Symmetric Key Cryptography

Symmetrical Key Cryptography also known as conventional or single-key encryption was the primary method of encryption before the introduction of public key cryptography in the 1970s. In symmetric-key algorithms, the same keys are used for data encryption and decryption. This type of cryptography plays a crucial role in securing data because the same key is used for both encryption and decryption.

In this article, we will cover the techniques used in symmetric key cryptography, its applications, principles on which it works, its types and limitations as well as what type of attacks in the digital world it gets to face.

Techniques Used in Symmetric Key Cryptography

Substitution and Transposition are two principal techniques used in symmetric-key cryptography.

Substitution Techniques

The symmetric key cryptographic method employs one secret key for the operations of encryption and decryption. Substitution techniques provide two significant approaches, wherein elements (letters, characters) from the plaintext message are replaced with new elements according to the rules based on the secret key.

• Caesar Cipher: Caesar cipher has since their predictability is so complete and no complexity is invested.

• Monoalphabetic Ciphers: This is where the ciphers use one rule of substitution throughout the message. This may involve replacing letters with numbers, symbols, or another set of letters in another order.

• Playfair Cipher: Implementation of repeated letters or letter pairs can expose patterns, and cryptanalysis techniques exist to exploit them.

• Hill Cipher: This cipher operates on blocks of letters (typically bigrams or trigrams) using a matrix multiplication approach. The Hill ciphers have a limitation on key size and susceptibility towards cryptanalysis for larger key sizes.

• Polyalphabetic Ciphers: This is the type of cipher where any one of the letters in the plaintext is substituted by a different letter to keep frequency analysis challenging. For example, the Vigenère cipher operates with a keyword that would determine the shift value for each letter in the plaintext.

• One-Time Pad (OTP): It is a theoretically impossible cipher where the key is a random string of characters that is exactly as long as the message itself. The key is used for a single encryption and then discarded.

Transposition Techniques

Transposition techniques rearrange the order of elements in the plaintext message without changing the elements themselves.

• Rail Fence Cipher: This is a simple cipher that rearranges the elements by writing the plaintext message in a zigzag pattern, with the different components written in rows (rails) of an imaginary fence and then reading through the columns in a standard order. The key to this is the number of rails used.

• Columnar Transposition: In the case of a plaintext message written in columns and then the columns rearranged according to a permutation determined by the key, this cipher is known as columnar transposition. Although it is still vulnerable to cryptanalysis techniques that exploit the statistical properties of the language.

Types of Symmetric Key Cryptography

1. Stream Ciphers
2. Block Ciphers

Stream Ciphers

The encryption process begins with the stream cipher's algorithm generating a pseudo-random keystream made up of the encryption key and the unique randomly generated number known as the nonce. The result is a random stream of bits corresponding to the length of the ordinary plaintext. Then, the ordinary plaintext is also deciphered into single bits.

These bits are then joined one by one to the keystream bits, gradually converting the ordinary plaintext into the ciphertext using the XOR bitwise operations. When the recipient wants to decrypt the encrypted plaintext, they must generate a new keystream made during the encryption. The encrypted plaintext is then deciphered one by one to derive the encrypted plaintext at the recipient's end.

The most common stream cipher algorithms are

Rivest Cipher 4 (RC4)

• Strengths: The initial appeal of RC4 came from its efficient design and capability to handle variable-length data streams.
• Current Status: Due to these identified weaknesses, RC4 is no longer considered secure for most applications. Its use is strongly prohobited by cryptographic standards bodies.

Salsa20

• Strengths: It's fast and efficient, with a simple and elegant design. Most importantly, the security it offers against known attacks is robust. Apart from that, Salsa20 serves as a building block for other cryptographic protocols, exhibiting its versatility.
• Current Status: Salsa20 is a very widely used and well-respected stream cipher. It's used for many applications where performance and security balance.

Grain-128

• Strengths of Grain-128 include efficiency, lightweight implementation, and the ability to perform well with limited processing power and memory, making it ideal for radio frequency identification (RFID) tags and sensor networks. Importantly, Grain-128 still provides strong security with such simplicity.
• Current Status: Grain-128 is useful in some resource-limited situations where an application needs to be run with huge restrictions in the amount of data that is available for use.

Block Cipher

The result of a block cipher is a sequence of blocks that are then encrypted with the key. The output is a sequence of blocks of encrypted data in a specific order. When the ciphertext travels to its endpoint, the receiver uses the same cryptographic key to decrypt the ciphertext blockchain to the plaintext message.

The most common block cipher algorithms are

Advanced Encryption Standard (AES)

• It has support for three-length keys: 128 bits, 192 bits, or 256 bits, the most commonly used one is a 128-bit key.
• It includes secure communication, data encryption in storage devices, digital rights management (DRM), and so on.

Data Encryption Standard (DES)

• In DES, the 64-bit blocks of plaintext are encrypted using a 56-bit key.
• This weakness caused by the small key size led to the development of a more secure algorithm, called AES.

Triple Data Encryption Algorithm (Triple DES)

• The development of the Triple DES, also called Triple-DES or TDEA, was triggered by the weak security resulting from the small key size in the DES.
• Triple DES denotes a method of three times applying the DES algorithm sequentially (encrypt-decrypt-encrypt) on every plaintext block.

Applications of Symmetric Key Cryptography

• Data encrypting/decrypting: SKC widely applies to protect sensitive data either statically stored in some device or transmitted through the network. Some of these applications include the authentication of users' credentials, encryption of email messages, and financial transactions.
• Secure communication: The majority of the communication protocols commonly used are SSL/TLS, which use the combination of symmetric and asymmetric key encryption to ensure the confidentiality and integrity of exchanged information between two parties. These messages will be encrypted and decrypted using symmetric key encryption using a shared key.
• Authenticity verification: In some places, SKC is applied using techniques like message authentication codes (MACs) and keyed-hash MACs (HMACs) to authenticate the messages by verifying their authenticity and integrity, thus ensuring tamper-resistant communication.
• File and disk encryption: Full-disk encryption software and file encryption tools also apply SKC to encrypt sensitive data stored in hard disks or portable storage devices.
• Virtual private networks: VPN technologies are technologies that aim to provide confidential communication channels free from eavesdropping. Some of these may use symmetric or asymmetric key encryption to connect remote users and corporate networks.

Principles of Symmetric Key Encryption

Basic principles which underpin the security of symmetric key encryption algorithms.

• Resistance to brute force attack: The most basic requirement for the security of an encryption cipher is that the keyspace size—in other words, the number of possible distinct keys from which someone using the algorithm could have chosen—is very large.
• Cryptographic attack resistance: The second fundamental requirement for symmetric or non-symmetrical encryption is the ability to generate information-influenced (i.e., non-random) encrypted messages. For this to happen, a critical but not sufficient requirement in an informational sense is that the encrypted message has high entropy.

It would, in other words, be impossible for an attacker to infer any information about the plain text or secret key from having tried to analyze the ciphertext by the use of frequency analysis or other statistical techniques. Resistance to general forms of cryptanalytic attacks sufficient to ensure semantic security is formalized via the notion of indistinguishability.

Advantages of Symmetric Key Cryptography

• Speed and efficiency: Symmetric key+ algorithms are better suited for encrypting large volumes of data or for use in real-time communication scenarios as they are faster and less resource-intensive than asymmetric cryptography. SKC algorithms do not involve algebraically mathematical operations.

• Scalability: Because symmetric key algorithms have relatively low computational overhead, they scale well with the number of users and the amount of data being encrypted.

• Simplicity: Symmetric encryption protocols are often more straightforward to implement and understand than asymmetric key methods, and this would go a long way in attracting developers and users.

Challenges and Limitations of Symmetric Key Cryptography

• Key management and distribution: Both the sender and the receiver in the SKC of a message need to have the same key, and the key should not be seen by a third party. In case the key is somehow captured or compromised by a third party, the security of the encrypted data is also lost.

• Non-repudiation: Non-repudiation refers to the ability to prove that a specific party has sent a message. In SKC, since the same key is used for encryption and decryption, it is impossible to find out which party created a particular cipher text.

Because of these challenges, symmetric key cryptography is often used in conjunction with asymmetric key cryptography.

Operation Modes in Symmetric Cryptography

Electronic Codebook (ECB)

• ECB is one of the simplest modes of operation for block ciphers.
• A major limitation of ECB is that the same plaintext block produces identical ciphertext blocks that can be used for subsequent attacks, and patterns in the plaintext are visible in the ciphertext.

Cipher Block Chaining (CBC)

• CBC mode links each plaintext block with the previous ciphertext block before encryption.
• Each plaintext block is XORed with the previous ciphertext block before encryption, adding randomness and preventing patterns in the plaintext from being apparent in the ciphertext.

Cipher Feedback (CFB)

• CFB mode operates like a stream cipher, generating a keystream to XOR with the plaintext block before encryption.
• One drawback of CFB mode is error propagation, if an error occurs in one ciphertext block, it will affect subsequent blocks.

Output Feedback (OFB)

• It is a method for switching a block cipher to a stream cipher, creating enciphering through interpolating the plaintext directly.
• It produces a separate keystream, which will be the XOR with the plaintext to derive the ciphertext.

Counter (CTR) mode

• CTR mode transforms a block cipher into a stream cipher by using a counter value as the input to the block cipher.
• CTR mode is highly parallelizable and efficient, making it suitable for scenarios where performance is critical, such as disk and network encryption.

Attacks on Symmetric Key Cryptography

There are two general approaches to attacking a Symmetric Key Cryptography scheme:

Cryptanalysis

Cryptanalytic attacks depend on the ciphertext characteristics of the algorithm plus possibly some knowledge of general characteristics of plaintext or perhaps even some sample plaintext–ciphertext pairs. This type of attack attempts to deduce some specific plaintext or to deduce the key being used through an examination of algorithmic characteristics.

Brute-Force Attack

The attacker attempts all the possible keys on the piece of encrypted data until they get a readable translation into plain text. It takes on average 50% of all the possible keys to get this far. If either of these attacks gets the key right, then all of the future and previous messages encrypted with this key are lost. This is why a large key size offers protection from brute force attacks by making them infeasible to compute. The below table lists the key space sizes of some well-known ciphers.

Conclusion

Symmetric Key Cryptography (SKC) is a powerful and efficient solution for protecting digital information. SKC is easy to use for both encrypting and decrypting data and is highly scalable. In modern symmetric key systems, confusion, diffusion, and randomness are combined with appropriate key sizes to provide semantic security. While secret key management is essential, it cannot be achieved by SKC on its own.