Using the CC2420 with AES Support

Eric Famiglietti and Simone Willett

WSN Encryption IssuesLack of physical protection

Resource constraintsPublic-key cryptography, RSA

Unsuitable for low power and limited memory

Key establishment/distribution problemSymmetric-key algorithms

WSN Encryption ExamplesIdentity-based encryption

http://eprint.iacr.org/2007/020.pdfSolves problem of public-key exchange

RSA, Elliptic curve cryptographyhttp://www.cs.wayne.edu/~weisong/papers/walters

05-wsn-security-survey.pdf

TinySecDES, RC5, Skipjack, AES

AESAdvanced Encryption Standard

Successor of DES

Symmetric key encryption standard

Used worldwide

Fixed block size of 128 bits

Key size of 128, 192, or 256 bits

Number of rounds of encryption10, 12, 14 respectively

AES: PerformanceFaster in hardware

Good performance was an explicit goalPerforms well on a variety of hardware

Trust ManagementWe are primarily concerned with authorization,

as opposed to authentication.

Authentication is the mechanism whereby systems may securely identify their users.

Authorization is the mechanism by which a system determines what level of access a particular authenticated user should have to secured resources controlled by the system.

Trust Management, the bottomline

Main GoalWe would like to set up a public key encryption

to authorize the sender and receiver then establish AES session keys.Verification takes 2 minutes. - unrealistic

Sprocket currently works with software created AES keys that are sent across the air and verified by the nodes.

Do all AES encryption on the chip.

Trust Mangement = RT0Flexible approach to access control in distributed

systems

Access control decisions are based on the policy statements, called credentials,

Permissions in RT0 are represented by roles.

Credentials are made by different principals and stored in a distributed manner

CredentialA credential is a statement

Signed by the issuerAbout a subjectContaining info about the subject.

Requirements:Unforgeable.Verifiable (belongs to the entity asking for the

service)SignedHave well defined semantics.

Example

CertificateContains credential information in an over the

air format.

A role A.r denotes the set of entities that are members of it.Example: UVM.studentId Alice

Entities can define roles, issue credentials, and make requests.

In our case identities are pubic keys.

Certificate ValidityCredentials contain private information and

should be treated as such (e.g. medical record).

Since an authorizer receives certificates from an unknown potentially untrustworthy entities, the validity must be checked

Signatures

Certificate must not have expired.

Show Verify ExampleForm 1 is A.r E

Form 1

Public Key 40 bytes.

Role 1

Pub Key2 40 bytes.

Signature 2(21) = 42 bytes.

Total 124 bytes.

Project Currently symmetric keys are being used to

encrypt with software.

We want to use the hardware to make this a faster process.CC2420 chip that has AES support.

Allow for multiple keys and decryptionMany motes talking at once

http://focus.ti.com/lit/ds/swrs041b/swrs041b.pdf

Steps1) Use the hardware version of AES in a simple

Blink program.

2) Use hardware AES to send and receive packets that are encrypted.

3) Integrate into the overall project.

4) Test and optimize.

Encryption on CC2420Use Hardware security in CC2420.

The encryption provides a plain AES encryption, with 128 bit plaintext and 128 bit keys.

To encrypt a plaintext, a node first writes the plaintext to the stand-alone buffer SABUF, and issues a SAES command to initiate the encryption.

When encryption is complete, the ciphertext is written to the buffer, overwriting the plaintext.

Interfacesinterface CC2420Register as SECCTRL0;

interface CC2420Register as SECCTRL1;

interface CC2420Ram as KEY0;

interface CC2420Ram as SABUF;

interface CC2420Strobe as SAES;

interface CC2420Strobe as SNOP;

How to startFirst power up the chip.

Initialize the key.

Encrypt.

Send

Decrypt.

Show AES Standalone example

Referenceswww.cs.purdue.edu/homes/ninghui/readings/trus

t/rt_slides.pdf.

Theory.stanford.edu/~ninghui/talks/rt_oakland02_slides.pdf.

Cis.sjtu.edu.cn/…/The_Standalone_AES_Encryption_of_CC2420_(TinyOS_2.10_and_MICAz)