A cryptosystem protects data with algorithms and keys.
Encryption
Encryption transforms plaintext into ciphertext using an encryption algorithm and a key. Decryption recovers the plaintext using a key.
- Plaintext
The original readable data. - Ciphertext
The encrypted data. - Encryption algorithm
The procedure that produces ciphertext. - Decryption algorithm
The procedure that recovers plaintext. - Key
A value that controls encryption or decryption.
An encryption algorithm can be public. A key must remain secret when secrecy depends on it.
Encoding
Encoding converts data into a format suited to storage or transmission. It does not provide secrecy.
Anyone who knows the encoding can reverse it.
Examples:
- Base64
Converts binary data into printable characters. - UTF-8
Converts text characters into bytes.
Kerckhoffs’s Principle
A cryptosystem must remain secure even if everything about the system, except the key, is public knowledge.
These rules are recommended by Auguste Kerckhoffs for military ciphers.
- Practical indecipherability
The system must be practically, if not mathematically, indecipherable. - Compromise tolerance
It should not require secrecy, and it should not be a problem if it falls into enemy hands. - Memorable, changeable keys
The key must be easy to communicate and remember, without requiring written notes, and correspondents must be able to change or modify it at will. - Telegraph compatibility
It must be applicable to telegraph communications. Due to historical reasons. - Portability
Must be easy to move, and its use must not require several people. - Ease of use
Given the circumstances in which it is used, the system must be easy to use, requiring neither mental strain nor knowledge of a long series of rules.
Security Through Obscurity
Relying on secrecy of the design, algorithm, or architecture itself for security.
Keeps the whole algorithm and architecture secret. Assumes the hacker knows nothing.
A single leak or reverse-engineering of the secret design compromises the entire system, and the key cannot be rotated to recover.
Proprietary or closed cryptosystems loses public peer review. Hidden flaws and backdoors stay undetected until attackers find and exploit them. Open, peer-reviewed standards are preferred.
Secret ciphers broken after reverse-engineering:
- DVD CSS
40-bit key, broken in 1999. - GSM A5/1
Stream cipher, design leaked in 1994, broken in practice. - TETRA:BURST
2023 disclosure of flaws in the secret TETRA radio ciphers, including a reduced-strength TEA1.
