Advanced Encryption Standard
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.
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.
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.
There are three sample programs. The first shows AES key and block sizes. The second and third use filters in a pipeline). Pipelining is a high level abstraction and it handles buffering input, buffering output and padding for you.
If you are benchmarking then you may want to visit Benchmarks | Sample Program . It shows you how to use StreamTransformation and its ProcessString method to process multiple blocks at a time. ProcessString eventually calls BlockTransformation and ProcessBlock. Calling a cipher's ProcessString or ProcessBlock eventually call ProcessAndXorBlock or AdvancedProcessBlocks, and they are the lowest level API you can use.
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
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; byte iv; // 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
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);
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)