[tor-commits] [tor/master] latest revision of cert normalization spec
nickm at torproject.org
nickm at torproject.org
Mon Feb 21 18:45:47 UTC 2011
commit 2f3648942202ca9d965344b8d2128a7a7b2056fb
Author: Jacob Appelbaum <jacob at appelbaum.net>
Date: Mon Feb 21 13:45:57 2011 -0500
latest revision of cert normalization spec
---
.../ideas/xxx-draft-spec-for-TLS-normalization.txt | 134 ++++++++++++--------
1 files changed, 82 insertions(+), 52 deletions(-)
diff --git a/doc/spec/proposals/ideas/xxx-draft-spec-for-TLS-normalization.txt b/doc/spec/proposals/ideas/xxx-draft-spec-for-TLS-normalization.txt
index 14546df..16484e6 100644
--- a/doc/spec/proposals/ideas/xxx-draft-spec-for-TLS-normalization.txt
+++ b/doc/spec/proposals/ideas/xxx-draft-spec-for-TLS-normalization.txt
@@ -1,6 +1,6 @@
Filename: xxx-draft-spec-for-TLS-normalization.txt
Title: Draft spec for TLS certificate and handshake normalization
-Author: Jacob Appelbaum
+Author: Jacob Appelbaum, Gladys Shufflebottom
Created: 16-Feb-2011
Status: Draft
@@ -94,32 +94,9 @@ An example of OpenSSL’s asn1parse over a typical Tor certificate:
308:d=2 hl=2 l= 0 prim: NULL
310:d=1 hl=3 l= 129 prim: BIT STRING
-I propose that the commonName field be generated to match a specific property
-of the server in question. It is reasonable to set the commonName element to
-match either the hostname of the relay, the detected IP address of the relay,
-or for the relay operator to override certificate generation entirely by
-loading a custom certificate. For custom certificates, see the Custom
-Certificates section.
-
-I propose that the value for the commonName field be populated with the
-fully qualified host name as detected by reverse and forward resolution of the
-IP address of the relay. If the host name is in the DNS, this host name should
-be set as the common name. When forward and reverse DNS is not available, I
-propose that the IP address alone be used.
-
-The commonName field for the issuer should be set to known issuer names,
-random words or omitted entirely.
-
-Since some host names may themselves trigger censorship keyword filters,
-it may be reasonable to provide an option to override the defaults and
-force certain values in the commonName field.
-
-Considerations for commonName normalization
-
-Any host name supplied for the commonName field should resolve - even if it
-does not resolve to the IP address of the relay[0]. If the commonName field
-does include an IP address, it should be the current IP address of the relay as
-seen by other Internet hosts.
+I propose that we match OpenSSL's default self-signed certificates. I hypothesise
+that they are the most common self-signed certificates. If this turns out not
+to be the case, then we should use whatever the most common turns out to be.
Certificate serial numbers
@@ -127,25 +104,35 @@ Currently our generated certificate serial number is set to the number of
seconds since the epoch at the time of the certificate's creation. I propose
that we should ensure that our serial numbers are unrelated to the epoch,
since the generation methods are potentially recognizable as Tor-related.
+
Instead, I propose that we use a randomly generated number that is
-subsequently hashed with SHA-512 and then truncated to a length chosen at
-random within a finite set of bounds. The length of the serial number should be
-chosen randomly at certificate generation time; it should be bound between the
-most commonly found bit lengths[1] in the wild. Random sixteen byte values
-appear to be the high bound for serial number as issued by Verisign and
-DigiCert. RapidSSL appears to be three bytes in length. Others common byte
-lengths appear to be between one and four bytes. I propose that we choose a
-byte length that is either 3, 4, or 16 bytes at certificate generation time.
-
-This randomly generated field may now serve as a covert channel that signals to
-the client that the OR will not support TLS renegotiation; this means that the
-client can expect to perform a V3 TLS handshake setup. Otherwise, if the serial
-number is a reasonable time since the epoch, we should assume the OR is
-using an earlier protocol version and hence that it expects renegotiation.
+subsequently hashed with SHA-512 and then truncate the data to eight bytes[1].
+
+Random sixteen byte values appear to be the high bound for serial number as
+issued by Verisign and DigiCert. RapidSSL appears to be three bytes in length.
+Others common byte lengths appear to be between one and four bytes. The default
+OpenSSL certificates are eight bytes and we should use this length with our
+self-signed certificates.
+
+This randomly generated serial number field may now serve as a covert channel
+that signals to the client that the OR will not support TLS renegotiation; this
+means that the client can expect to perform a V3 TLS handshake setup.
+Otherwise, if the serial number is a reasonable time since the epoch, we should
+assume the OR is using an earlier protocol version and hence that it expects
+renegotiation.
+
+We also have a need to signal properties with our certificates for a possible
+v3 handshake in the future. Therefore I propose that we match OpenSSL default
+self-signed certificates (a 64-bit random number), but reserve the two least-
+significant bits for signaling. For the moment, these two bits will be zero.
+
+This means that an attacker may be able to identify Tor certificates from default
+OpenSSL certificates with a 75% probability.
As a security note, care must be taken to ensure that supporting this
covert channel will not lead to an attacker having a method to downgrade client
-behavior.
+behavior. This shouldn't be a risk because the TLS Finished message hashes over
+all the bytes of the handshake, including the certificates.
Certificate fingerprinting issues expressed as base64 encoding
@@ -190,16 +177,6 @@ query. I propose that we ensure that we test our certificates to ensure that
they do not have these kinds of statistical similarities without ensuring
overlap with a very large cross section of the internet's certificates.
-Other certificate fields
-
-It may be advantageous to also generate values for the O, L, ST, C, and OU
-certificate fields. The C and ST fields may be populated from GeoIP information
-that is already available to Tor to reflect a plausible geographic location
-for the OR. The other fields should contain some semblance of a word or
-grouping of words. It has been suggested[2] that we should look to guides for
-certificate generation that use OpenSSL as a reasonable baseline for
-understanding these fields, as well as other certificate properties.
-
Certificate dating and validity issues
TLS certificates found in the wild are generally found to be long-lived;
@@ -231,6 +208,58 @@ The expiration time should not be a fixed time that is simple to calculate by
any Deep Packet Inspection device or it will become a new Tor TLS setup
fingerprint.
+Proposed certificate form
+
+The following output from openssl asn1parse results from the proposed
+certificate generation algorithm. It matches the results of generating a
+default self-signed certificate:
+
+ 0:d=0 hl=4 l= 513 cons: SEQUENCE
+ 4:d=1 hl=4 l= 362 cons: SEQUENCE
+ 8:d=2 hl=2 l= 9 prim: INTEGER :DBF6B3B864FF7478
+ 19:d=2 hl=2 l= 13 cons: SEQUENCE
+ 21:d=3 hl=2 l= 9 prim: OBJECT :sha1WithRSAEncryption
+ 32:d=3 hl=2 l= 0 prim: NULL
+ 34:d=2 hl=2 l= 69 cons: SEQUENCE
+ 36:d=3 hl=2 l= 11 cons: SET
+ 38:d=4 hl=2 l= 9 cons: SEQUENCE
+ 40:d=5 hl=2 l= 3 prim: OBJECT :countryName
+ 45:d=5 hl=2 l= 2 prim: PRINTABLESTRING :AU
+ 49:d=3 hl=2 l= 19 cons: SET
+ 51:d=4 hl=2 l= 17 cons: SEQUENCE
+ 53:d=5 hl=2 l= 3 prim: OBJECT :stateOrProvinceName
+ 58:d=5 hl=2 l= 10 prim: PRINTABLESTRING :Some-State
+ 70:d=3 hl=2 l= 33 cons: SET
+ 72:d=4 hl=2 l= 31 cons: SEQUENCE
+ 74:d=5 hl=2 l= 3 prim: OBJECT :organizationName
+ 79:d=5 hl=2 l= 24 prim: PRINTABLESTRING :Internet Widgits Pty Ltd
+ 105:d=2 hl=2 l= 30 cons: SEQUENCE
+ 107:d=3 hl=2 l= 13 prim: UTCTIME :110217011237Z
+ 122:d=3 hl=2 l= 13 prim: UTCTIME :120217011237Z
+ 137:d=2 hl=2 l= 69 cons: SEQUENCE
+ 139:d=3 hl=2 l= 11 cons: SET
+ 141:d=4 hl=2 l= 9 cons: SEQUENCE
+ 143:d=5 hl=2 l= 3 prim: OBJECT :countryName
+ 148:d=5 hl=2 l= 2 prim: PRINTABLESTRING :AU
+ 152:d=3 hl=2 l= 19 cons: SET
+ 154:d=4 hl=2 l= 17 cons: SEQUENCE
+ 156:d=5 hl=2 l= 3 prim: OBJECT :stateOrProvinceName
+ 161:d=5 hl=2 l= 10 prim: PRINTABLESTRING :Some-State
+ 173:d=3 hl=2 l= 33 cons: SET
+ 175:d=4 hl=2 l= 31 cons: SEQUENCE
+ 177:d=5 hl=2 l= 3 prim: OBJECT :organizationName
+ 182:d=5 hl=2 l= 24 prim: PRINTABLESTRING :Internet Widgits Pty Ltd
+ 208:d=2 hl=3 l= 159 cons: SEQUENCE
+ 211:d=3 hl=2 l= 13 cons: SEQUENCE
+ 213:d=4 hl=2 l= 9 prim: OBJECT :rsaEncryption
+ 224:d=4 hl=2 l= 0 prim: NULL
+ 226:d=3 hl=3 l= 141 prim: BIT STRING
+ 370:d=1 hl=2 l= 13 cons: SEQUENCE
+ 372:d=2 hl=2 l= 9 prim: OBJECT :sha1WithRSAEncryption
+ 383:d=2 hl=2 l= 0 prim: NULL
+ 385:d=1 hl=3 l= 129 prim: BIT STRING
+
+
Custom Certificates
It should be possible for a Tor relay operator to use a specifically supplied
@@ -308,6 +337,7 @@ simply avoided by the censors.
The Rakshasa prime approach may cause censors to specifically allow only
certain known and accepted DH parameters.
+
Appendix: Other issues
What other obvious TLS certificate issues exist? What other static values are
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