From d9ebbb6c4a92a2391977c0d252ed87da725d8e28 Mon Sep 17 00:00:00 2001
From: tomsmeding <tom.smeding@gmail.com>
Date: Thu, 13 Oct 2016 18:31:51 +0200
Subject: Comment envelope.{cpp,h}

---
 envelope.cpp | 52 +++++++++-------------------------------------------
 1 file changed, 9 insertions(+), 43 deletions(-)

diff --git a/envelope.cpp b/envelope.cpp
index 4971358..abf6e8f 100644
--- a/envelope.cpp
+++ b/envelope.cpp
@@ -14,8 +14,12 @@ using namespace std;
 
 namespace Envelope{
 
+	// Envelope format: [AES encrypted data | RSA encrypted AES key | length of AES key]
+	// The encrypted AES key is in big-endian format, as is the length tag. The length tag
+	// is an unsigned 16-bit integer.
+
 	bool safeKey(const string &key){
-		//checks against keys 0 and 1, because they don't undergo change in RSA
+		//checks against keys 0 and 1, because they don't undergo change in RSA enryption
 		int i;
 		for(i=0;i<(int)key.size();i++)if(key[i]!=0)break;
 		if(i==(int)key.size())return false; //key is 0
@@ -30,36 +34,20 @@ namespace Envelope{
 		do {
 			for(int i=0;i<keylen;i++)*(uint32_t*)&aeskey[4*i]=crng.get();
 		} while(!safeKey(aeskey));
-#ifdef DEBUG
-		cerr<<"decrkey="<<Base64::encode(aeskey)<<endl;
-#endif
 		
 		string payload(AES::encrypt(data,aeskey,AES::AES_256_CBC));
-#ifdef DEBUG
-		cerr<<"payload="<<Base64::encode(payload)<<endl;
-#endif
 		Bigint rsadata;
 		for(int i=0;i<(int)aeskey.size();i++){
-			if(i!=0)rsadata<<=8;
+			if(i!=0)rsadata<<=8; //we see aeskey as being in big-endian format
 			rsadata+=(uint8_t)aeskey[i];
 		}
-#ifdef DEBUG
-		cerr<<"rsadata="<<rsadata<<endl;
-		cerr<<"pubkey = {"<<pubkey.mod<<" , "<<pubkey.exp<<'}'<<endl;
-#endif
 		Bigint res(RSA::encrypt(rsadata,pubkey));
-#ifdef DEBUG
-		cerr<<"rsaencr="<<res<<endl;
-#endif
 
-		vector<uint8_t> bytes; //bytes in little-endian order (so, reverse!)
+		vector<uint8_t> bytes; //bytes in little-endian order (so, reversed!)
 		while(res!=0){
 			bytes.push_back(res.lowdigits()&0xff);
 			res>>=8;
 		}
-#ifdef DEBUG
-		cerr<<"encrkey="<<Base64::encode(string(bytes.rbegin(),bytes.rend()))<<endl;
-#endif
 
 		payload.reserve(payload.size()+bytes.size()+2);
 		for(int i=bytes.size()-1;i>=0;i--)payload.push_back(bytes[i]); //append in big-endian order again
@@ -70,38 +58,22 @@ namespace Envelope{
 	}
 
 	string decrypt(const string &data,const RSA::Key &privkey){
-#ifdef DEBUG
-		cerr<<"=== DECRYPT ==="<<endl;
-#endif
 		if(data.size()<2)throw invalid_argument("Envelope data length invalid");
 		int encrkeylen=((uint16_t)(uint8_t)data[data.size()-2]<<8)+(uint8_t)data.back();
 		assert(encrkeylen<(1<<16));
-#ifdef DEBUG
-		cerr<<"encrkeylen="<<encrkeylen<<endl;
-#endif
 		if((int)data.size()<encrkeylen+2)throw invalid_argument("Envelope key format invalid");
 
 		string encrkey(encrkeylen,'\0'); //in big-endian
 		for(int i=0;i<encrkeylen;i++){
 			encrkey[i]=data[data.size()-2-encrkeylen+i];
 		}
-#ifdef DEBUG
-		cerr<<"encrkey="<<Base64::encode(encrkey)<<endl;
-#endif
 
 		Bigint rsadata;
 		for(int i=0;i<encrkeylen;i++){
-			if(i!=0)rsadata<<=8;
+			if(i!=0)rsadata<<=8; //encrkey is also in big-endian
 			rsadata+=(uint8_t)encrkey[i];
 		}
-#ifdef DEBUG
-		cerr<<"rsaencr="<<rsadata<<endl;
-		cerr<<"privkey = {"<<privkey.mod<<" , "<<privkey.exp<<'}'<<endl;
-#endif
 		Bigint res(RSA::decrypt(rsadata,privkey));
-#ifdef DEBUG
-		cerr<<"rsadata="<<res<<endl;
-#endif
 
 		vector<uint8_t> bytes; //bytes in little-endian order
 		for(int i=0;i<32;i++){ //32 is the number of bytes in a 256-bit AES key (256/8=32)
@@ -112,14 +84,8 @@ namespace Envelope{
 			throw invalid_argument("Envelope RSA private key incorrect");
 		}
 		string decrkey(bytes.size(),'\0');
-		for(int i=0;i<(int)bytes.size();i++)decrkey[bytes.size()-1-i]=bytes[i];
-#ifdef DEBUG
-		cerr<<"decrkey="<<Base64::encode(decrkey)<<endl;
-#endif
+		for(int i=0;i<(int)bytes.size();i++)decrkey[bytes.size()-1-i]=bytes[i]; //save in decrkey in big-endian order again
 
-#ifdef DEBUG
-		cerr<<"payload="<<Base64::encode(data.substr(0,data.size()-2-encrkeylen))<<endl;
-#endif
 		try {
 			return AES::decrypt(data.substr(0,data.size()-2-encrkeylen),decrkey,AES::AES_256_CBC);
 		} catch(invalid_argument){
-- 
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