SPEECH CRYPTOGRAPHY

Submitted By

Anurag Kumar (2012IS05)Pratyush Pachauri (2012IS25)

INTRODUCTION : SPEECH encryption seeks to perform a completely reversible operation on a portion of speech, such that it s totally unintelligible to an unauthorized listener. The three most important criteria used to evaluate speech scramblers are:

The scrambler’s ability to produce encrypted speech with low residual intelligibility.

The extent to which the encryption and decryption processes affect the quality of the speech recovered by the intended recipient.

The scrambler’s immunity to cryptanalytic attack.

Several firsts were The first realization of enciphered telephony

The first transmission of speech by Pulse Code Modulation (PCM)

The first examples of multilevel Frequency Shift Keying (FSK)

The first useful realization of speech bandwidth compression.

Origin of Speech Cryptology : SIGSALY

The Bandsplitter

Discrete Fourier Transform based Encryption

CRYPTANALYSISCryptanalysis of any incoming speech data

may be carried out in two steps:

Identification of the scrambling/ciphering scheme i.e. system identification.

Descrambling/deciphering using the various parameters of speech to obtain intelligible speech.

Time domain parameters:• Short time energy function• Short time autocorrelation function• Short time average magnitude difference function

Frequency domain parameters:• FFT of spectrum estimation• Filter bank methods• Homophonic speech processing• Model based techniques• LP-based spectral estimation• Maximum entropy spectral estimation

Transform Domain : These scramblers operate on speech which has

been sampled, digitized partitioned into frames of equal length, each containing N speech samples. Transformation is applied on them and then permuted.

CRYPTANALYSIS OF DFT BASED SYSTEM

Plaintext/Ciphertext Attack:

Ciphertext Only Attack on a Fixed Permutation System:

Ciphertext Only Attack on a Varying Permutation System :

CRYPTANALYSIS OF THE BANDSPLITTER

Plaintext/Ciphertext Attack:

Ciphertext Only Attack on a Fixed Permutation System:

Ciphertext Only Attack on a Varying Permutation System :

SRTP : STANDARD OF INTERNET TELEPHONY

1) Data Encryption

Payload_Dataencrypted = DES( keyDES, Payload_Data )2) Hashing

Authen_Tag = SHA(Payload_Dataencrypted)

DIGITAL WATERMARKING A secret digital watermark, speech_watermark,

used for Non –Repudiation and Authenticity. Both the sender and receiver share one reference

watermark (comm_watermark) from an independent trustworthy party.

SPEECH WATERMARKING

Regular Pulse Excitation, Long-Term Prediction (RPE-LTP) is slightly varied to include the watermark

A GSM 610 coder generates 52 RPE pulses of 3 bits each for a speech frame of 20 ms duration. An RPE pulse pair in one frame is chosen to embed one bit of the digital watermark, and the pulse pair is selected according to key.

ADVANTAGE