket via: stream.write(cipher.doFinal(message));

end for

• Disconnect

My logic for server works as following:

for each message received

• Read IV from socket (16 bytes).
• Init the cipher by: cipher.init(Cipher.DECRYPT_MODE, key, new IvParameterSpec(iv));
• Read the message from socket (its a fixed size). (176bytes)
• Decipher message via cipher.doFinal(message)

end for

Strange thing is that it works perfectly locally with a java client and server. But when I put the server on my VPS and have an andriod client it breaks.

Update: It appears that it may actually be that I am getting an extra block at the beginning of message - doing more testing.

Answer 1

You probably get the IV at the beginning. You should use the first bytes as IV, then decrypt with the key and the given IV. This is a common technique, and will display exactly the problem as you are describing. You might even skip the first output block and simply use the following blocks as plain text (although that's called hacking). One thing is sure: if your first block is garbage, then the prolbem is the IV.