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We have identified a potential for spoofing when using
rfc5083. Rfc5083 computes a message HMAC over the content using the authenticated
encryption algorithms. The message HMAC is calculated based on the content and
the content encryption key (CEK). A number of allowed key management techniques derive a key encryption
key (KEK) which can be used to authenticate the sender e.g. Static-Static
Diffie Hellman, Kerberos shared secret, shared password. The KEK is used to encrypt
the CEK per recipient. However when the integrity of the message is validated via the
message HMAC, the sender only demonstrates knowledge of the CEK to the
recipient. Any other recipient knows the CEK so can construct another message
using the same CEK and compute the new HMAC and reuse the encrypted KEK data from
the original message. For example: Alice
sends a message to Bob and Mallet. Alice uses different KEK’s for
both Bob and Mallet to encrypt the CEK. Bob decrypts the CEK using his KEK.
The message decrypts and the HMAC checks out so Bob think the message is from
Alice because only Bob and Alice know the KEK. Mallet
is the recipient of the original message so he can use his KEK to learn the
message CEK. Mallet then constructs a new message and encrypts it using
the same CEK as the original message. Mallet copies Bobs KEK recipient
Info from Alice's message which contains the same encrypted CEK. Because Mallet
knows the CEK he can also compute the correct HMAC for the new message. Mallet
sends the new message to Bob. Bob again uses his KEK to discover the CEK and
successfully decrypts the message and the HMAC check out so Bob thinks this new
message is also from Alice. This
issue only applies when there are multiple recipients. The motivation to use the
new authenticated encryption is to improve performance so solutions like bifurcating
messages to use different CEK or signing the message are counterproductive. We need an additional structure to allow the sender to demonstrate
knowledge of both the KEK and CEK per recipient when sending to multiple recipients. What is the consensus to resolve this? We can version 5083 to document the new structure or we can
do document the new structure via a standalone document. |