I don't know anything about this case, so everything I say is pure
supposition.
Let's suppose you have Alice and Bob who are working together on some
sort of business, and they are using some OpenPGP [1] software to
encrypt their emails that pertain to that business. Let's suppose
that the authorities then decide to raid Bob. Let us then suppose
that they go to Alice's ISP and get a lot of encrypted email, by
warrant, subpoena, etc. It doesn't matter for our purposes what ISPs
Alice and Bob are using, nor what OpenPGP software they are using.
* Let us consider the case where Bob turns state's evidence. If those
emails were encrypted to both Alice's key and Bob's key, after Bob
turns state's evidence, the authorities can decrypt all the messages
they seized from Alice's ISP. It doesn't matter what Alice did with
her key or what Alice's ISP did with it. They can be decrypted
because Bob's key has been compromised.
* Let us consider the same basic scenario where all the messages are
encrypted to the sender's, but not the recipient's key. In this case,
the authorities can decrypt all of Alice's messages to Bob, but not
Bob's messages to Alice. After they have compromised Bob, all of
Alice's messages to Bob can be decrypted. The fact that Alice's
security is untouched is mostly irrelevant. Alice is likely toast,
not because of the cryptography, but because Bob has been
compromised, and Bob's key decrypts mail Alice has sent.
* Let us consider a slightly different scenario in which neither
Alice nor Bob are compromised, but Bob is detained. If the
authorities raid Alice's ISP, despite the fact that they cannot
decrypt the messages, they may be able to show a connection between
Alice and Bob. If they have been CCing themselves, then you'll find
the same undecryptable message in each mailbox. If they have been
using "reply," there's probably metadata in the plaintext headers
that shows that M_n is a reply to M_{n-1} ... M_1, and thus you have
a chain of messages. If there is other evidence, such as Bob sending
checks to Alice every so often, the cryptography may be moot or worse
than moot. (If those messages are harmless, why don't you decrypt
them? Yes, this can get into many interesting discussions like the
applicability of Amendments 4 and 5, but these are also not
cryptographic. I really don't want to discuss them because I'll bet
we agree.)
Cryptography is not magic pixie dust that you can sprinkle on a
security problem and make it go away. If your adversary is a major
national government, you have operational security issues, as well.
If your adversary is a major national government that has direct
authority over where you live, then you have a much larger problem.
The adversary is going to use forensic analysis, traffic analysis,
and anything else they can think of. They are also not dumb. You also
have to expect that third parties, including ISPs, are unlikely to
see why they should fail to comply with legal documents like
subpoenas and warrants because of what you did. Smart cryptographers
make sure there are no backdoors in the crypto, because if there
were, then every beat cop and two-bit mafioso will want you to break
just that one message -- or else. If the system is strong, it all
comes down to your operational security.
Jon
[1] I have to give a now-usual rant. PGP is a trademark of PGP
Corporation and refers to software it makes. OpenPGP is an IETF
standard that covers encryption, certificates, and digital
signatures. There are many products that implement the OpenPGP
standard. PGP software is one of those. But other products, such as
GnuPG, Hushmail, Bouncy Castle, and so on also implement the OpenPGP
standard. Futhermore, PGP software implements other standards than
OpenPGP. For example, PGP software implements the S/MIME and X.509
standards as well as the OpenPGP standard.
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