crypto

Overview ¶

Package crypto implements XBPS' signature signing and verification using crypto/rsa.

XBPS SHA1-SHA256 workaround ¶

xbps specifies sha1 as algorithm for RSA_{sign,verify} but uses a sha256 message and length.

xbps still uses full sha256 hashes for the verification, the describes issue is just a parsing issue which with strict ASN1/PKCS1 implementations.

As a workaround this implementation disables golangs PKCS1v15 prefix by using crypto.Hash(0) as hash argument for rsa.VerifyPKCS1v15 and rsa.SignPKCS1v15 and uses a precomputed ASN1 prefix instead.

The ANS1 prefix comes from xbps generated signature, dumped using the openssl command:

openssl rsautl -verify -in foo-1.0_1.noarch.xbps.sig  -inkey ~/.ssh/id_rsa -raw -hexdump

XBPS implementation:

https://github.com/void-linux/xbps/blob/b4eebaf/lib/verifysig.c#L66

Original bug report:

https://github.com/voidlinux/xbps/issues/146

Note: golang also hardcodes the ASN1 prefix for performance reasons:

https://github.com/golang/go/blob/dca707b/src/crypto/rsa/pkcs1v15.go#L210