TOFU code available

[Neal H. Walfield]

Hi,

I've now pushed the TOFU code to master.  It will be part of the next
release.  It would be great to get some feedback on it.  To enable
TOFU, either set --trust-model=tofu or --trust-model=tofu+pgp.
(Alternatively, you can also add the line 'trust-model MODEL' to your
gpg.conf).  To get a feel for how TOFU works, try the following.

Encrypt a message:

  $ gpg2 -e -a -r EE37CF96
* gpg: Verified 0 messages signed by "Testing (insecure!)" (key: 362D 3527 F53A AD19 71AA  FDE6 5885 9975 EE37 CF96, policy auto).
* gpg: Warning: we've have yet to see a message signed by this key!
* gpg: Warning: if this value is unexpectedly low, this might indicate that this key is a forgery!  Carefully examine the email address for small variations (e.g., additional white space).  If the key is suspect, then use 'gpg --tofu-policy bad "362D 3527 F53A AD19 71AA  FDE6 5885 9975 EE37 CF96"' to mark the key as being bad.
  gpg: 6EA74366: There is limited assurance this key belongs to the named user
  -----BEGIN PGP MESSAGE-----
  ...

Verify a message:

  $ gpg2 --verify EE37CF96-1.txt 
  gpg: Signature made Fri 18 Sep 2015 02:05:37 PM CEST using RSA key ID EE37CF96
* gpg: Good signature from "Testing (insecure!)" [marginal]
* gpg: Verified 0 messages signed by "Testing (insecure!)" (key: 362D 3527 F53A AD19 71AA  FDE6 5885 9975 EE37 CF96, policy auto).
* gpg: Warning: we've have yet to see a message signed by this key!
* gpg: Warning: if this value is unexpectedly low, this might indicate that this key is a forgery!  Carefully examine the email address for small variations (e.g., additional white space).  If the key is suspect, then use 'gpg --tofu-policy bad "362D 3527 F53A AD19 71AA  FDE6 5885 9975 EE37 CF96"' to mark the key as being bad.
  gpg: WARNING: This key is not certified with sufficiently trusted signatures!
  gpg:          It is not certain that the signature belongs to the owner.
  Primary key fingerprint: 362D 3527 F53A AD19 71AA  FDE6 5885 9975 EE37 CF96

And, encrypt to a key with the same email (well, in this case, name
since there is no email):

  $ gpg2 -e -a -r BC15C85A
  The binding <439D 954F 18F7 9CC4 F71B  ED91 CACE D996 BC15 C85A, testing (insecure!)> is NOT known.  Please indicate whether you believe the binding is legitimate (the key belongs to the stated owner) or a forgery (bad).
  
  Statistics for keys with the email 'testing (insecure!)':
    439D 954F 18F7 9CC4 F71B  ED91 CACE D996 BC15 C85A (this key):
      0 signed messages.
    362D 3527 F53A AD19 71AA  FDE6 5885 9975 EE37 CF96 (policy: auto):
      1 message signed over the past 1 month.
  
  Normally, there is only a single key associated with an emailaddress.  However, people sometimes generate a new key iftheir key is too old or they think it might be compromised.Alternatively, a new key may indicate a man-in-the-middle attack!Before accepting this key, you should talk to or call the personto make sure this new key is legitimate.
  
  (G)ood/(A)ccept once/(U)nknown/(R)eject once/(B)ad? b
  gpg: Verified 0 messages signed by "Testing (insecure!)" (key: 439D 954F 18F7 9CC4 F71B  ED91 CACE D996 BC15 C85A, policy bad).
  gpg: invalid trustlevel 3 returned from validation layer
  gpg: C6D85A87: There is no assurance this key belongs to the named user
  sub  rsa1024/C6D85A87 2015-09-18 Testing (insecure!)
   Primary key fingerprint: 439D 954F 18F7 9CC4 F71B  ED91 CACE D996 BC15 C85A
        Subkey fingerprint: F5A6 985C B48D BF6C 241B  6172 6B2A BB68 C6D8 5A87
  
  It is NOT certain that the key belongs to the person named
  in the user ID.  If you *really* know what you are doing,
  you may answer the next question with yes.
  
  Use this key anyway? (y/N) n
  gpg: [stdin]: encryption failed: Unusable public key

Also, take a look at the manual for a description the related commands
/ options (search for 'tofu').

:) Neal