Which of the following encryption methods is known to be unbreakable?

A.
Symmetric ciphers.

B.
DES codebooks.

C.
One-time pads.

D.
Elliptic Curve Cryptography.

Explanation:
The one-time pad encryption scheme is considered unbreakable only if:
The pad is used only one time.
The pad is as long as the message.
The pad is securely distributed and protected at its destination.
The pad is made up of truly random values.
Incorrect Answers:
A, B: Symmetric ciphers and DES electronic code books are part of symmetric encryption, which are
susceptible to brute force and cryptanalysis attacks.
D: Elliptic curve cryptography is not known to be unbreakable, as it is susceptible to a modified Shor’s algorithm
for solving the discrete logarithm problem on elliptic curves.

Harris, Shon, All In One CISSP Exam Guide, 6th Edition, McGraw-Hill, 2013, pp. 771-773
http://www.encryptionanddecryption.com/encryption/symmetric_encryption.html
https://en.wikipedia.org/wiki/Elliptic_curve_cryptography#Security