EAX mode
Encyclopedia
EAX mode is a mode of operation
for cryptographic block ciphers. It is an Authenticated Encryption with Associated Data (AEAD
) algorithm designed to simultaneously provide both authentication
and privacy
of the message (authenticated encryption
) with a two-pass scheme, one pass for achieving privacy and one for authenticity for each block.
EAX mode was submitted on October 3, 2003 to the attention of NIST in order to replace CCM
as standard AEAD mode of operation, since CCM mode lacks some desirable attributes of EAX and is more complex.
-using two-pass AEAD scheme with no restrictions on block cipher primitive to be used, nor on block size, and supports arbitrary-length messages. Authentication tag length is arbitrarily sizeable up to the used cipher's block size.
The block cipher primitive is used in CTR mode for encryption and as OMAC
for authentication over each block through the EAX composition method, that may be seen as a particular case of a more general algorithm called EAX2 and described in "The EAX Mode of Operation" paper.
The reference implementation in the aforementioned paper uses AES in CTR mode for encryption combined with AES OMAC for authentication.
EAX mode has several desirable attributes, notably:
Notably, CCM mode lacks the last 2 attributes.
, Phillip Rogaway
, and David Wagner placed the work under public domain and have stated that they were unaware of any patents covering this technology. Thus, EAX mode of operation is believed to be free and unencumbered for any use.
C12.22 standard for transport of meter-based data over a network.
Block cipher modes of operation
In cryptography, modes of operation is the procedure of enabling the repeated and secure use of a block cipher under a single key.A block cipher by itself allows encryption only of a single data block of the cipher's block length. When targeting a variable-length message, the data must first be...
for cryptographic block ciphers. It is an Authenticated Encryption with Associated Data (AEAD
AEAD block cipher modes of operation
Authenticated Encryption with Associated Data is a class of block cipher modes which encrypt the message and authenticate the message simultaneously. Interest in these modes was sparked by the publication of Charanjit Jutla's IACBC and IAPM modes in 2000. As of 2005 this is an active area of...
) algorithm designed to simultaneously provide both authentication
Authentication
Authentication is the act of confirming the truth of an attribute of a datum or entity...
and privacy
Privacy
Privacy is the ability of an individual or group to seclude themselves or information about themselves and thereby reveal themselves selectively...
of the message (authenticated encryption
Authenticated encryption
Authenticated Encryption is a block cipher mode of operation which simultaneously provides confidentiality, integrity and authenticity assurances on the data. It became readily apparent that securely compositing a confidentiality mode with an authentication mode could be error prone and difficult...
) with a two-pass scheme, one pass for achieving privacy and one for authenticity for each block.
EAX mode was submitted on October 3, 2003 to the attention of NIST in order to replace CCM
CCM mode
CCM mode is a mode of operation for cryptographic block ciphers. It is an authenticated encryption algorithm designed to provide both authentication and confidentiality. CCM mode is only defined for block ciphers with a block length of 128 bits...
as standard AEAD mode of operation, since CCM mode lacks some desirable attributes of EAX and is more complex.
Encryption and authentication
EAX is a flexible nonceCryptographic nonce
In security engineering, nonce is an arbitrary number used only once to sign a cryptographic communication. It is similar in spirit to a nonce word, hence the name. It is often a random or pseudo-random number issued in an authentication protocol to ensure that old communications cannot be reused...
-using two-pass AEAD scheme with no restrictions on block cipher primitive to be used, nor on block size, and supports arbitrary-length messages. Authentication tag length is arbitrarily sizeable up to the used cipher's block size.
The block cipher primitive is used in CTR mode for encryption and as OMAC
One-key MAC
OMAC is a message authentication code constructed from a block cipher much like the PMAC algorithm.Officially there are two OMAC algorithms which are both essentially the same except for a small tweak....
for authentication over each block through the EAX composition method, that may be seen as a particular case of a more general algorithm called EAX2 and described in "The EAX Mode of Operation" paper.
The reference implementation in the aforementioned paper uses AES in CTR mode for encryption combined with AES OMAC for authentication.
Performance
Being a two-pass scheme, EAX mode is slower than a well designed one-pass scheme based on the same primitives.EAX mode has several desirable attributes, notably:
- provable security (dependent on the security of the underlying primitive cipher);
- message expansion is minimal, being limited to the overhead of the tag length;
- using CTR mode means the cipher need be implemented only for encryption, in simplifying implementation of some ciphers (especially desirable attribute for hardware implementation);
- the algorithm is "on-line", that means that can process a stream of data, using constant memory, without knowing total data length in advance;
- the algorithm can process static Associated Data (AD), useful for encryption/decryption of communication session parameters (where session parameters may represent the Associated Data).
Notably, CCM mode lacks the last 2 attributes.
Patent status
The authors of EAX mode, Mihir BellareMihir Bellare
Mihir Bellare is a cryptographer and professor at the University of California, San Diego. He has published several seminal papers in the field of cryptography , many coauthored with Phillip Rogaway. Bellare has published a number of papers in the field of Format-Preserving Encryption...
, Phillip Rogaway
Phillip Rogaway
Phillip Rogaway is a professor of computer science at the University of California, Davis. He graduated with an BA in computer science from UC Berkeley and completed his PhD in cryptography at MIT, in the Theory of Computation group. He has taught at UC Davis since 1994.Dr...
, and David Wagner placed the work under public domain and have stated that they were unaware of any patents covering this technology. Thus, EAX mode of operation is believed to be free and unencumbered for any use.
Use
A modification of the EAX mode, so called EAX' , is used in the ANSIAnsi
Ansi is a village in Kaarma Parish, Saare County, on the island of Saaremaa, Estonia....
C12.22 standard for transport of meter-based data over a network.
See also
- Authenticated Encryption with Associated Data (AEAD)AEAD block cipher modes of operationAuthenticated Encryption with Associated Data is a class of block cipher modes which encrypt the message and authenticate the message simultaneously. Interest in these modes was sparked by the publication of Charanjit Jutla's IACBC and IAPM modes in 2000. As of 2005 this is an active area of...
- Authenticated Encryption (AE)Authenticated encryptionAuthenticated Encryption is a block cipher mode of operation which simultaneously provides confidentiality, integrity and authenticity assurances on the data. It became readily apparent that securely compositing a confidentiality mode with an authentication mode could be error prone and difficult...
- CCM modeCCM modeCCM mode is a mode of operation for cryptographic block ciphers. It is an authenticated encryption algorithm designed to provide both authentication and confidentiality. CCM mode is only defined for block ciphers with a block length of 128 bits...
- CTR mode
- OMACOne-key MACOMAC is a message authentication code constructed from a block cipher much like the PMAC algorithm.Officially there are two OMAC algorithms which are both essentially the same except for a small tweak....
External links
- NIST: proposed modes of operation
- A Critique of CCM - February 2003
- EAX: A Conventional Authenticated-Encryption Mode - Apr 13, 2003, last revised Sep 9, 2003
- The EAX Mode of Operation - January 18, 2004
- ANSI C12 22 site