diff --git a/Makefile b/Makefile
index 41cdbcb2..9be9a9e6 100644
--- a/Makefile
+++ b/Makefile
@@ -104,7 +104,18 @@ BASE64_SRC =					ft_base64.c 								\
 
 DES_SRC =						ft_des_initial_permutation.c 				\
 								ft_des_final_permutation.c 					\
-								ft_des_expansion_box.c
+								ft_des_expansion_box.c 						\
+								ft_des_s_box.c 								\
+								ft_des_s_box_1.c 							\
+								ft_des_s_box_2.c 							\
+								ft_des_s_box_3.c 							\
+								ft_des_s_box_4.c 							\
+								ft_des_s_box_5.c 							\
+								ft_des_s_box_6.c 							\
+								ft_des_s_box_7.c 							\
+								ft_des_s_box_8.c 							\
+								ft_des_feistel_function_permutation.c 		\
+								ft_des_feistel_function.c
 
 SRC =							main.c										\
 								ft_ssl_usage.c
diff --git a/inc/ft_des.h b/inc/ft_des.h
index 50e53b6b..68add0ee 100644
--- a/inc/ft_des.h
+++ b/inc/ft_des.h
@@ -15,7 +15,11 @@
 
 # define FT_DES_BIT_BLOCK_SIZE 64
 # define FT_DES_EXPANDED_HALF_BLOCK_SIZE 48
-# define FT_DES_S_BOX_SIZE 6
+# define FT_DES_S_BOX_INPUT_SIZE 6
+# define FT_DES_S_BOX_OUTPUT_SIZE 4
+# define FT_DES_S_BOX_TABLE_ROWS 4
+# define FT_DES_S_BOX_TABLE_COLUMNS 16
+# define FT_DES_FEISTEL_FUNCTION_KEY_SIZE 48
 
 typedef unsigned char	t_byte1;
 
@@ -37,4 +41,72 @@ void					ft_des_expansion_box
 	t_byte1 expanded[FT_DES_EXPANDED_HALF_BLOCK_SIZE]
 );
 
+void					ft_des_s_box
+(
+	t_byte1 input[FT_DES_S_BOX_INPUT_SIZE],
+	t_byte1 output[FT_DES_S_BOX_OUTPUT_SIZE],
+	t_byte1 table[FT_DES_S_BOX_TABLE_ROWS][FT_DES_S_BOX_TABLE_COLUMNS]
+);
+
+void					ft_des_s_box_1
+(
+	t_byte1 input[FT_DES_S_BOX_INPUT_SIZE],
+	t_byte1 output[FT_DES_S_BOX_OUTPUT_SIZE]
+);
+
+void					ft_des_s_box_2
+(
+	t_byte1 input[FT_DES_S_BOX_INPUT_SIZE],
+	t_byte1 output[FT_DES_S_BOX_OUTPUT_SIZE]
+);
+
+void					ft_des_s_box_3
+(
+	t_byte1 input[FT_DES_S_BOX_INPUT_SIZE],
+	t_byte1 output[FT_DES_S_BOX_OUTPUT_SIZE]
+);
+
+void					ft_des_s_box_4
+(
+	t_byte1 input[FT_DES_S_BOX_INPUT_SIZE],
+	t_byte1 output[FT_DES_S_BOX_OUTPUT_SIZE]
+);
+
+void					ft_des_s_box_5
+(
+	t_byte1 input[FT_DES_S_BOX_INPUT_SIZE],
+	t_byte1 output[FT_DES_S_BOX_OUTPUT_SIZE]
+);
+
+void					ft_des_s_box_6
+(
+	t_byte1 input[FT_DES_S_BOX_INPUT_SIZE],
+	t_byte1 output[FT_DES_S_BOX_OUTPUT_SIZE]
+);
+
+void					ft_des_s_box_7
+(
+	t_byte1 input[FT_DES_S_BOX_INPUT_SIZE],
+	t_byte1 output[FT_DES_S_BOX_OUTPUT_SIZE]
+);
+
+void					ft_des_s_box_8
+(
+	t_byte1 input[FT_DES_S_BOX_INPUT_SIZE],
+	t_byte1 output[FT_DES_S_BOX_OUTPUT_SIZE]
+);
+
+void					ft_des_feistel_function_permutation
+(
+	t_byte1 input[FT_DES_BIT_BLOCK_SIZE / 2],
+	t_byte1 output[FT_DES_BIT_BLOCK_SIZE / 2]
+);
+
+void					ft_des_feistel_function
+(
+	t_byte1 right_half[FT_DES_BIT_BLOCK_SIZE / 2],
+	t_byte1 key[FT_DES_FEISTEL_FUNCTION_KEY_SIZE],
+	t_byte1 output[FT_DES_BIT_BLOCK_SIZE / 2]
+);
+
 #endif
diff --git a/src/des/ft_des_feistel_function.c b/src/des/ft_des_feistel_function.c
new file mode 100644
index 00000000..98293e6a
--- /dev/null
+++ b/src/des/ft_des_feistel_function.c
@@ -0,0 +1,39 @@
+#include "ft_des.h"
+
+static void merge_key
+(
+	t_byte1 expanded_half[FT_DES_EXPANDED_HALF_BLOCK_SIZE],
+	t_byte1 key[FT_DES_FEISTEL_FUNCTION_KEY_SIZE],
+	t_byte1 output[FT_DES_EXPANDED_HALF_BLOCK_SIZE]
+)
+{
+	int i = 0;
+	while(i < FT_DES_EXPANDED_HALF_BLOCK_SIZE)
+	{
+		output[i] = expanded_half[i] ^ key[i];
+		i++;
+	}
+}
+
+void	ft_des_feistel_function
+(
+	t_byte1 right_half[FT_DES_BIT_BLOCK_SIZE / 2],
+	t_byte1 key[FT_DES_FEISTEL_FUNCTION_KEY_SIZE],
+	t_byte1 output[FT_DES_BIT_BLOCK_SIZE / 2]
+)
+{
+	t_byte1 expanded[FT_DES_EXPANDED_HALF_BLOCK_SIZE];
+	t_byte1 all_s_boxes[FT_DES_BIT_BLOCK_SIZE / 2];
+
+	ft_des_expansion_box(right_half, expanded);
+	merge_key(expanded, key, expanded);
+	ft_des_s_box_1(expanded, all_s_boxes);
+	ft_des_s_box_2(expanded + 6, all_s_boxes + 4);
+	ft_des_s_box_3(expanded + 12, all_s_boxes + 8);
+	ft_des_s_box_4(expanded + 18, all_s_boxes + 12);
+	ft_des_s_box_5(expanded + 24, all_s_boxes + 16);
+	ft_des_s_box_6(expanded + 30, all_s_boxes + 20);
+	ft_des_s_box_7(expanded + 36, all_s_boxes + 24);
+	ft_des_s_box_8(expanded + 42, all_s_boxes + 28);
+	ft_des_feistel_function_permutation(all_s_boxes, output);
+}
\ No newline at end of file
diff --git a/src/des/ft_des_feistel_function_permutation.c b/src/des/ft_des_feistel_function_permutation.c
new file mode 100644
index 00000000..d254f0e1
--- /dev/null
+++ b/src/des/ft_des_feistel_function_permutation.c
@@ -0,0 +1,24 @@
+#include "ft_des.h"
+
+void ft_des_feistel_function_permutation
+(
+	t_byte1 input[FT_DES_BIT_BLOCK_SIZE / 2],
+	t_byte1 output[FT_DES_BIT_BLOCK_SIZE / 2]
+)
+{
+	static t_byte1 table[FT_DES_BIT_BLOCK_SIZE / 2] = {
+		16,  7, 20, 21, 29, 12, 28, 17,
+		 1, 15, 23, 26,  5, 18, 31, 10,
+		 2,  8, 24, 14, 32, 27,  3,  9,
+		19, 13, 30,  6, 22, 11,  4, 25,
+	};
+	int i;
+
+	i = 0;
+	while(i < FT_DES_BIT_BLOCK_SIZE / 2)
+	{
+		output[i] = input[table[i] - 1];
+		i++;
+	}
+
+}
\ No newline at end of file
diff --git a/src/des/ft_des_s_box.c b/src/des/ft_des_s_box.c
new file mode 100644
index 00000000..3cacf7da
--- /dev/null
+++ b/src/des/ft_des_s_box.c
@@ -0,0 +1,21 @@
+#include "ft_des.h"
+
+void	ft_des_s_box
+(
+	t_byte1 input[FT_DES_S_BOX_INPUT_SIZE],
+	t_byte1 output[FT_DES_S_BOX_OUTPUT_SIZE],
+	t_byte1 table[FT_DES_S_BOX_TABLE_ROWS][FT_DES_S_BOX_TABLE_COLUMNS]
+)
+{
+	t_byte1 row;
+	t_byte1 column;
+	t_byte1 selected;
+
+	row = input[0] << 1 | input[5];
+	column = input[1] << 3 | input[2] << 2 | input[3] << 1 | input[4];
+	selected = table[row][column];
+	output[0] = selected >> 3 & 1;
+	output[1] = selected >> 2 & 1;
+	output[2] = selected >> 1 & 1;
+	output[3] = selected & 1;
+}
\ No newline at end of file
diff --git a/src/des/ft_des_s_box_1.c b/src/des/ft_des_s_box_1.c
new file mode 100644
index 00000000..82b2fe85
--- /dev/null
+++ b/src/des/ft_des_s_box_1.c
@@ -0,0 +1,16 @@
+#include "ft_des.h"
+
+void ft_des_s_box_1
+(
+	t_byte1 input[FT_DES_S_BOX_INPUT_SIZE],
+	t_byte1 output[FT_DES_S_BOX_OUTPUT_SIZE]
+)
+{
+	static t_byte1 tbl[FT_DES_S_BOX_TABLE_ROWS][FT_DES_S_BOX_TABLE_COLUMNS] = {
+		{ 14,  4, 13,  1,  2, 15, 11,  8,  3, 10, 6,  12,  5,  9,  0,  7 },
+		{  0, 15,  7,  4, 14,  2, 13,  1, 10,  6, 12, 11,  9,  5,  3,  8 },
+		{  4,  1, 14,  8, 13,  6,  2, 11, 15, 12,  9,  7,  3, 10,  5,  0 },
+		{ 15, 12,  8,  2,  4,  9,  1,  7,  5, 11,  3, 14, 10,  0,  6, 13 }
+	};
+	ft_des_s_box(input, output, tbl);
+}
\ No newline at end of file
diff --git a/src/des/ft_des_s_box_2.c b/src/des/ft_des_s_box_2.c
new file mode 100644
index 00000000..ec513508
--- /dev/null
+++ b/src/des/ft_des_s_box_2.c
@@ -0,0 +1,16 @@
+#include "ft_des.h"
+
+void	ft_des_s_box_2
+(
+	t_byte1 input[FT_DES_S_BOX_INPUT_SIZE],
+	t_byte1 output[FT_DES_S_BOX_OUTPUT_SIZE]
+)
+{
+	static t_byte1 tbl[FT_DES_S_BOX_TABLE_ROWS][FT_DES_S_BOX_TABLE_COLUMNS] = {
+		{15,  1,  8, 14,  6, 11,  3,  4,  9,  7,  2, 13, 12,  0,  5, 10},
+		{ 3, 13,  4,  7, 15,  2,  8, 14, 12,  0,  1, 10,  6,  9, 11,  5},
+		{ 0, 14,  7, 11, 10,  4, 13,  1,  5,  8, 12,  6,  9,  3,  2, 15},
+		{13,  8, 10,  1,  3, 15,  4,  2, 11,  6,  7, 12,  0,  5, 14,  9},
+	};
+	ft_des_s_box(input, output, tbl);
+}
\ No newline at end of file
diff --git a/src/des/ft_des_s_box_3.c b/src/des/ft_des_s_box_3.c
new file mode 100644
index 00000000..7fb8955b
--- /dev/null
+++ b/src/des/ft_des_s_box_3.c
@@ -0,0 +1,16 @@
+#include "ft_des.h"
+
+void	ft_des_s_box_3
+(
+	t_byte1 input[FT_DES_S_BOX_INPUT_SIZE],
+	t_byte1 output[FT_DES_S_BOX_OUTPUT_SIZE]
+)
+{
+	static t_byte1 tbl[FT_DES_S_BOX_TABLE_ROWS][FT_DES_S_BOX_TABLE_COLUMNS] = {
+		{10,  0,  9, 14,  6,  3, 15,  5,  1, 13, 12,  7, 11,  4,  2,  8},
+		{13,  7,  0,  9,  3,  4,  6, 10,  2,  8,  5, 14, 12, 11, 15,  1},
+		{13,  6,  4,  9,  8, 15,  3,  0, 11,  1,  2, 12,  5, 10, 14,  7},
+		{ 1, 10, 13,  0,  6,  9,  8,  7,  4, 15, 14,  3, 11,  5,  2, 12},
+	};
+	ft_des_s_box(input, output, tbl);
+}
\ No newline at end of file
diff --git a/src/des/ft_des_s_box_4.c b/src/des/ft_des_s_box_4.c
new file mode 100644
index 00000000..8e6540dc
--- /dev/null
+++ b/src/des/ft_des_s_box_4.c
@@ -0,0 +1,16 @@
+#include "ft_des.h"
+
+void	ft_des_s_box_4
+(
+	t_byte1 input[FT_DES_S_BOX_INPUT_SIZE],
+	t_byte1 output[FT_DES_S_BOX_OUTPUT_SIZE]
+)
+{
+	static t_byte1 tbl[FT_DES_S_BOX_TABLE_ROWS][FT_DES_S_BOX_TABLE_COLUMNS] = {
+		{ 7, 13, 14,  3,  0,  6,  9, 10,  1,  2,  8,  5, 11, 12,  4, 15},
+		{13,  8, 11,  5,  6, 15,  0,  3,  4,  7,  2, 12,  1, 10, 14,  9},
+		{10,  6,  9,  0, 12, 11,  7, 13, 15,  1,  3, 14,  5,  2,  8,  4},
+		{ 3, 15,  0,  6, 10,  1, 13,  8,  9,  4,  5, 11, 12,  7,  2, 14},
+	};
+	ft_des_s_box(input, output, tbl);
+}
\ No newline at end of file
diff --git a/src/des/ft_des_s_box_5.c b/src/des/ft_des_s_box_5.c
new file mode 100644
index 00000000..e8a5f5ff
--- /dev/null
+++ b/src/des/ft_des_s_box_5.c
@@ -0,0 +1,16 @@
+#include "ft_des.h"
+
+void	ft_des_s_box_5
+(
+	t_byte1 input[FT_DES_S_BOX_INPUT_SIZE],
+	t_byte1 output[FT_DES_S_BOX_OUTPUT_SIZE]
+)
+{
+	static t_byte1 tbl[FT_DES_S_BOX_TABLE_ROWS][FT_DES_S_BOX_TABLE_COLUMNS] = {
+		{ 2, 12,  4,  1,  7, 10, 11,  6,  8,  5,  3, 15, 13,  0, 14,  9},
+		{14, 11,  2, 12,  4,  7, 13,  1,  5,  0, 15, 10,  3,  9,  8,  6},
+		{ 4,  2,  1, 11, 10, 13,  7,  8, 15,  9, 12,  5,  6,  3,  0, 14},
+		{11,  8, 12,  7,  1, 14,  2, 13,  6, 15,  0,  9, 10,  4,  5,  3},
+	};
+	ft_des_s_box(input, output, tbl);
+}
\ No newline at end of file
diff --git a/src/des/ft_des_s_box_6.c b/src/des/ft_des_s_box_6.c
new file mode 100644
index 00000000..5f6f3f0f
--- /dev/null
+++ b/src/des/ft_des_s_box_6.c
@@ -0,0 +1,16 @@
+#include "ft_des.h"
+
+void	ft_des_s_box_6
+(
+	t_byte1 input[FT_DES_S_BOX_INPUT_SIZE],
+	t_byte1 output[FT_DES_S_BOX_OUTPUT_SIZE]
+)
+{
+	static t_byte1 tbl[FT_DES_S_BOX_TABLE_ROWS][FT_DES_S_BOX_TABLE_COLUMNS] = {
+		{12,  1, 10, 15,  9,  2,  6,  8,  0, 13,  3,  4, 14,  7,  5, 11},
+		{10, 15,  4,  2,  7, 12,  9,  5,  6,  1, 13, 14,  0, 11,  3,  8},
+		{ 9, 14, 15,  5,  2,  8, 12,  3,  7,  0,  4, 10,  1, 13, 11,  6},
+		{ 4,  3,  2, 12,  9,  5, 15, 10, 11, 14,  1,  7,  6,  0,  8, 13},
+	};
+	ft_des_s_box(input, output, tbl);
+}
\ No newline at end of file
diff --git a/src/des/ft_des_s_box_7.c b/src/des/ft_des_s_box_7.c
new file mode 100644
index 00000000..60d580b2
--- /dev/null
+++ b/src/des/ft_des_s_box_7.c
@@ -0,0 +1,16 @@
+#include "ft_des.h"
+
+void	ft_des_s_box_7
+(
+	t_byte1 input[FT_DES_S_BOX_INPUT_SIZE],
+	t_byte1 output[FT_DES_S_BOX_OUTPUT_SIZE]
+)
+{
+	static t_byte1 tbl[FT_DES_S_BOX_TABLE_ROWS][FT_DES_S_BOX_TABLE_COLUMNS] = {
+		{ 4, 11,  2, 14, 15,  0,  8, 13,  3, 12,  9,  7,  5, 10,  6,  1},
+		{13,  0, 11,  7,  4,  9,  1, 10, 14,  3,  5, 12,  2, 15,  8,  6},
+		{ 1,  4, 11, 13, 12,  3,  7, 14, 10, 15,  6,  8,  0,  5,  9,  2},
+		{ 6, 11, 13,  8,  1,  4, 10,  7,  9,  5,  0, 15, 14,  2,  3, 12},
+	};
+	ft_des_s_box(input, output, tbl);
+}
\ No newline at end of file
diff --git a/src/des/ft_des_s_box_8.c b/src/des/ft_des_s_box_8.c
new file mode 100644
index 00000000..954e3476
--- /dev/null
+++ b/src/des/ft_des_s_box_8.c
@@ -0,0 +1,16 @@
+#include "ft_des.h"
+
+void	ft_des_s_box_8
+(
+	t_byte1 input[FT_DES_S_BOX_INPUT_SIZE],
+	t_byte1 output[FT_DES_S_BOX_OUTPUT_SIZE]
+)
+{
+	static t_byte1 tbl[FT_DES_S_BOX_TABLE_ROWS][FT_DES_S_BOX_TABLE_COLUMNS] = {
+		{13,  2,  8,  4,  6, 15, 11,  1, 10,  9,  3, 14,  5,  0, 12,  7},
+		{ 1, 15, 13,  8, 10,  3,  7,  4, 12,  5,  6, 11,  0, 14,  9,  2},
+		{ 7, 11,  4,  1,  9, 12, 14,  2,  0,  6, 10, 13, 15, 03,  5,  8},
+		{ 2,  1, 14,  7,  4, 10,  8, 13, 15, 12,  9,  0,  3,  5,  6, 11},
+	};
+	ft_des_s_box(input, output, tbl);
+}
\ No newline at end of file
diff --git a/t/des_tests.c b/t/des_tests.c
index ed2cce1d..741e0a63 100644
--- a/t/des_tests.c
+++ b/t/des_tests.c
@@ -3,6 +3,8 @@
 #include "ft_des.h"
 #include "libft.h"
 
+#define S_BOX_CASES_NUMBER 3
+
 int perform_initial_permutation()
 {
 	// all 64 bits:
@@ -159,7 +161,222 @@ int perform_expansion_in_feistel_function()
 	_end("perform expansion in feistel function");
 }
 
+static int s_box_check
+(
+	t_byte1 inputs[S_BOX_CASES_NUMBER][FT_DES_S_BOX_INPUT_SIZE],
+	t_byte1 expected[S_BOX_CASES_NUMBER][FT_DES_S_BOX_OUTPUT_SIZE],
+	void (*box)(t_byte1 in[FT_DES_S_BOX_INPUT_SIZE],
+				t_byte1 out[FT_DES_S_BOX_OUTPUT_SIZE])
+)
+{
+	t_byte1 output[FT_DES_S_BOX_OUTPUT_SIZE];
+	int j = 0;
+	while(j < S_BOX_CASES_NUMBER)
+	{
+		box(inputs[j], output);
 
+		int i = 0;
+		while(i < FT_DES_S_BOX_OUTPUT_SIZE)
+		{
+			if(output[i] != expected[j][i])
+				return 0;
+			i++;
+		}
+		j++;
+	}
+	return 1;
+}
+
+int	s_boxes_confuse()
+{
+	t_byte1 s1_inputs[S_BOX_CASES_NUMBER][FT_DES_S_BOX_INPUT_SIZE] = {
+		{1, 0, 0, 1, 0, 1},
+		{0, 0, 0, 0, 0, 0},
+		{0, 1, 1, 1, 1, 0},
+	};
+	t_byte1 s1_expected[S_BOX_CASES_NUMBER][FT_DES_S_BOX_OUTPUT_SIZE] = {
+		{1, 0, 0, 0},
+		{1, 1, 1, 0},
+		{0, 1, 1, 1},
+	};
+
+	_is(s_box_check(s1_inputs, s1_expected, ft_des_s_box_1));
+
+	t_byte1 s2_inputs[S_BOX_CASES_NUMBER][FT_DES_S_BOX_INPUT_SIZE] = {
+		{0, 0, 0, 0, 0, 0},
+		{1, 0, 0, 0, 0, 1},
+		{1, 1, 0, 0, 0, 0},
+	};
+
+	t_byte1 s2_expected[S_BOX_CASES_NUMBER][FT_DES_S_BOX_OUTPUT_SIZE] = {
+		{1, 1, 1, 1},
+		{1, 1, 0, 1},
+		{0, 1, 0, 1},
+	};
+
+	_is(s_box_check(s2_inputs, s2_expected, ft_des_s_box_2));
+
+	t_byte1 s3_inputs[S_BOX_CASES_NUMBER][FT_DES_S_BOX_INPUT_SIZE] = {
+		{0, 0, 0, 0, 0, 0},
+		{1, 0, 0, 0, 0, 1},
+		{1, 1, 0, 0, 0, 0},
+	};
+
+	t_byte1 s3_expected[S_BOX_CASES_NUMBER][FT_DES_S_BOX_OUTPUT_SIZE] = {
+		{1, 0, 1, 0},
+		{0, 0, 0, 1},
+		{1, 0, 1, 1},
+	};
+
+	_is(s_box_check(s3_inputs, s3_expected, ft_des_s_box_3));
+
+	t_byte1 s4_inputs[S_BOX_CASES_NUMBER][FT_DES_S_BOX_INPUT_SIZE] = {
+		{0, 0, 0, 0, 0, 0},
+		{1, 0, 0, 0, 0, 1},
+		{1, 1, 0, 0, 0, 0},
+	};
+
+	t_byte1 s4_expected[S_BOX_CASES_NUMBER][FT_DES_S_BOX_OUTPUT_SIZE] = {
+		{0, 1, 1, 1},
+		{0, 0, 1, 1},
+		{1, 1, 1, 1},
+	};
+
+	_is(s_box_check(s4_inputs, s4_expected, ft_des_s_box_4));
+
+	t_byte1 s5_inputs[S_BOX_CASES_NUMBER][FT_DES_S_BOX_INPUT_SIZE] = {
+		{0, 0, 0, 0, 0, 0},
+		{1, 0, 0, 0, 0, 1},
+		{1, 1, 1, 1, 1, 1},
+	};
+
+	t_byte1 s5_expected[S_BOX_CASES_NUMBER][FT_DES_S_BOX_OUTPUT_SIZE] = {
+		{0, 0, 1, 0},
+		{1, 0, 1, 1},
+		{0, 0, 1, 1},
+	};
+
+	_is(s_box_check(s5_inputs, s5_expected, ft_des_s_box_5));
+
+	t_byte1 s6_inputs[S_BOX_CASES_NUMBER][FT_DES_S_BOX_INPUT_SIZE] = {
+		{0, 0, 0, 0, 0, 0},
+		{1, 0, 0, 0, 0, 1},
+		{1, 1, 1, 1, 1, 1},
+	};
+
+	t_byte1 s6_expected[S_BOX_CASES_NUMBER][FT_DES_S_BOX_OUTPUT_SIZE] = {
+		{1, 1, 0, 0},
+		{0, 1, 0, 0},
+		{1, 1, 0, 1},
+	};
+
+	_is(s_box_check(s6_inputs, s6_expected, ft_des_s_box_6));
+
+	t_byte1 s7_inputs[S_BOX_CASES_NUMBER][FT_DES_S_BOX_INPUT_SIZE] = {
+		{0, 0, 0, 0, 0, 0},
+		{1, 0, 0, 0, 0, 1},
+		{1, 1, 1, 1, 1, 1},
+	};
+
+	t_byte1 s7_expected[S_BOX_CASES_NUMBER][FT_DES_S_BOX_OUTPUT_SIZE] = {
+		{0, 1, 0, 0},
+		{0, 1, 1, 0},
+		{1, 1, 0, 0},
+	};
+
+	_is(s_box_check(s7_inputs, s7_expected, ft_des_s_box_7));
+
+	t_byte1 s8_inputs[S_BOX_CASES_NUMBER][FT_DES_S_BOX_INPUT_SIZE] = {
+		{0, 0, 0, 0, 0, 0},
+		{1, 0, 0, 0, 0, 1},
+		{1, 1, 1, 1, 1, 1},
+	};
+
+	t_byte1 s8_expected[S_BOX_CASES_NUMBER][FT_DES_S_BOX_OUTPUT_SIZE] = {
+		{1, 1, 0, 1},
+		{0, 0, 1, 0},
+		{1, 0, 1, 1},
+	};
+
+	_is(s_box_check(s8_inputs, s8_expected, ft_des_s_box_8));
+
+	_end("s boxes confuse");
+}
+
+int perform_premutation_in_feistel_function()
+{
+	t_byte1 input[FT_DES_BIT_BLOCK_SIZE / 2] = {
+		1, 0, 1, 0, 1, 0, 1, 0,
+		0, 1, 0, 1, 0, 1, 0, 1,
+		1, 0, 1, 0, 1, 0, 1, 0,
+		0, 1, 0, 1, 0, 1, 0, 1,
+	};
+
+	t_byte1 expected[FT_DES_BIT_BLOCK_SIZE / 2] = {
+		1, 1, 0, 1, 0, 1, 1, 1,
+		1, 0, 1, 1, 1, 0, 0, 1,
+		0, 0, 0, 1, 1, 0, 1, 0,
+		1, 0, 1, 0, 0, 0, 0, 0,
+	};
+
+	t_byte1 actual[FT_DES_BIT_BLOCK_SIZE / 2];
+
+	ft_des_feistel_function_permutation(input, actual);
+
+	int i = 0;
+	while(i < FT_DES_BIT_BLOCK_SIZE / 2)
+	{
+		_is(actual[i] == expected[i]);
+		i++;
+	}
+	_end("should perform permutation in feistel function");
+}
+
+int perform_feistel_function()
+{
+	t_byte1 key[FT_DES_FEISTEL_FUNCTION_KEY_SIZE] = {
+		0, 0, 0, 1, 1, 0,
+		1, 1, 0, 0, 0, 0,
+		0, 0, 1, 0, 1, 1,
+		1, 0, 1, 1, 1, 1,
+		1, 1, 1, 1, 1, 1,
+		0, 0, 0, 1, 1, 1,
+		0, 0, 0, 0, 0, 1,
+		1, 1, 0, 0, 1, 0,
+	};
+	t_byte1 right_half[FT_DES_BIT_BLOCK_SIZE / 2] = {
+		1, 1, 1, 1,
+		0, 0, 0, 0,
+		1, 0, 1, 0,
+		1, 0, 1, 0,
+		1, 1, 1, 1,
+		0, 0, 0, 0,
+		1, 0, 1, 0,
+		1, 0, 1, 0,
+	};
+	t_byte1 expected[FT_DES_BIT_BLOCK_SIZE / 2] = {
+		0, 0, 1, 0,
+		0, 0, 1, 1,
+		0, 1, 0, 0,
+		1, 0, 1, 0,
+		1, 0, 1, 0,
+		1, 0, 0, 1,
+		1, 0, 1, 1,
+		1, 0, 1, 1,
+	};
+
+	t_byte1 output[FT_DES_BIT_BLOCK_SIZE / 2];
+
+	ft_des_feistel_function(right_half, key, output);
+
+	int i = 0;
+	while(i < FT_DES_BIT_BLOCK_SIZE / 2)
+	{
+		_is(output[i] == expected[i]);
+		i++;
+	}
+	_end("feistel function should encode half of a block");
+}
 
 int des_tests()
 {
@@ -167,5 +384,8 @@ int des_tests()
 	_should(perform_final_permutation);
 	_should(final_permutation_is_reverse_of_initial);
 	_should(perform_expansion_in_feistel_function);
+	_should(s_boxes_confuse);
+	_should(perform_premutation_in_feistel_function);
+	_should(perform_feistel_function);
 	return 0;
 }
\ No newline at end of file