Symmetric Encryption

Section 2.6 Symmetric Encryption

The image, captioned ’symmetric,’ is a flowchart demonstrating the symmetric encryption process. It displays a sequence of five stages, from left to right, connected by arrows, illustrating data transformation.

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The process begins with a document icon labeled ’Plaintext’. An arrow points to a key-shaped icon labeled ’A’ and also ’Encryption’, representing the encryption step with Key A. This transformation results in a document icon labeled ’Ciphertext’. Following this, an arrow points from the ciphertext to the same key-shaped icon, indicating the decryption step. A final arrow leads to a document icon labeled ’Plaintext’, signifying the recovery of the original data.

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This diagram clearly illustrates the core principle of symmetric encryption: the identical key (Key ’A’) is utilized for both encrypting the plaintext to ciphertext and subsequently decrypting the ciphertext back to the original plaintext.

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Figure 2.6.1. Workflow of Symmetric Encryption

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Symmetric encryption is probably the simplest encryption to understand in that it only uses a single key (in this case our key is labelled ’A’) to encrypt or decrypt data. Both parties need to know the private key in order to communicate. It does pose a security risk in that if the channel used for key exchange is insecure, all of the messages can be decrypted. That being said, given that it is simpler than many other forms of encryption, it is often used for secure communication or storage.

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Example 2.6.2. One-time-pad.

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One-time-pad (OTP) is a rare example of a pen and paper, symmetric encryption scheme that cannot be cracked. The difficulty in OTP mirrors the difficulty with all symmetric encryption, namely that pre-shared keys need to be exchanged at some point.

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Imagine that a prisoner wishes to send encrypted messages to someone outside the prison. To do so, they will make use of a copy of Harry Potter and the Sorcerer’s Stone that they have in their cell. The message they want to send is "DIG UP THE GOLD". They turn to "Chapter One: The Boy Who Lived" and look up the first twelve letters in the chapter: MR AND MRS DURS. For each letter of their message, they convert it to its number in the alphabet: 4 9 7 21 16 20 8 5 7 15 12 4 (DIG UP THE GOLD). They do the same for the key they looked up in their book: 13 18 1 14 4 13 18 19 4 21 18 19 (MR AND MRS DURS). Finally they add the two numbers to get their ciphertext: 17 27 8 35 20 33 26 24 11 36 30 23.

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If the prisoner sends that ciphertext to someone on the outside who knows that they key is the first chapter of Harry Potter and the Sorcerer’s Stone, they will be able to subtract the key from each of the numbers in the ciphertext and discover the plaintext message. While theoretically unbreakable, anybody else who has the key can recover the text as well. This means that using common keys like popular books make it trivial for a man-in-the-middle to decode the ciphertext. After all, the warden probably knows every book that the prisoner has in their cell.

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OTP has been used by spy agencies, often for communications between individuals via dead-drops. In this situation tables of random characters printed in duplicate are exchanged as the key.

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