A Scalable and Format-Compliant Encryption Scheme for H.264 ...

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Figure 10: The flowchart of bitstream decryption.4.4. The Flow of DecryptionSimilar to the encryption architectures, Figure 10 illustrates the decryptionprocess which is the reverse of the encryption flow elaborated in Subsection4.3.(1) Parse the bitstream into NALUs, then decrypt each NALU as follows.• For any SEI NALU (6) whose payload type is 5 and which followsa prefix NALU (14) , construct a VCL NALU (1) or NALU (5) withDID = QID = TID =0.• For any PACSI NALU (30) whose DID=0 and QID=0, copy theprefix NALU (14) , and recover NALU (1) or NALU (5) based on theencrypted NALU (30) .18
• For any PACSI NALU (30) whose DID or QID is not zero, recoveran NALU (20) based on the NALU header and the SEI NALUs ofPACSI NALU.• For any PACSI NALU, if its SEI payload includes the header ofPACSI NALU (20) , recover the existing PACSI NALU.(2) Decrypt the visual sample data in the NALU (1) , NALU (5) or NALU (20)with the method in Subsection 4.1.2, and replace the NALU of theprotected SVC bitstream with the new NALU.Following the above decryption process, an authorized user can recover theSVC bitstream with his/her decryption key and then decode the decryptedbitstream.5. Discussion5.1. SecurityUsing a secret key which is transmitted under secure socket layer (SSL,http://www.openssl.org) protocol, a stream cipher such as RC4 [25] withInitialization Vector (IV ) is used for encryption, IV should be unique foreach message such that the cipertexts do not repeat themselves even if thecorresponding plaintexts are the same. This ensures the encryption schemeis secure against chosen plaintext attack (CPA).In our encryption scheme, IV is generated asIV = F(H n ,H s )where F is a one-way function such as SHA1 [26], H n represents the SVCNALU header such as DID, QID, and TID at different scalability layers, andH S denotes the slice header for each VCL NALU at the same scalability layer.Because the proposed scheme only encrypts the visual sample data, and theheader information is in clear text, IV can be deduced from the protectedbitstream at the decoder/decryptor side.5.2. Packet fragmentationAs mentioned in Subsection 3.1, a PACSI NALU must be encapsulatedin a RTP packet. However, when an SVC bitstream is delivered over heterogeneousnetworks, the RTP packet may be fragmented due to network MTU(Maximum Transmission Unit) limit. Therefore, with regard to Figure 11,19
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Page 5 and 6: Table 1: NALU TYPESNALU type Descri
Page 7 and 8: Figure 2: The structure of base lay
Page 9 and 10: poral base layer is used as referen
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Page 13 and 14: PACSIunit NALPrefixunit NALNRI 14 R
Page 15 and 16: • A new PACSI NALU (30) including
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Page 21 and 22: an SVC-aware router is required to
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Page 29 and 30: References[1] D. L. Goeckel, Adapti
Page 31: [22] H. Hellwagner, R. Kuschnig, T.

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