adding a project API description

Author: GRISHNOV

README.md

@@ -24,3 +24,112 @@ Implementation of PKCS7 padding in C. It can be used to pre-prepare data before
    including input values that are already a multiple of the block size,
    and no padding string is a suffix of another.  This padding method is
    well defined if and only if k is less than 256.
+
+# Quick start
+
+Download this repository and go to directory with project.
+
+```
+git clone https://github.com/GRISHNOV/PKCS7-Padding.git
+cd PKCS7-Padding
+```
+
+Run the project build using the make utility.
+
+```
+make
+```
+
+Now the `build` folder contains the `PKCS7.o` object file. Use it together with the `PKCS7.h` header file from `include` folder to work with PKCS7 padding in your project.
+
+For PKCS7 padding, use this function with appropriate structure:
+
+```C
+/* 
+    A pointer to this structure is returned from the function addPadding().
+    The structure contains result of adding PKCS7 padding.
+*/
+typedef struct {
+    void*    dataWithPadding;        /* result of adding padding to the data */
+    uint64_t dataLengthWithPadding;  /* length of the result */
+    uint8_t  valueOfByteForPadding;  /* used for padding byte value */
+} PKCS7_Padding; 
+
+/* 
+    Applies PKCS7 padding to data.
+    Your data at the provided address does not change. A copy is created, to which the adding padding is applied.
+    WARNING: use only 0 < BLOCK_SIZE < 256
+*/
+PKCS7_Padding* addPadding(const void* const data, const uint64_t dataLength, const uint8_t BLOCK_SIZE);
+```
+
+For PKCS7 unpadding, use this function with appropriate structure:
+
+```C
+/* 
+    A pointer to this structure is returned from the function removePadding().
+    The structure contains result of removing PKCS7 padding.
+*/
+typedef struct {
+    void*    dataWithoutPadding;         /* result of remove padding from data */
+    uint64_t dataLengthWithoutPadding;   /* length of the result */
+    uint8_t  valueOfRemovedByteFromData; /* value of byte that was used for padding */
+} PKCS7_unPadding;                              
+
+/* 
+    Remove PKCS7 padding from data.
+    Your data at the provided address does not change. A copy is created, to which the removing padding is applied.
+*/
+PKCS7_unPadding* removePadding(const void* const data, const uint64_t dataLength);
+```
+
+Also note the `paddingBlockSize` constants from the `PKCS7.h` file, which can be used to pass to addPadding() as the third parameter. For convenience, you can set your own `BLOCK_SIZE_CUSTOM_VALUE` value to this enum:
+
+```C
+/* 
+    Examples of commonly used block sizes for data padding.
+    WARNING: block size for PKCS7 padding can be 0 < BLOCK_SIZE < 256 bytes.
+*/
+typedef enum {
+    BLOCK_SIZE_128_BIT      = 128 / 8,  /* 16 bytes block */
+    BLOCK_SIZE_256_BIT      = 256 / 8,  /* 32 bytes block */
+    BLOCK_SIZE_CUSTOM_VALUE = 0         /* you can set your own constant to use */
+} paddingBlockSize;                     /* can be used as third argument to the function addPadding() */
+```
+
+# Demonstration
+
+You can also find `PKCS7_test.out` in the `build` folder. Use it to see how algorithm PKCS7 works.
+
+For example (3 bytes data and 128 bits block size):
+
+```
+./build/PKCS7_test.out 24 128
+```
+
+Output:
+
+```
+************************************
+
+ORIGINAL DATA (size is 3 bytes):
+
+ff	fe	fd	
+
+************************************
+
+WITH PADDING (now size is 16 bytes):
+
+ff	fe	fd	d
+d	d	d	d
+d	d	d	d
+d	d	d	d
+
+************************************
+
+REMOVE PADDING (size is 3 bytes):
+
+ff	fe	fd	
+
+************************************
+```