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TLS-PSK-WITH-AES-256-CBC-SHA Cipher Suite
A breakdown of the Cipher Suite TLS_PSK_WITH_AES_256_CBC_SHA, its strengths, and its weaknesses.
Key Exchange Mechanism
Pre-Shared Key - PSK
Grade - A
A pre-shared key (PSK) in a TLS cipher suite is a symmetric key shared in advance between the client and server. It serves as the basis for establishing a secure connection without the need for public key infrastructure (PKI). PSKs are typically used in scenarios where both parties have agreed upon a key beforehand, such as in IoT devices, VPNs, or certain enterprise networks. They provide mutual authentication and confidentiality by encrypting communication using a shared secret, ensuring that only authorized parties can access the encrypted data exchanged during the TLS session.
Authentication
Pre-Shared Key - PSK
Grade - A
PSK (Pre-Shared Key) cipher suites are used for authentication in secure communication protocols like TLS. They allow parties to establish a shared secret beforehand, ensuring confidentiality and integrity of data exchanges without the overhead of public key infrastructure (PKI), suitable for constrained environments or specific security requirements.
Cipher
Advanced Encryption Standard - AES
Grade - A
AES should be used in cipher suites because it offers strong security with efficient performance, large block size (128 bits), and resistance to known attacks. Its widespread adoption and thorough analysis by the cryptographic community ensure reliability and robustness for encrypting sensitive data.
Hash
Secure Hash Algorithm - SHA
Grade - D
Chosen prefix attacks for SHA1 are feasible at an accessible cost to a well-funded adversary. This level of expense, while significant, does not pose a substantial barrier to attackers with sufficient resources, making such attacks a credible threat.
Cipher Mode
Cipher Block Chaining - CBC
Grade - D
Cipher Block Chaining (CBC) mode is vulnerable to the Lucky13 and POODLE (in TLS v1.2 and below) attacks. The Lucky13 attack exploits timing discrepancies in padding validation, allowing attackers to gradually reveal plaintext. The POODLE attack leverages padding errors to decrypt ciphertext by repeatedly modifying and sending it to the server, observing the error responses. These vulnerabilities arise from CBC’s handling of padding and error messages, making it less secure than modern encryption modes like Galois Counter Mode (GCM), which offer stronger integrity and confidentiality guarantees.
