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If the recipient does not know the decryption key in advance, then the
sender generates and sends it. The key can be protected in transit by
encrypting method or key agreement.
If the plaintext data to encrypt is an XML element or content, you
encode it using UTF-8 and perform any necessary transforms to it,
otherwise, if it is an external resource, you simply consider it as an
octet sequence. You then encrypt the data, creating CipherValue, which
you place in EncryptedData.
Care must be taken when signing content that may later be encrypted;
clearly; the content must be restored to exactly the original plaintext
form for the signature to validate properly. To restore the plaintext
in the signed content, use the decryption transform method for XML
signature defined by the XML encrypt joint W3C and IETF working group.
This transform also allows specifications of XML fragments that were
encrypted and then signed with rest of the document and, therefore, are
not decrypted to validate the signature. Often, encrypted fragments are
removed from the signed information by using the XPATH transform in the
reference element, since the meaningful information is the plaintext.
We can sign the plaintext version of an encrypted element by including
the appropriate reference element pointing to it. When the signed
document is confidential and encrypted after being signed, you should
also protect against surreptitious forwarding in which the recipient
forwards the signed confidential document to a competitor, encrypted by
the competitor public key, trying to make it look as if the sender sent
the confidential information. To prevent surreptitious forwarding, the
signer should append the recipient identities to the document being
signed.
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