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RC6 encryption and decryption

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14 Jul 20023 min read 357.8K   11K   44   43
RC6 is an evolutionary improvement of RC5, designed to meet the requirements of the Advanced Encryption Standard (AES).

Sample Image -
screenshot.jpg

Introduction

RC6 is an evolutionary improvement of RC5, designed to meet the requirements of the Advanced Encryption Standard (AES). Like RC5, RC6 makes essential use of data-dependent rotations. New features of RC6 include the use of four working registers instead of two, and the inclusion of integer multiplication as an additional primitive operation. The use of multiplication greatly increases the diffusion achieved per round, allowing for greater security, fewer rounds, and increased throughput. I found an article about it online and fulfilled the algorithm using C++ for fun. Hope it'd be helpful to some interested people.

Details of RC6

Like RC5, RC6 is a fully parameterized family of encryption algorithms. A version of RC6 is more accurately specified as RC6-w/r/b where the word size is w bits, encryption consists of a nonnegative number of rounds r, and b denotes the length of the encryption key in bytes. Since the AES submission is targeted at w = 32 and r = 20, we shall use RC6 as shorthand to refer to such versions. When any other value of w or r is intended in the text, the parameter values will be specified as RC6-w/r. Of particular relevance to the AES effort will be the versions of RC6 with 16-, 24-, and 32-byte keys. For all variants, RC6-w/r/b operates on units of four w-bit words using the following six basic operations. The base-two logarithm of w will be denoted by lgw.

  • a + b: integer addition modulo 2^w
  • a - b: integer subtraction modulo 2^w
  • a @ b: bitwise exclusive-or of w-bit words
  • a * b: integer multiplication modulo 2^w
  • a <<< b: rotate the w-bit word a to the left by the amount given by the least significant lgw bits of b
  • a >>> b: rotate the w-bit word a to the right by the amount given by the least significant lgw bits of b

Note that in the description of RC6 the term "round" is somewhat analogous to the usual DES-like idea of a round: half of the data is updated by the other half; and the two are then swapped. In RC5, the term "half-round" was used to describe this style of action, and an RC5 round was deemed to consist of two half-rounds. This seems to have become a potential cause of confusion, and so RC6 reverts to using the term "round" in the more established way.

To get the detailed algorithm description of RC6-w/r/b. Please read the article "The RC6 Block Cipher" by Ronald L. Rivest, M.J.B. Robshaw, R. Sidney and, Y.L. Yin.

Details of Code

In my program, I fulfilled RC6-32/16. Since the integer addition, subtraction and multiplication don't exceed 2^32 in my program, I don't let their results modulo 2^32 like the operations described above. Anyway, the encryption and decryption go well.

I wrapped the bits rotation operations in two functions

DWORD
CHexDoc::LeftRotate(DWORD dwVar, DWORD dwOffset)
and DWORD CHexDoc::RightRotate(DWORD dwVar, DWORD dwOffset).

DWORD CHexDoc::LeftRotate(DWORD dwVar, DWORD dwOffset)
{
    DWORD temp1, temp2;

    temp1 = dwVar >> (W - dwOffset);
    temp2 = dwVar << dwOffset;
    temp2 = temp2 | temp1;

    return temp2;
}

DWORD CHexDoc::RightRotate(DWORD dwVar, DWORD dwOffset)
{
    DWORD temp1, temp2;

    temp1 = dwVar << (W - dwOffset);
    temp2 = dwVar >> dwOffset;
    temp2 = temp2 | temp1;

    return temp2;
}

The key generation part is like

void CHexDoc::KeyGen(DWORD dwKey)
{
    DWORD P32 = 0xB7E15163;
    DWORD Q32 = 0x9E3779B9;
    DWORD i, A, B;
    DWORD dwByteOne, dwByteTwo, dwByteThree, dwByteFour;

    dwByteOne = dwKey >> 24;
    dwByteTwo = dwKey >> 8;
    dwByteTwo = dwByteTwo & 0x0010;
    dwByteThree = dwKey << 8;
    dwByteThree = dwByteThree & 0x0100;
    dwByteFour = dwKey << 24;

    dwKey = dwByteOne | dwByteTwo | dwByteThree
        | dwByteFour;

    m_dwS[0] = P32;

    for(i = 1; i < 2 * R + 4; i++)
        m_dwS[i] = m_dwS[i - 1] + Q32;

    i = A = B = 0;

    int v = 3 * max(1, 2 * R + 4);

    for(int s = 1; s <= v; s++)
    {
        A = m_dwS[i] = LeftRotate(m_dwS[i] + A + B, 
            OffsetAmount(3));
        B = dwKey = LeftRotate(dwKey + A + B, 
            OffsetAmount(A + B));

        i = (i + 1) % (2 * R + 4);
    }
}

Finally, the core parts of encryption and decryption are as following:

// encrypt the file
void CHexDoc::EncodeFile()
{
    DWORD* pdwTemp;

    for(UINT i = 0; i < m_nDocLength; i += 16)
    {
        pdwTemp = (DWORD*)&m_pFileData[i];

        pdwTemp[0] = (pdwTemp[0] - m_dwS[2 * R + 2]);
        pdwTemp[2] = (pdwTemp[2] - m_dwS[2 * R + 3]);

        for(int j = R; j >= 1; j--)
        {
            DWORD temp = pdwTemp[3];
            pdwTemp[3] = pdwTemp[2];
            pdwTemp[2] = pdwTemp[1];
            pdwTemp[1] = pdwTemp[0];
            pdwTemp[0] = temp;

            DWORD t = 
                LeftRotate((pdwTemp[1] * (2 * pdwTemp[1] + 1)),
                OffsetAmount((DWORD)(log((double)W)/log(2.0))));
            DWORD u = 
                LeftRotate((pdwTemp[3] * (2 * pdwTemp[3] + 1)),
                OffsetAmount((DWORD)(log((double)W)/log(2.0))));
            pdwTemp[0] =
                (RightRotate((pdwTemp[0] - m_dwS[2 * j]),
                OffsetAmount(u))) ^ t;
            pdwTemp[2] = 
                (RightRotate((pdwTemp[2] - m_dwS[2 * j + 1]),
                OffsetAmount(t))) ^ u;
        }

        pdwTemp[1] = (pdwTemp[1] - m_dwS[0]);
        pdwTemp[3] = (pdwTemp[3] - m_dwS[1]);
    }
    pdwTemp = NULL;
    SetModifiedFlag(TRUE);

    POSITION pos = GetFirstViewPosition();
    while(pos != NULL)
    {
        CView* pView = GetNextView(pos);
        pView->RedrawWindow();
    }
}

// decrypt the file
void CHexDoc::DecodeFile()
{
    DWORD* pdwTemp;

    for(UINT i = 0; i < m_nDocLength; i += 16)
    {
        pdwTemp = (DWORD*)&m_pFileData[i];

        pdwTemp[1] = (pdwTemp[1] + m_dwS[0]);
        pdwTemp[3] = (pdwTemp[3] + m_dwS[1]);

        for(int j = 1; j <= R; j++)
        {
            DWORD t = 
                LeftRotate((pdwTemp[1] * (2 * pdwTemp[1] + 1)),
                OffsetAmount((DWORD)(log((double)W)/log(2.0))));
            DWORD u = 
                LeftRotate((pdwTemp[3] * (2 * pdwTemp[3] + 1)),
                OffsetAmount((DWORD)(log((double)W)/log(2.0))));
            pdwTemp[0] = 
                (LeftRotate(pdwTemp[0] ^ t, OffsetAmount(u)) +
                m_dwS[2 * j]);
            pdwTemp[2] = 
                (LeftRotate(pdwTemp[2] ^ u, OffsetAmount(t)) +
                m_dwS[2 * j + 1]);

            DWORD temp = pdwTemp[0];
            pdwTemp[0] = pdwTemp[1];
            pdwTemp[1] = pdwTemp[2];
            pdwTemp[2] = pdwTemp[3];
            pdwTemp[3] = temp;
        }

        pdwTemp[0] = (pdwTemp[0] + m_dwS[2 * R + 2]);
        pdwTemp[2] = (pdwTemp[2] + m_dwS[2 * R + 3]);
    }
    pdwTemp = NULL;
    SetModifiedFlag(TRUE);

    POSITION pos = GetFirstViewPosition();
    while(pos != NULL)
    {
        CView* pView = GetNextView(pos);
        pView->RedrawWindow();
    }
}

Conclusion

In the view window, I showed the Hex and Char content of the loaded file and their addresses. You can see the changes every time you encrypt/decrypt it. Thanks!

License

This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

A list of licenses authors might use can be found here


Written By
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United States United States
Hey you,
Out there in the cold,
Getting lonely, getting old,
Can you feel me?
Hey you,
Standing in the aisle,
With itchy feet and fading smile,
Can you feel me?
Hey you,
Don't help them to bury the light.
Don't give in without a fight.

Comments and Discussions

 
QuestionOpen source license type Pin
Song Huang17-Dec-15 18:58
Song Huang17-Dec-15 18:58 
Generalrc6 in jar Pin
endra hermawan27-May-09 22:55
endra hermawan27-May-09 22:55 
GeneralThe bug with overrun Pin
mircea13131325-Apr-09 1:24
mircea13131325-Apr-09 1:24 
Questionis there a rc6 algorithm in vb.net Pin
moonshaddow30-Oct-08 15:35
moonshaddow30-Oct-08 15:35 
AnswerRe: is there a rc6 algorithm in vb.net Pin
didit adja5-Aug-15 5:23
didit adja5-Aug-15 5:23 
GeneralHep block error... Pin
Roel Virgel L. Baldon6-Oct-08 23:28
Roel Virgel L. Baldon6-Oct-08 23:28 
GeneralI have a errors, help me [modified] Pin
Srgteam22-Nov-07 14:58
Srgteam22-Nov-07 14:58 
GeneralPls Help~~; Pin
ofuka14-Mar-06 22:07
ofuka14-Mar-06 22:07 
GeneralDES cryptanalysis with 6 rounds Pin
nasor6-May-05 23:14
nasor6-May-05 23:14 
GeneralRC3, RC4, RC5, RC6 Pin
Mansureh Shahraki3-Feb-05 6:00
sussMansureh Shahraki3-Feb-05 6:00 
GeneralRe: RC3, RC4, RC5, RC6 Pin
Mingming Lu3-Feb-05 6:14
Mingming Lu3-Feb-05 6:14 
QuestionPlease help me ? Pin
lonelywind198218-Mar-04 21:47
lonelywind198218-Mar-04 21:47 
QuestionHow to setup RC6 with C# ? Pin
lonelywind198214-Mar-04 0:12
lonelywind198214-Mar-04 0:12 
GeneralThis program is not run in all Windows OS Pin
Brahma12-Dec-03 1:33
Brahma12-Dec-03 1:33 
Questionhelp me?? Pin
xxhimanshu4-Nov-03 23:16
xxhimanshu4-Nov-03 23:16 
Generalweak keys detection Pin
krishni13-Aug-03 4:22
krishni13-Aug-03 4:22 
GeneralKey Length Pin
brad_brady13-Apr-03 10:43
brad_brady13-Apr-03 10:43 
Generalkey gen Pin
abritto24-Mar-03 5:48
abritto24-Mar-03 5:48 
Generalopening a file Pin
abritto20-Mar-03 6:42
abritto20-Mar-03 6:42 
GeneralRe: opening a file Pin
Mingming Lu20-Mar-03 12:07
Mingming Lu20-Mar-03 12:07 
Generalvsc++6.0 Pin
abritto26-Feb-03 3:16
abritto26-Feb-03 3:16 
GeneralRe: vsc++6.0 Pin
Mingming Lu26-Feb-03 5:53
Mingming Lu26-Feb-03 5:53 
when that happened to me before, I just deleted that .pch file and compiled it again. Then, everything is fine. you can try it. good luck!

Come on! Decode me.
Questioncan this be changed easily for RC5 Pin
abritto25-Feb-03 3:43
abritto25-Feb-03 3:43 
AnswerRe: can this be changed easily for RC5 Pin
Mingming Lu25-Feb-03 4:39
Mingming Lu25-Feb-03 4:39 
GeneralOverrun Pin
BossShot23-Oct-02 2:18
BossShot23-Oct-02 2:18 

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