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We'll fix that be computing the "master secret. We then concatenate the random values that were sent in the ClientHello and ServerHello from Amazon messages that we saw at the beginning. Half of the input is sent to each hash function.
It's clever because it is quite resistant to attack, even in the face of weaknesses in MD5 and SHA This process can feedback on itself and iterate forever to generate as many bytes as we need. Now that both sides have a "master secrets", the spec shows us how we can derive all the needed session keys we need using the PRF to create a "key block" where we will pull data from:.
Therefore, we just need two Message Authentication Code MAC keys for each side that are 16 bytes bits each since the specified MD5 hash digest size is 16 bytes. In addition, the RC4 cipher uses a 16 byte bit key that both sides will need as well. The last handshake message the client sends out is the " Finished message.
We take the result and add a record header byte "0x14" to indicate "finished" and length bytes "00 00 0c" to indicate that we're sending 12 bytes of verify data. Then, like all future encrypted messages, we need to make sure the decrypted contents haven't been tampered with. Some people get paranoid when they hear MD5 because it has some weaknesses.
I certainly don't advocate using it as-is. This means that instead of using MD5 m directly, we calculate:. The sequence number foils attackers who might try to take a previously encrypted message and insert it midstream. If this occurred, the sequence numbers would definitely be different than what we expected.
This also protects us from an attacker dropping a message. This tells us that we need to use Ron's Code 4 RC4 to encrypt the traffic. Ron Rivest developed the RC4 algorithm to generate random bytes based on a byte key. The algorithm is so simple you can actually memorize it in a few minutes. RC4 begins by creating a byte "S" byte array and populating it with 0 to You then iterate over the array by mixing in bytes from the key.
You do this to create a state machine that is used to generate "random" bytes.
To generate a random byte, we shuffle around the "S" array. To encrypt a byte, we xor this pseudo-random byte with the byte we want to encrypt. Remember that xor'ing a bit with 1 causes it to flip. Since we're generating random numbers, on average the xor will flip half of the bits. This random bit flipping is effectively how we encrypt data. As you can see, it's not very complicated and thus it runs quickly. I think that's why Amazon chose it. The server does almost the same thing. It sends out a "Change Cipher Spec" and then a "Finished Message" that includes all handshake messages, including the decrypted version of the client's "Finished Message.
Now, milliseconds after we started, we're finally ready for the application layer.
At this point, the handshake is over. Our TLS record's content type is now 23 0x Encrypted traffic begins with "17 03 01" which indicate the record type and TLS version. These bytes are followed by our encrypted size, which includes the HMAC hash. The only other interesting fact is that the sequence number increases on each record, it's now 1 and the next record will be 2, etc. We see its response, including the tell-tale application data header:.
Cneonction: close Transfer-Encoding: chunked. TLS is just below the application layer. The HTTP server software can act as if it's sending unencrypted traffic.
The only change is that it writes to a library that does all the encryption. The connection will stay open while both sides send and receive encrypted data until either side sends out a " closure alert " message and then closes the connection. If we reconnect shortly after disconnecting, we can re-use the negotiated keys if the server still has them cached without using public key operations, otherwise we do a completely new full handshake. It's important to realize that application data records can be anything.
It's certainly better to use TLS than inventing your own solution. Additionally, you'll benefit from a protocol that has withstood careful security analysis. We covered just one single path in our observation of the millisecond dance between Firefox and Amazon's server. It's a reasonable choice that slightly favors speed over security.
As we saw, if someone could secretly factor Amazon's "n" modulus into its respective "p" and "q", they could effectively decrypt all "secure" traffic until Amazon changes their certificate. Amazon counter-balances this concern this with a short one year duration certificate:.https://thomacksyssesi.tk
Pin by Vitthal Jadhav on Speed Math (Magic Math) | Resume template free, Math magic, Resume
One downside to this algorithm is that it requires more math with big numbers, and thus is a little more computationally taxing on a busy server. It's different than RC4 in that it works on 16 byte "blocks" at a time rather than a single byte. Since its key can be up to bits, many consider this to be more secure than RC4. In just milliseconds, two endpoints on the Internet came together, provided enough credentials to trust each other, set up encryption algorithms, and started to send encrypted traffic.
I wrote a program that walks through the handshake steps mentioned in this article. Join a community of over , senior developers. View an example. You need to Register an InfoQ account or Login or login to post comments. But there's so much more behind being registered. Your message is awaiting moderation.
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Thank you for participating in the discussion. A very nice summary of the TLS exchange. In my opinion the two purposes of the TLS digital certificate needs to be separated. I just wanted to thank you for this post. Also, I would like to encourage you to write more on this subject in the form of online books. I was hoping you could write a simple and practical book for the InfoQ bookshelf.
Best wishes. Thank you, this is a very good article about https protocol I've ever seen. It's really useful for me. Implementation of this protocol is still a major challenge - if one would start from scratch, I'd imagine it will take a couple of months at least. I wonder if it can be reduced in complexity - just leaving the active ingredient in. Nice article.
Art and Craft of Mathematical Problem Solving
Very informative. I cannot understand how this value is computed and how both parties client and server computes the same values. Is your profile up-to-date? Please take a moment to review and update. Like Print Bookmarks. Jul 27, 25 min read by Jeff Moser. A new page is loaded when proceeding to checkout: In the milliseconds that flew by, a lot of interesting stuff happened to make Firefox change the address bar color and put a lock in the lower right corner.
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