Investigating the OpenPGP Web of Trust
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1 Investigating the OpenPGP Web of Trust Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle Diskrete Mathematik Universität Tübingen Netzarchitekturen und Netzdienste Technische Universität München ESORICS 2011 Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 1
2 Introducing the Web of Trust PGP/GnuPG (GPG) Widely used implementations of OpenPGP (authentication & encryption) Often used for Web of Trust (WoT) PKI: everyone can certify anyone else Decentralized Certification Authorities (CAs) allowed: just very active users Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 2
3 Web of Trust (WoT): Directed Graph Oliver George Bob Nate Paul Charlie Daniel Quentin Emile Ivan Alice Frank Henry Jane Karla Laura "signs" Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 3
4 Web of Trust (WoT): Directed Graph Oliver Bob George Charlie Nate Daniel Paul Quentin Ivan Emile Alice Henry Frank Jane Karla Laura Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 3
5 Web of Trust (WoT): Directed Graph Oliver Bob George Nate Paul Charlie Daniel Quentin Emile Alice Ivan Frank Henry Jane Karla Laura Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 3
6 Our Questions (Problem Statement) Analyze the Web of Trust s graph w.r.t. Macro structure How can users profit from the WoT? Usefulness to users How effectively can the WoT used? Robustness How does the WoT react to changes? Further Aspects Social structures? Crypto algorithms? Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 4
7 Our Questions (Problem Statement) Analyze the Web of Trust s graph w.r.t. Macro structure How can users profit from the WoT? Usefulness to users How effectively can the WoT used? Robustness How does the WoT react to changes? Further Aspects Social structures? Crypto algorithms? Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 4
8 Our Questions (Problem Statement) Analyze the Web of Trust s graph w.r.t. Macro structure How can users profit from the WoT? Usefulness to users How effectively can the WoT used? Robustness How does the WoT react to changes? Further Aspects Social structures? Crypto algorithms? Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 4
9 Our Questions (Problem Statement) Analyze the Web of Trust s graph w.r.t. Macro structure How can users profit from the WoT? Usefulness to users How effectively can the WoT used? Robustness How does the WoT react to changes? Further Aspects Social structures? Crypto algorithms? Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 4
10 Our Questions (Problem Statement) Analyze the Web of Trust s graph w.r.t. Macro structure How can users profit from the WoT? Usefulness to users How effectively can the WoT used? Robustness How does the WoT react to changes? Further Aspects Social structures? Crypto algorithms? Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 4
11 Background: OpenPGP Certification Public/private key pair: pub 2048R/69B003EF User ID: [Ralph Holz, Issue a certificate = sign(user ID, public key) Web of Trust (WoT) Network of key servers to upload keys Synchronizing Keyservers (SKS) protocol Complete history of the network (SKS knows no delete operation!) Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 5
12 Trust in OpenPGP Owner Trust Alice: I trust Bob [very much/somewhat/not] to properly identify a person before signing. Private assessment stored locally Valid keys in GnuPG default settings Path length 5 Either full trust in all owners on path Or 3 distinct paths with marginal trust in owners Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 6
13 Deriving Requirements A good WoT should... have certification paths between many (all) keys else it is not useful have short certification paths less entities to trust chances of accurately assessing key authenticity have redundant paths between keys beneficial for GnuPG trust metric be robust removal of a key must have little impact on reachability capture social relations between users well trust assessment is easier in communities Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 7
14 Deriving Requirements A good WoT should... have certification paths between many (all) keys else it is not useful have short certification paths less entities to trust chances of accurately assessing key authenticity have redundant paths between keys beneficial for GnuPG trust metric be robust removal of a key must have little impact on reachability capture social relations between users well trust assessment is easier in communities Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 7
15 Deriving Requirements A good WoT should... have certification paths between many (all) keys else it is not useful have short certification paths less entities to trust chances of accurately assessing key authenticity have redundant paths between keys beneficial for GnuPG trust metric be robust removal of a key must have little impact on reachability capture social relations between users well trust assessment is easier in communities Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 7
16 Deriving Requirements A good WoT should... have certification paths between many (all) keys else it is not useful have short certification paths less entities to trust chances of accurately assessing key authenticity have redundant paths between keys beneficial for GnuPG trust metric be robust removal of a key must have little impact on reachability capture social relations between users well trust assessment is easier in communities Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 7
17 Deriving Requirements A good WoT should... have certification paths between many (all) keys else it is not useful have short certification paths less entities to trust chances of accurately assessing key authenticity have redundant paths between keys beneficial for GnuPG trust metric be robust removal of a key must have little impact on reachability capture social relations between users well trust assessment is easier in communities Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 7
18 Deriving Requirements A good WoT should... have certification paths between many (all) keys else it is not useful have short certification paths less entities to trust chances of accurately assessing key authenticity have redundant paths between keys beneficial for GnuPG trust metric be robust removal of a key must have little impact on reachability capture social relations between users well trust assessment is easier in communities Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 7
19 Let s Start: Obtaining Our Dataset Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 8
20 Used Dataset Obtained full snapshot of SKS database Stored relevant key properties in SQL DB Snapshot contains complete history of network Time stamps of key creation, signatures, expiry, revocations,... Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 9
21 Resulting Key Set Many keys available on the servers All keys 2.7 millions Expired, revoked, broken keys 570,000 But not many used for signatures Keys with incoming or outgoing signatures 325,000 Resulting signatures 817,000 Majority of available keys are not verifiable: no signature chains. Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 10
22 Macro Structure Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 11
23 Macro Structure Strongly Connected Components (SCCs) Bob Nate Paul Charlie Daniel Alice Frank Henry Jane Karla Laura Within an SCC, there is 1 signature chain between any key pair. Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 12
24 Macro Structure SCCs are important: mutual authentication only within the same SCC SCCs in the Web of Trust Largest SCC (LSCC) of just 45,000 keys (!) But there are 240,283 SCCs > 100,000 are single nodes (trivial sub-graphs)... 10,000 node pairs Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 13
25 Macro Structure: SCC Sizes quantity 1e+00 1e+01 1e+02 1e+03 1e+04 1e component size Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 14
26 Macro Structure: SCCs and LSCC SCCs of size > 8 LSCC in the middle Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 15
27 Macro Structure: Pecularities Links in/out of LSCC (uni-directional!) 18,000 LSCC (45,000) 92,000 Certification Authorities Prominent: Heise, CACert and DFN-Verein (4,200 keys signed in LSCC) Heise signed 21,000 keys outside LSCC, too Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 16
28 Impact on Usability 2.7m keys just 45,000 really profit from the WoT Significant user activity only in LSCC Ratio edges/nodes in LSCC is 9.85, and in whole WoT 2.51 Recommendation to new users: Get a signature from someone in the LSCC Get a signature from a CA Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 17
29 Focusing on LSCC The remainder of this talk will focus on the LSCC We investigate Usefulness (distances, paths, clustering) Robustness Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 18
30 Usefulness: Distances and Node Degrees Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 19
31 Nodes reachable via 1,..., 5 hops CDF for 1-, 2-,..., 5-neighborhoods Fn(x) h=2 h=3 h=4 h= number of keys in h-neighborhood Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 20
32 Nodes reachable via 1,..., 5 hops The LSCC is well meshed 2-neighborhood (2 hops) Mostly very small neighborhood Very few keys can reach a few hundred keys 5-neighborhood (5 hops) 50% chance that a key can reach 22,000 keys Some keys can reach up to almost 38,000 keys Significance Good finding: path lengths not a problem But recall: availability of paths is important, too Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 21
33 Node Degrees GnuPG views redundant paths as beneficial High indegree: key more likely to be verifiable High outdegree: higher likeliness of redundant paths Mutual signatures are also beneficial Improves overall verifiability of keys Strengthens indegree and outdegree Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 22
34 Node Indegrees quantity indegree Note: Outdegrees have practically the same distribution Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 23
35 Majority of nodes: low in/outdegree This is a bad finding Almost half of keys have indegree 1 or 2 About 1/3 of nodes have outdegree 1 or 2 Mutual signatures: only in 50% of cases... This means: redundant paths are too rare Verify another key: needs direct signatures Be verifiable: only via very few other keys Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 24
36 Robustness: Resilience Against Change Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 25
37 Robustness What happens when keys expire, are revoked,... Paths over these keys become invalid Simulated this by randomly removing nodes Targeted attacks... Difficult: either compromise the key or delete it on all SKS servers Simulated this: remove nodes with high degree first Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 26
38 Remove keys, recompute LSCC size size LSCC random targeted number of removed keys Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 27
39 Removing keys Random removal (expiry, revocation,...) Very robust Need to remove 1/3 of keys to cut LSCC by half Targeted removal (attack) Quite robust decay not too bad Remove all nodes of degree: > 160 ( 0.5% of nodes) LSCC shrinks to 88% > 18 ( 11% of nodes) LSCC shrinks to 50% Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 28
40 Removing keys Assume CA keys are compromised/revoked The LSCC does not care: new size at 94.4% Average distances stay the same Many paths around the CAs: they are not critical components Key removal is not an efficient attack There are many hubs, and they are inter-connected Not a typical scale-free network A very good finding for a WoT Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 29
41 Further Aspects Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 30
42 Communities Analysis of community structure The LSCC shows a clear Small World Effect Used two algorithms for community detection Findings: Very strong community structure Communities often dominated by a top-level domain Second-level domains less clearly identifiable Details in paper Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 31
43 Crypto strength Algorithms in LSCC Hash Algorithm Occ. SHA % MD5 9.34% SHA % Key Algorithm Occ. DSA % RSA % RSA % Not too much to criticize here Some RSA keys of 1,024 bit are well-connected Length of < 768 bit occurs 500 times (problematic) 1,024 bit not a problem today, but maybe tomorrow Thankfully, few MD5-based signatures Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 32
44 Conclusions Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 33
45 Conclusions We have found light and dark Macro structure Only users in LSCC really profit from WoT CAs are useful, but not critical Usefulness Good reachability via 5 hops Redundant paths too rare! Robustness Very robust against expiration, revocation,... Key removal is not an efficient attack WoT works well in close neighborhoods of active nodes but not otherwise. Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 34
46 Conclusions We have found light and dark Macro structure Only users in LSCC really profit from WoT CAs are useful, but not critical Usefulness Good reachability via 5 hops Redundant paths too rare! Robustness Very robust against expiration, revocation,... Key removal is not an efficient attack WoT works well in close neighborhoods of active nodes but not otherwise. Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 34
47 Conclusions We have found light and dark Macro structure Only users in LSCC really profit from WoT CAs are useful, but not critical Usefulness Good reachability via 5 hops Redundant paths too rare! Robustness Very robust against expiration, revocation,... Key removal is not an efficient attack WoT works well in close neighborhoods of active nodes but not otherwise. Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 34
48 Conclusions We have found light and dark Macro structure Only users in LSCC really profit from WoT CAs are useful, but not critical Usefulness Good reachability via 5 hops Redundant paths too rare! Robustness Very robust against expiration, revocation,... Key removal is not an efficient attack WoT works well in close neighborhoods of active nodes but not otherwise. Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 34
49 Conclusions We have found light and dark Macro structure Only users in LSCC really profit from WoT CAs are useful, but not critical Usefulness Good reachability via 5 hops Redundant paths too rare! Robustness Very robust against expiration, revocation,... Key removal is not an efficient attack WoT works well in close neighborhoods of active nodes but not otherwise. Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 34
50 Thank you! Download dataset from pki.net.in.tum.de Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 35
51 Backup Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 36
52 Network History Number of keys in WoT and LSCC number of keys time Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 37
53 Network History RSA and DSA keys number of new keys DSA RSA time Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 38
54 Communities Dissection COPRA and BL: 94% vs. 99% of nodes in communities of size > 3. Figure: COPRA dissection for communities > 5. Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 39
55 Related Work Capkun et al., 2001 LSCC at 12,000 keys only; claims Small-World Effect and Power Law distribution Arenas et al., 2004 Investigated network as undirected graph Degree distribution, clustering: Power Law Community Dissection: also claim Power Law wotsap, Penning Continous snapshots and some statistics of LSCC Distances, degree distribution, robustness Less in-depth; wotsap extraction algorithm is faulty Alexander Ulrich, Ralph Holz, Peter Hauck, Georg Carle: Investigating the OpenPGP Web of Trust 40
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