1) Symmetric-key cryptography uses the same key to encrypt and decrypt data but requires a secure way to share the secret key between parties. 2) Public-key cryptography addresses this issue by using different but mathematically linked public and private keys, allowing secure communication without pre-shared keys. 3) Digital signatures use public-key cryptography to authenticate senders, working by encrypting a digest of the message with the sender's private key that can be decrypted and verified using their public key.

1. Pertemuan-13
Enkripsi and Authentication
Jaringan Komputer
Teknik Informatika
Universitas Yudharta Pasuruan
2012
MOCHAMAD SIRODJUDIN, S.Kom, MM
www.sirodjudin.com

2. Symmetric-key Cryptography
• Data encrypted and decrypted with same key
• Classical examples: Caesar cipher, one-time pad,
Enigma Machine

3. Symmetric-key Cryptography: Drawbacks
• How do the parties get the shared, secret key?
• How do we transmit this or establish it securely?
• Must have some secret or ‘offline’ way of sending the secret.
• This is really hard to do in some situations…
• You could read it to them over the phone, but someone
could be tapping your phone in addition to your internet
connection.
• How can we both get the shared secret?

4. Public Key Cryptography
Each user has a keypair, consisting of a
public and private key
• Anything encrypted with one key may only be
decrypted by the other.
• To make message readable only by B, encrypt
message using B's public key

5. Where we are now…
• We can send coded messages without having to
establish any shared secret keys between us
ahead of time.
• There’s another exciting application of this
technology from the fact that Public and Private
keys can be used to decode one another (no
matter which is used to encode in the first place).
• But why would someone code a message with
their private key. Anyone in the world could use
their public key to decode it…

6. Digital Signatures I
• Well, what would the receiver know about the
sender of a message if Bob Smith’s public key
can decode the message?
• Whoever sent the message had Bob Smith’s
private key… (So, it was probably Bob Smith.)
• We’re no longer keeping the contents of a
message secret. Now we have found a way to
verify who was the sender of a message.
• Also, we know that nobody but Bob Smith
modified the contents of the message along the
way. (So, it’s intact and how Bob last saw it.)

7. Digital Signatures II
Using Public-Key Crypto for Strong Authentication
• Switch the roles of the keys
• Encrypt with Private key ("signing")
• Decrypt with Public key ("verifying" )
• Anyone (B) can read the message,
• But only A could have generated it

8. Digital Signatures III
• But there’s a problem… The Encoding/Decoding
step for public/private key cryptography is really
slow.
• For secret agents and governments and people
who really care about the secrecy of their
message, this isn’t a big deal.
• But for a lot of people who’d just like a digital
signature, this slowness almost makes it not
worthwhile to use the technology.

9. Digital Signatures IV
• What if we only encrypt a part of the message?
But then someone could go in and change the
non-signed part, and we’d never know if Bob
Smith really did that.
• What if there was some good way to calculate
some compressed or smaller form of the text and
then encrypt/sign that?
• But the smaller piece of text (or “digest”) would
have to reflect the whole text in some way or else
we have the same forgery problem as above.
There are ways to do this…

10. Digital Signatures V
• Remember our original digital signature picture?

11. Digital Signatures VI
Signature Generation:
Signature Verification:

12. A Digitally Signed Message (PGP)
-----BEGIN PGP SIGNED MESSAGE----Dear Alice: I'm getting very tired of cryptographers talking
about us behind our back. Why can't they keep their
nosesin their own affairs?!
Really, it's enough to make me paranoid.
Sincerely,
Bob
-----BEGIN PGP SIGNATURE----Version: 2.6.2
iQB1AwUBL4XFS2F2HFbSU7RpAQEqsQMAvo3mETurtUnLBL
zCj9/U8oOQg/T7iQcJvzMedbCfdR6ah8sErMV+3VRid64o2h2
XwlKAWpfVcC+2v5pba+BPvd86KIP1xRFIe3ipmDnMaYP+iV
bxxBPVELundZZw7IRE=Xvrc
-----END PGP SIGNATURE-----

13. But we’ve made an assumption here…
• We’ve assumed that there’s an easy and accurate
way to look up the public key for someone.
• What if some imposter just makes a web page,
claims to be Bob Smith, and publishes a public
key that is supposedly for Bob Smith (but it’s
really for them).
• Now this imposter could send us e-mails, sign
them as Bob Smith, and we might now know the
difference.

14. Verify a Public Key…
• How can we know that the public key we look up
for someone really is the correct public key for
that person. (And not just some key put up by an
imposter who’s pretending to be Bob Smith.)
• Well, there’s companies that make a big business
out of this. They keep secure registries of
listings of actual people and corporations and
store a copy of their official public key.
• You can go to this trusted company and know
you’re getting the right public key for Bob Smith.

15. A lot of business for one company…
• But there could be millions of times a day that
people need to check digital signatures. This
could just overwhelm some company.
• So, the one company can also verify that a
second company is also a trustworthy place to
ask about people’s public keys.
• So, now future requests for verification of public
keys can go to these sub-companies.

16. Chain of Trust I
• There’s a “Chain of Trust.” Start with a ‘root’ and
grow the trust tree/chain until we find a company
that is willing to verify Bob Smith’s public key.
VeriSign
Microsoft
MSN
Bob Smith

17. Chain of Trust II
• There’s a “Chain of Trust.” Start with a ‘root’ and
grow the trust tree/chain until we find a company
that is willing to verify Bob Smith’s public key.
VeriSign
Microsoft
Sheila Roy
MSN
Bob Smith

18. That slowness problem…
• So, we’ve seen:
• Symmetric Cryptography – Fast.
• Asymmetric (or Public Key) Cryptography – Slow.
• Digital Signatures (which use Private/Public Keys)
—Chains of Trust of public key verification.
• We also saw how to deal with the slowness issue
for digital signatures. (Using a “digest.”)
• Is there any way we can compensate for
slowness in the general message encoding task?
• Can we get the speed of symmetric cryptography?
• With the convenience of public key cryptography?

19. What’s the problem with each?
• Asymmetric is slower than symmetric.
• Symmetric is hard to use because you need a
secret/secure way to agree on your shared key.
• What if we use the slow asymmetric cryptography
to send a very short message:
We send the secret shared key for symmetric.
• Then we use symmetric crypto from then on.
“Secure Socket Layer” Coding.

20. Secure Socket Layer
• Used by most websites for secure connections
and for financial transactions to keep info safe.
• Encrypts the info you send to the site and the
info it sends to you. It also authenticates that the
site you are connected to is really who you think
it is.
• You can tell that this is being used when you see
the little yellow padlock icon in Internet Explorer.
• SSL uses Symmetric crypto, Asymmetric crypto,
and Digital Signatures.

21. How does SSL Work? (1)
• Go to a website for a financial transaction.
• It sends you a ‘certificate’ claiming to be some
organization and claiming to have some public
key.
• Your browser uses a chain of trust until it finds a
site you trust to will “vouch” for the accuracy of
the certificate the website sent you.
• Now, you know that the Amazon.com site you are
looking at is really authentic. You also know the
public key for Amazon; so, you can send it stuff.

22. How does SSL Work? (2)
• Now you can send stuff to Amazon securely
using asymmetric public/private key
cryptography. But this is a bit slow.
• What do you send them? SHARED SECRET KEY.
• If you both have this shared secret key, you can
now use symmetric cryptography to do the rest
of the transaction and send info in both
directions. Symmetric crypto is a lot faster than
asymmetric.

23. Look at a web page with a certificate…
• Check out www.citizensbank.com.
• Go to the Personal Banking Log-in Page.
• File: Properties: Certificates.
• This webpage is digitally signed by the bank so
that you know it is the official bank web page and
not some kind of imposter.
• In this case, a ‘root’ has directly verified the
bank’s public key.

24. Key length and security in real use
• How could we break each part of this?
• Factoring is the method to break public/private
keys; to break a 1024-bit private key, one would
need to factor an integer of length 1024 bits (or
over 300 decimal digits), which is well beyond
what anybody has done to date (currently people
can factor numbers of about 130 digits with lots
of computing power and time ….)
(from RSA Security)

25. Key length and security in real use II
But one can also attack encryption by trying to break the
symmetric key…. Here, there’s no math trick to break it.
You just try all the possible keys.
But adding just one bit to the length of a symmetric key
doubles the number of possible keys and the amount of
time that is needed to find the right one.
For example, the number of possible keys in a 56-bit
encrypted message is about 72 quadrillion keys, or
72,057,594,037,927,936.
Symmetric keys typically have lengths between 40 and 128
bits. Public keys typically have lengths between 512 and
2048 bits. Both the symmetric and public keys need to be
long enough to withstand an attack.
(from RSA Security)

26. So We’ve Seen…
• Symmetric Crypto – Fast, but hard to share secret
• Asymmetric Crypto – Slow but easier to set up
• Digital Signatures –
Uses Asymmetric, Digests, Chains of Trust
• Secure Socket Layer – Uses all three of the above
techniques to allow people to authenticate the
sender of a web page and conduct secure
business with it without having to use a lot of
slow asymmetric cryptography.