First presentation of a Cryptography series, it aims to provide a high level overview of cryptography, clarify its objectives, define the terminology and explain the basics of how digital security systems, like Bitcoin, are built. Mike Dance is a web developer and Bitcoin advocate. ---------- Presented at the BitcoinSYD Meetup on 11 February 2015

1. Cryptology
The practice and study of hiding information.
michael.dance@gmail.com
@mikedance

2. Presentation Goals
• Provide a high level overview of cryptography
• Clarify the objectives of cryptography
• Define the terminology
• Explain the basic concepts to build an
understanding of how digital security systems,
like Bitcoin, are built

3. Branches
• Cryptography
• Symmetric Algorithms
• Asymmetric (Public Key) Algorithms
• Cryptography Protocols
• Cryptanalysis

4. Cryptography
Objectives
There are four things we are trying to achieve with
cryptography.

5. Confidentiality
Information is kept secret from all but the authorised
parties.

6. Message
Authentication
The sender of a message is authentic.

7. Message Integrity
The message has not been tampered with during
transmission.

8. Non Repudiation
The sender of a message cannot deny that they created
or sent the message.

9. Terminology
There are around 21 terms you have to know.

10. Cryptography
The science of secret writing with the goal of hiding the
meaning of a message.

11. Cryptanalysis
The science and art of breaking crypto systems.

12. Cipher/Cypher
An algorithm for performing encryption and decryption.

13. Encryption
The process of hiding the meaning of a message so only
authorised parties can read it.

14. Decryption
The process of restoring a cipher text to the original
message.

15. Channel
A communication medium, for example, the internet,
mobile/wireless networks, radio waves, etc.

16. Secure Channel
A secure communication medium which can be trusted, used to exchange
secret key information, for example, face to face communication.

17. Insecure Channel
An insecure channel is an untrusted communication
medium, for example, the internet.

18. Message
The plaintext (readable) message you would like to keep
secret.

19. Cipher Text
The encrypted message.

20. Symmetric Algorithms
When encryption and decryption is performed using the
same secret key.

21. Secret Key
The data used to encrypt and decrypt in a symmetric algorithm. For example, a password.
This must be communicated over a secure channel and kept secret.

22. Asymmetric/Public
Key Algorithms
When encryption and decryption is performed using
different keys.

23. Private Key
The key used to decrypt a cipher text in a public key
algorithm. This must be kept secret.

24. Public Key
The key used to encrypt a message in a public key
algorithm. This can be made public.

25. Key Pair
The related public and private keys.

26. Key Space
The set of all possible keys.

27. Key Size/Length
The size measured in bits of the key, this determines the
key space required to brute force attack the key.

28. RNG
Random number generator.

29. PRNG
Pseudo Random Number Generator is a RNG that is not truly random,
used when security is not a concern, for example with video games.

30. CSPRNG
A cryptographically secure pseudo-random number generator. Used for
cryptography applications, for example generating a bitcoin private key.

31. THE END.
Of the terminology.

32. Real World Crypto
Systems

33. Kerckhoffs’ Principle
A crypto system should be secure even if the attacker knows all
the details about the system, with the exception of the secret key.

34. Symmetric Crypto
Systems
Uses one secret key for encryption and decryption

35. Analogy
A safe with one lock where each person with access has
a copy of the same key

36. DES
• Data Encryption Standard
• The most popular and best studied block cipher of the last 30
years
• Proposed in 1974 in response to a NIST (National Institute of
Standards and Technology) request for a standardised cipher
to secure government and national security communications
• Proposed by IBM cryptographers, with input from the NSA
with no public discourse
• Insecure due to small key space

37. 3DES
• Triple DES
• Performs DES encryption 3 times, which yields a
more secure cipher
• Still widely used primarily due to legacy
implementations and hardware
• Software implementations are not very efficient

38. AES
• Advanced Encryption Standard
• The most widely used symmetric cipher today
• In 1997, an open worldwide competition was held where cryptography algorithms
were submitted and reviewed by the international scientific community
• In 2001, the Rinjdael cipher, created by two young Belgian cryptographers was
selected for AES over submissions from IBM, RSA, and several famous
cryptographers
• In 2003, the NSA announced it will allow AES encryption to be used for classified
top secret documents
• Used in Blockchain.info wallets, Mac FileVault, Skype, WIFI encryption, IPSec, TLS,
SSH, etc.
• No known attacks have been found

39. Symmetric Pitfalls
• Key Distribution Problem
• A copy of the key must be exchanged over a
secure channel, problematic to do over the
internet!
• The number of keys required can get very large
• Non repudiation is not possible because several
people may use the same key

40. Asymmetric/Public
Key Crypto Systems
Uses a public key for encryption and a private key for
decryption.

41. Analogy
A mailbox on the street, everyone can put a letter in (encrypt), but only
a person with a private (secret) key can retrieve the letters (decrypt).

42. Background
• Symmetric cryptography has been around for as
long as written language, public key
cryptography is very new
• Public key cryptography was publicly introduced
in 1976 by Whitfield Diffie, Martin Hellman, and
Ralph Merkle

43. Overview
• Solves the key exchange problem because the encryption key
can be public (hence the name, public key)
• Can be used to prove the authenticity and integrity of a message
using digital signatures, and a digital signature algorithm (non
repudiation)
• Can be used to identify entities using challenge and response
protocols together with digital signatures, for example, electronic
car keys, passports, or bank cards
• Relies on one way functions, which is easy to compute in one
direction, but the inverse computation is computationally
infeasible

44. RSA
• Published in 1977 by Ron Rivest, Adi Shamir and
Leonard Adleman
• Security is provided by the integer factorisation
problem
• Given two large primes, it is easy to compute the
product, but difficult to factor the resulting product
• Requires large key sizes to be secure with modern
day computers, which is problematic for smaller
devices like mobile phones, smart cards, etc.

45. Elliptic Curve Cryptography
• Introduced in 1985 by Neal Koblitz and Victor S. Miller,
algorithms entered wide use in 2004, and 2005
• Uses elliptic curve over finite field mathematics to generate
public and private keys
• Security is provided by the elliptic curve discrete logarithm
problem
• Provides roughly the same security of RSA with large key sizes,
with much smaller key sizes, which is leading to adoption in
mobile phones, and smart cards
• Bitcoin uses private and public key pairs based on ECC

46. Asymmetric Pitfalls
• Encryption is significantly more computationally
expensive than symmetric algorithms

47. In reality
A hybrid approach is used by using a public key algorithm (which is slower) to
encrypt and exchange a symmetric key (which is small), a symmetric algorithm
(which is fast) can then be used to encrypt the message (which is big).

48. The End
Of part 1

49. What’s Next?
• Digital Signatures and Hash Functions
• Elliptic Curve Cryptography
• Bitcoin Cryptography

50. Questions?
michael.dance@gmail.com
@mikedance

51. Further Reading
• Understanding Cryptography by Christof Paar and
Jan Pelzl (http://www.crypto-textbook.com/)
• Cryptography Lectures
• https://www.youtube.com/playlist?
list=PLoJC20gNfC2gAB-eg7oaUTheB_JgQY4-q
• Public Key Cryptography: Diffie-Hellman Key
Exchange
• https://www.youtube.com/watch?v=3QnD2c4Xovk