gpg - encryption & signing

1 David Morgan 2006-2013

GPG GPG encryption & signingencryption & signing

David Morgan

David Morgan 2006-2013

Functional purposes of Functional purposes of cryptograhycryptograhy

Confidentiality ensuring illegibility to outsiders

Authentication ensuring ostensible and actual sender are one

and the same

Data integrity ensuring non-alteration in transit


GPG GPG GNU Privacy GuardGNU Privacy Guard An implementation of OpenPGP

Open-PGP software uses a combination of strong public-key and symmetric cryptography to provide security services for electronic communications and data storage. These services include confidentiality, key management, authentication, and digital signatures.

Abstract, OpenPGP RFC4880


Cryptographic processingCryptographic processing

plaintext

cipher

cryptogram

cryptogram

reverse cipher

plaintext

Encryption(data sender)

Decryption(data receiver)


Cryptographic strengthCryptographic strength Difficulty of recovering plaintext from ciphertext

without the key

Does not depend on the cipher (algorithm) standard algorithms are published

So everything depends on the key key loss/compromise/interception=Achilles heel


Key distribution managementKey distribution management Good: protect it Better: dont distribute it


Best key distribution philosophyBest key distribution philosophy DONT What good would it do after all to develop impenetrable cryptosystems, if their users were forced to share their keys with a Key Distribution Center that could be compromised by either burglary or subpoena? Whitfield Diffie


2 broad categories2 broad categories Secret-key cryptography

versus

Public-key cryptographynew!

( 1970 )


Known synonymously as:Known synonymously as:

One technology single-key private-key symmetric secret-key shared-key conventional

Versus the other dual-key public-key asymmetric


good

DecryptDecrypt--key distribution demandskey distribution demands Secret-key

distribute but protect low risk

Public-key none, you dont need to distribute it no risk b

etter!


Keys: secretKeys: secret--key cryptokey crypto

plaintext

cipher

cryptogram

cryptogram

reverse cipher

plaintext

(same key)




Keys: publicKeys: public--key cryptokey crypto

plaintext

cipher

cryptogram

cryptogram

reverse cipher

plaintext

Encryption Decryption

(differentkey)


Wait a minuteWait a minute who sends the key(s) to whom? what/which key(s) does he send?


SecretSecret--key crypto: key crypto: data sender is key senderdata sender is key sender

plaintext

cipher

cryptogram

cryptogram

reverse cipher

plaintext

(same key)



Key sender

Key sent


What if itWhat if its publics public--key? key? more guys/keysmore guys/keys

if there are 2 guys, there are 2 key pairs (4 keys) who sends the key? what key does he send? what does that accomplish?


WellWell only public keys can be sent! so either guy could be the key sender and he would send his public key (only! ever!) sepending who sends, accomplishes

confidentiality, or authentication


Data receiver as key senderData receiver as key sender

plaintext

cipher

cryptogram

cryptogram

inverse cipher

plaintext



(data receiversprivate key)

Key sender

Key sent(data receivers public key)


Functional achievement checklistFunctional achievement checklist

Data receiver as key senderData receiver as key sender

Confidentiality

Authentication

Data integrity

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Data sender as key senderData sender as key sender

plaintext

cipher

cryptogram

cryptogram

inverse cipher

plaintext



(datasendersprivate key)

Key sender

Key sent(data senders public key)


Confidentiality

Authentication

Data integrity

Data sender as key senderData sender as key sender

Functional achievement checklistFunctional achievement checklist

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But canBut cant we have both together?t we have both together?

Confidentiality

Authentication

Data integrity

Certainly! if you just encrypt and decrypt twice


Encrypting the whole message Encrypting the whole message twice is too expensive!!twice is too expensive!!

Make a little token1 from a big message with a hash function2

Encrypt the token instead of the message

1also called a message digest or hash2also called a digest function, like MD5 or SHA1 or RIPEMD-160

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Confidential Confidential andand authenticauthentic**

plaintext

cipher

cryptogram

cryptogram

reverse cipher



H

S

plaintext S

H H

senders private

senders public

receivers public

receivers private

H - hashS - signature OK if same

*gpg encrypt and sign


authentic but not confidentialauthentic but not confidential**

plaintext



H

S

plaintext S

H H

senders private senders public

H - hashS - signature

OK if same

*gpg sign only,also useful

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gpggpg default directories and filesdefault directories and files/

tom dick harry

home

.gnupg

.gnupg .gnupg

gpg.conf pubring secring

gpg.conf pubring secring gpg.conf pubring secring


gpggpg key management optionskey management options --gen-key

create keypair and store on disk --export

take public key from disk and output to file --import

take public key from file and output to disk

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gpggpg encryption optionsencryption options --encrypt --recipient

encrypt file with users pubkey from disk --decrypt

decrypt file using private key from disk that matches public key with which file was encrypted


gpggpg signing optionssigning options --sign

create digest of file, encrypt it with private key --verify

decrypt senders digest, generate your own, check theyre the same

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Example: believing in fedoraExample: believing in fedora

1) this files digests,for the other files,make them believable

2) signature ondigests file makesit believable


Get fedora projectGet fedora projects public keys public key

16


Add fedoraAdd fedoras key to your s key to your keyringkeyring


Use it: file really from fedora?Use it: file really from fedora?

if the key is really fedoras, the file is really from them

we believe so

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Do downloads check out?Do downloads check out?o

sten

sible

per

fedo

ra a

nd

we

belie

ve

it!ac

tual

OK, exceptwhats up with disc2 ??


gpagpa GUI frontend to GUI frontend to gpggpg

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EnigmailEnigmail integrates integrates GPG+emailGPG+email

Others: http://www.gnupg.org/related_software/frontends.html


InfoInfo official page

http://www.gnupg.org GPG Mini HowTo

good, quick bare essentials http://www.dewinter.com/gnupg_howto/

english/GPGMiniHowto.html GNU Privacy Handbook

more thorough and explanatory http://www.gnupg.org/gph/en/manual.html

RFC4880 (OpenPGP message format)

gpg - encryption & signing

Documents

key management

key pairs

key distribution center

key senderdata sender

transit2 david morgan

it4 david morgan

low risk publickey

openpgp rfc4880 david