check md5 and length of downloads #6
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Since the quicklisp release contains the md5 and length of files, I figured we might as well check it when downloading. In the same spirit of
ql-httpsjust shelling out to curl, I just shell out tomd5sumwhich is present on 99.9% of unix boxes.If md5 was secure it would add significant security. Rather than getting a certificate authority to issue a false certificate or temporarily compromising the quicklisp server and backdooring some package, an attacker would have to also edit the dist to match their new hash and maintain the compromise unnoticed for months until you download the new dist and update some package. Unfortunately md5 is not secure and it's possible to make collisions with the same length, but it at least makes it a little harder for an attacker, and if quicklisp ever changes to use sha256sum or something then it will be trivial to change here and actually provide significant extra security. The quicklisp dist also contains a sha1 which would be better than md5, but that is generated by git which uses the time and author of the commit among other information we don't have in the simple release .tgz, so I don't think we can check it.