Serpent

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Serpent is a block cipher designed by Ross Anderson, Eli Biham, and Lars Knudsen. It was ranked 2nd in the Advanced Encryption Standard contest. The Serpent homepage is located at http://www.cl.cam.ac.uk/~rja14/serpent.html.

If your project is using encryption alone to secure your data, encryption alone is usually not enough. Please take a moment to read Authenticated Encryption and understand why you should prefer to use CCM, GCM, or EAX over other modes, such as CBC or CTR.

Sample Programs

The first snippet dumps the minimum, maximum, and default key lengths used by Serpent.

cout << "key length: " << Serpent::DEFAULT_KEYLENGTH << endl;
cout << "key length (min): " << Serpent::MIN_KEYLENGTH << endl;
cout << "key length (max): " << Serpent::MAX_KEYLENGTH << endl;
cout << "block size: " << Serpent::BLOCKSIZE << endl;

Output from the above snippet produces the following. Notice the default key size is 128 bits or 16 bytes. The minimum key size of 0 is probably a bug in Crypto++.

key length: 16
key length (min): 0
key length (max): 32
block size: 16

The following program demonstrates CBC encryption using Serpent. The key is declared on the stack using a SecByteBlock to ensure the sensitive material is zeroized. Similar could be used for both plain text and recovered text.

AutoSeededRandomPool prng;

SecByteBlock key(Serpent::DEFAULT_KEYLENGTH);
prng.GenerateBlock(key, key.size());

byte iv[Serpent::BLOCKSIZE];
prng.GenerateBlock(iv, sizeof(iv));

string plain = "CBC Mode Test";
string cipher, encoded, recovered;

/*********************************\
\*********************************/

try
{
    cout << "plain text: " << plain << endl;

    CBC_Mode< Serpent >::Encryption e;
    e.SetKeyWithIV(key, key.size(), iv);

    // The StreamTransformationFilter adds padding
    //  as required. ECB and CBC Mode must be padded
    //  to the block size of the cipher.
    StringSource ss1(plain, true, 
        new StreamTransformationFilter(e,
            new StringSink(cipher)
        ) // StreamTransformationFilter
    ); // StringSource
}
catch(const CryptoPP::Exception& e)
{
    cerr << e.what() << endl;
    exit(1);
}

/*********************************\
\*********************************/

// Pretty print
StringSource ss2(cipher, true,
    new HexEncoder(
        new StringSink(encoded)
    ) // HexEncoder
); // StringSource

cout << "cipher text: " << encoded << endl;

/*********************************\
\*********************************/

try
{
    CBC_Mode< Serpent >::Decryption d;
    d.SetKeyWithIV(key, key.size(), iv);

    // The StreamTransformationFilter removes
    //  padding as required.
    StringSource ss3(cipher, true, 
        new StreamTransformationFilter(d,
            new StringSink(recovered)
        ) // StreamTransformationFilter
    ); // StringSource

    cout << "recovered text: " << recovered << endl;
}
catch(const CryptoPP::Exception& e)
{
    cerr << e.what() << endl;
    exit(1);
}

A typical output is shown below. Note that each run will produce different results because the key and initialization vector are randomly generated.

$ ./Driver.exe
key: BE4295539F6BD1752FD0A80229EF8847
iv: 00963F59224794D5AD4252094358FBC3
plain text: CBC Mode Test
cipher text: CF2CF2547E02F6D34D97246E8042ED89
recovered text: CBC Mode Test

By switching to EAX mode, authenticity assurances can placed on the cipher text for nearly no programming costs. Below the StreamTransformationFilter was replaced by AuthenticatedEncryptionFilter and AuthenticatedDecryptionFilter.

EAX< Serpent >::Encryption e;
e.SetKeyWithIV(key, key.size(), iv);

StringSource ss1(plain, true, 
    new AuthenticatedEncryptionFilter(e,
        new StringSink(cipher)
    ) // StreamTransformationFilter
); // StringSource

...

EAX< Serpent >::Decryption d;
d.SetKeyWithIV(key, key.size(), iv);

StringSource ss2(cipher, true, 
    new AuthenticatedDecryptionFilter(d,
        new StringSink(recovered)
    ) // StreamTransformationFilter
); // StringSource

Typical output is as follows. Notice the additional cipher text bytes due to the MAC bytes. See EAX Mode for details.

$ ./Driver.exe
key: 7494A57648FB420043BFBFC5639EB82D
iv: 6DF94638B83E01458F3E30C9A1D6AF1C
plain text: EAX Mode Test
cipher text: 96C521F32DC5E9BBC369DDE4914CB13B710EEBBAB7D706D3ABE06A99DC
recovered text: EAX Mode Test

To manually insert bytes into the filter, perform multiple Puts. Though Get is used below, a StringSink could easily be attached and save the administrivia.

const size_t SIZE = 16 * 4;
string plain(SIZE, 0x00);

for(size_t i = 0; i < plain.size(); i++)
    plain[i] = 'A' + (i%26);
...

CBC_Mode < Serpent >::Encryption encryption(key, sizeof(key), iv);
StreamTransformationFilter encryptor(encryption, NULL);

for(size_t j = 0; j < plain.size(); j++)
    encryptor.Put((byte)plain[j]);

encryptor.MessageEnd();
size_t ready = encryptor.MaxRetrievable();

string cipher(ready, 0x00);
encryptor.Get((byte*) &cipher[0], cipher.size());

Downloads

Serpent-CBC-Filter.zip - Demonstrates encryption and decryption using Serpent in CBC mode with filters (confidentiality only)

Serpent-CTR-Filter.zip - Demonstrates encryption and decryption using Serpent in CTR mode with filters (confidentiality only)

Serpent-GCM-Filter.zip - Demonstrates encryption and decryption using Serpent in GCM mode with filters (confidentiality and authenticity)

Serpent-EAX-Filter.zip - Demonstrates encryption and decryption using Serpent in EAX mode with filters (confidentiality and authenticity)