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