36 lines
761 B
Markdown
36 lines
761 B
Markdown
Here's a home-brewed stream cipher:
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```
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uint64_t next(uint64_t x)
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{
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return (x << 12) ^ ((x >> 12) + 1);
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}
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typedef uint8_t (*hash_func_t)(uint64_t x);
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hash_func_t hash;
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uint64_t state;
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void initialize(hash_func_t h, uint64_t iv)
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{
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hash = h;
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state = iv;
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}
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uint8_t get_byte(void)
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{
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uint8_t b = hash(state);
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state = next(state);
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return b;
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}
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```
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It's extra secure because even some of its code is secret.
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Here's a (ASCII) message, containing the flag, encrypted with this method: the ciphertext is the plaintext xor-ed with a stream of bytes generated using these functions, starting with a secret, well-chosen hash function and a random initialization vector.
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Flag: the format as commonly used, eg. CCTF{flagitself}
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//Type: Cryptography
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//Author: SI
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