Why can't we rely on the symmetric encryption schemes used 1000 years ago or 100 years ago?

Why can't we rely on the symmetric encryption schemes used 1000 years ago or 100 years ago?

• A. They are too slow for modern computers.
• B. They require too much memory.
• C. Older encryption schemes are easy to crack with computers, since a computer can quickly perform frequency analysis and try out many keys.
• D. They are not standards anymore.

Answer: (D)

Security rests on resisting cryptanalysis, and older symmetric ciphers aren’t built to withstand modern computing power. In simple substitution ciphers, each plaintext symbol is replaced by a fixed ciphertext symbol. Because languages have uneven letter frequencies (E and T occur a lot, while Q and Z are rare), a computer can quickly analyze the ciphertext to infer the letter mapping. That makes the cipher easy to crack. Even older polyalphabetic or more complex historical ciphers can be broken with more advanced statistical methods and pattern analysis. Computers can test many candidate keys, look for outputs that resemble natural language, and pinpoint the correct key or reveal the plaintext. The key issue isn’t whether a scheme is a standard; the issue is whether its design can be broken with the kinds of analyses and computing power available today. Modern symmetric encryption uses large keyspaces and designs that resist these attacks, which is why it’s trusted for current security.

Security rests on resisting cryptanalysis, and older symmetric ciphers aren’t built to withstand modern computing power. In simple substitution ciphers, each plaintext symbol is replaced by a fixed ciphertext symbol. Because languages have uneven letter frequencies (E and T occur a lot, while Q and Z are rare), a computer can quickly analyze the ciphertext to infer the letter mapping. That makes the cipher easy to crack.

Even older polyalphabetic or more complex historical ciphers can be broken with more advanced statistical methods and pattern analysis. Computers can test many candidate keys, look for outputs that resemble natural language, and pinpoint the correct key or reveal the plaintext. The key issue isn’t whether a scheme is a standard; the issue is whether its design can be broken with the kinds of analyses and computing power available today.

Modern symmetric encryption uses large keyspaces and designs that resist these attacks, which is why it’s trusted for current security.