Re:What does a PGP signature on a Git commit prove?
by kazinator on Thursday April 01, 2021 @11:34AM
(#26661792)
> With git, a cryptographic signature on a commit provides strong integrity guarantees of the entire history of that branch going backwards, including all metadata and all contents of the repository, all the way back to the initial commit.
The purpose of a signature is to add authenticity assurances on top of integrity guarantees (which are necessary to authenticity, but independent). A document is signed by producing a digest of it, and that digest provides an integrity guarantee. The signature of the digest is then an additional authenticity guarantee.
"Integrity guarantees" means that it is vanishingly unlikely that someone can make a maliciously altered clone of that repository, including its history, all the way to the initial commit, including that signature.
The git hashes already provide pretty strong integrity guarantees of this sort. We can be confident that someone re-creating a fake history cannot end up at the same baseline at the HEAD, with the same commit hash. The strength of that confidence could be increased simply by adding additional, stronger hashes as part of a commit's content.
In terms of authenticity, the signature provides confidence (not a "guarantee") that whoever signed that commit held some beliefs about the repository, sufficient to want to put their signature to it
Someone could be duped into signing a corrupted repository, believing it to be genuine, in which case their beliefs were wrong. (And do not necessarily match their present beliefs!)
Someone could be coerced into signing corrupted repository, in which case their beliefs at the time of signing are that the repository is malicious, and that that they will be harmed if they don't comply.
Someone's private key could be compromised, so that an unauthorized agent perpetrates their signature without their knowledge or consent.
An authenticity assurance is not as easily quantifiable as an integrity guarantee. The integrity guarantee rests in the digest algorithm (how difficult is it to produce a document matching a given digest), but authencity assurances involve people problems.
The purpose of a signature is to add authenticity assurances on top of integrity guarantees (which are necessary to authenticity, but independent). A document is signed by producing a digest of it, and that digest provides an integrity guarantee. The signature of the digest is then an additional authenticity guarantee.
"Integrity guarantees" means that it is vanishingly unlikely that someone can make a maliciously altered clone of that repository, including its history, all the way to the initial commit, including that signature.
The git hashes already provide pretty strong integrity guarantees of this sort. We can be confident that someone re-creating a fake history cannot end up at the same baseline at the HEAD, with the same commit hash. The strength of that confidence could be increased simply by adding additional, stronger hashes as part of a commit's content.
In terms of authenticity, the signature provides confidence (not a "guarantee") that whoever signed that commit held some beliefs about the repository, sufficient to want to put their signature to it
Someone could be duped into signing a corrupted repository, believing it to be genuine, in which case their beliefs were wrong. (And do not necessarily match their present beliefs!)
Someone could be coerced into signing corrupted repository, in which case their beliefs at the time of signing are that the repository is malicious, and that that they will be harmed if they don't comply.
Someone's private key could be compromised, so that an unauthorized agent perpetrates their signature without their knowledge or consent.
An authenticity assurance is not as easily quantifiable as an integrity guarantee. The integrity guarantee rests in the digest algorithm (how difficult is it to produce a document matching a given digest), but authencity assurances involve people problems.