# AuthenticatedDecryptionFilter

 Documentation #include

AuthenticatedDecryptionFilter is the concrete object for authenticated encryption (AE) and authenticated encryption with additional data (AEAD). The filter combines a block cipher operated in an appropriate mode with a HashFilter for authenticated encryption. Currently, the two modes of operation that can be utilized by this filter are CCM and GCM.

The filter allows input of both confidential data (data to be encrypted or decrypted) and additional authenticated data (plain text data to be authenticated). The confidential data, on the primary channel, has both encryption and authentication applied to it. The additional authenticated data (aad), presented to the filter on the AAD channel, has only authentication assurances.

Unlike StreamTransformationFilter, the AuthenticatedDecryptionFilter requires a counterpart for the encryption and authentication process - the AuthenticatedEncryptionFilter. The wiki also has a AadSource example to pump both confidential data and aad to an AuthenticatedDecryptionFilter.

Crypto++ 8.2 and earlier had a bug in AuthenticatedDecryptionFilter where using a FileSource would cause an exception; but a StringSource was OK. The bug was fixed at Commit ff110c6e183e. Also see Issue 817.

Sources, filters and sinks are discussed at Pipelining. The pipeline article explains the design and shows you how to use them.

## Constructor

AuthenticatedDecryptionFilter(AuthenticatedSymmetricCipher &c,
BufferedTransformation *attachment = NULL, word32 flags = DEFAULT_FLAGS,
int truncatedDigestSize=-1, BlockPaddingScheme padding = DEFAULT_PADDING);

The AuthenticatedSymmetricCipher will be a CCM mode, EAX mode or GCM mode object. As is customary with Crypto++, a BufferedTransformation is available for pipelining as the second parameter.

The third parameter, flags, can be any of the following; however MAC_AT_BEGIN and MAC_AT_END are mutually exclusive.

• DEFAULT_FLAGS = THROW_EXCEPTION | MAC_AT_END
• MAC_AT_END (0)
• specifies the mac is inserted after the additional authenticated data (aad) and cipher text data
• MAC_AT_BEGIN (1)
• specifies the mac is inserted before the additional authenticated data (aad) and cipher text data
• THROW_EXCEPTION (16)
• intructs the AuthenticatedDecryptionFilter to throw a HashVerificationFailed exception upon verification failure

The fourth parameter, truncatedDigestSize, is used by the HashFilter to truncate the digest size. Only GCM mode should use this parameter, as simple truncation works as expected. CCM, which uses a formatting function, requires the digest size to be known at compile time and declared as a template parameter. So CCM mode should not change the default value.

The final parameter, padding, allows you to specify padding. Depending on the mode, the value may (or may not) have an effect.

The tag sizes are not always in the realm of construction (due to CCM's formatting function), however, it is appropriate to list their default values when discussing constructors. The default tag size for an AuthenticatedDecryptionFilter using both CCM and GCM is 16 bytes.

### CCM Mode

The first sample demonstrates using the AuthenticatedDecryptionFilter with CCM mode. Recall that the tag size must be a template parameter when using CCM.

const int TAG_SIZE = 12 /*96 bits*/;

CCM< AES, TAG_SIZE >::Decryption d;
d.SetKeyWithIV(key, key.size(), iv, iv.size());
d.SpecifyDataLengths( ... );

AuthenticatedDecryptionFilter df( d,
new StringSink( recovered)
/* THROW_EXCEPTION | MAC_AT_END is default */
); // AuthenticatedEncryptionFilter

...

// If verification fails, catch a
//  HashVerificationFailed exception


### GCM Mode

The second sample demonstrates using the AuthenticatedDecryptionFilter with GCM mode. Recall that the tag size is passed as a parameter to the AuthenticatedDecryptionFilter during construction.

const int TAG_SIZE = 12 /*96 bits*/;

GCM< AES >::Decryption d;
d.SetKeyWithIV(key, key.size(), iv, iv.size());

AuthenticatedDecryptionFilter df( d,
new StringSink( recovered ),
DEFAULT_FLAGS, TAG_SIZE
); // AuthenticatedEncryptionFilter

...