Cryptographic protocol

What is a cryptographic protocol in simple words

A cryptographic protocol is a set of rules and procedures that use cryptographic algorithms for secure data exchange between parties. In simple words, it is the “language” on which computers agree on how to encrypt information, verify each other and make sure that the data was not forged.

Imagine that you and your friend exchange secret letters. You agree that you will use a special cipher, change the key every time and check the seal on the envelope. This is the protocol — a set of rules that make your correspondence safe. In the digital world, cryptographic protocols perform this role.

Cryptographic protocols underlie all modern digital security. Without them, internet banking, e-commerce, secure email, VPN and even ordinary browsing of websites via HTTPS would be impossible. Every time you see the “padlock” in the browser's address bar, this means that your computer uses the TLS cryptographic protocol for secure data exchange with the website. Read about how encryption works in the article Data encryption.

Main tasks of cryptographic protocols

Cryptographic protocols solve four key tasks that ensure the security of digital interaction:

• Authentication: Confirmation of the authenticity of the interaction participants. The system verifies that you really are who you claim to be. This prevents “man-in-the-middle” (MITM) attacks and identity forgery.
• Confidentiality: Protection of data from interception and unauthorized reading. Only the recipient can read the message thanks to encryption. Even if an attacker intercepts the data, it will be meaningless to them.
• Integrity: The guarantee that the information was not changed during transmission. If the data was substituted, the recipient will see it thanks to checksums (hashes) and digital signatures.
• Non-repudiation (authorship): The impossibility for the sender to deny that they sent this message. This is achieved through the use of an electronic signature that links the message to a specific sender.

Popular cryptographic protocols

In the modern digital world, many cryptographic protocols are used, each solving its own security tasks:

• TLS/SSL (Transport Layer Security / Secure Sockets Layer): Used for secure data transmission on the internet. These are the same “padlocks” in the browser's address bar that ensure HTTPS security. TLS is used to protect websites, mail servers, messengers and mobile applications. The modern version is TLS 1.3 (2018), which is significantly faster and more secure than previous ones.
• SSH (Secure Shell): Used for secure remote management of servers and file transfer. Popular among system administrators for managing Linux servers. Replaces the insecure Telnet and FTP protocols.
• IPsec (Internet Protocol Security): A protocol for protecting network traffic at the network level. Often used in corporate VPNs to protect all traffic between offices or remote employees.
• PGP/GPG (Pretty Good Privacy / GNU Privacy Guard): Used for encrypting email and files, ensuring the confidentiality of correspondence. Popular among journalists, activists and security specialists.
• Kerberos: An authentication protocol used in corporate networks (especially in Windows Active Directory) to provide single sign-on (SSO).

Read about the security of network connections in the article VPN.

How a cryptographic protocol works

A typical cryptographic protocol consists of a sequence of steps (rounds or “handshakes”) that are performed to establish a secure connection:

• Step 1. Establishing the connection: The parties agree on the protocol version and the encryption algorithms they will use (for example, AES-256 for encryption and RSA for key exchange).
• Step 2. Authentication: The parties verify each other using certificates (in TLS) or keys (in SSH). Certificates are verified through certification authorities (CAs).
• Step 3. Key exchange: The parties agree on a secret key for encrypting data. In TLS, the Diffie-Hellman algorithm is used for this, which allows securely exchanging keys even over an open channel.
• Step 4. Secure data transmission: All data is encrypted and transmitted with integrity verification. Each data packet contains a checksum (hash) to detect substitution.
• Step 5. Closing the connection: The session is closed, the keys are deleted, which prevents their compromise in the future.

Read about the key pair in the article Key pair.

Main threats to cryptographic protocols

Despite the high reliability of cryptographic protocols, there are threats that are important to consider when using them:

• Man-in-the-middle attack (MITM): An attacker intercepts and changes messages between parties, impersonating each of them. Protection — use of certificates and verification of their authenticity.
• Cryptanalysis: Breaking the encryption algorithm using mathematical methods. For example, with an insufficient key length (less than 128 bits), the algorithm can be broken by brute force. Therefore, it is recommended to use AES-256 and RSA-2048 and higher.
• Implementation attack: Using errors in the protocol code (for example, the Heartbleed vulnerability in OpenSSL). It is important to regularly update the software.
• Social engineering: Deceiving users to obtain keys, certificates or passwords. Protection — employee training and use of multi-factor authentication.
• Side-channel attack: Using side effects of the algorithm's operation (execution time, power consumption, electromagnetic radiation).

Read about protection from threats in the article Information security.