GnuPG 2.1.22 リリースと寄付の募集

This release fixes a keyring corruption bug introduced with last release. Users of 2.1.20, who are using the old "pubring.gpg" file to store their public keys, are asked to update to this new release.

Note that this side-channel attack requires that the attacker can run arbitrary software on the hardware where the private RSA key is used. Allowing execute access to a box with private keys should be considered as a game over condition, anyway. Thus in practice there are easier ways to access the private keys than to mount this side-channel attack. However, on boxes with virtual machines this attack may be used by one VM to steal private keys from another VM.

It is well known that constant-time implementations of modular exponentiation cannot use sliding windows. However, software libraries such as Libgcrypt, used by GnuPG, continue to use sliding windows. It is widely believed that, even if the complete pattern of squarings and multiplications is observed through a side-channel attack, the number of exponent bits leaked is not sufficient to carry out a full key-recovery attack against RSA. Specifically, 4-bit sliding windows leak only 40% of the bits, and 5-bit sliding windows leak only 33% of the bits.