Shen: A Security Scheme for the World Wide Web

Encryption Mechanisms

It is to be expected that any data transmitted in encrypted form is likely to be stored in encrypted form also. In the case of servers this may be considered advisable, in the case of clients this may be considered mandatory.

HTTP header lines are used for both potentialy confidential data such as content type information and information required by the protocol. In addition when a message passes through a gateway or proxy it may be necessary to modify header items or incorporate additional ones.

Summary of Tags

The tag Proc-Type: 4 tag is implicit in the HTTP/1.1 version identifier. The Proc-Type tag may not be used to override the PEM version nummber. The keys MIC-only and MIC-clear are redundant since the content encoding may be given separately.

Date:Weekday, DD-Mon-YY HH:MM:SS GMT

All authentificated messages should be dated to guard against replay type attacks. The recipient should check that the date is within permitted limits. Note that the timezone GMT is mandatory. This field should not be encrypted since the server may wish to perform checking on this field prior to decryption as a guard against denial of service type attacks.

Originator-ID-Symmetric uuencoded..recipient..id

Originator-ID-Asymmetric uuencoded..recipient..id

Recipient-ID-Asymmetric uuencoded..recipient..id

MIC-Info RSA-MD5, RSA, uuencoded..signature===

Contains a message digest of the body of the message signed with the secret component of the issuers signature key.

Originator-Certificate

Issuer-Certificate

DEK-Info: DES-CBC,hexadecimalparameter

Key and parameters for the encryption algorithm used.

Key-Info RSA, uuencoded..encrypted..session.key...==

The session key in uuencoded form encrypted with the public key of the preceeding recipient id.

The following tags have been added.

Prohibit: [cache] [storage] [modification]

Explicitly forbids caching, modification or storage of the data.

Authorized-Role: rolename [,rolename]

Specifies that the sender claims the stated role names. Such claims should not be accepted as valid unless certified as such by the signature of an authorised garantor.

MIC-Head:

MIC signature of message head up to an including the CRLF pair preceeding this header item. All MIC elements are evaluated within the scope. If multiple MIC-Head elements are used the boundaries of each one will be different.

Secret-Header: [parameter]

The parameter contains a series of encrypted header items. The items should be decrypted and the decrypted text read as if they were ordinary headers placed at that point. the substitution should hold for all purposes except the calculation of MIC digests which shall used the original encoded form..

Anonymous-ID : session_id, thread, serial

The session_id is a secure cryptographic hash of a number of parameters guaranteed (bar random chance) to provide a unique session identified. The serial number is guaranteed to increase monotonically for a given thread and session id value pair.

Digest-Boundary: [parameter]

Calculate digest boundaries from the current line. This tag allows the calculation of seaprate digests for parts of the header generated separately. An optional parameter may be used to prime the digest with a randomly generated value.

Summary of Responses

405 Out of time.

In order to facilitate the debugging of problems due to baddly matched clocks on communicating machines the out of time response may be issued instead of 401 Unauthorized. Such a response should have a header with a Date: field with the server time value.