Secret codes

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عدد المتطلبات السابقة الإلزامية (حتى الأساس): 3

ستتعلّم

• Encode and decode messages with a Caesar shift using an alphabet wheel.
• Break a Caesar cipher by trying every shift, and a simple substitution cipher by counting letter frequencies.
• Explain why shifting past Z wraps around to A.

تعريف

• A cipher is a rule for turning a message into a disguised form and back again.
• A Caesar shift replaces each letter by the letter a fixed number of places later in the alphabet, wrapping round from Z to A.
• The key is the secret information needed to decode a message, such as the size of the shift.

مبرهنة

• A 26-letter alphabet has only 25 non-trivial Caesar shifts, so the cipher can be broken by trying them all.
• A shift forward by k is undone by a shift forward by 26 − k.
• In English text some letters, such as E, are much more common than others, which helps to break substitution ciphers.

مفاهيم خاطئة شائعة

• Thinking a code is secure because the message looks scrambled.
• Forgetting to wrap around at the end of the alphabet.

اكتشافات اختيارية

• Clock arithmetic: remainders and modular arithmetic A Caesar shift is addition modulo 26.
• Cryptography Modern cryptography gets its security from number theory and computational hardness, not from keeping the method secret.

قراءات إضافية

• Simon Singh, The Code Book (1999)