How to decrypt data at Application Layer of HTTPS?

Question

I'm writing an web server with Lisp to handler HTTPS request. I followed TLS 1.2 and already completed the handshake process. The Cipher Suite I chose is TLS_RSA_WITH_RC4_128_SHA. I already calculated client_write_MAC_secret, server_write_MAC_secret, client_write_key, server_write_key. These keys seems are correct, because I can decrypt "Finished" message from browser and validate the data inside. I also validate the HMAC of the record layer. Then I send a "Change Cipher Spec" and "Finished" from server. So far everything seems working fine.

Then I got the message from browser start with #(23 3 3 1 61 ...). 23 means it's an application data. #(3 3) means TLS 1.2. #(1 61) means length is 256+61=317 which is correct because the data left is really 317 bytes long. Here comes my question: I decrypted these 317 bytes with RC4 using the "client_write_key" then I got data like #(148 104 81 182 67 111 28 201 202 50 207 57 126 209 19 ...) which can't be converted to text. I thought I should get something like GET / HTTP/1.1. What do I get wrong?

Thanks.

You should decrypt with the server_write_key, not the client_write_key. — President James K. Polk, May 25 '14 at 20:36
@GregS Thanks for the reply. Why use server_write_key? According to , server write key: The key used to encrypt data written by the server. — Wise Simpson, May 26 '14 at 03:38
My bad, I missed that you were writing ther server, I thought you were writing the client. Yeah, you need to decrypt with the client write key. I don't know what the problem is, sorry. — President James K. Polk, May 26 '14 at 13:10
@GregS You're genius! That's the problem that I reset the state. Thank you so much! — Wise Simpson, May 27 '14 at 11:32

score 1 · Accepted Answer · edited Oct 07 '21 at 06:16

RC4 is a stream cipher and, according to RFC 5246, section 6.2.3.1, "For stream ciphers that do not use a synchronization vector (such as RC4), the stream cipher state from the end of one record is simply used on the subsequent packet."

So, the first record you decrypt is the FINISHED message and the first application data you decrypt should be with the RC4 state as it was left after decrypting the FINISHED message.

How to decrypt data at Application Layer of HTTPS?

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