Sensitive data must be protected from eavesdropping and malicious tampering. An obfuscated transfer object [Steel 2005] that is strongly encrypted can protect data. This approach is known as sealing the object. To guarantee object integrity, apply a digital signature to the sealed object.
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This noncompliant code example signs the object as well as seals it. According to Abadi and Needham [Abadi 1996],
When a principal signs material that has already been encrypted, it should not be inferred that the principal knows the content of the message. On the other hand, it is proper to infer that the principal that signs a message and then encrypts it for privacy knows the content of the message.
Any malicious party can intercept the originally signed encrypted message from the originator, strip the signature, and add its own signature to the encrypted message. Both the malicious party and the receiver have no information about the contents of the original message as it is encrypted and then signed (it can be decrypted only after verifying the signature). The receiver has no way of confirming the sender's identity unless the legitimate sender's public key is obtained over a secure channel. One of the three Internal Telegraph and Telephone Consultative Committee (CCITT) X.509 standard protocols was susceptible to such an attack [CCITT 1988].
Because the signing occurs after the sealing, it cannot be assumed that the signer is the true originator of the object.
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SER02-EX0: A reasonable use for signing a sealed object is to certify the authenticity of a sealed object passed from elsewhere. This represents a commitment about the sealed object itself rather than about its content [Abadi 1996].
SER02-EX1: Signing and sealing is required only for objects that must cross a trust boundary. Objects that never leave the trust boundary need not be signed or sealed. For example, when an entire network is contained within a trust boundary, objects that never leave that network need not be signed or sealed. Another example would be objects that are only sent down a signed binary stream.
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Bibliography
[API 2006] |
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9.10, Sealing Objects | |
Chapter 11, Object serialization, sealed objects | |
Item 64, Use | |
| Item 65, Use |
Chapter 10, Securing the Business Tier, Obfuscated Transfer Object |
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Rule 14: Serialization (SER)
SER03-J. Do not serialize unencrypted, sensitive data