Demo-caesar

Overview

This library serves as the support material for TrustInSoft Analyzer tutorial. The purpose of this tutorial is to successively perform:

• The verification of the test suite
• The verification of the generalized test suite with the exhaustive API verification

Tags

For the purpose of the tutorial, each verification step is marked with a git tag. This allows to easily follow the verification methodology.

• Tag STEP_0: Verification of the test suite
• Tag STEP_1: Fixes of the test suite
• Tag STEP_2: Verification of the generalized test suite
• Tag STEP_3: Fixes of the generalized test suite
• Tag STEP_4: Exhaustive verification

Caesar Cipher

This library is based on the Caesar cipher.

The Caesar cipher is one of the simplest and most widely known encryption techniques. It is a type of substitution cipher in which each letter in the plain text is replaced by a letter some fixed number of positions down the alphabet.

Caesar cipher

For example, with a fix number 3, the letter A is replaced by D.

Plain text:   c a e s a r
Cipher text:  f d h v d u

API Reference

The user provides the string to encrypt as well as the corresponding shifting value, and the same applies for the decrypt version.

The specificity of the caesar_encrypt function is that it always performs a shift to the right while the caesar_decrypt function always performs a shift to the left.

/* \brief Encrypt a string with a right shift specified by the user
   \param [in] str The string to encrypt
   \param [in] str_len The length of the string str
   \param [in] shift The right shift to perform
   \return The pointer to the encrypted string
*/
char *caesar_encrypt(char *str, int str_len, int shift);

/* \brief Decrypt a string with a left shift specified by the user
   \param [in] str The string to decrypt
   \param [in] str_len The length of the string str
   \param [in] shift The left shift to perform
   \return The pointer to the decrypted string
*/
char *caesar_decrypt(char *str, int str_len, int shift);

Test suite

A test driver has been written to check the functional correctness of the source code:

∀ string str, int d, caesar_decrypt(caesar_encrypt(str, d), d) = str

The test driver performs two tests:

1. encrypt and decrypt a string with a negative shift value
2. encrypt and decrypt a string with a positive shift value

The two tests aim at covering all the source code and checking the source code specification (i.e. passing a negative value to caesar_encrypt should still perform a shift to the right).

Results

$ gcc -I. -fprofile-arcs -ftest-coverage caesar.c main.c && ./a.out

Test 1: Shift with a negative input
Encrypt text 'People of Earth, your attention please'
Result:       Shrsoh ri Hduwk, brxu dwwhqwlrq sohdvh
Decrypt text 'Shrsoh ri Hduwk, brxu dwwhqwlrq sohdvh'
Result:       People of Earth, your attention please

Test 2: Shift with a positive input
Encrypt text 'People of Earth, your attention please'
Result:       Wlvwsl vm Lhyao, fvby haaluapvu wslhzl
Decrypt text 'Wlvwsl vm Lhyao, fvby haaluapvu wslhzl'
Result:       People of Earth, your attention please

$ gcov caesar.c

File 'caesar.c'
Lines executed:100.00% of 38
Creating 'caesar.c.gcov'