Signature with ECDSA keys on Mac OSX use always SHA384 hash

[daniele_infoc]

Hi all,

we're using on OSX an application that interfaces with the local keychain and intercepts the signature request issued from Acrobat (like a click on a signature field).

When testing this with ECDSA keys of the common secp family and different lenghts we noticed a strange behaviour, Acrobat always use the SHA384 hash algorithm indepentently from the actual key length.

To be more clear, for signatures we expected on the "keychain side" a digest fit for an "optimal cryptography", that is something like:

- key secp256 -> digest SHA256

- key secp384 -> digest SHA384

- key secp521 -> digest SHA512

Insetad we got:

- key secp256 -> digest SHA384

- key secp384 -> digest SHA384

- key secp521 -> digest SHA384

So, is possible that there is a bug somewhere and on OSX Acrobat tries to use incorrectly only the ecdsaSignatureDigestX962SHA384/ecdsaSignatureMessageX962SHA384 methods?

[daniele_infoc]

One more bit: we probably have undestood that Acrobat on Mac always choose the digest algorithm "looking" at the one that was used to sign the user certificate.

I.E., having a user certificate with secp256 keys that is signed by the RootCA with a "SHA384ECDSA" algortihm, when trying to make a signature the digest algorithm chosen by Acrobat will be "SHA384" (and not the expected "SHA256", as from the user's key length), thus will be based on the RootCA signature alg....

[richard_3563]

Richard m Singleton