Chris Lamb (via Hacker News):
Accessing mirrors over HTTPS would not prevent a compromised mirror from tampering with packets, so APT already has other mechanisms to guard against this.
HTTPS would also not stop a particular attacker from extracting which effective packages your system downloaded (though this becomes more difficult).
A switch to HTTPS would also mean that you could not take advantage of local proxy servers for faster access and would also prohibit many kinds of peer-to-peer mirroring where files are stored on servers that are not directly managed by your distribution. This will disproportionately affect users in remote locations.
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