Crypto Application PGP Pretty Good Privacy Issue Date: Revision: 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 2 Security issues for E-mail • Confidentiality – Sys Admins/Email providers can read – Anyone on your LAN can capture and read • 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 Attack To: your e-mail address From: Fakrul Alam [email protected] 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 ! Spoofed Mail Cryptography - PGP • PGP is based on asymmetric (public-key) encryption Asymmetric 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 Sign with Encrypt with SENDER sender’s receiver’s private key public key � Document Signed & Encrypted document � Verify (decrypt) Decrypt RECEIVER with sender’s with local public key private key � Document Signed & Encrypted document 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 Eve Carlos Bob Tashi • 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 line gpg --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) – If 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 pub-pvt key pair • Generate the GPG public-private key pair • We will opt for the default algorithm: RSA and RSA Generate pub-pvt key pair • 1024 bit key is not strong enough (key space). – 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 pub-pvt 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 pub-pvt key pair Generate pub-pvt 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 pub-pvt 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 of cleartext signature – 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 [email protected] has been signed by [email protected] Publishing signed keys • You can now push the signed key to a 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 [email protected]’s private key • encrypted with [email protected]’s public key References: • https://www.gnupg.org/gph/en/manual/book1.html 42 Lab Exercise 43 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 this slide Lab Exercise 3 • Sign each others key and publish – Build your web of trust – Make sure to: • Verify the Key fingerprint • Verify any form of ID card Lab Exercise 4 • Setup mail client: – Thunderbird with Enigmail (follow the next slides) • Send Signed/Encrypted email 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 • 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.