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Crypto application:

PGP (Pretty Good Privacy)

PGP vs GPG vs OpenPGP

• Pretty Good Privacy (PGP) written by Phil Zimmerman and released in 1991

• OpenPGP is an IETF standard with which both pieces of software are compliant

• Gnu Privacy Guard (GPG) similar software released in 1999 under the GPL open source license.– Based on the OpenPGP standards

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Security issues for E-mail• Confidentiality

– Sys Admin, Email provider can read– Could be captured and read by anyone on your LAN

• Integrity– contents could be changed by anyone on the network.

• Authenticity– Easy to insert any e-mail header: “From”– Difficult to know if it was sent by the “Sender”

Targeted Attack - Recap

• Targeting a specific organization/group or person

• Email with malicious attachments– Executable– Word document

Targeted AttackTo: your e-mail address

From: Fakrul Alam fakrul@dhaka.com

Subject: my request

Hello,

I have been looking for someone who can answer questions in the attached file. I hope you can help me.

Thanks !

Example of Spoof Mail

Cryptography - PGP

• PGP is based on asymmetric (public-key) encryption

Asymmetric Encryption Recap

• Solves the problem of securely sharing (generating) secret keys and key explosion

• Public and private key mathematically related to each other– Cannot derive one from the other

• Encrypt with one and decrypt with the other– Encrypt with private, only public can decrypt– Encrypt with public, only private can decrypt

Signing & Encrypting

• Data can be signed with private key to be verified by anyone who has the public key– Remember digital signature?– Does not always have to be hash and encrypt!

• Since public keys are also data, they can be signed too!– Digital Certificates?

How PGP Works

Encrypt with receiver’s public key

Sign with sender’s

private key 🔐🔐Document Signed & Encrypted

document

🔐🔐🔐🔐

Decrypt with local private key

Verify (decrypt) with sender’s

public key

Document Signed & Encrypted document

SENDER

RECEIVER

Trust

• Chain of Trust (centralized/hierarchical)– Certain globally trusted bodies sign the public keys for

everyone

• Web of Trust (decentralized)– You pick whom you trust, and decide if you trust who they

trust

• Helps verify/associate a public key to an individual

• Which one is better?

Sample Web of Trust

Alice

Bob

Eve

Tashi

Carlos

• You can share your “trust-information” by publishing someone’s public key with your PGP sign – signed with your private key

PGP by GnuPG

• Create your keys– Public and Private keys

• Identify a key by– Key ID (like 0x23AD8EF6)

• Verify someone’s public key by– Key fingerprint

• Find public keys on public key servers– Like http://pgp.mit.edu

Key Management

• Use graphical tools– GPG Keychain for OS X– Kleopatra/GPA for Windows

• Use the command linegpg --list-keys

Key Management

• On printed media– Business cards

• Digital– Email– Sneakernet

• Online– OpenPGP key servers (pgp.mit.edu)

• But does not say anything about whether you trust a key

Key Management

• Make sure you specify the expiry (validity period)– Ensures that in case your private key is compromised, the

key-pair can only be used until it expires

• The expiry period can be changed anytime after creating the key

• However, before a key-pair expires, it is important you create a new key pair, sign it with the old one, and publish the signed new key to everyone in your web of trust– So that they can sign your new key

Key Management - Revocation

• Used to mark a key as invalid– Either before an expiry, or– If the private key has been compromised

• Always generate a revocation certificate as soon as you create your key– Don’t keep you revocation cert together with your private

key– gpg --gen-revoke IDENTITY

Key Management - Partying

• Key signing parties– To build your web of trust

• Each participant carries some form of ID, and a copy of their key fingerprint

– and maybe some 🍻🍻

• Each participant decides whether to sign someone’s keys– Based on their personal policies

How PGP Works

• To check your GPG version

How PGP Works

• Use “gpg --help” or “man gpg”

Generate Public-Private key pair

• Create the GPG public-private key pair

• We will opt for the default algorithm: RSA and RSA

Generate Public-Private key pair

• We will opt for 2048 bits

It is important to select key expiry period• You do not want a key that never expires• Many organizations operate with a 1 year key validity• Need to inform others when you change your keys

Generate Public-Private key pair

• Type your name and email address– Along with the comment will be used as USER-ID

Anyone can generate keys with your email!How can you identify your key uniquely?• Fingerprint!

Generate Public-Private key pair

Generate Public-Private key pair

• GPG will create the key pair

Read the messages carefully and take note of the contents:• Key ID• Key Fingerprint• Expiry• User ID

Generate Public-Private key pair

• List your keys:

Sign data & verify

• Create a file for signing

• Sign the data (type the passphrase)

Sign data & verify

• Have a look inside the signed file

Sign data & verify it

• ”Good signature from” indicates that the file was signed by the private key correctly

• The “WARNING” message is because we signed it as a cleartext signature – cleartext signature is readable without any special tool– If signed as a detached signature (--detach-sign), it would

create a separate file just for the signature

Export key (public)

• You can export a key to a file– for backup or further submission to public key servers

• The -a option generates the output in ASCII format

Export key (public) – key server• You can either use the CLI:

– Need to specify the key ID

– You can also specify a key server

• Or submit the keys (ASCII) directly on a key server

Find keys – key server

Import keys (public)

• You can import keys in a file

• Or import from public key servers

• Find the imported key

Verify the fingerprint

• Make sure to verify the fingerprint of every imported key

Signing keys• You can sign someone’s public key (trust)

• Make sure you verify their key fingerprint and any form of identification

Verify signatures

• The tashi.netsec@gmail.com has been signed by tashi@apnic.net

Publishing signed keys• You can now push the signed key to public key

server– Publishing your web of trust

• Verify on the key server

Encrypt Message• You must have the public keys of the intended

recipient– Create a file to encrypt

– Sign (with your private key) and Encrypt (with recipient’s public key) the file

Decrypt Message

• Try to read the encrypted message

Decrypt Message

• Decrypt the file

– The file was:• Signed with tashi.netsec@gmail.com’s private key• encrypted with tashi@apnic.net’s public key

References:

• https://www.gnupg.org/gph/en/manual/book1.html

Lab Exercise

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Lab Exercise - 1

• Download and install PGP software– Windows: gpg4Win (https://www.gpg4win.org)– OS X: GPG Suite (https://www.gpgtools.org)

Lab Exercise - 2

• Generate and publish key pair– Windows: Follow the lab manual pgp-lab.pdf– OS X/Linux: Follow the slide

Lab Exercise - 3

• Setup mail client:– Thunderbird with Enigmail (follow the slides)

• Send Signed/Encrypted email (follow the slides)

Thunderbird with Enigmail• Download and install Thunderbird

– Add Enigmail (Tools > Add-ons)– Restart Thunderbird

Thunderbird with Enigmail

• Setup Enigmail

Thunderbird with Enigmail

• Setup Enigmail– Select the key-pair you generated earlier

Thunderbird with Enigmail

• Setup Enigmail

Thunderbird with Enigmail

• Send signed email

Thunderbird with Enigmail

• Send signed and encrypted email– You will need the public key of the recipient