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169 lines
4.6 KiB
Markdown
169 lines
4.6 KiB
Markdown
---
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id: seminar-05-06
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title: 5th and 6th seminar
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description: |
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200IQ encryption.
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---
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For this bonus you can get at maximum 2.5 K₡.
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[Source](pathname:///files/pb071/bonuses/05-06/)
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## Introduction
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In this bonus you will implement few functions that will be used together for
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implementing a very special cipher.
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### Task no. 1: Reverse (0.5 K₡)
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Write a function `char* reverse(const char* text)` that returns copy of the input
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string in reversed order (also uppercase).
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In case you are given `NULL`, return `NULL`.
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Example (more in tests):
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```c
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char* reversed = reverse("Hello world!");
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printf("%s\n", reversed);
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// "!DLROW OLLEH"
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if (reversed != NULL) {
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free(reversed);
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}
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```
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### Task no. 2: Vigenère (0.5 K₡)
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Vigenère cipher is similar to the Caesar cipher, but you also have a key that is
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used for encrypting (or decrypting).
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Your task is to write two functions:
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- `char* vigenere_encrypt(const char* key, const char* text)` for encrypting
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- `char* vigenere_decrypt(const char* key, const char* text)` for decrypting
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In both of those you should return uppercase characters.
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Meaning of the parameters you are given:
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- `key` - String that represents key that is used for \*crypting. It consists of
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one word and can have only characters of the alphabet. Does not matter if they
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are uppercase or lowercase.
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- `text` - String that is to be \*crypted.
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Function returns address of the encrypted (or decrypted) string. Or `NULL` in case
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error occurs.
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Example:
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```c
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char *encrypted = vigenere_encrypt("CoMPuTeR", "Hello world!");
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printf("%s\n", encrypted);
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// "JSXAI PSINR!"
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if (encrypted != NULL) {
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free(encrypted)
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}
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```
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#### Bonus part (0.5 K₡)
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If you can utilize helper function that would do both encrypting and decrypting,
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you can gain 0.5 K₡.
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Usage of `true`/`false` to decide path in code is prohibited. It leads to merging
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of both functions into one. Point of this part is to discover a way to do this
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generically in such way that there are no separate paths for one or the other. One
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function with no branching for both of them, parametrization is your friend :)
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### Task no. 3: Bit madness (0.5 K₡)
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This is a state of the art crypto. Please do not share :)
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For encrypting:
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1. Split the character that is to be encrypted in halves (4 and 4 bits each).
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2. Bits in 1st half are to be split into pairs. Swap bits in those pairs.
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3. Then use the 4 bits that you created in the 2nd step for `XOR` with the other
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4 bits.
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This simple and ingenious principle will be illustrated on the following example.
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String we want to encrypt is `Hello world!`. We need to encrypt each letter separately,
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so we will demonstrate on letter `H`:
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1. Letter `H` is represented in ASCII as `72`.
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`72` represented in binary is: `01001000`. So first 4 bits are: `0100` and last
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4 bits are `1000`.
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2. First half of bits (`0100`) consists of 2 pairs (`01` and `00`) which we swap
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(`01 ~> 10` and `00 ~> 00`). That way we get `1000`.
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3. That half is used for xor with the other 4 bits:
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```
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1000 // second half
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XOR 1000 // first half after 2nd step
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--------
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0000
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```
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4. Now we combine both halves (first one is `1000`, which we got from the 2nd step
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and second one is `0000`, which we got from the 3rd step) and get `10000000`,
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which is encrypted character `H` using this method.
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In case of decryption, reverse those steps.
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Your task is to implement functions:
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- `unsigned char* bit_encrypt(const char* text)`
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- `char* bit_decrypt(const unsigned char* text)`
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Example:
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```c
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unsigned char* encrypted = bit_encrypt("Hello world!");
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for (int i = 0; i < 12;i++) {
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printf("%x ", encrypted[i]);
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//80 9c 95 95 96 11 bc 96 b9 95 9d 10
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}
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if (encrypted != NULL) {
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free(encrypted);
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}
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```
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### Task no. 4: All combined to BMP (0.5 K₡)
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Authors of the BMP cipher are non-disclosed :)
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Create pair of functions:
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- `unsigned char* bmp_encrypt(const char* key, const char* text)`
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- `char* bmp_decrypt(const char* key, const unsigned char* text)`
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BMP cipher consists of following steps for encrypting:
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1. Reverse the input string
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2. Use Vigenere on the string you got from step #1
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3. Use bit madness on the string you got from step #2
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For decrypting, reverse the steps.
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## Submitting
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In case you have any questions, feel free to reach out to me.
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---
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Ideally submit the assignment through the merge request. Step-by-step tutorial is
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present [here](../mr). For setting assignee my xlogin is `xfocko`.
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In case you do not want to experiment on GitLab, send me the source code via email,
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but please prefix subject with: `[PB071/14][seminar-05-06]`
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Deadline for the submission of the bonus is **April 21th 24:00**.
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