Advanced Encryption Standard

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The Advanced Encryption Standard, or AES is a NIST approved block cipher specified in FIPS 197, Advanced Encryption Standard (AES). When using AES, one typically specifies a mode of operation and optionally a padding scheme. AES provides confidentiality only using most modes of operation (such as ECB and CBC). When operating the cipher in CCM, GCM, or EAX mode, the mode provides both confidentiality and authenticity.

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.

Usage

Default Key Length

AES can use 16, 24, or 32 byte keys (128, 192, and 256 bits respectively). The default key length in Crypto++ is 16 bytes and specified by AES::DEFAULT_KEYLENGTH. essay2u help

Block Size

The block size is determined by AES::BLOCKSIZE. For AES, this is always 16 bytes.

Reference to block cipher object

You can also create a mode object that holds a reference to a block cipher object rather than an instance of it:

AES::Encryption aesEncryption(key, AES::DEFAULT_KEYLENGTH);
CFB_Mode_ExternalCipher::Encryption cfbEncryption(aesEncryption, iv);

ECB and CBC mode remarks

For ECB and CBC mode, you must process data in multiples of the block size. Alternatively, you can wrap StreamTransformationFilter around the mode object and use it as a filter object. The StreamTransformationFilter will buffer data into blocks and pad as required.

Sample Programs

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

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

Output from the above snippet produces the following. Notice the default key size is 128 bits or 16 bytes.

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

Encrypting and Decrypting Using AES

The following example uses CFB mode and in-place encryption and decryption. Since the mode is not ECB or CBC, the data's length does not need to be a multiple of AES's block size.

AutoSeededRandomPool rnd;

// Generate a random key
SecByteBlock key(0x00, AES::DEFAULT_KEYLENGTH);
rnd.GenerateBlock( key, key.size() );

// Generate a random IV
byte iv[AES::BLOCKSIZE];
rnd.GenerateBlock(iv, AES::BLOCKSIZE);

char plainText[] = "Hello! How are you.";
int messageLen = (int)strlen(plainText) + 1;

//////////////////////////////////////////////////////////////////////////
// Encrypt

CFB_Mode<AES>::Encryption cfbEncryption(key, key.size(), iv);
cfbEncryption.ProcessData((byte*)plainText, (byte*)plainText, messageLen);

//////////////////////////////////////////////////////////////////////////
// Decrypt

CFB_Mode<AES>::Decryption cfbDecryption(key, key.size(), iv);
cfbDecryption.ProcessData((byte*)plainText, (byte*)plainText, messageLen);

Here is an example of getting the encryptor and decryptor using an externally generated key and IV:

byte aes_key[16];
byte iv[16];

// stub for how you really get it, e.g. reading it from a file, off of a network socket encrypted with an asymmetric cipher, or whatever
read_key(aes_key, sizeof(aes_key));

// stub for how you really get it, e.g. filling it with random bytes or reading it from the other side of the socket since both sides have to use the same IV as well as the same key
read_initialization_vector(iv);

// the final argument is specific to CFB mode, and specifies the refeeding size in bytes. This invocation corresponds to Java's Cipher.getInstance("AES/CFB8/NoPadding")
auto enc = new CFB_Mode<AES>::Encryption(aes_key, sizeof(aes_key), iv, 1);

// the final argument is specific to CFB mode, and specifies the refeeding size in bytes. This invocation corresponds to Java's Cipher.getInstance("AES/CFB8/NoPadding")
auto dec = new CFB_Mode<AES>::Decryption(aes_key, sizeof(aes_key), iv, 1);

(After that they're used in exactly the same manner as in the previous example.)

Generating an AES Key from a Diffie-Hellman Session Key

See Using Diffie-Hellman to generate an AES key.

Encrypting a string using AES

byte key[AES::DEFAULT_KEYLENGTH], iv[AES::BLOCKSIZE];
string plainText;

// ... populate key, iv, plainText here

string cipher;
StringSink* sink = new StringSink(cipher);
Base64Encoder* base64_enc = new Base64Encoder(sink);
CBC_Mode<AES>::Encryption aes(key, sizeof(key), iv);
StreamTransformationFilter* aes_enc = new StreamTransformationFilter(aes, base64_enc);
StringSource source(plainText, true, aes_enc);

Downloads

AES-GCM-Test.zip - Demonstrates encryption and decryption using AES in GCM mode (confidentiality and authentication)

AES-CCM-Test.zip - Demonstrates encryption and decryption using AES in CCM mode (confidentiality and authentication)

AES-EAX-Test.zip - Demonstrates encryption and decryption using AES in EAX mode (confidentiality and authentication)

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

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

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

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

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