The problem is that RFC 2716 specifies the use of TLS-PRF-128. If TLS v1.2
negotiates a PRF where PRF-64 is not the same as the first 64 octets of PRF-128
(the IKEv2 PRF is an example of such a PRF), then RFC 2716bis implementations
will not interoperate with RFC 2716 implementations.
Subject: RE: [Emu] Issue with RFC 2716bis key generationDate: Thu, 7 Jun 2007
17:02:40 -0400From: [EMAIL PROTECTED]: [EMAIL PROTECTED]; [EMAIL PROTECTED]:
Is there any reason not to use the approach in RFC2716Bis? This guarantees
backward compatibility and require less computation.
From: Bernard Aboba [mailto:[EMAIL PROTECTED] Sent: Thursday, June 07, 2007
3:05 PMTo: [EMAIL PROTECTED]: [Emu] Issue with RFC 2716bis key generation
It has been pointed out by Paul Funk that the key generation formula specified
in RFC 2716bis could cause backward compatibility problems once TLS v1.2 is
introduced. The formula in -09 is as follows: MSK(0,63) = TLS-PRF-64(TMS,
"client EAP encryption", client.random || server.random)
EMSK(0,63) = second 64 octets of: TLS-PRF-128(TMS, "client
EAP encryption", client.random || server.random) IV(0,63)
= TLS-PRF-64("", "client EAP encryption", client.random
|| server.random)The issue here is that RFC 2716 Section 3.5 specifies a
different approach: MSK(0,63) = first 64 octets of:
TLS-PRF-128(TMS, "client EAP encryption", client.random ||
server.random) EMSK(0,63) = second 64 octets of:
TLS-PRF-128(TMS, "client EAP encryption", client.random ||
server.random) IV(0,63) = TLS-PRF-64("", "client EAP encryption",
client.random || server.random) With the current TLS PRF, these two
approaches are identical. However, this is not necessarily true for all PRFs
that could conceivably be negotiated within TLS v1.2. The text from RFC 2716
Section 3.5 reads as follows: Since the normal TLS keys are used in the
handshake, and therefore should not be used in a different context, new
encryption keys must be derived from the TLS master secret for use with PPP
encryption. For both peer and EAP server, the derivation proceeds as follows:
given the master secret negotiated by the TLS handshake, the pseudorandom
function (PRF) defined in the specification for the version of TLS in use,
and the value random defined as the concatenation of the handshake message
fields client_hello.random and server_hello.random (in that order), the value
PRF(master secret, "client EAP encryption", random) is computed up to 128
bytes, and the value PRF("", "client EAP encryption", random) is computed up
to 64 bytes (where "" is an empty string). The peer encryption key (the
one used for encrypting data from peer to EAP server) is obtained by
truncating to the correct length the first 32 bytes of the first PRF of these
two output strings. The EAP server encryption key (the one used for
encrypting data from EAP server to peer), if different from the client
encryption key, is obtained by truncating to the correct length the second 32
bytes of this same PRF output string. The client authentication key (the one
used for computing MACs for messages from peer to EAP server), if used, is
obtained by truncating to the correct length the third 32 bytes of this same
PRF output string. The EAP server authentication key (the one used for
computing MACs for messages from EAP server to peer), if used, and if
different from the peer authentication key, is obtained by truncating to the
correct length the fourth 32 bytes of this same PRF output string. The peer
initialization vector (IV), used for messages from peer to EAP server if a
block cipher has been specified, is obtained by truncating to the cipher's
block size the first 32 bytes of the second PRF output string mentioned
above. Finally, the server initialization vector (IV), used for messages
from peer to EAP server if a block cipher has been specified, is obtained by
truncating to the cipher's block size the second 32 bytes of this second PRF
output._______________________________________________
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