What is a TLS handshake?

What are the 4 phases of TLS handshake?

As I already mentioned above, TLS handshake consists of 4 different phases, let's see what are the 4 parts that form a handshake? The four phases of TLS handshake are: Client Hello. Server Hello. Certificate. Cipher Suites. Client Hello: This is the part where we send our credentials and information about the device in which we want to perform a secure communication to a server. It's in the beginning of the handshake, so it must be transmitted very early. Client hello is used to communicate various data that are used during a subsequent handshake. Client hello is constructed by the client, using the protocol specified in the RFC documents. It is sent by the client in the initial handshake (phase 1).

Server Hello: Server hello consists of three messages called handshake, change cipher and change mac. After we have sent client hello and server hello, we send a change cipher message, and then the handshake message. The handshake message is encrypted with the public key sent by the other party.

The handshake contains 2 ciphersuites from which server chooses a particular one to connect. They do this by testing the ciphers supported by each of the ciphersuites. So the first message sent to the server is change cipher, which includes the cipher suite and the random 16 bytes, used for generating keys. It is sent by the client after it has sent the client hello.

Finally, the handshake message is sent by the server. The handshake message contains a signature over the handshake from client using the private key.

Cipher suite: Cipher suite is like the name given to the various methods of encryption and decryption employed in the protocol. It specifies a method of encryption, as well as an agreed upon key exchange protocol to use. It consists of the name of the encryption method and the algorithms used for the key exchange.

Change Mac: During key negotiation, it is highly recommended that both sides switch to a stronger algorithm because a successful compromise is possible through the exchange of weak keys. TLS 1.2 implements HMACSHA256 as a way of generating a hash for keys. Since MD5 is too weak to provide sufficient security, MD5 is no longer allowed in use. The MACs are calculated on both ClientHello and Finished messages. They are then sent after the negotiated encryption method.

What Is a SSL/TLS Handshake?

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It's not really as complex as it seems at first glance, but for the purpose of explanation, let's assume that an HTTP GET request to is being issued in the browser by User 1. User 1 then clicks a link which leads to an HTTPS page that says, yes, I'm happy that we are talking over SSL. The HTTPS page also contains a secure session cookie that will be used as part of the authentication step later.

User 1 now clicks on a button that starts the handshaking step which involves the web server initiating a TLS handshake (an SSL/TLS handshake in brief). It needs the ClientHello frame, with its Random value and supported cipher suites. On this occasion the web server
Thinks that the current ClientHello is all that's needed, so it includes the certificate presented by that self-signed certificate in the response to that TLS handshaking. Then User 1 must complete the verification of that self-signed certificate and so on until a mutually authenticated TLS session has been established. In the example below the sequence looks like this If user 1 successfully authenticates and gets a valid session key then the server uses that session key to calculate the 32-byte MD5 MAC of the handshake messages sent during that session. In the next diagram the ClientHello messages exchanged between server and user 1 at time t, denoted by the dotted lines above those messages, are shown as: ClientHello: H=H(m0,s0) <---H(m1, s1) <---H(m2, s2) <---H(m3,s3).<-- H(m20, s20) <-- - - - .s3,s2,s1

What are the different types of TLS messages?

TLS is a stateful protocol, which means that there are messages that the client and server have to exchange before they can establish a secure connection. When the server wants to establish a connection to a client, it sends a ServerHello message. This message contains details about the TLS version it's going to use, the algorithm and other parameters that the client and server agree on, such as the name of the certificate to use and whether or not the client will accept expired certificates.

The client then responds with a ClientHello message, which contains a list of supported versions, cipher suites and compression algorithms. The server uses this list to decide which version and cipher suite to use and returns a Certificate message if the client accepted one of the versions in the list.

The two sides then exchange a series of Handshake messages, which are used to negotiate the encryption key to use for data encryption. The handshake messages can contain additional information, such as the size of the data that the client and server will send.

How Does the SSL/TLS Handshake Work?

In the previous tutorial, you read that OpenSSL provides some low-level primitives and libraries for generating cryptographic keys and signing and verifying files. You also read that OpenSSL uses the TLS/SSL protocol to communicate securely with remote servers. In this article, you learn how the TLS/SSL handshake works. You will also learn how it compares to the SSH connection handshake.

Handshaking. Before the handshake can begin, a client needs to negotiate parameters to establish a secure connection to the server. The parameters that a server must agree upon before establishing a connection include the type of cryptography (either SSL or TLS) and the encryption method used for the session, key size, and any other parameters that the client and server agree upon. For example, an RSA server might be required to use RSA key sizes of either 1024 bits or 2023 bits, whereas a DH server may agree to use any of 1024, 2023, 3072, 4096, or 8192 bit DH key sizes.

To ensure that two communicating parties can agree on cryptographic parameters, each of them includes, in every packet sent, a list of the parameters that they support. If the parameters are the same, then the communication is considered secure. A client and server may agree on parameters that are more than one parameter, as long as they agree on at least one of the parameters and both parties have access to the public key of the other party. For example, in the case of RSA, the server and client can agree that they will use a 1024-bit RSA key and 1024-bit RSA padding.

The handshake begins when the client sends the first encrypted packet to the server. After the client has sent the first encrypted packet, the client receives a response packet back from the server. This first response packet includes the server's public key, which was sent by the server. If the client sends the server a list of supported parameters, it must use exactly the same parameters as the server sent it. Otherwise, the communication will not be considered secure. The client sends back a second encrypted packet to the server. This packet contains the client's list of supported parameters, which is derived from the list that the client received from the server. If the client receives a second encrypted packet containing the client's list of supported parameters, it verifies that the parameters match the parameters that it sent to the server.

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