Symmetric, Asymmetric, and Hybrid Encryption

Symmetric Encryption: Symmetric encryption, also known as single-key and/or private-key encryption, uses a secret key (could be a number, a word, a random string of characters) as a means to modify or mask the content of a given message. A “key” in cryptography simply refers to a piece of information used in completing the operation of a cryptographic algorithm. The key is a necessary tool for encrypting messages and decrypting cipher text. It should be noted, private-key encryption schemes are generally more efficient and less computationally expensive.

Symmetric encryption is the oldest form of encryption and has been used for thousands of years. Former Roman emperor, Julius Caesar, often used various symmetric encryption methods to conceal messages from his enemies. One such method, the rotation cipher, is now commonly referred to as the “Caesar Cipher”. The rotation cipher simply substitutes letters from the alphabet with other letters based on a certain key length.

Symmetric Encryption Example: Rotation Cipher – Key - 2

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What is most important to understand with symmetric encryption is simply that the same key is used for both the purpose of encrypting and decrypting messages. The use of one key can often make the encryption/decryption process far less complicated. However, there is potential danger in using only one key. If an attacker or man in the middle is able to intercept a symmetrically encrypted message and determine the key, he/she now has the ability to both encrypt and decrypt messages. With this knowledge, an attacker can deceive both the original sender and receiver.

Asymmetric Encryption: Asymmetric encryption, also known as public key encryption, is a cryptographic method in which a user has both a public key and private key. The private key is undisclosed while the public key can be widely distributed. These two keys are related mathematically, often through use of large prime numbers. Despite their mathematical relation, the private key cannot be derived from the public key, at least not by feasible means.

Public key encryption is convenient because a sender and receiver can communicate securely without both needing a common secret key. However, public-key cryptosystems are often computationally expensive due to the manipulation of large primes.

With asymmetric encryption the public key is used as a tool to encrypt the message. The message can then only be decrypted with its corresponding private key. An analogy commonly used to represent the asymmetric encryption scheme is a locked mailbox with a mail slot. A mailbox with a mail slot is accessible to the public allowing anyone to drop off a message. The locked mailbox can be thought of as a public encryption tool safeguarding the message. Only the receiver possessing their private key to the mailbox has access to the message.

In regards to security, it is absolutely necessary that the private key remain private. If the private key were disclosed to an attacker he/she would have the ability to decrypt any messages encrypted with the corresponding public key. Message senders using the public key would have no way of knowing their data was being compromised by a malicious attacker with the private key.

Hybrid Encryption: Hybrid cryptosystems incorporate aspects from both symmetric and asymmetric encryption schemes. These hybrid systems amalgamate the convenience of public-key with the efficiency of private-key.

A hybrid system is basically broken down into two separate cryptosystems; the key encapsulation system and the data encapsulation system. The data encapsulation system which holds the message data is encrypted and decrypted by means of private-key encryption, meaning that both the sender and receiver have the same key. The key encapsulation system on the other hand uses public-key encryption as a means to encrypt/decrypt the key data. This key data, obtained through public-key encryption, is used as the private-key for the data encapsulation system.

For long, complex messages, the majority of the encrypting/decrypting work is done by the more efficient private-key scheme, while the lesser efficient, public-key method, is used to encrypt/decrypt the short key value.

Differences in Symmetric, Asymmetric, and Hybrid Encryption Methods

The differences between the three systems are quite apparent and easily distinguishable. Firstly, symmetric encryption methods use just one key. Both the sender and receiver have the same private key which is used for both encrypting and decrypting messages.

Asymmetric encryption on the other hand uses two different keys. One key is public and accessible to all. This public key allows senders to encrypt their messages. The other key used is a receiver’s private key. This private key is used to decrypt a sender’s corresponding publicly encrypted message.

Lastly, hybrid encryption, being its own entity, uses characteristics from both symmetric and asymmetric encryption schemes. Hybrid uses public-key (asymmetric) encryption for key encapsulation and private-key (symmetric) encryption for data encapsulation.