Which of the following is NOT a technique used in symmetric encryption?

Symmetric encryption relies on a single key for both encryption and decryption of data. The techniques used within symmetric encryption are designed to manipulate the plaintext to produce ciphertext and then reverse that process using the same key.

Asymmetric decryption stands out as the correct answer because it pertains to asymmetric encryption, which relies on a pair of keys (a public key and a private key) rather than a single shared key. In asymmetric encryption, one key is used for encryption, and a different key is used for decryption, making it distinctly different from symmetric techniques. Consequently, asymmetric decryption does not belong in the context of symmetric encryption methods.

In contrast, techniques like Exclusive OR (XOR), Arbitrary Substitution, and Permutation are all fundamental components of symmetric encryption. XOR is a basic operation that can create ciphertext by combining binary data with a key. Arbitrary substitution involves replacing elements of plaintext with elements from a defined set, and permutation rearranges the order of bits or characters, both functioning in ways that contribute to the security of symmetric encryption. Thus, recognizing the role of these techniques is crucial in understanding how symmetric encryption operates.