Encrypting E-mail with GnuPG, Thunderbird and Enigmail

On Monday, 2 December 2013 I'm giving a presentation at KWLUG on Encrypting E-mail with GnuPG, Thunderbird and Enigmail which will be followed by a Formal Keysigning.

I'm using this page to develop my presentation notes. If you have comments, criticisms, or suggestions please put them on the Talk page.

--Bob.

I've started to convert this page to presentation slides BobJonkman 19:52, 19 November 2013 (UTC)

Intro to Crypto

Why use Encrypted E-mail?

Security
- Encrypted mail cannot be read by a Man In The Middle (MITM)
Authenticity (Integrity)
- Signed mail cannot be modified in transit, accidentally by mis-configured servers, or maliciously by MITM
Non-repudiability
- Signed mail can only come from the sender

Why NOT use Encrypted E-mail?

Rubber Hose Cryptanalysis (using a $5 wrench). http://xkcd.com/538

Need the other party to use the same encryption
Locked-in format
- Lose your secret key, lose your mail
- Need to keep revoked keys to read old mail
- If your key is compromised, all your old mail is compromised (no forward secrecy)
Non-repudiability
- Politicians? CEOs?
Rubber Hose Cryptanalysis (or $5 Wrench Cryptanalysis)
It's hard!
- and looks geeky...

Unencrypted, unsigned message

Date: Mon, 25 Nov 2013 18:27:22 -0500
From: Crypto Guy <cryptoguy@sobac.com>
To: Crypto Guy <cryptoguy@sobac.com>
Subject: Hello!

Hello World!

Unencrypted, signed message

Date: Mon, 25 Nov 2013 18:27:22 -0500
From: Crypto Guy <cryptoguy@sobac.com>
To: Crypto Guy <cryptoguy@sobac.com>
Subject: Hello!

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

Hello World!


-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.14 (GNU/Linux)
Comment: Using GnuPG with Thunderbird - http://www.enigmail.net/

iQEcBAEBAgAGBQJSk9zaAAoJENrSxFs55fZVAAgIAK2BKzV/qUTXCu0gEWJq2U3z
mZ6nsfzjs8aXJe8CT/c7kr7HSgSV57kvukIbvcUP5sCGwpIfwK04qA0Af4J9jXp7
Wq1/k0wjA1WzhCWUEEjdBs/05bAbQ78ulTbhFlII2ywLgH6BYxgjceZa9abgF8Di
xHkRWEAI6q+scoEhi0rCGlCX3UX/pkiZ1GlaxxxMBu1J5DbFaAJZ1MiPUDOLQN9w
5LzIqi4rKTtnCQo6G3WWRg5HvPMHHmUJoaZfZpnPrszf4ttG0vrFLxJKUqZszDyr
V5Lx/IjKAZBwoRRjfpRZILFHWoveaw5MG8487jM76W7LiVTEsX2AFGI3R6uy+KA=
=0p58
-----END PGP SIGNATURE-----

Encrypted, signed message

Date: Date: Mon, 25 Nov 2013 18:27:22 -0500
From: Crypto Guy <cryptoguy@sobac.com>
To: Crypto Guy <cryptoguy@sobac.com>
Subject: Hello!

-----BEGIN PGP MESSAGE-----
Charset: ISO-8859-1
Version: GnuPG v1.4.14 (GNU/Linux)
Comment: Using GnuPG with Thunderbird - http://www.enigmail.net/

hQEMA7bHREkg/1GxAQf/dmPoG27m33QGlUEZ0F9rlndQJFgeAyPBpXVEV2uCgaGw
rCzeBOkcE7FMmo2MjiozS64QaynPAGuHYoKwRIYS06Aa3ll2FwFO4O2tGiC1jsGQ
EUgjIE8h4iqIG4D5uzsv/iQ14QWJm4vaACxLCEtSaIRYcNB6Kxvy0phydltc3cp1
Ri1OlYPV799yiT7bcdT3ntTew5UF853mUrEIPt3NRfEDrC/m8ScTCT9SD2VRzlfA
qMviBt20XgApQscLBOWCAHv+1u2eUQ8AM3OsAZ/K4mw+s/jcWNNqLG9DRrrHJzMK
YXiZtLuE02eqpXjxrgfKmUloWAu7p9uQHMRwJjIy89LAugFdmW8HY1eZbpu7kUQU
9PvGydWd0QUPjkG7CXEeh63hmmbJgmJdjqOVjDuXhuHI1xU9+psmLPEszAY1TP9f
V4WR7SAIIVtE5r2S3YRwgyXWsPF+VgesBiUtQ7mroSGeVJ6GRR/rn0IdnLabrJsb
WXiCAtNajDx/x1QGcxFDIQjNsnqvzraSHwcpX6XelRCIz3jyS3SKDdR0T/x5YkmA
mkmXDZlLUntyjbJjZF0LmB96PTAi8JT/OG4QCxTsPMIpkLJZOcQ4SnHsaxfVgLqg
gOt5bxT9ybGt17xrw/j/mKpcq9cbyXFVJoRr73OVE8Wg2IYBye4TPu/HfXIGUh70
s4dE1Jc+iqdVLqJ2w3qGo4v8B2pg6bPLde7TT/7NrrvEWg0Zxlr+SFr+60xiyPGu
8eJHtQYSXe4OOc2FZev9mb0M2IIr0zpd7q5MvUqE+4b/VUtmadUFcExuUdaayS0P
rgLVNx3285jGXzelaFB2/+Gzx/m7MMXsKMBX7Q==
=jfdJ
-----END PGP MESSAGE-----

Crypto Theory

Symmetric Key Encryption

Substitution cipher, Caesar cipher
- Key = -1
  - Encrypt: IBM ⇒ -1 = HAL
  - Decrypt: Khmtw Trdq Fqnto ⇒ +1 = Linux User Group
- Key = 13 (ROT13)
  - Encrypt: Linux User Group ⇒ +13 = Yvahk Hfre Tebhc
  - Decrypt: Yvahk Hfre Tebhc ⇒ +13 = Linux User Group
Need a secure way to share key

Public/Private Key Encryption

The Math

Based on One-way function: Easy to do, hard to reverse

59 x 61 = ????
- 59 x 61 = 3599
3551 = ?? x ??
- 3551 = 53 x 67

The Theory

A Key Pair

Generate a keypair

Add name, e-mail and comment (doesn't have to be your real name or e-mail)
Public Key, Private (Secret) Key
Anything encrypted by one key is decrypted by the other
- Encrypt: Hello World ⇒ Public Key = |-|3110 '//0|21|)
- Decrypt: |-|3110 '//0|21|) ⇒ Secret Key = Hello World

Encrypt: Linux Is Cool ⇒ Secret Key = 1!/\/|_|>< !5 (001
Decrypt: 1!/\/|_|>< !5 (001 ⇒ Public Key = Linux Is Cool

In Practice

GnuPG/PGP uses a symmetric key, not Public/Secret keys to encrypt a message
Public/Secret keys are combined to make a Shared Secret
Shared Secret generates a Symmetric Key

Bob encrypts a message to Alice
- Alice's Public Key ⊗ Bob's Secret Key = Alice⊗Bob Shared Secret
- Alice⊗Bob Shared Secret generates Symmetric Key
- Message ⇒ Symmetric Key = |\/|355463

Alice decrypts a message from Bob
- Alice's Secret Key ⊗ Bob's Public Key = Alice⊗Bob Shared Secret
- Alice⊗Bob Shared Secret generates Symmetric Key
- |\/|355463 ⇒ Symmetric Key = Message

Bob signs a message
- Message ⇒ Hash = ABC
- ABC ⇒ Bob's Secret Key = 4|>(

Alice checks Bob's signature
- 4|>( ⇒ Bob's Public Key = ABC
- Message ⇒ Hash = ABC
- Same result? Message is untampered!

Practical GnuPG/PGP

Keep your Secret Key secret!
But you can distribute your Public Key widely
- Upload your Public Key to keyservers
- Send your Public Key by e-mail
Use a Public Key to encrypt a message
- Only that person's Secret Key can decrypt it
- You can only encrypt a message to someone whose Public Key you have
Use your Secret Key to sign a message
- Anyone can use your Public Key to verify the signature
- You can sign messages for everyone!
  - But only people who have your Public Key can verify the signature
  - Shows others that encryption is not such a weird thing
If I download your Public Key, how do I know it's really yours?

Keysigning!

You tell me what your Key Fingerprint is.
I verify that's the same Key Fingerprint on the Public Key I download
If I believe that's your Public Key, I sign it.

Alice's Public Key ⇒ Bob's Secret Key = 5!6|>0|>
Alice's Public Key + 5!6|>0|> = Alice's Public Key5!6|>0|>
Alice is popular: Alice's Public Key5!6|>0|>5!6(4|2015!6|\/|41(01|\/|

Web of Trust
- Your Public Key has been signed by others
- I already trust their Public Keys, some of that trust transfers to your Public Key

Demonstration

Install Enigmail

Use Wizard

Generate keypair

If your Key Fingerprint is 04F7 742B 8F54 C40A E115 26C2 B912 89B0 D2CC E5EA

Then your Long KeyID is 0xB91289B0D2CCE5EA
And your Short KeyID is 0xD2CCE5EA
Short KeyID is unique to about 1 in 10 billion (2^32)
Long KeyID is unique to about in 10^20 (2^64)
Key Fingerprint is unique to about 10^48 (2^160)
Number of atoms in the universe is about 10^80

Configure Enigmail

Generally, use the defaults.

I chose DSA for my key because of patent restrictions in 1999
Use PGP/MIME to hide signature blocks
- But displaying signature blocks may encourage others to use encryption
- Some mailing lists may remove PGP/MIME signature attachments

Retrieve a Public Key

E-mail address is convenient, but may return multiple keys
KeyID search will return one key. Prefix KeyIDs with "0x"

Sending a Message

OpenPGP → Default Composition Options → Signing/Encryption Options
OpenPGP → Per-Recipient Options

Receiving a Message

How to use PGP to verify an e-mail is authentic

If you want to be extra safe, check that there's a big block of jumbled characters at the bottom.
from http://xkcd.com/1181

Fortunately, Enigmail checks a little more thoroughly!

Keysigning with Enigmail

OpenPGP → Key Management → Edit → Sign Key

Resources

What do to with encrypted mail and who to do it with http://xkcd.com/1269

Other tools

Evolution — Built-in support for GnuPG
Claws — Plugins PGP/Core, PGP/Inline, PGP/MIME
KMail — use gnupg2 package
Mutt — Built-in support
GMail — Use Chromium and plugin cr-gpg
Other Webmail (Yahoo!, Hotmail, &c.) — Cut'n'paste with Firefox plugin WebPG (doesn't work for me)