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Artifact ID: | f1886a2353b2551aaa3d5cc9ec7bd7cdd15b9d29 |
---|---|
Page Name: | keyrevokation |
Date: | 2014-04-28 00:33:03 |
Original User: | bernd |
Parent: | ba1ff1982ac9eda4085114d2357b393055c6cda2 (diff) |
Next | 616431c3c2901599d5bb63b1315ae39e550a0765 |
Key Revocation
Key revocation (for PKIs without certification authorities) usually is done
with a signature of the lost key, i.e. both the owner and the adversary can
revoke a compromised key. However, the important function in case of a
revocation is declaring the successor key, which reestablishs trust. To do
that, you must actually proof that you are the legitimate owner of the exposed
secret key, so how do you do that? After all, the adversary has stolen
it!
Therefore, the requirements are as follows:
- Only the creator of the secret key can revoke it
- A thief of the secret key can't (i.e. further information is necessary)
- Revocation must present a trustworthy replacement key
- Third parties must trust both the revocation and the replacement key without another trustworthy instance, i.e. trusting only their communication partner
I keep s2 as offline copy (it's just 64 hex digits), and s as protected online copy in my device; s is subject to attacks and backdoors, and therefore at risk. To revoke a key, I publish p2, which the recipient can validate by p1*(p2)==p.
To sign a new key, I use s2 as signature key, i.e. the recipient can use the just published p2 to verify the transition to the replacement key. Of course, the new key also has a signature with s, the old key. The format of the revocation attribute is actually <new pubkeys: pnew, p1new><p2, sig using s2><sig using s>. Both signatures must have the same signing date, and a never expiration date (a revocation doesn't expire).
An alternative way would be to create a signature key (which would be s2), and use that to sign the working key. Cross-signing would still prevent identity theft if just the signature key is stolen.