Public Key Infrastructures

Transcription

1 Public Key Infrastructures Certcoin Cryptography and Computer Algebra Prof. Johannes Buchmann Dr. Johannes Braun

2 Background Blockchain Distributed database, consisting of a list of blocks Decentralized and modification-resistant by design Best known example: Bitcoin blockchain, and many other cryptocurrencies Other (upcoming) use-cases: e.g. SmartContracts 2

3 Background Blockchain: Structure A block contains A hash of the previous block (for chaining) The data (representation varies by implementation) Block Hash of previous Block Data The blockchain consists of An origin (genesis block, green) A main chain (black) Orphaned side chains (invalidated, purple) [Quelle: File:Blockchain.svg] 3

4 Background Blockchain: Consensus process Adding new blocks: A miner performs a consensus process (mostly Proof-of-Work) Proof is expensive to generate the proof But easy to verify Concurrently generated blocks multiple chains that exist in parallel, called forks To ensure, everybody has the same chain Trust in the majority (majority of computing power) The fork with the highest value becomes the (new) main chain Value usually accumulative, so the longest fork will likely win 4

5 Namecoin Cryptocurrency like Bitcoin Main purpose: Alternative, distributed Domain-Name-System (DNS) Based on Blockchains no controlling authority No censorship No single point-of-failure DNS records for.bit domains are stored in the Blockchain Many other usages besides DNS in various stages of planning or implementation 5

6 Certcoin 6

7 Motivation Hierarchical PKIs and the Web of Trust have several drawbacks Hierarchical PKIs Trusted Third Parties (CAs) needed single points of failure & censorship Web of Trust High entry barrier & usability issues management of keys by user Trust in individual users required Certcoin is meant to overcome these issues. 7

8 Certcoin New PKI-concept based on Namecoin Public and decentralized Only requires trust in majority of other users new trust model Namecoin provides the needed infrastructure (called auxiliary information ) Offers typical PKI-functionalities: Registration, update, look up (verification), revocation 8

9 Registration Append the following to the Blockchain Keytype: either online or offline Public Key: Either the Online-Key for everyday use Or the Offline-Key as backup in case of compromise Signature: The Identity signed by the Private Key of type Keytype 9

10 Update Append the following to the Blockchain Keytype: online or offline Old Key/New Key: The public keys of type Keytype Signatures: Identity and New Key signed by Old Key Identity signed by New Key 10

11 Look up (verification) Look up of the public key of a given identity Traverse the complete blockchain and check that The identity is only registered once There is no revocation for the identity If both of those hold, the key in the last update is returned Verification of a given public key of an identity Perform look up Compare the returned key with the key that shall be verified 11

12 Revocation Only needed if online- and offline- secret keys are compromised (otherwise an update is possible) Append the following to the Blockchain Keytype: online or offline Online/Offline Key: The public keys to be revoked Signatures: Identity, Revoke and Keytype signed by both private keys respectively 12

13 Problems Some operations require the traversal of the whole chain very inefficient better versions exist, which use additional data structures for caching Hierarchical PKIs offer additional functionalities like: Verification of the identities (Certcoin uses first come, first serve) Policies and their enforcement Multiple revocation systems (OCSP, ) Supervision, e.g. use of secure key lengths, proper algorithms etc. 13