55 |
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56 |
.. Digital signatures for updateable documents |
.. Digital signatures for updateable documents |
57 |
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|
58 |
A simple approach would be to use |
A simple approach would be to use digital signatures. |
59 |
|
A document could be identified by a cryptographic |
60 |
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public key; all versions signed by the corresponding |
61 |
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private key would be considered versions of the |
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document. |
63 |
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|
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- Keys expire and are revoked |
.. Keys expire and are revoked |
65 |
- The usual approach: Certificate Authorities (CAs) |
|
66 |
plus Time Stamping Services (TSSs) |
However, public key cryptography relies on the |
67 |
- For a complete system, also need a |
keeping of secrets-- the private keys. Since there |
68 |
Key Archival Service (KAS), which stores the |
is no perfect way of guarding a secret, a key |
69 |
certificate associated with an identity at |
may be stolen; then, there must be a way for |
70 |
one time, in order to verify that key K was really |
the publisher to 'revoke' the key-- i.e., to |
71 |
the certified public key of entity E at time T |
say that signatures given with this key cannot |
72 |
- Together: KASTS design; non-centralized: Prokopius |
be trusted any longer. |
73 |
- Good TSS mechanisms patented; the need for |
|
74 |
something new |
However, if all signatures given with a key |
75 |
- We present an alternative design which |
become invalid, then old versions of a document |
76 |
relies on non-patented technology |
could not be retrieved any longer. |
77 |
- We show that attacks on our system have |
|
78 |
equivalents in a KASTS system |
.. The usual approach: Certificate Authorities (CAs) |
79 |
|
plus Time Stamping Services (TSSs) |
80 |
|
|
81 |
|
The usual solution to this problem is to use |
82 |
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digital timestamping, which certifies that |
83 |
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a certain digital document existed at a given time. |
84 |
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If, using timestamping, we can verify that a signature |
85 |
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was provided before the corresponding key was revoked, |
86 |
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the signature continues to be valid even after |
87 |
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revocation. |
88 |
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|
89 |
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Unfortunately, while there are good methods to |
90 |
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timestamp documents in a secure, trusted way, |
91 |
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these methods are patented. |
92 |
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|
93 |
|
In this document, we describe a system |
94 |
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providing the equivalent of digital signatures |
95 |
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(verifiable statements of the form: "Authority A |
96 |
|
approved of message B") without relying on |
97 |
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patented timestamping technology. |
98 |
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|
99 |
|
.. |
100 |
|
- For a complete system, also need a |
101 |
|
Key Archival Service (KAS), which stores the |
102 |
|
certificate associated with an identity at |
103 |
|
one time, in order to verify that key K was really |
104 |
|
the certified public key of entity E at time T |
105 |
|
- Together: KASTS design; non-centralized: Prokopius |
106 |
|
- Good TSS mechanisms patented; the need for |
107 |
|
something new |
108 |
|
- We present an alternative design which |
109 |
|
relies on non-patented technology |
110 |
|
- We show that attacks on our system have |
111 |
|
equivalents in a KASTS system |
112 |
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|
113 |
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|
114 |
An "A said that B said that C said..." approach |
An "A said that B said that C said..." approach |