Multiple DataReference elements

"prop3" states that "Multiple DataReference elements can occur if multiple
EncryptedData elements exist that are encrypted by the same key".

The use of multiple DataReference elements allows the following flawed
relationship;  Alice must encrypt EncryptedData-A and EncryptedData-B with
the same symmetric key, which is encrypted with Bob's public key in
EncryptedKey-Bob.  If Eve is a second recipient of EncryptedData-A, she
gains indirect access to EncryptedData-B, which Alice did not intend.  (Same
applies to KeyReferences.)

Although a similarly flawed linkage could be created also with
EncryptedKeyReferences or KeyRetrievalMethods instead of DataReferences, the
use of multiple DataReferences invites the application developer to create
it, by requiring re-use of encryption keys.

            +----------------------+
            | XML Document         |
            |   +------------+     |
 DataReference  | Encrypted  |  DataReference 
    +---<----<- |  Key-Bob   | --->---->--->---+
    |       |   +------------+     |           |
    |       |   +------------+     |  URI      |
    |       |   | Encrypted  | ---<----<----+  |
    |       |   |  Key-Eve   |     |        |  |
    |       |   +------------+     |        |  |
    |       |   +----------------+ |        |  |
    |       |   | Encrypted      | -->-->---+  |
    |       |   |  Data-A        | -<--<--<--<-+
    |       |   +----------------+ |
    |       |   +----------------+ |
    |       |   | Encrypted      | |
    +-->--->--->|  Data-B        | |
            |   +----------------+ |
            +----------------------+

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