Add digital envelope encryption

1 files changed, 88 insertions, 0 deletions

diff --git a/envelope.cpp b/envelope.cpp
new file mode 100644
index 0000000..a8cd793
--- /dev/null
+++ b/envelope.cpp
@@ -0,0 +1,88 @@
+#include <cassert>
+#include "aes.h"
+#include "base64.h" //TODO: remove
+#include "envelope.h"
+#include "rng.h"
+
+using namespace std;
+
+namespace Envelope{
+
+	bool safeKey(const string &key){
+		//checks against keys 0 and 1, because they don't undergo change in RSA
+		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
+		if(i==(int)key.size()-1&&key[i]==1)return false; //key is 1
+		return true; //fine
+	}
+
+	string encrypt(const string &data,const RSA::PublicKey &pubkey){
+		const int keylen=8; //256-bit
+		CryptoRng crng;
+		string key(4*keylen,'\0');
+		do {
+			for(int i=0;i<keylen;i++)*(uint32_t*)&key[4*i]=crng.get();
+		} while(!safeKey(key));
+		//cerr<<"WARNING: using predetermined envelope key"<<endl;
+		//key="kaasKAASkaasKAAShalloHALLOhallo!";
+
+		cerr<<"decrkey="<<Base64::encode(key)<<endl;
+		
+		string payload(AES::encrypt(data,key,AES::AES_256_CBC));
+		cerr<<"payload="<<Base64::encode(payload)<<endl;
+		Bigint rsadata;
+		for(int i=0;i<(int)key.size();i++){
+			if(i!=0)rsadata<<=8;
+			rsadata+=(uint8_t)key[i];
+		}
+		cerr<<"rsadata="<<rsadata<<endl;
+		// cerr<<"We encrypt:"<<endl<<"  "<<rsadata<<endl;
+		Bigint res(RSA::encrypt(rsadata,pubkey));
+		// cerr<<"to:"<<endl<<"  "<<res<<endl;
+		vector<uint8_t> bytes; //bytes in little-endian order
+		while(res!=0){
+			bytes.push_back(res.lowdigits()&0xff);
+			res>>=8;
+		}
+		cerr<<"encrkey="<<Base64::encode(string(bytes.rbegin(),bytes.rend()))<<endl;
+		payload.reserve(payload.size()+bytes.size()+2);
+		for(int i=bytes.size()-1;i>=0;i--)payload.push_back(bytes[i]);
+		payload.push_back(bytes.size()>>8);
+		payload.push_back((uint8_t)bytes.size()&0xff);
+
+		return payload;
+	}
+
+	string decrypt(const string &data,const RSA::PrivateKey &privkey){
+		cerr<<"=== DECRYPT ==="<<endl;
+		if(data.size()<2)throw invalid_argument("Envelope data length invalid");
+		int encrkeylen=((uint16_t)data[data.size()-2]<<8)+(uint16_t)data.back();
+		assert(encrkeylen<(1<<16));
+		cerr<<"encrkeylen="<<encrkeylen<<endl;
+		if((int)data.size()<encrkeylen+2)throw invalid_argument("Envelope key format invalid");
+		string encrkey(encrkeylen,'\0');
+		for(int i=0;i<encrkeylen;i++){
+			encrkey[i]=data[data.size()-2-encrkeylen+i];
+		}
+		cerr<<"encrkey="<<Base64::encode(encrkey)<<endl;
+		Bigint rsadata;
+		for(int i=0;i<encrkeylen;i++){
+			if(i!=0)rsadata<<=8;
+			rsadata+=(uint8_t)encrkey[i];
+		}
+		Bigint res(RSA::decrypt(rsadata,privkey));
+		cerr<<"rsadata="<<res<<endl;
+		vector<uint8_t> bytes; //bytes in little-endian order
+		while(res!=0){
+			bytes.push_back(res.lowdigits()&0xff);
+			res>>=8;
+		}
+		string decrkey(bytes.size(),'\0');
+		for(int i=0;i<(int)bytes.size();i++)decrkey[bytes.size()-1-i]=bytes[i];
+		cerr<<"decrkey="<<Base64::encode(decrkey)<<endl;
+		cerr<<"payload="<<Base64::encode(data.substr(0,data.size()-2-encrkeylen))<<endl;
+		return AES::decrypt(data.substr(0,data.size()-2-encrkeylen),decrkey,AES::AES_256_CBC);
+	}
+
+}