CODEWITHSUNDEEP
openssl
Alex
Alex

Reputation: 2402

Openssl refuses to verify signature

I'm sure I'm being stupid, but I'm trying to install GPGSuite on macos. I've downloaded the file and the signature, as well as their public key (https://gpgtools.org/GPGTools-00D026C4.asc). I have then tried these commands:

openssl base64 -d -in <signature> -out /tmp/sign.sha256
openssl dgst -sha256 -verify <pub-key> -signature /tmp/sign.sha256 <file>

Where pub-key is a file into which I've copied and pasted the public key from the link.

The first one works fine, the second one gives the rather unhelpful response of 'unable to load key file'. I've tried to find an answer online, but all the answers seem to be related to people trying to use a cert in place of the public key, which I don't believe I'm doing.

I appreciate that this is a noob question, but could somebody please point me in the right direction?

Upvotes: 1

Views: 627

Answers (1)

dave_thompson_085
dave_thompson_085

Reputation: 38771

Meta: this is not a programming question, and probably offtopic.

The key formats (both privatekey and publickey) used by GnuPG, and other implementations of OpenPGP, are completely different from those used by OpenSSL. The PGP formats (binary and 'armored') are defined in RFC4880 et pred; OpenSSL uses RSA-specific formats defined by PKCS1 (RFC8017 et pred) based on ASN.1 encoding defined now by X.680 et seq and 'PEM' (at least PEM-like) wrapping defined by RFC1421 or RFC7468, and generic formats defined by PKCS8 (RFC5208) and X.509 (available at RFC5280 et pred and RFC3279) also based on ASN.1 and 'PEM'.

There are quite a few Qs on several Stacks about these differences, but I can't find any that cover the direction you would need, which is PGP to OpenSSL public. https://superuser.com/questions/360507/are-gpg-and-ssh-keys-interchangable comes closest, and at least indicates the difficulties. (Although not clearly stated in the Q, on systems that have GPG, SSH usually means OpenSSH, which at the time the Q was asked used OpenSSL formats for privatekey though not publickey.)

However, this would not solve your problem, because that signature is a PGP (detached) signature, which like the PGP public key block uses PGP message/packet formatting whereas openssl dgst -sign/verify simply uses a raw (k/8)-byte value. And more importantly the PGP signature algorithm includes some metadata in addition to the actual message/file, specifically the 'hashed subpackets' of the message (as opposed to the un-hashed subpackets); you'd have to write a good bit of code (which would be ontopic!) to handle that.

I suggest you instead use the SHA256 provided over (and thus authenticated by) HTTPS to verify your download.

Upvotes: 1

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