Protecting Stored Data with Cryptography
Introduced| English | Meaning |
|---|---|
| cryptography/krɪpˈtɒɡrəfi/ | cryptography |
| plaintext/ˈpleɪntekst/ | Unencrypted data supplied to an encryption operation or recovered by decryption; it need not be human-readable text. |
| key/kiː/ | A value used by a cryptographic algorithm to encrypt, decrypt or authenticate information. |
| ciphertext/ˈsaɪfətekst/ | Information transformed by encryption into a form intended to be unreadable without the appropriate key. |
| keyspace/ˈkiːspeɪs/ | The set of key values available to a cryptographic scheme; its size is the number of possible keys. |
| symmetric encryption/sɪˈmetrɪk enˈkrɪpʃn/ | Encryption in which the same secret key is used to encrypt and decrypt data. |
| AES/ˌeɪ iː ˈes/ | A symmetric block cipher with 128-bit blocks and keys of 128, 192 or 256 bits. |
| block cipher/blɒk ˈsaɪfə/ | A reversible, keyed cryptographic transformation operating on fixed-size blocks of data. |
Encryption basics
- Cryptography 密码学 hides information.
- Encryption combines plaintext 明文 with a key 密钥 to make ciphertext 密文.
- The keyspace 密钥空间 is the number of possible keys — an n-bit key has $2^n$.
Symmetric encryption
- Symmetric encryption 对称加密 uses the same key to encrypt and decrypt.
- The standard is AES 高级加密标准, a block cipher 分组密码.
- Both sides need the same secret key — sharing it safely is the challenge.
The Enigma machine scrambled messages with rotors— an early, breakable example of encryption
Symmetric encryption uses...
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Symmetric = one shared key.
The standard modern symmetric block cipher is...
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AES is the symmetric standard.
The scrambled output of an encryption algorithm is called ____.
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Encryption turns plaintext into ciphertext.
Bigger keys, more security
- A larger keyspace takes an adversary longer to guess.
- Longer keys are more secure but slower.
- AES works on 128-bit blocks and secures Wi-Fi, browsing, and files.
Symmetric encryption is fast and strong, but it has one hard problem: both sides must already share the same secret key. Sending that key over an insecure channel would expose it — which is exactly what asymmetric cryptography solves next.
Encrypt by shifting letters
Encryption combines plaintext with a key. Here the key is the shift amount; only someone who knows it can decrypt.
A larger keyspace makes an encryption key harder to guess.
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More possible keys = more guessing time.
How many possible keys does a 4-bit key have? (2^4)
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$2^4 = 16$ possible keys.
A simple Caesar cipher shifts each letter by a fixed key. With key = 3, CAT becomes FDW. Only someone who knows the shift (the key) can turn FDW back into CAT. Real algorithms like AES use far larger keyspaces, but the idea is the same.
Cryptography turns plaintext into ciphertext using a key. A bigger keyspace ($2^n$ for an n-bit key) is harder to break. Symmetric encryption (e.g. AES) uses one shared key — fast and strong, but the shared key must be exchanged safely.