From fa5696d2a1ec781bc3f7ff197ebf0c0019bb56c4 Mon Sep 17 00:00:00 2001
From: tomsmeding <tom.smeding@gmail.com>
Date: Wed, 12 Oct 2016 22:38:33 +0200
Subject: Comment aes.{cpp,h}

---
 aes.cpp | 31 +++++++++++++++++++------------
 aes.h   |  3 ++-
 2 files changed, 21 insertions(+), 13 deletions(-)

diff --git a/aes.cpp b/aes.cpp
index 7430717..f36a69f 100644
--- a/aes.cpp
+++ b/aes.cpp
@@ -1,5 +1,3 @@
-#include <iostream>
-#include <iomanip>
 #include <stdexcept>
 #include <cstring>
 #include <cassert>
@@ -17,16 +15,12 @@ namespace AES{
 	//State is represented in bytes, as is the key schedule (which is represented in words in the AES spec).
 	//State is in column-major order.
 
+	//Most functions (and the three tables below) refer to similarly named functions and tables in the AES
+	//spec. The core AES algorithm is not documented here.
+
 	uint32_t roundconstant[10]={};
 	uint8_t sbox[256]={};
-	uint8_t invsbox[256]={};
-
-	void printstate(const uint8_t *state){
-		for(int i=0;i<16;i++){
-			cout<<setw(2)<<setfill('0')<<hex<<(int)state[i]<<dec;
-		}
-		cout<<endl;
-	}
+	uint8_t invsbox[256]={}; //look-up table of sbox
 
 	void initTables(){
 		//generated tables have been checked with AES spec
@@ -140,6 +134,7 @@ namespace AES{
 	}
 
 
+	//The AES block encryption algorithm specified in the AES spec
 	void encryptBlock(uint8_t *state,const uint8_t *keysched,const uint8_t *data,int numrounds){
 		memcpy(state,data,16);
 
@@ -155,6 +150,7 @@ namespace AES{
 		addRoundKey(state,keysched+16*numrounds);
 	}
 
+	//The AES block decryption algorithm specified in the AES spec
 	void decryptBlock(uint8_t *state,const uint8_t *keysched,const uint8_t *data,int numrounds){
 		memcpy(state,data,16);
 
@@ -170,6 +166,14 @@ namespace AES{
 		addRoundKey(state,keysched);
 	}
 
+	//Encrypt variably sized data with a key (assumed to be of valid length).
+	//This applies the CBC algorithm: first generate an IV and add it to the
+	//ciphertext as the first block. Then when encrypting the next block, first
+	//xor it with the previous encrypted block. With this encryption method, all
+	//of the encrypted text is influenced by the random IV, and therefore
+	//different each time.
+	//Padding is standard: if n>0 bytes are needed to reach a multiple of 16 bytes,
+	//n bytes of padding are added with the value n. This is easily reversible.
 	string encryptCBC(const string &data,const string &key,int numrounds){
 		if(roundconstant[0]==0)initTables();
 
@@ -200,13 +204,14 @@ namespace AES{
 		}
 
 		if(padding<16)memcpy(inbuf,data.data()+16*blocks,16-padding);
-		memset(inbuf+16-padding,padding,padding);
+		memset(inbuf+16-padding,padding,padding); //apply the padding
 		for(int j=0;j<16;j++)inbuf[j]^=res[res.size()-16+j]; //the CBC xor step
-		encryptBlock(buf,keysched,inbuf,numrounds);
+		encryptBlock(buf,keysched,inbuf,numrounds); //encrypt the final padded block
 		res.insert(res.size(),(char*)buf,16);
 		return res;
 	}
 
+	//The inverse of encryptCBC
 	string decryptCBC(const string &data,const string &key,int numrounds){
 		if(roundconstant[0]==0)initTables();
 
@@ -228,6 +233,7 @@ namespace AES{
 		return res;
 	}
 
+	//Public interface to encryptCBC
 	string encrypt(const string &data,const string &key,Algorithm algo){
 		int increment;
 		switch(algo){
@@ -240,6 +246,7 @@ namespace AES{
 		return encryptCBC(data,key,10+2*increment);
 	}
 
+	//Public interface to decryptCBC
 	string decrypt(const string &data,const string &key,Algorithm algo){
 		int increment;
 		switch(algo){
diff --git a/aes.h b/aes.h
index 9274d21..77dc9d2 100644
--- a/aes.h
+++ b/aes.h
@@ -12,7 +12,8 @@ namespace AES{
 
 	std::string encrypt(const std::string &data,const std::string &key,Algorithm algo);
 
-	//throws invalid_argument for an invalid ciphertext (length not a multiple of block size, or padding malformed) or an invalid key (invalid length)
+	//throws invalid_argument for an invalid ciphertext (length not a multiple of block
+	//size, or padding malformed) or an invalid key (invalid length)
 	std::string decrypt(const std::string &data,const std::string &key,Algorithm algo);
 
 }
-- 
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