/*
Licensed to the Apache Software Foundation (ASF) under one
or more contributor license agreements.  See the NOTICE file
distributed with this work for additional information
regarding copyright ownership.  The ASF licenses this file
to you under the Apache License, Version 2.0 (the
"License"); you may not use this file except in compliance
with the License.  You may obtain a copy of the License at

  http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing,
software distributed under the License is distributed on an
"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
KIND, either express or implied.  See the License for the
specific language governing permissions and limitations
under the License.
*/
//
//  gcm.swift
//  
//
//  Created by Michael Scott on 23/06/2015.
//  Copyright (c) 2015 Michael Scott. All rights reserved.
//

import Foundation

/*
* Implementation of the AES-GCM Encryption/Authentication
*
* Some restrictions..
* 1. Only for use with AES
* 2. Returned tag is always 128-bits. Truncate at your own risk.
* 3. The order of function calls must follow some rules
*
* Typical sequence of calls..
* 1. call GCM_init
* 2. call GCM_add_header any number of times, as long as length of header is multiple of 16 bytes (block size)
* 3. call GCM_add_header one last time with any length of header
* 4. call GCM_add_cipher any number of times, as long as length of cipher/plaintext is multiple of 16 bytes
* 5. call GCM_add_cipher one last time with any length of cipher/plaintext
* 6. call GCM_finish to extract the tag.
*
* See http://www.mindspring.com/~dmcgrew/gcm-nist-6.pdf
*/

final class GCM {
    static let NB:Int=4
    static let GCM_ACCEPTING_HEADER:Int=0
    static let GCM_ACCEPTING_CIPHER:Int=1
    static let GCM_NOT_ACCEPTING_MORE:Int=2
    static let GCM_FINISHED:Int=3
    static let GCM_ENCRYPTING:Int=0
    static let GCM_DECRYPTING:Int=1

    private var table=[[UInt32]](count:128,repeatedValue:[UInt32](count:4,repeatedValue:0)) /* 2k bytes */
    private var stateX=[UInt8](count:16,repeatedValue:0)
    private var Y_0=[UInt8](count:16,repeatedValue:0)

    private var counter:Int=0
    private var lenA=[UInt32](count:2,repeatedValue:0)
    private var lenC=[UInt32](count:2,repeatedValue:0)
    private var status:Int=0
    private var a=AES()

    private static func pack(b: [UInt8]) -> UInt32
    { /* pack bytes into a 32-bit Word */
        var r=((UInt32(b[0])&0xff)<<24)|((UInt32(b[1])&0xff)<<16)
        r = r|((UInt32(b[2])&0xff)<<8)|(UInt32(b[3])&0xff)
        return r
    }

    private static func unpack(a: UInt32) -> [UInt8]
    { /* unpack bytes from a word */
        let b:[UInt8]=[UInt8((a>>24)&0xff),UInt8((a>>16)&0xff),UInt8((a>>8)&0xff),UInt8(a&0xff)];
        return b
    }

    private func precompute(H: [UInt8])
    {
        var b=[UInt8](count:4,repeatedValue:0)
        var j=0
        for var i=0;i<GCM.NB;i++
        {
            b[0]=H[j]; b[1]=H[j+1]; b[2]=H[j+2]; b[3]=H[j+3];
            table[0][i]=GCM.pack(b);
            j+=4
        }
        for var i=1;i<128;i++
        {
            var c:UInt32=0
            for var j=0;j<GCM.NB;j++ {table[i][j]=c|(table[i-1][j])>>1; c=table[i-1][j]<<31;}
            if c != 0  {table[i][0]^=0xE1000000} /* irreducible polynomial */
        }
    }

    private func gf2mul()
    { /* gf2m mul - Z=H*X mod 2^128 */
        var P=[UInt32](count:4,repeatedValue:0)

        for var i=0;i<4;i++ {P[i]=0}
        var j=8; var m=0;
        for var i=0;i<128;i++
        {
            let c=(stateX[m]>>UInt8(--j))&1;
            if c != 0 {for var k=0;k<GCM.NB;k++ {P[k]^=table[i][k]}}
            if (j==0)
            {
				j=8; m++;
                if (m==16) {break}
            }
        }
        j=0
        for var i=0;i<GCM.NB;i++
        {
            var b=GCM.unpack(P[i])
            stateX[j]=b[0]; stateX[j+1]=b[1]; stateX[j+2]=b[2]; stateX[j+3]=b[3];
            j+=4
        }
    }
    private func wrap()
    { /* Finish off GHASH */
        var F=[UInt32](count:4,repeatedValue:0)
        var L=[UInt8](count:16,repeatedValue:0)

    /* convert lengths from bytes to bits */
        F[0]=(lenA[0]<<3)|(lenA[1]&0xE0000000)>>29
        F[1]=lenA[1]<<3;
        F[2]=(lenC[0]<<3)|(lenC[1]&0xE0000000)>>29
        F[3]=lenC[1]<<3;
        var j=0
        for var i=0;i<GCM.NB;i++
        {
            var b=GCM.unpack(F[i]);
            L[j]=b[0]; L[j+1]=b[1]; L[j+2]=b[2]; L[j+3]=b[3]
            j+=4
        }
        for var i=0;i<16;i++ {stateX[i]^=L[i]}
        gf2mul()
    }

    private func ghash(plain: [UInt8],_ len: Int) -> Bool
    {
    //    var B=[UInt8](count:16,repeatedValue:0)

        if status==GCM.GCM_ACCEPTING_HEADER {status=GCM.GCM_ACCEPTING_CIPHER}
        if (status != GCM.GCM_ACCEPTING_CIPHER) {return false}

        var j=0;
        while (j<len)
        {
            for var i=0;i<16 && j<len;i++
            {
				stateX[i]^=plain[j++];
                lenC[1]++; if lenC[1]==0 {lenC[0]++}
            }
            gf2mul();
        }
        if len%16 != 0 {status=GCM.GCM_NOT_ACCEPTING_MORE}
        return true;
    }

    /* Initialize GCM mode */
    func init_it(key: [UInt8],_ niv: Int,_ iv: [UInt8])
    { /* iv size niv is usually 12 bytes (96 bits). AES key size nk can be 16,24 or 32 bytes */
        var H=[UInt8](count:16,repeatedValue:0)

        for var i=0;i<16;i++ {H[i]=0; stateX[i]=0}

        a.init_it(AES.ECB,key,iv)
        a.ecb_encrypt(&H);    /* E(K,0) */
        precompute(H)

        lenA[0]=0;lenC[0]=0;lenA[1]=0;lenC[1]=0;
        if (niv==12)
        {
            for var i=0;i<12;i++ {a.f[i]=iv[i]}
            var b=GCM.unpack(UInt32(1))
            a.f[12]=b[0]; a.f[13]=b[1]; a.f[14]=b[2]; a.f[15]=b[3];  /* initialise IV */
            for var i=0;i<16;i++ {Y_0[i]=a.f[i]}
        }
        else
        {
            status=GCM.GCM_ACCEPTING_CIPHER;
            ghash(iv,niv) /* GHASH(H,0,IV) */
            wrap()
            for var i=0;i<16;i++ {a.f[i]=stateX[i];Y_0[i]=a.f[i];stateX[i]=0}
            lenA[0]=0;lenC[0]=0;lenA[1]=0;lenC[1]=0;
        }
        status=GCM.GCM_ACCEPTING_HEADER;
    }

    /* Add Header data - included but not encrypted */
    func add_header(header: [UInt8],_ len: Int) -> Bool
    { /* Add some header. Won't be encrypted, but will be authenticated. len is length of header */
        if status != GCM.GCM_ACCEPTING_HEADER {return false}

        var j=0
        while (j<len)
        {
            for var i=0;i<16 && j<len;i++
            {
				stateX[i]^=header[j++];
                lenA[1]++; if lenA[1]==0 {lenA[0]++}
            }
            gf2mul();
        }
        if len%16 != 0 {status=GCM.GCM_ACCEPTING_CIPHER}
        return true;
    }
    /* Add Plaintext - included and encrypted */
    func add_plain(plain: [UInt8],_ len: Int) -> [UInt8]
    {
        var B=[UInt8](count:16,repeatedValue:0)
        var b=[UInt8](count:4,repeatedValue:0)

        var cipher=[UInt8](count:len,repeatedValue:0)
        var counter:UInt32=0
        if status == GCM.GCM_ACCEPTING_HEADER {status=GCM.GCM_ACCEPTING_CIPHER}
        if status != GCM.GCM_ACCEPTING_CIPHER {return [UInt8]()}

        var j=0
        while (j<len)
        {

            b[0]=a.f[12]; b[1]=a.f[13]; b[2]=a.f[14]; b[3]=a.f[15];
            counter=GCM.pack(b);
            counter++;
            b=GCM.unpack(counter);
            a.f[12]=b[0]; a.f[13]=b[1]; a.f[14]=b[2]; a.f[15]=b[3]; /* increment counter */
            for var i=0;i<16;i++ {B[i]=a.f[i]}
            a.ecb_encrypt(&B);        /* encrypt it  */

            for var i=0;i<16 && j<len;i++
            {
				cipher[j]=(plain[j]^B[i]);
				stateX[i]^=cipher[j++];
                lenC[1]++; if lenC[1]==0 {lenC[0]++}
            }
            gf2mul();
        }
        if len%16 != 0 {status=GCM.GCM_NOT_ACCEPTING_MORE}
        return cipher;
    }
    /* Add Ciphertext - decrypts to plaintext */
    func add_cipher(cipher: [UInt8],_ len: Int) -> [UInt8]
    {
        var B=[UInt8](count:16,repeatedValue:0)
        var b=[UInt8](count:4,repeatedValue:0)

        var plain=[UInt8](count:len,repeatedValue:0)
        var counter:UInt32=0

        if status==GCM.GCM_ACCEPTING_HEADER {status=GCM.GCM_ACCEPTING_CIPHER}
        if status != GCM.GCM_ACCEPTING_CIPHER {return [UInt8]()}

        var j=0
        while (j<len)
        {

            b[0]=a.f[12]; b[1]=a.f[13]; b[2]=a.f[14]; b[3]=a.f[15];
            counter=GCM.pack(b);
            counter++;
            b=GCM.unpack(counter);
            a.f[12]=b[0]; a.f[13]=b[1]; a.f[14]=b[2]; a.f[15]=b[3]; /* increment counter */
            for var i=0;i<16;i++ {B[i]=a.f[i]}
            a.ecb_encrypt(&B);        /* encrypt it  */
            for var i=0;i<16 && j<len;i++
            {
				plain[j]=(cipher[j]^B[i]);
				stateX[i]^=cipher[j++];
                lenC[1]++; if lenC[1]==0 {lenC[0]++}
            }
            gf2mul()
        }
        if len%16 != 0 {status=GCM.GCM_NOT_ACCEPTING_MORE}
        return plain;
    }

    /* Finish and extract Tag */
    func finish(extract: Bool) -> [UInt8]
    { /* Finish off GHASH and extract tag (MAC) */
        var tag=[UInt8](count:16,repeatedValue:0)

        wrap();
        /* extract tag */
        if (extract)
        {
            a.ecb_encrypt(&Y_0);        /* E(K,Y0) */
            for var i=0;i<16;i++ {Y_0[i]^=stateX[i]}
            for var i=0;i<16;i++ {tag[i]=Y_0[i];Y_0[i]=0;stateX[i]=0;}
        }
        status=GCM.GCM_FINISHED;
        a.end();
        return tag;
    }

    static func hex2bytes(s: String) -> [UInt8]
    {
        var array=Array(arrayLiteral: s)
        let len=array.count;
        var data=[UInt8](count:len/2,repeatedValue:0)

        for var i=0;i<len;i+=2
        {
            data[i / 2] = UInt8(strtoul(String(array[i]),nil,16)<<4)+UInt8(strtoul(String(array[i+1]),nil,16))
        }
        return data;
    }


}