A cryptographic key is a parameter selecting a particular transformation from a family of algorithms in cryptography. For an encryption family, correctness requiresfor every allowed plaintext and appropriate encryption/decryption keys. In symmetric-key cryptography, the communicating parties share a secret key. In public-key cryptography, the public encryption or verification key is paired with a private decryption or signing key.
For a one-bit example, and recover the message because applying exclusive or twice with the same bit cancels it. The one-time pad extends this to a uniformly random independent key as long as the message. Its secrecy depends on the key distribution and use, not on hiding the XOR algorithm. The number of possible keys gives a brute-force search space, but key length alone does not establish the security of a cryptographic construction.
New to topics? Read the docs here!