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What uses a key of the same length as the message where…

What uses a key of the same length as the message where each bit or character from the plaintext is encrypted
by a modular addition?

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A.
Running key cipher

B.
One-time pad

C.
Steganography

D.
Cipher block chaining

Explanation:
In cryptography, the one-time pad (OTP) is an encryption technique that cannot be cracked if used correctly. In
this technique, a plaintext is paired with a random secret key (also referred to as a one-time pad). Then, each
bit or character of the plaintext is encrypted by combining it with the corresponding bit or character from the pad
using modular addition. If the key is truly random, is at least as long as the plaintext, is never reused in whole or
in part, and is kept completely secret, then the resulting ciphertext will be impossible to decrypt or break.
However, practical problems have prevented one-time pads from being widely used.
The “pad” part of the name comes from early implementations where the key material was distributed as a pad
of paper, so that the top sheet could be easily torn off and destroyed after use.
The one-time pad has serious drawbacks in practice because it requires:
Truly random (as opposed to pseudorandom) one-time pad values, which is a non-trivial requirement.
Secure generation and exchange of the one-time pad values, which must be at least as long as the
message. (The security of the one-time pad is only as secure as the security of the one-time pad
exchange).
Careful treatment to make sure that it continues to remain secret, and is disposed of correctly preventing
any reuse in whole or part—hence “one time”.
Because the pad, like all shared secrets, must be passed and kept secure, and the pad has to be at least as
long as the message, there is often no point in using one-time padding, as one can simply send the plain text
instead of the pad (as both can be the same size and have to be sent securely).
Distributing very long one-time pad keys is inconvenient and usually poses a significant security risk. The pad is
essentially the encryption key, but unlike keys for modern ciphers, it must be extremely long and is much too
difficult for humans to remember. Storage media such as thumb drives, DVD-Rs or personal digital audio
players can be used to carry a very large one-time-pad from place to place in a non-suspicious way, but even
so the need to transport the pad physically is a burden compared to the key negotiation protocols of a modern
public-key cryptosystem, and such media cannot reliably be erased securely by any means short of physical
destruction (e.g., incineration).
The key material must be securely disposed of after use, to ensure the key material is never reused and to
protect the messages sent. Because the key material must be transported from one endpoint to another, and
persist until the message is sent or received, it can be more vulnerable to forensic recovery than the transient
plaintext it protects.
Incorrect Answers:
A: Running key cipher does not use a key of the same length as the message.
C: Steganography is a method of hiding data in another media type so the very existence of the data is
concealed. This is not what is described in the question.
D: Cipher block chaining is an encryption method where each block of text, the key, and the value based on the
previous block are processed in the algorithm and applied to the next block of text. This is not what is described
in the question.

https://en.wikipedia.org/wiki/One-time_pad


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