Key Exchange Definition

A key exchange is a cryptographic process in which two parties create or share secret keys for use with an encryption algorithm. Its purpose is to ensure that only the intended parties obtain the keys, keeping protected data confidential. Before any secure communication begins, the parties agree on how the keys will be established. A key exchange may occur over the same channel as the communication itself (in-band) or over a separate, trusted channel (out-of-band).

How Key Exchange Works

1. Agree on public setup information: Both sides begin by agreeing on public setup information, which can be shared openly because it doesn’t reveal sensitive data.
2. Generate private secret values: Each side creates a random private value on its own device. This value remains secret and is never sent over the network.
3. Compute public values: Using its private value, each side computes a corresponding public value. The math allows the public value to be created easily, but makes it extremely difficult to reverse it to recover the private value from it.
4. Exchange public values: The public values are exchanged over the network. An attacker can see these values, but can’t check the private values behind them.
5. Derive a shared secret independently: Each device combines its private value with the other party’s public value to arrive at the same shared secret. This happens independently on both sides, without exposing private data.
6. Create encryption keys: The shared secret is processed into one or more encryption keys used to protect the session. These keys are typically unique.
7. Encrypt further communication: All subsequent data is encrypted using the keys derived from the exchange.

Common Key Exchange Methods

Key exchange methods follow the same general idea but use different mathematical approaches and offer different levels of security and performance.

• Diffie–Hellman (DH): A classic method where both sides create private values, exchange public values, and compute the same shared secret. It relies on large-number math and is still widely supported.
• Elliptic Curve Diffie–Hellman (ECDH): A modern variant of DH that uses elliptic curve math, which provides strong security with much smaller keys. It’s faster and more efficient.
• RSA key transport (Legacy): The client generates a random key and encrypts it using the server’s public RSA key. This approach is largely outdated because it doesn’t provide forward secrecy.
• Pre-shared key (PSK) methods: Both sides already know a shared secret before connecting. The protocol uses that secret to authenticate the parties and derive fresh session keys.

Read More

• What Is a Cryptographic Key?
• What Is Encryption?
• What Is a Man-in-the-Middle Attack?

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