GnuPG/Kryptografie

PGP/GPG - Pretty Good Privacy

The OpenPGP protocol defines a set of asymmetric- and symmetric encryption algorithms, signature methods and compression protocols. GnuPG, a FOSS implementation of the OpenPGP standard, is widely used for mail encryption. GnuPG signs a message, encrypts it symmetrically and encrypts the symmetric key and the hash with Bob’s public key asymmetrically. Research on SHA-1 conducted back in 2005 (see: SHA-1 Broken) as well as the first practical successful collision in early 2017 (see: SHAttered) has made clear that collision attacks are a real threat to the security of the SHA-1 hash function. Since SHA-1 is defined as a must implementation by the OpenPGP specification, GnuPG is still using it. Currently settings should be adapted to preferably avoid using SHA-1. When using GnuPG, there are a couple of things to take care of:* keylengths (see: Keylengths)

randomness (see: Random Number Generators)
preference of symmetric encryption algorithm (see: Architectural overview)
preference of hash function (see: Architectural overview)

Properly dealing with key material, passphrases and the web-of-trust is outside of the scope of this document. The GnuPG website has a good tutorial on GnuPG. After 31 December 2017 GnuPG version 2.0.x is no longer supported and shall not be used anymore. Use the new long term version 2.1 instead.

Hashing

Avoid SHA-1 by preferring better hashing methods. GnuPG. Edit $HOME/.gnupg/gpg.conf:

Digest selection in GnuPG

personal-digest-preferences SHA512 cert-digest-algo SHA512 default-preference-list AES256 CAMELLIA256 AES192 CAMELLIA192 AES CAMELLIA128 TWOFISH SHA512 SHA384 SHA256 BZIP2 ZLIB ZIP

Key Generation

Because of lack of forward secrecy (see: [pfs]) in OpenPGP it is preferable to use large asymmetric keys for long term communication protection. A RSA key of 4096 bits should provide enough confidentiality for the next 10 years (see: Cryptographic Key Length Recommendation).

New key generation with GnuPG version 2.1

$ gpg --batch --full-gen-key $HOME/Desktop/params.txt`

Parameters for key generation with GnuPG version 2.1

Key-Type: RSA Key-Length: 4096 Subkey-Type: RSA Subkey-Length: 4096 Name-Real: <your-name> Name-Email: <your-email-address> Passphrase: <password> Expires: 2y

# My preferences: AES256, CAMELLIA256, AES192, CAMELLIA192, AES128, CAMELLIA128, TWOFISH, SHA512, SHA384, SHA256, BZIP2, ZLIB and ZIP

Preferences: S9 S13 S8 S12 S7 S11 S10 H10 H9 H8 Z3 Z2 Z1

	The preferences parameters S9 to Z1 correspond to AES256, CAMELLIA256, AES192, CAMELLIA192, AES, CAMELLIA128, TWOFISH, SHA512, SHA384, SHA256, BZIP2, ZLIB and ZIP. The parameters 3DES, SHA-1 and uncompressed are set automatically by GnuPG.

ECC - Elliptic Curve Cryptography

Since the release of GnuPG version 2.1 end-2014 ECC is supported. Older versions though are still widely used therefore ECC is not yet applicable in practice.