An Introduction to Mobile VoiceCall Interception and Security

Cellcrypt Security BriefAugust 2009

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Table of Contents

1. Key Requirements............................................................................................................................ 3

2. Cryptography ................................................................................................................................... 4

3. Random Numbers............................................................................................................................ 5

6. Contact Cellcrypt:............................................................................................................................. 6

Figures

Figure 1: Cellcrypt Key Security Requirements...................................................................................... 3

Figure 2: Public-key Cryptography.......................................................................................................... 4

Figure 3: Symmetric Cryptography.......................................................................................................... 4

Figure 4: Hashing Algorithm.................................................................................................................... 4

Figure 5: Random Number...................................................................................................................... 5

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1. KEY REQUIREMENTS

In order to provide a secure voice communication between two parties several requirements have to be

met. The list below displays the key requirements to establish a secure voice channel between Alice (A)

and Bob (B).

Requirement Details

Confidentiality The communication must be confidential. No 3rd party can listen to the communication between Alice and Bob.

Authentication Both parties need to be able to strongly authenticate each other. Alice must be sure she is speaking with Bob.

Integrity What Alice is communicating must be what Bob is listening to. No 3rd party must be able to alter the communication, without the change being detected by Bob.

Forward secrecy Past communications cannot be decrypted even if the private keys or session keys are leaked.

Figure 1: Cellcrypt Key Security Requirements

Cellcrypt uses internationally recognised encryption algorithms to maintain confidentiality.

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delay between words spoken and words heard – to enable full-duplex conversations between callers.

High latency results in the undesirable effects of callers talking over each other or resorting to less

productive half-duplex conversations (“walkie-talkie style”).

Cellcrypt meets its security requirements at the same time as achieving very high performance by

implementing Encrypted Mobile Content Protocol (EMCP) developed by Cellcrypt to specifically solve

the performance issues associated with delivering encrypted real-time content between mobile devices

across wireless networks.

At a high level, EMCP addresses three areas:

High-speed mechanisms to establish encrypted data streams in real-time using standard

encryption algorithms.

Establishment of an end-to-end secure channel between mobile handsets, and authentication

and routing of encrypted data streams between them without the requirement for a key server

(or other technology that can be a security back door).

Mechanisms that ensure high performance data stream delivery over low bandwidth networks.

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2. CRYPTOGRAPHY

Cellcrypt uses public key cryptography to authenticate Alice and Bob, and for secure key exchange to

establish a session key. The voice traffic is encrypted using symmetric algorithms, using a different

session key for each call.

Figure 2: Public-key Cryptography

Symmetric Cryptography

• AES with 256 bit key

• RC4 with 256 bit key

Description: Both encryption algorithms are used at the same time. The data packet is first encrypted with RC4 and the crypto-data is then encrypted again with AES, in Counter Mode (CTR). Both algorithms are initialized with different session keys derived from the exchanged secrets. The first 4096 bytes are discarded from the key stream to prevent state initialization attacks.

Figure 3: Symmetric Cryptography

Hashing Algorithm

• SHA384 used with ECDSA

• SHA512 used with RSA

• SHA512, MD5 used for session key derivation

Description: Hash functions are used in the generation of random numbers and in the authentication algorithms.

Figure 4: Hashing Algorithm

Asymmetric (Public-key) Cryptography

• RSA with 2048 bit key

• ECDSA using NIST approved curves p-384, with 384-bit prime modulus

• ECDH using NIST approved curves p-384, with 384-bit prime modulus

Description: RSA and ECDSA are used to mutually authenticate parties in each call. The key pairs are generated on the handset when the application is installed, so are unique to each phone. A private key is never shared. Elliptic Curve Diffie-Hellman (ECDH) is used for key exchange to generate a session key that is valid for only one phone call. The application securely destroys the session key after use. On BlackBerry, the software uses RIM’s BlackBerry Cryptographic Kernel libraries.

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3. RANDOM NUMBERS

The quality of cryptographic keys underpins the security of all cryptography. Good keys must be

unpredictable which only results from use of a high-quality source of random numbers.

Random numbers are also essential to ensure that protocol messages are unique, thereby protecting

against man-in-the-middle and playback attacks. Good random numbers must be derived from

physically random sources that have inherent unpredictability, and be distributed uniformly across a

range of values.

Random Number

• 2048 bit seed pool

• Microphone noise

Description: A 2048 bit seed pool is generated during the installation. The seed pool is periodically updated. The initial seed is derived from the microphone input.

Figure 5: Random Number