EMSK (Extended Master Session Key)

Extended Master Session Key (EMSK) is a protocol used to derive session keys in the Transport Layer Security (TLS) protocol. TLS is a cryptographic protocol used to secure communication between clients and servers over the internet. EMSK is used to derive session keys that are used to encrypt and decrypt data transmitted between clients and servers.

In this article, we will discuss the basics of the EMSK protocol, its purpose, and its implementation. We will also discuss the security benefits of using EMSK.

The Basics of EMSK

When a client connects to a server using TLS, the client and server negotiate a shared secret, known as the master secret. The master secret is used to derive session keys that are used to encrypt and decrypt data transmitted between the client and server. The session keys are generated using the master secret, along with other parameters such as the client and server random values and the cipher suite negotiated by the client and server.

The session keys are used to encrypt and decrypt data transmitted between the client and server. The session keys are typically only used for the duration of the TLS session, which ends when the client and server terminate the connection.

EMSK extends the functionality of the master secret by allowing it to be used to derive session keys for multiple TLS sessions. This is achieved by deriving a set of keys from the master secret, rather than a single key. These keys can then be used to establish multiple TLS sessions between the same client and server, without the need to renegotiate the master secret for each session.

EMSK derives a set of session keys from the master secret using a key derivation function (KDF). The KDF takes the master secret as input, along with a label that identifies the purpose of the derived keys, and other parameters such as the client and server random values and the cipher suite negotiated by the client and server.

The KDF used by EMSK is similar to the KDF used in the TLS protocol, but with some additional parameters. The EMSK KDF takes a label parameter that identifies the purpose of the derived keys, along with an optional context parameter that can be used to provide additional information to the KDF. The label parameter is used to ensure that the derived keys are used only for their intended purpose.

EMSK Implementation

EMSK is implemented in TLS using the TLS Session Resumption protocol. The TLS Session Resumption protocol allows a client to resume a previous TLS session with a server, without the need to renegotiate the master secret. This is achieved by storing the session parameters, including the derived session keys, on the client and server.

When a client wants to resume a TLS session with a server, it sends a Session Ticket to the server. The Session Ticket includes the session parameters, including the derived session keys, encrypted using a key derived from the master secret. The server can then decrypt the Session Ticket using the same key derivation function, and use the derived session keys to resume the TLS session with the client.

Security Benefits of EMSK

EMSK provides several security benefits over traditional TLS session key derivation methods. First, EMSK reduces the number of times the master secret is used to derive session keys. This reduces the risk of the master secret being compromised, as it is used less frequently. Additionally, EMSK allows the session keys to be derived in a way that is resistant to certain types of attacks, such as precomputation attacks.

Another security benefit of EMSK is that it allows the session keys to be derived in a way that is unique to each session. This reduces the risk of session key compromise, as a compromised session key can only be used to decrypt data transmitted during that session. If a different session key is used for each session, the risk of data being compromised is reduced.

Finally, EMSK allows for more efficient use of resources, as it reduces the need for frequent renegotiations of the master secret. This is especially beneficial for high-volume applications, where renegotiation can cause significant overhead.

EMSK also provides enhanced forward secrecy. Forward secrecy is a property of cryptographic protocols that ensures that the compromise of a long-term key (such as the master secret) does not compromise the confidentiality of past sessions. With EMSK, the use of a new session key for each session provides greater forward secrecy than traditional TLS session key derivation methods.

However, it is important to note that EMSK is not a silver bullet for all TLS security issues. It only addresses certain aspects of the TLS protocol, such as session key derivation and forward secrecy. Other security issues, such as certificate verification and protocol vulnerabilities, must be addressed separately.

Conclusion

In summary, Extended Master Session Key (EMSK) is a protocol used to derive session keys in the Transport Layer Security (TLS) protocol. EMSK allows for more efficient use of resources, reduces the risk of session key compromise, and provides enhanced forward secrecy. EMSK is implemented in TLS using the TLS Session Resumption protocol and is resistant to certain types of attacks, such as precomputation attacks. While EMSK provides several security benefits over traditional TLS session key derivation methods, it is important to note that it is not a silver bullet for all TLS security issues.