Security+ Flashcards: Cryptography Concepts, Symmetric, Asymmetric, Hashing, Certificates

Encryption converts readable plaintext into unreadable ciphertext using a mathematical algorithm and a key, ensuring that only authorized parties with the correct key can read the data. It protects confidentiality of data at rest on storage devices, data in transit across networks, and increasingly data in use during processing. Without encryption, anyone who intercepts network traffic or gains access to storage can read sensitive information.

The Advanced Encryption Standard, or AES, is the current US government standard for symmetric encryption, adopted in 2001 to replace the Data Encryption Standard. AES is a block cipher that processes data in 128-bit blocks and supports three key sizes: 128, 192, and 256 bits. Longer keys provide stronger security but require more processing. AES-256 is required for top secret government data and is considered resistant to all known attacks including theoretical quantum computing threats.

A digital certificate is an electronic document issued by a Certificate Authority that binds a public key to the identity of its owner, such as a person, organization, or server. Certificates enable trust in public key cryptography by providing a verified link between a key and an identity. When your browser connects to a website using HTTPS, it checks the site's certificate to verify the server's identity and obtain its public key for encrypted communication.

In asymmetric cryptography, the public key and private key are a mathematically related pair. The public key is shared openly and used by others to encrypt messages intended for the key owner or to verify digital signatures the owner created. The private key is kept secret and used to decrypt messages encrypted with the matching public key or to create digital signatures. If someone encrypts a message with your public key, only your private key can decrypt it. If you sign a message with your private key, anyone with your public key can verify the signature.

A cryptographic hash function takes input of any size and produces a fixed-length output called a digest. It must have three properties: it must be one-way, meaning you cannot derive the input from the output; it must be collision-resistant, meaning it is computationally infeasible to find two different inputs that produce the same output; and it must exhibit the avalanche effect, where a tiny change in input produces a dramatically different output. SHA-256 produces a 256-bit digest. Hashing verifies file integrity, stores passwords securely, and is the foundation of digital signatures.

Secure Sockets Layer, or SSL, was the original protocol for encrypting web communications, but all versions including SSL 3.0 have known vulnerabilities and are deprecated. Transport Layer Security, or TLS, is the successor protocol that provides the same function with stronger security. Current recommended versions are TLS 1.2 and TLS 1.3. TLS 1.3 removed support for older insecure cipher suites, reduced the handshake to one round trip for faster connections, and made forward secrecy mandatory. Although people still say "SSL certificate," the actual protocol used is TLS.

Key escrow is the practice of storing copies of encryption keys with a trusted third party so that authorized entities, typically law enforcement or the organization, can access encrypted data if the key holder is unavailable or uncooperative. It is controversial because it creates a centralized target for attackers, introduces the risk of unauthorized access, and weakens the fundamental security promise of encryption. Critics argue that any backdoor can be exploited by malicious actors. Proponents argue it enables lawful access for criminal investigations.

Perfect forward secrecy, or PFS, is a property of key exchange protocols that ensures session keys are not compromised even if the server's long-term private key is later stolen. Each session generates unique ephemeral keys using Diffie-Hellman or Elliptic Curve Diffie-Hellman that are discarded after the session ends. Without PFS, an attacker who records encrypted traffic and later obtains the private key can decrypt all past sessions. With PFS, each session's keys are independent and destroying them makes past traffic permanently unrecoverable. ---