If ClientHello has an Encrypt-Then-Mac extension, the server will compute the MAC after encrypting the data. If the ClientHello message does not contain an Encrypt-Then-Mac extension, then the default is MAC-then-encrypt mode. Encrypt-then-MAC: The cipher-text is generated by encrypting the plaintext and then appending a MAC of the encrypted plaintext.MAC-then-encrypt: This method calculates the MAC of the plain text, concatenates it with the plain text, and runs the encryption algorithm over it.Once the negotiation is done, the client and the server send each other a ChangedCipherSpec message, after which the traffic is encrypted with the negotiated algorithms.Įncrypted data is sent in one of two ways along with the message authentication code (MAC) in SSL/TLS. Today we examine the high-severity bug CVE-2017-3733, the Encrypt-Then-MAC renegotiation crash that can cause a denial of service.īefore SSL/TLS encrypts data, it runs the Handshake and ChangeCipherSpec protocols.ĭuring the Handshake phase, the client and server decide which encryption algorithms to use. We have written about “CVE-2017-3731: Truncated Packets Can Cause Denial of Service in OpenSSL” and “SSL Death Alert (CVE-2016-8610) Can Cause Denial of Service to OpenSSL Servers” among others. OpenSSL, the popular general-purpose cryptographic library that implements SSL/TLS protocols for web authentication, has recently suffered from several vulnerabilities.
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