The Most Difficult Cipher to Crack, Ranked

Voting rules: Choose the cipher you think is the most difficult!

Updated on May 4, 2024 06:23

In the complex world of cryptography, certain ciphers pose significant challenges, even to the most skilled codebreakers. Identifying the toughest cipher can be a great aid to both novices and experts in enhancing their understanding and strategies. By classifying these ciphers from easiest to hardest, enthusiasts can better focus their energies and research on the nuances of cryptographic security. This interactive ranking system is a public resource where you can cast your vote on which cipher you believe is the most formidable. Each vote helps shape the list, reflecting the current consensus on the complexity and robustness of various encryption methods. This evolving ranking not only serves as a guide but also encourages a collaborative environment where insights are shared and discussed.

What Is the Most Difficult Cipher to Crack?

1

86
votes
One-time pad

The one-time pad is considered the most secure cipher as it uses a random key that is only used once. As long as the key remains secret, the message cannot be deciphered.
The One-time pad is a cryptographic encryption technique that is considered to be mathematically unbreakable if used correctly. It uses a randomly generated key of the same length as the plaintext. Each character of the plaintext is encrypted by combining it with the corresponding character of the key using modular addition, resulting in the ciphertext. The key is only used once and is never reused.

Key Length: Same length as the plaintext

Mathematical Unbreakability: If used correctly

Perfect Secrecy: When key is truly random, never reused, and kept secret

Key Distribution: Requires secure distribution of the key

Key Management: Keys must be securely stored and destroyed after use
2

29
votes
RSA

RSA is a public-key encryption algorithm widely used in secure data transmission. It is considered secure because of the difficulty of factoring large prime numbers.
RSA is a widely used asymmetric encryption algorithm that is considered one of the most difficult ciphers to crack. It is based on the mathematical problem of factorization, which involves factoring large composite numbers into their prime factors.

Key Size: RSA supports key lengths typically ranging from 1024 bits to 4096 bits, with longer key lengths providing increased security.

Asymmetric Encryption: RSA uses a pair of keys - a public key for encryption and a private key for decryption. Information encrypted with the public key can only be decrypted using the corresponding private key.

Security: The security of RSA relies on the difficulty of factoring large numbers. As of now, no efficient algorithm exists to factor large numbers, making RSA a secure encryption scheme.

Digital Signatures: RSA can be used for creating digital signatures, which provide data integrity and authenticity by verifying the signer's identity.

Secure Key Exchange: RSA can be employed in key exchange protocols, allowing two parties to securely exchange a symmetric encryption key over an insecure network.
3

30
votes
AES

AES is a symmetric encryption algorithm that is widely used in securing data. It uses a 128-bit block size and a 256-bit key size. It is considered secure because of the complexity of its key schedule and the difficulty of attacks such as brute force.
AES (Advanced Encryption Standard) is a symmetric key encryption algorithm widely used for its high level of security and efficiency. It replaces the Data Encryption Standard (DES) and is considered one of the most secure ciphers currently available.

Key Sizes: AES supports key sizes of 128, 192, and 256 bits.

Block Size: AES operates on fixed block sizes of 128 bits.

Rounds: AES performs a varying number of rounds depending on the key size: 10 rounds for 128-bit keys, 12 rounds for 192-bit keys, and 14 rounds for 256-bit keys.

Key Expansion: AES uses a key expansion algorithm to generate round keys from the original encryption key.

Substitution-Permutation Network: AES employs a substitution-permutation network (SPN) structure, consisting of multiple rounds of byte substitution, row shifting, column mixing, and key mixing operations.
4

21
votes
Enigma

Enigma was a cipher machine used by the Germans during World War II. It was considered very difficult to crack because of its complex wiring and the constant changes in its cipher settings.
The Enigma machine was a complex electro-mechanical encryption device used by the Germans during World War II. It was designed to encipher and decipher secret messages, providing a level of encryption considered highly secure at the time.

Rotor-based encryption: Enigma used a series of rotors to scramble the input text, creating a different substitution cipher for each letter.

Multiple rotors and positions: Enigma had multiple rotors that could be configured in various positions, increasing the complexity of the encryption.

Plugboard: The machine featured a plugboard that allowed pairs of letters to be swapped before entering the rotors, further enhancing the encryption process.

Stecker connections: The Enigma allowed a variation called 'Stecker connections,' where additional letter pairs could be swapped to increase the complexity of the encryption.

Regular rotor changes: The rotors would be changed regularly according to a predetermined schedule, making the encryption even harder to crack.
5

17
votes
Vigenère cipher

Giovan Battista Bellaso

The Vigenère cipher is a polyalphabetic substitution cipher that uses a keyword to encrypt the message. It is considered difficult to crack because of its multiple alphabets and the length of the keyword.
The Vigenère cipher is a symmetric polyalphabetic substitution cipher that was invented by Giovan Battista Bellaso in the 16th century. It is considered one of the most difficult historical ciphers to crack, primarily due to its polyalphabetic nature.

Polyalphabetic Substitution: The cipher uses multiple alphabets to encode the plaintext, making it more resistant to frequency analysis attacks.

Key: The cipher uses a keyword as the key, which is repeated to match the length of the plaintext.

Encryption: Each letter of the plaintext is shifted according to its corresponding letter in the key, using different alphabets.

Decryption: Each letter of the ciphertext is shifted back according to its corresponding letter in the key, using different alphabets.

Periodicity: The cipher has a repeating pattern, known as the key's period, which can aid in cracking it.
6

22
votes
patents.google.com · Public domain

Hill cipher

Lester S. Hill

The Hill cipher is a polygraphic substitution cipher that uses matrix algebra to encrypt the message. It is considered secure because of its use of linear algebra to encrypt the message.
The Hill cipher is a polygraphic substitution cipher that operates on blocks of letters. It was developed by Lester S. Hill in 1929 and is based on linear algebra. It is known for its resistance to frequency analysis attacks, making it one of the more challenging ciphers to crack.

Encryption Block Size: n x n (typically 2x2 or 3x3)

Key Size: n x n matrix (where n is the block size)

Encryption Process: The plaintext is divided into blocks of size n. Each block is represented numerically and multiplied by the encryption key matrix to obtain the ciphertext.

Decryption Process: The ciphertext blocks are multiplied by the inverse of the encryption key matrix to obtain the original plaintext.

Key Space: The number of possible keys is determined by the size of the matrix used as the key.
7

13
votes
Davidtheterp · CC0

Playfair cipher

Charles Wheatstone

The Playfair cipher is a polygraphic substitution cipher that uses a 5x5 grid of letters to encrypt the message. It is considered secure because of the complexity of its key and the multiple routes that can be taken to encrypt the message.
The Playfair cipher is a symmetric encryption technique that uses a substitution and transposition method. It was invented by Charles Wheatstone and later promoted by Lord Playfair.

Encryption type: Symmetric

Substitution technique: Yes

Transposition technique: Yes

Key length: Single keyword

Alphabet size: 25 letters (excluding 'J')
8

14
votes
Rail fence cipher

The Rail fence cipher is a transposition cipher that rearranges the letters of the message in a zigzag pattern. It is considered secure because it is difficult to detect patterns in the ciphertext.
The Rail fence cipher is a transposition cipher that rearranges the letters of a message by writing them in a zigzag pattern along a set number of rails. It is named after its resemblance to a fence made of rails. Each rail represents a row, and the letters are written diagonally upwards and downwards along the rails.

Type: Transposition cipher

Difficulty Level: Moderate

Security: Low

Key Length: 1

Key Type: Numeric
9

11
votes
Historicair · CC BY-SA 3.0

Beale cipher

The Beale cipher is a cipher that allegedly encodes the location of a treasure buried in the United States. It is considered difficult to crack because of the complexity of its key and the lack of information about the cipher.
The Beale cipher is a mysterious coded message that supposedly leads to a hidden treasure. It consists of three ciphertexts, referred to as Papers 1, 2, and 3, each encoded using a book cipher. The papers contain descriptions of the location, contents, and value of the treasure.

Number of Ciphertexts: 3

Cipher Type: Book cipher

Ciphertext Names: Papers 1, 2, and 3

Encrypted Content: Descriptions of treasure location, contents, and value

Treasure: Allegedly hidden and valued at over USD 20 million (as per the cipher's creator)
10

7
votes
Four-square cipher

Felix Delastelle

The Four-square cipher is a polygraphic substitution cipher that uses two 5x5 grids of letters to encrypt the message. It is considered secure because of the complexity of its key and the multiple routes that can be taken to encrypt the message.
The Four-square cipher is a polygraphic substitution cipher that uses four 5x5 matrices filled with random letters to encrypt and decrypt the message. It was invented in 1854 by Felix Delastelle.

Key length: Two 5x5 matrices

Encryption process: Involves two stages - first, the message is divided into pairs of letters, then each pair is encrypted using the matrices

Letter selection: The letter 'J' is usually combined with 'I'

Alphabet arrangement: The letters are arranged in a grid without repeating any letter, with 'J' usually omitted from one of the matrices

Matrix selection: One matrix is used for the first letter of the pair, while the other is used for the second letter

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Discussion

Ranking factors for difficult cipher

Encryption algorithm

The strength of the cipher depends on the encryption algorithm used. Modern algorithms such as AES, RSA, and ChaCha20 are considered to be strong and secure. The complexity of the algorithm and resistance to various attack methods should be taken into account.
Key length

The length of the encryption key directly impacts the strength of the cipher. The larger the key, the more difficult it becomes to crack the cipher using brute-force attacks.
Implementation

Even the strongest encryption algorithms can become weak if they are implemented incorrectly. Vulnerabilities in the software or hardware implementing the cipher can expose it to various attacks, such as side-channel attacks or buffer overflows.
Resistance to attacks

A strong cipher should resist various types of attacks, including brute force, differential and linear cryptanalysis, and meet-in-the-middle attacks. The longer it takes to crack the cipher through these attacks, the stronger it is.
Forward secrecy

Forward secrecy ensures that the compromise of a single encryption key does not expose past or future encrypted messages. Ciphers that provide forward secrecy should be ranked higher in terms of difficulty to crack.
Standardization and peer review

Ciphers that have been standardized by organizations such as NIST and have undergone extensive analysis and peer review by the cryptographic community are more likely to be secure and difficult to crack.
Usage history and vulnerability disclosure

Ciphers that have been widely used for a long time without any known successful attacks or vulnerabilities being discovered are usually considered to be more secure. It is also essential to consider how vulnerabilities are disclosed and handled when they are discovered.
Adaptability and upgradability

A strong cipher should be adaptable and upgradable to resist new and emerging threats and attacks. This includes the ability to upgrade key lengths and algorithms in response to computational advancements.

About this ranking

This is a community-based ranking of the most difficult cipher to crack. We do our best to provide fair voting, but it is not intended to be exhaustive. So if you notice something or cipher is missing, feel free to help improve the ranking!

Statistics

3124 views
225 votes
10 ranked items

Voting Rules

A participant may cast an up or down vote for each cipher once every 24 hours. The rank of each cipher is then calculated from the weighted sum of all up and down votes.

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More information on most difficult cipher to crack

When it comes to cryptography, there are many different types of ciphers that have been developed over the years to keep information secure. However, some ciphers are much more difficult to crack than others. The reason for this is that certain ciphers are designed with specific features that make them more resistant to attacks by hackers and other malicious actors. In this article, we will explore some of the most difficult ciphers to crack and what makes them so challenging to decipher. From the complex mathematical algorithms used in the Enigma machine to the one-time pad cipher, we will delve into the fascinating world of cryptography and the ongoing battle to keep information secure.