diff --git a/cipher/caesar_cipher.c b/cipher/caesar_cipher.c
new file mode 100644
index 0000000000..f846cf653f
--- /dev/null
+++ b/cipher/caesar_cipher.c
@@ -0,0 +1,96 @@
+/**
+ * @file
+ * @brief A [caesar cipher](https://en.wikipedia.org/wiki/Caesar_cipher) is a
+ * substitution cipher that replaces each letter in a text by another located
+ * a given number of positions along the alphabet.
+ * @author [Andrey Gonçalves Barreto Isidoro](https://github.com/agbi623)
+ */
+
+#include <string.h>  /// for strcmp
+#include <stdio.h>   /// for Input/Output
+#include <ctype.h>   /// for isalpha, isupper and tolower
+#include <assert.h>  /// for assert
+
+/**
+ * @brief calculates the encrypted char for the encryption
+ * @param c the character to be used
+ * @param key the key to be used
+ * @returns the encrypted char (case-sensitive)
+ */
+int calculate_encrypted_char(char c, int key)
+{
+	int new_c;
+
+	// calculates the offset
+	new_c = ((tolower(c) - 'a') + key) % 26;
+
+	// (because the % operator doesn't handle negative left inputs)
+	// puts new_c back on the 0-26 range if it turned negative
+	if (new_c < 0) new_c += 26;
+
+	return isupper(c) ? new_c + 'A' : new_c + 'a';
+}
+
+/**
+ * @brief encrypts the passed text by calling and assigning the return value
+ * of calculate_encrypted_char for each alphabetical character in the passed
+ * text string
+ * @param text the text to be encrypted
+ * @param key used for shifting a letter - shifts left when negative, right
+ * when positive
+ * @returns void
+ */
+void caesar_cipher(char *text, int key)
+{
+	for (int i = 0; text[i] != '\0'; i++)
+	{
+		if (isalpha(text[i]))
+		{
+			text[i] = calculate_encrypted_char(text[i], key);
+		}
+	}
+}
+
+/**
+ * @brief tests the caesar_cipher function by passing some strings to it
+ * @returns void
+ */
+void test(void)
+{
+	char first[] = "ILOVETV";
+	char second[] = "ILOVETV";
+	char third[] = "The smoke... was blue.";
+	char fourth[] = "They think they are the fuckin big shot, like the world revolves around them. It\'s funny at times though x\'D";
+	char fifth[] = "My compass was swallowed by the sea...";
+
+	caesar_cipher(first, 15);
+	assert(strcmp(first, "XADKTIK") == 0);
+	printf("First test OK\n");
+
+	caesar_cipher(second, -15);
+	assert(strcmp(second, "TWZGPEG") == 0);
+	printf("Second test OK\n");
+
+	caesar_cipher(third, 35);
+	assert(strcmp(third, "Cqn bvxtn... fjb kudn.") == 0);
+	printf("Third test OK\n");
+
+	caesar_cipher(fourth, -81);
+	assert(strcmp(fourth, "Qebv qefkh qebv xob qeb crzhfk yfd pelq, ifhb qeb tloia obslisbp xolrka qebj. Fq\'p crkkv xq qfjbp qelrde u\'A") == 0);
+	printf("Fourth test OK\n");
+
+	caesar_cipher(fifth, 0);
+	assert(strcmp(fifth, "My compass was swallowed by the sea...") == 0);
+	printf("Fifth test OK\n");
+}
+
+/**
+ * @brief main function; calls the test function
+ * @returns 0 if all tests pass
+ */
+int main(void)
+{
+	test();
+
+	return 0;
+}
diff --git a/numerical_methods/newton_raphson_root_2.c b/numerical_methods/newton_raphson_root_2.c
new file mode 100644
index 0000000000..057ab4c2a2
--- /dev/null
+++ b/numerical_methods/newton_raphson_root_2.c
@@ -0,0 +1,156 @@
+/**
+ * @file
+ * 
+ * 
+ * @brief The [Newton-Raphson method](https://en.wikipedia.org/wiki/Newton%27s_method)
+ * is a root-finding algorithm which successively produces better
+ * approximations to the roots of a real-valued function.
+ * 
+ * 
+ * @author [Andrey Gonçalves Barreto Isidoro](https://github.com/agbi623)
+ */
+
+#include <stdio.h> 		// for Input/Output
+#include <math.h>		// for fabs, sqrtf and logf
+#include <ctype.h>		// for tolower
+#include <assert.h>		// for assert
+
+/**
+ * @brief Produces better approximations of the root of a.
+ * 
+ * @details Assigns the absolute value of the a parameter to itself. Inside
+ * the loop, assigns the next approximation to x_nplus1. If the absolute
+ * difference between the next approximation and the current is less than
+ * or equal to the diff parameter, the for loop breaks. Else, the value of
+ * x_nplus1 gets assigned to x_n and the loop continues. If the visualize
+ * parameter is 1 (true), each approximation is printed.
+ * 
+ * 
+ * @param a the positive value to find the approximate root of
+ * 
+ * @param diff the maximum deviation to be accepted
+ * 
+ * @param visualize whether the user chose to visualize the process
+ * 
+ * 
+ * @returns the best approximation of the root
+ */
+double newtons_method(double a, double diff, int visualize)
+{
+    a = fabs(a);
+
+    double x_n = a;
+    double x_nplus1;
+    
+    int i;
+
+	if (visualize) printf("Start of approximations\n");
+
+    for (i = 1; i <= logf(a) * 20; i++)
+    {
+		if (visualize) printf("x_%d: %.12lf\n", i - 1, x_n);
+		
+        x_nplus1 = 0.5 * (x_n + a / x_n);
+
+        if (fabs(x_nplus1 - x_n) <= diff) break;
+
+        x_n = x_nplus1;
+    }
+    
+    if (visualize) printf("Final approximation (x_%d): %.12lf\n\n", i, x_nplus1);
+
+    return x_nplus1;
+}
+/**
+ * @brief Tests if the implementation of the Newton-Raphson method is valid.
+ * 
+ * @details Checks if the absolute difference between the return value of
+ * newtons_method and the square root is less than or equal to the passed
+ * deviation tolerance within the test[] parameter.
+ * 
+ * 
+ * @param test[] an array containing two values; the first is the number to
+ * get the root of and the second is the maximum deviation from the square
+ * root to be accepted
+ * 
+ * @param visualize whether the user chose to visualize the process
+ * 
+ * 
+ * @returns whether the aforementioned check is true
+ */
+int individual_test(double test[], int visualize)
+{
+	double function_return = newtons_method(test[0], test[1], visualize);
+	double square_root = sqrtf(fabs(test[0]));
+	double abs_diff = fabs(function_return - square_root);
+	
+    return abs_diff <= test[1];
+}
+
+/**
+ * @brief Tests a given number of values.
+ * 
+ * @details Iterates through the tests[] array and passes each element to
+ * individual_test(). Calculates the number of elements inside the tests[]
+ * array. Inside the loop, if the visualize parameter is 1 (true), the current
+ * test, the number to take the root of and the accepted deviation are
+ * printed. Then, the current element of the tests[] array and the visualize
+ * parameter are passed to the individual_test() function, in that order. If
+ * this function returns 0, the program halts because of assert. If not, the
+ * loop continues.
+ * 
+ * 
+ * @param visualize whether the user chose to visualize the process
+ * 
+ * 
+ * @returns void
+ */
+void test(int visualize)
+{
+	double test1[] = {625, 0.1};
+    double test2[] = {2, 0.001};
+    double test3[] = {1024, 0};
+    double test4[] = {-25, 0};
+    double test5[] = {4294967296, 0};
+    
+    double *tests[] = {test1, test2, test3, test4, test5};
+    
+	int tests_num = sizeof(tests) / sizeof(tests[0]);
+	
+	for (int i = 0; i < tests_num; i++)
+	{
+		if (visualize)
+		{
+			printf("-------------------\n");
+			printf("Test %d\n", i + 1);
+			printf("a: %.12lf\n", tests[i][0]);
+			printf("Precision: %.12lf\n\n", tests[i][1]);
+		}
+		
+	    assert(individual_test(tests[i], visualize));
+	    printf("Test %d OK\n", i + 1);
+	    
+		if (visualize) printf("\n");
+	}
+	
+	printf("\nAll tests passed\n");
+}
+
+/**
+ * @brief asks if the user would like to visualize then calls the test() function
+ * 
+ * 
+ * @returns 0 if all tests passed
+ */
+int main(void)
+{
+	int visualize = 0;
+	
+	printf("Would you like to see the process? (y/N) - ");
+	if (tolower(getchar()) == 'y') visualize = 1;
+	printf("\n");
+	
+	test(visualize);
+
+    return 0;
+}