Campbell R. Harvey Duke University and NBER Innovation and Cryptoventures Digital Signatures Campbell R. Harvey Duke University and NBER January 26, 2019

Campbell R. Harvey 2019

Definition Cryptography is the science of communication in the presence of an adversary. Part of the field of cryptology. Campbell R. Harvey 2019

Goals of Adversary Alice sends message to Bob Eve is the adversary Campbell R. Harvey 2019

Goals of Adversary Eve’s goals could be: Eavesdrop Steal secret key so that all future messages can be intercepted Change Alice’s message to Bob Masquerade as Alice in communicating to Bob Campbell R. Harvey 2019

Symmetric Keys Early algorithms were based on symmetric keys. This meant a common key encrypted and decrypted the message You needed to share the common key and this proved difficult Campbell R. Harvey 2019

Secret Keys Symmetric key DES (Data Encryption Standard) was a popular symmetric key method, initially used in SET (first on-line credit card protocol) DES has been replaced by AES (Advanced Encryption Standard) Campbell R. Harvey 2019

Diffie-Hellman Key Exchange Breakthrough in 1976 with Diffie-Hellman-Merkle key exchange There is public information that everyone can see. Each person, say Alice and Bob, have secret information. The public and secret information is combined in a way to reveal a single secret key that only they know https://www.youtube.com/watch?v=YEBfamv-_do Campbell R. Harvey 2019

Diffie-Hellman Key Exchange Will use prime numbers and modulo arithmetic We already encountered one example of modular arithmetic simple ciphers (also the SHA-256 which uses mod=232 or 4,294,967,296) https://www.youtube.com/watch?v=YEBfamv-_do Campbell R. Harvey 2019

Symmetric Key Exchange Numerical example “5 mod 2” = 1 Divide 5 by 2 the maximum number of times (2) 2 is the modulus The remainder is 1 Remainders never larger than (mod-1) so for mod 12 (clock) you would never see remainders greater than 11. EXCEL function = mod(number, divisor) e.g., mod(329, 17) = 6 “mod” Campbell R. Harvey 2019

Symmetric Key Exchange Alice and Bob decide on two public pieces for information A modulus (say 17) A generator (or the base for an exponent) (say 3) Alice has a private key (15) Bob has a private key (13) Is it possible for them to share a common secret that is unlikely to be intercepted? Campbell R. Harvey 2019 https://www.khanacademy.org/computing/computer-science/cryptography/modern-crypt/v/diffie-hellman-key-exchange-part-2

Symmetric Key Exchange Alice: Calculates 315 mod 17 = 6 (i.e., =mod(3^(15), 17)) Alice send the message “6” to Bob Campbell R. Harvey 2019

Symmetric Key Exchange Alice: Calculates 315 mod 17 = 6 (i.e., =mod(3^(15), 17)) Alice send the message “6” to Bob Eve intercepts the message! Campbell R. Harvey 2019

Symmetric Key Exchange Bob: Calculates 313 mod 17 = 12 (i.e., =mod(3^(13), 17)) Bob send the message “12” to Alice Campbell R. Harvey 2019

Symmetric Key Exchange Bob: Calculates 313 mod 17 = 12 (i.e., =mod(3^(13), 17)) Bob send the message “12” to Alice Eve intercepts the message! Now Eve has the 6 and the 12. Campbell R. Harvey 2019

Symmetric Key Exchange Alice: She takes Bob’s message of 12 and raises it to the power of her private key. Calculates 1215 mod 17 = 10 (i.e., =mod(12^(15), 17))* This is their common secret Campbell R. Harvey 2019 *EXCEL only does 15 digits so this will not work

Symmetric Key Exchange Bob: He takes Alice’s message of 6 and raises it to the power of his private key. Calculates 613 mod 17 = 10 (i.e., =mod(6^(13), 17)) This is their common secret Campbell R. Harvey 2019

Symmetric Key Exchange Eve She has intercepted their message. However, without the common secret key, there is little chance she can recover the shared secret. Campbell R. Harvey 2019

Symmetric Key Exchange Common secret Alice can now encrypt a message with the common secret and Bob can decrypt it with the common secret. Notice this is a common secret. Next we will talk private/public keys. That is, both and Alice have separate public keys and separate private keys. Campbell R. Harvey 2019

Asymmetric Keys: RSA - High Level Overview RSA stands for Rivest, Shamir and Adleman. Discovered earlier by UK Communications-Electronics Security Group (CESG) – but kept secret. Receiver generates two public pieces of information and a private key One piece of public information is just the product of two prime numbers, N=p*q (called “max”) The other is the public key, e, is just another prime that is greater than 2 and less than the product, N The prime numbers, p and q, that are used are huge. The private key is mathematically linked to public keys. Sender encrypts with the two public keys, e and N Receiver can easily decrypt Campbell R. Harvey 2019

Asymmetric Keys: RSA - High Level Overview See my Cryptography 101 (linked) deck for much more detail. Two prime numbers are chosen and they are secret (say 7 and 13, p, q). Multiply them together. The product (N=91) is public but people don’t know the prime numbers used to get it. A public key, e, is chosen (say 5). Given the two prime numbers, 7 and 13, and the public key, 5, we can derive the private key, which is 29. Campbell R. Harvey 2019

Asymmetric Keys: RSA - High Level Overview Issues with RSA RSA relies on factoring N is public (our example was 91) as is e If you can guess the factors, p, q, then you can discover the private key Campbell R. Harvey 2019

Asymmetric Keys: RSA - High Level Overview Issues with RSA Factoring algorithms have become very efficient To make things worse, the algorithms become more efficient as the size of the N increases Hence, larger and larger numbers are needed for N (moving to 2,048 bits) This creates issues for mobile and low power devices that lack the computational power Campbell R. Harvey 2019 http://www.slate.com/articles/health_and_science/science/2016/01/the_world_s_largest_prime_number_has_22_338_618_digits_here_s_why_you_should.html

Elliptic Curve Cryptography Mathematics of elliptic curves Does not rely on factoring Curve takes the form of y2 = x3 + ax + b Note that diagram is “continuous” but we will be using discrete versions of this arithmetic Note: 4a3 + 27b2 ≠ 0 Bitcoin uses a=0 and b=7 Campbell R. Harvey 2019

Elliptic Curve Cryptography Properties Symmetric in x-axis Any non-vertical line between two points intersects in three points Algebraic representation Campbell R. Harvey 2019

Elliptic Curve Cryptography Properties: Addition Define a system of “addition”. To add “P” and “Q” pass a line through and intersect at third point “R”. Drop a vertical line down to symmetric part. This defines P+Q (usually denoted 𝑃⊕𝑄) R P Q P+Q Denote Elliptic Curve as E Campbell R. Harvey 2019

Elliptic Curve Cryptography Properties: Doubling Define a system of “addition”. To add “P” and “P” use a tangent line and intersect at third point. Drop a vertical line down to symmetric part. This definite 2P (usually denoted 𝑃⊕𝑃) P Denote Elliptic Curve as E 2P Campbell R. Harvey 2019

Elliptic Curve Cryptography (Optional slide) Properties: Other P + O = O + P = P for all P ∈ E. (existence of identity) (b) P + (−P) = O for all P ∈ E. (existence of inverse) (c) P + (Q + R) = (P + Q) + R for all P, Q, R ∈ E. (associative) (d) P + Q = Q + P for all P, Q ∈ E (communativity) Denote Elliptic Curve as E Campbell R. Harvey 2019

Elliptic Curve Cryptography Why use in cryptography? Suggested by Koblitz and Miller in 1985 Implemented in 2005 Key insight: Adding and doubling on the elliptic curve is easy but undoing the adding is very difficult 256 bit ECC public key provides about the same security as 3,072 bit RSA public key Bitcoin uses a particular type of ECC known as secp256k1 http://www.nicolascourtois.com/bitcoin/groups_ECC_7B.pdf Campbell R. Harvey 2019

ECDSA Private key is a number called “signing key” (SK). It is secret. Public key is the “verification key” and is mathematically linked to the private key EC SK VK Private key: (number) Elliptic curve operations: Need base point, modulus, order Public key: coordinate (x, y) Note: Easy to generate a public key with a private key. Not easy to go the other way. Campbell R. Harvey 2019

ECDSA Digital signature EC DS SK Private key: (number) Nonce: (random number) Nonce EC Message DS SK Private key: (number) Elliptic curve operations: Need base point, modulus, order (n) Digital signature: coordinate (r, s) Campbell R. Harvey 2019

ECDSA Verification s EC (x’, y’) VK Elliptic curve operations: coordinates r Yes (verified) s EC (x’, y’) r = x’ mod n ? Message No (rejected) VK Elliptic curve operations: Need base point, order (n) Derive new point on elliptic curve Check x coordinate of new point and DS Public key: (x, y) Note r not used until verification step Campbell R. Harvey 2019

How DSAs Work Notice Proves that the person with the private key (that generated the public key) signed the message. Interestingly, digital signature is different from a usual signature in that it depends on the message, i.e., the signature is different for each different message. In practice, we do not sign the message, we sign a cryptographic hash of the message. This means that the size of the input is the same no matter how long the message is. Campbell R. Harvey 2019

ECDSA in Action Campbell R. Harvey 2019 https://kjur.github.io/jsrsasign/sample/sample-ecdsa.html

ECDSA in Action OP_CHECKSIG uses Public Key + Digital Signature + Hash of Transaction Verifies whether this transaction has been signed by the owner of the Private Key https://www.youtube.com/watch?v=ir4dDCJhdB4 (advanced by Matt Thomas) Campbell R. Harvey 2019

Application: PGP Email My public key for secure email You can encrypt an email to me with my public key and only I can decrypt with my private key. Campbell R. Harvey 2019

Application: PGP Email Steps Message compressed Random session key (based on mouse movements and keystrokes) is generated. Message encrypted with session key Session key is encrypted with receiver’s public key Encrypted message + encrypted session key sent via email Recipient uses their private key to decrypt the session key Session key is used to decrypt the message Message decompressed Campbell R. Harvey 2019 http://www.pgpi.org/doc/pgpintro/

References The Math Behind Bitcoin [recommended] Elliptic Curve Digital Signature Algorithm (Bitcoin) What does the curve used in Bitcoin, secp256k1, look like? Elliptic Curve Digital Signature Algorithm (Wikipedia) Elliptic Curve Cryptography (UCSB) Elliptic Curve Cryptography and Digital Rights Management (Purdue) Zero to ECC in 30 minutes (Entrust) The Elliptic Curve Cryptosystem Goldwasser, Shaffi and Mihir Bellare, 2008, Lecture Notes on Cryptography Dan Boneh, Stanford University, Introduction to Cryptography Dan Boneh, Stanford University, Cryptography II https://arstechnica.com/security/2013/10/a-relatively-easy-to-understand-primer-on-elliptic-curve-cryptography/ Campbell R. Harvey 2019