The Secure Realtime Transport Protocol SRTP Authors David

Outline • • • Introduction The format of an SRTP packet SRTP Packet Processing

The format of an SRTP packet 原本RTP封包 SRTP封包 4

SRTP Packet Processing • Sender: – 1. Determine which cryptographic context to use. Cryptographic

SRTP Packet Processing(cont. ) – 2. Determine the index of the SRTP packet. index=216*ROC+SEQ(48

SRTP Packet Processing(cont. ) – 3. Determine the master key and master salt. 根據步驟

SRTP Packet Processing(cont. ) – 5. Encrypt the RTP payload AES(Advanced Encryption Standard)屬於對稱性密碼技術，加

SRTP Packet Processing(cont. ) IV =(index) XOR (salt_key) XOR(SSRC) 9

SRTP Packet Processing(cont. ) – 6. If the MKI indicator is set to one,

SRTP Packet Processing(cont. ) – 8. If necessary, update the ROC. Receiver: 1~4 the

SRTP Packet Processing(cont. ) – 6. Decrypt the Encrypted Portion of the packet –

Reference • [1] RFC 3550 RTP: A Transport Protocol for Real-Time Applications. • [2]

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The Secure Real-time Transport Protocol (SRTP) Author(s): David Oran 、 Rolf Blom 1

Outline • • • Introduction The format of an SRTP packet SRTP Packet Processing Conclusion Reference 2

The format of an SRTP packet 原本RTP封包 SRTP封包 4

SRTP Packet Processing • Sender: – 1. Determine which cryptographic context to use. Cryptographic context: 1. ROC (Rollover Counter) SEQ歸 0時ROC+1(16 bit) 2. s_l: SEQ最大值(receiver only) 3. The master key(s) 4. A replay list 5. Encryption algorithm(AES_CM 和 AES_F 8) 6. Authentication algorithm(預設HMA C_S H A 1) 5

SRTP Packet Processing(cont. ) – 2. Determine the index of the SRTP packet. index=216*ROC+SEQ(48 bit) SEQ: RTP封包的sequence number(16 bit) 當送出 248個封包時必須重新傳送密鑰 6

SRTP Packet Processing(cont. ) – 3. Determine the master key and master salt. 根據步驟 2推導或是MKI 指定產生master key and master salt – 4. Determine the session keys and session salt 7

SRTP Packet Processing(cont. ) – 5. Encrypt the RTP payload AES(Advanced Encryption Standard)屬於對稱性密碼技術，加密鑰匙為同一把，其優點是加解密的速度快，而缺點則為如何安全地讓雙方共享此一密鑰。 AES定義了兩種加密轉換模式(Encryption Transforms) a. Segmented Integer Counter Mode(AES_CM) b. F 8 Mode(AES _F 8) 8

SRTP Packet Processing(cont. ) IV =(index) XOR (salt_key) XOR(SSRC) 9

SRTP Packet Processing(cont. ) – 6. If the MKI indicator is set to one, append the MKI to the packet. – 7. Compute and append the authentication tag to the packet. • 可以藉此標籤判斷，該訊息是否來自同一持有該密鑰的發送端所發送。 10

SRTP Packet Processing(cont. ) – 8. If necessary, update the ROC. Receiver: 1~4 the same with Sender. – 5. For message authentication and replay protection 確認packet是否replay如果是則丟棄並記錄根據session key 確認authentication tag是否正確 11

SRTP Packet Processing(cont. ) – 6. Decrypt the Encrypted Portion of the packet – 7. Update the rollover counter and highest sequence number, s_l, in the cryptographic context – 8. remove the MKI and authentication tag 還原RTP封包 12

RTP波形圖 14

Reference • [1] RFC 3550 RTP: A Transport Protocol for Real-Time Applications. • [2] RFC 3771 The Secure Real-time Transport Protocol (SRTP) 18