Patent Publication Number: US-7917756-B2

Title: E-mail stamping with from-header validation

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional application No. 60/686,686, filed on Jun. 1, 2005, and incorporated by reference herein in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to e-mail security. In particular, the present invention is directed toward the registration and subsequent authentication of senders of e-mail. 
     2. Description of Background Art 
     The wide availability of domain name addresses for purchase has led to the nefarious practice of “phishing”, in which a bad actor sends e-mails that appear to be from a reputable source and typically encourage the recipient to click on a link within the e-mail message and provide personal information to the sender. For example, the domain names “bank-of-america-online.com”, “chasebank.cc”, “my-citibank.us” are a sample of legitimate-sounding names that are currently available to the public for purchase. 
     Because the DNS records of a domain are under the control of the domain owner, messages sent from the domain will be marked as legitimate by conventional domain authentication schemes such as the Sender Policy Framework (SPF), the Sender ID Framework, DomainKeys, and Identified Internet Mail. That is, if a sender purchases the “bank-of-america-online.com” domain and sends an e-mail with a From header of “customerservice@bank-of-america-online.com”, the message will be authenticated because it is indeed from who it claims to be from—the owner/operator of the bank-of-america-online.com domain. 
     Additionally, some e-mail programs (such as Microsoft&#39;s Hotmail, for example) display messages in an inbox using only the “display” string of an e-mail address, which is configurable by the sender. So, for example, an e-mail from phisher@bank-of-america-online.com could be configured by the sender to have a display name of “B of A Customer Service”, in order to entice the recipient to open the message and follow links contained within. 
     It would be of great assistance to e-mail recipients if a system and method existed for reliably accrediting senders of electronic mail messages. 
     SUMMARY OF THE INVENTION 
     The present invention combines the effective aspects of stamping outbound e-mail with a registration system for e-mail senders. A person or other entity (known henceforth as a mail sender, or simply a sender) participates by registering From: line information with a trusted third-party repository known as a stamp authority. When the sender sends an e-mail message, a hash of the From: line is compared against the From: line information for the sender stored in the repository. If the result is a match, the e-mail is allowed to be sent, subject to any other restrictions imposed by the stamping process. If there is not a match, then the e-mail is prevented from being stamped. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of the overall architecture of an embodiment of the present invention. 
         FIG. 2  is a flowchart illustrating a method of stamping e-mail messages with From: header authentication in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The use of stamps is a powerful, economically-driven solution to address the damage caused by spam and its negative after-effects such as false positives, spoofed messages and viruses. It shifts the burden of the high cost of fighting spam from the recipients to the senders of email. Stamping identifies and labels the “good” mail with stamps paid for by responsible high-volume senders who are subject to sender-level accreditation and made accountable for trusted email practices. Various stamping systems and methods are in conventional use, for example as described in U.S. Pat. No. 5,999,967 to Sundsted, which is incorporated by reference herein in its entirety. 
     The present invention makes use of stamping technology by making the availability of a stamp dependent on a match between the From: header in an outbound e-mail and a From: header previously registered with a stamp authority. 
       FIG. 1  is a block diagram that illustrates the interaction of various components of an e-mail stamping system.  FIG. 1  includes a stamp authority  102  having a registration database  114  and a stamp generator  106 ; a mail sender  104  having an imprinter  108 ; and a mailbox provider  110 , having a stamp checker  112  and a mailbox  116 . 
     Mail sender  104  sends e-mails to one or more recipients—typically to a large number of recipients, though for clarity of description we assume a single recipient in this instance without any loss of generality. When mail sender  104  sends an e-mail that he wishes to have stamped, the e-mail preferably is transmitted to imprinter  108 , which creates appropriate stamp header fields as described below, calculates a hash of the message, and sends the hash to stamp generator  106  to be signed and returned as a stamp. After receiving a stamp back from stamp generator  106 , imprinter  108  then sends the e-mail to its intended recipient. 
     When stamp generator  106  receives the hash to be stamped from imprinter  108 , it verifies that the mail sender  104  is authorized to use a stamp—for example, it verifies that the mail sender  104  is up to date on payments, has stamps in his account, has not violated any business rules that limit his ability to stamp e-mail, etc. Preferably imprinter  108  connects in real time to stamp authority  102  to have stamp generator  106  perform the verification; alternatively, outgoing e-mails can be queued and the verification process can take place during a batch update when connection to stamp authority  102  is available. 
     In addition to verifying that the mail sender  104  is authorized to use a stamp, stamp generator  106  also determines whether the From: header information included in the message matches the header information stored in registration database  114 , as described further below. 
     Once stamp generator  106  determines that mail sender  104  is authorized to use a stamp and that the From: header information in the messages is legitimate, it generates a stamp and provides the stamp to imprinter  108  to allow the e-mail to be sent. 
     The e-mail then travels in a conventional method to a mailbox provider  110 . Upon arrival, stamp checker  112  examines the stamp to determine whether the stamp is valid as described further below. If the stamp is valid, then the e-mail is delivered to the mailbox  116  of the addressed recipient, subject to any additional rules imposed by the mailbox provider or the recipient herself. If the stamp is not valid, this indicates that there might have been external tampering, or a transmission or other error, and the e-mail is rejected. 
     Stamp authority  102  is an agent that oversees the stamping process by issuing stamps to mail senders and enforcing business rules related to the sending of e-mail by the mail senders. In addition, a mail sender  104  registers complete From: line information with stamp authority  102 , and the information is then maintained in a registration database  114  for subsequent verification when mail sender  104  attempts to obtain a stamp for an e-mail message. 
       FIG. 2  illustrates a method of e-mail stamping with From: header validation in accordance with an embodiment of the present invention. 
     Mail sender  104  initially sends  202  a message to be stamped to imprinter  108 . In a preferred embodiment, a stamp is unique for each message, and is a cryptographic object contained within the header of the e-mail message. The stamp includes a variety of header fields, for example:
     X-StampAuthority-Rcpto: joe@example.com   X-StampAuthority-Reply-To: mary@example.com   X-StampAuthority-Sender: amy@example.com   X-StampAuthority: 1; i=“12345”;   s=“0000001C0000001C0001000141D32376000000010000001300000002”;   e=“20040612T123256”; d=“20040608T082310”; o=“342AC5”; t=“2”;   h=“4Io7sVcs55HmRWhSE3QucCKHc1U=”;   f=“QmlnIFRydWNrcyBvZmZlcnNAYmlndHJ1Y2tzlmNvbQ==”;   b=“6MdkylkSixEEfv7oh38fO6O2uic=”;   X-StampAuthority-Sig:   MfowCwYJKoZIhvcNAQEBA0sAMEgCQQDNZ+V7wcxLqyAQR   iHtMySKtD5UfT/rdFzaGehCmp8QECDKhPKqRC2EMbvBXZVdNIo500yrPayUKBYxfj   Mcxc5AgMBAAE=
 
The particular header fields chosen to implement the present invention may be determined according to the needs of the implementer. In a preferred embodiment, one header field includes a parameter containing a base 64 -encoded normalized From: header value.
   

     To normalize, in one embodiment the following rules are observed: the comment part must have any surrounding full quotation marks and surrounding parentheses removed. All whitespace characters must be preserved. The address part must not include a comma, more than one @ character, or a colon. The address part must have any surrounding angle braces preserved. If the address part lacks surrounding angle braces, surrounding angle braces must be added. In the “f” parameter, the comment part precedes the address part and is separated from the address part by exactly one space character. The result is base 64  encoded for transport. 
     Imprinter  108  also adds a header transit hash to the message, represented by the “h=” string in the example shown above, and a body transit hash, represented by the “b=” string in the example shown above. In a preferred embodiment, the “h” parameter contains the base 64  encoded SHA1 hash of data specific to the email message stamped. The inclusion of the hash in the stamp binds the stamp to the message headers, and it protects message headers during transit by allowing filters to detect if message headers have been modified. The “b” parameter contains the base 64  encoded SHAL hash of data specific to the email message stamped. The inclusion of the hash in the stamp binds the stamp to the message body, and it protects the message body during transit by allowing filters to detect if the message has been modified. 
     Once the stamp fields and hash are created  204 , imprinter  108  forwards the hash to stamp generator  106 , which verifies  206  that the sender is authorized to issue a stamp. A mail sender  104  may be ineligible to issue a stamp if, for example, the sender has used up all of the stamps purchased from stamp authority  102 . 
     Stamp generator  106  also verifies From: header information against the From: header registered in registration database  114 . Preferably, if  208  there is not a match between what is registered and what is in the e-mail header, stamp generator  106  will reject  210  the message. If there is a match and the mail sender  104  is otherwise allowed to send a stamped message, stamp generator  106  adds  212  its signature to the header and returns the message to imprinter  108 , which then sends  214  the message to the message&#39;s specified recipient. 
     In one embodiment, stamp generator  106  has a private/public key pair generated in a conventional manner. Stamp generator  106  uses the parameters such as those listed below and its private key to create a transit signature using a cryptographic algorithm, for example RSASSA-PKCS1-V1 — 5. The parameters used by the stamp generator  106  to create the stamp in one embodiment are: a version number of the stamping protocol; a unique ID for that stamp; an indication of a stamp type (adult, commercial, etc.); a hash of the message created from the message and the stamp fields (obtained from imprinter  108  as described above); Sender: and From: information (obtained from the message envelope by imprinter  108 ); and RCPT TO: information (obtained from the message envelope by imprinter  108 ). Other parameters could also be used as deemed appropriate by an implementer of such a system. 
     The creation of the transit signature in a preferred embodiment first involves the creation of a hash of all of the fields being signed (which includes all stamp fields and the message hash), and then the signing algorithm is implied. Thus, there are essentially two hash operations being performed; the first operation is of the entire message and the stamp fields, which yields a value that is then inserted as one of the fields in the stamp. This field, along with all other stamp authority  102  fields, is then hashed in a second hash operation, the value of which is then signed using cryptography. In this way, these stamp authority  102  fields can be validated without the entire message being present. In a preferred embodiment, the above parameters including the transit signature, combined with the certificate, create a fully-formed stamp. 
     When the e-mail is received by mailbox provider  110 , stamp checker  112  checks  216  the signature on the stamped e-mail to determine whether it is valid. 
     In one embodiment, the certificate is verified by stamp checker  112  as follows. Stamp checker  112  uses the stamp authority&#39;s public key previously obtained. Next, stamp checker  112  determines a hash of the fields in the certificate. Stamp checker  112  then takes the hash, the stamp authority public key, and the certificate signature and performs a signature verification operation to check whether the signature of the certificate (and hence the certificate) is valid. 
     If  218  the certificate is not valid, stamp checker  112  rejects  220  the message. If the message is rejected, additional steps can be taken, for example the sender of the message could be notified that a message was received claiming to be from the sender  104  but was not successfully validated. Preferably, a report is also made  222  to the stamp authority  102 . 
     If  218  the certificate is valid, then the e-mail is delivered  224  by mailbox provider  110  to the mailbox  116  of the specified recipient, subject to any other delivery rules that the mailbox provider or owner may have set up for mail handling. Preferably, a report is then made  222  to the stamp authority  102 , so that the stamp can be cancelled and not reused. 
     In an alternative embodiment of the invention, rather than using a hash of the From: line as described above, a mail sender  104  instead obtains from stamp authority  102  a signed certificate, issued by the stamp authority  102 , certifying that the sender  104  is accredited, i.e. that the sender is legitimate. The mail sender  104  then attaches the certificate to an e-mail message, for example using Domain Keys Identified Mail (DKIM); by embedding the certificate; or using other methods of domain authentication known in the art. When mailbox provider  110  receives the e-mail message, it compares the embedded certificate against the From: header in the message. If there is a match, then the message is delivered to the mailbox  116  of the addressee. If not, the message is rejected. 
     In another alternative embodiment, mail sender  104  registers its From: address with stamp authority  102 . Regardless of whether mail sender  104  indicates to mailbox provider  110  that it should query the stamp authority&#39;s registration database  114  of registered mail senders, mailbox provider  110  makes such a query, and determines whether the From: header in the received message matches a From: header registered with stamp authority  102  as being associated with that mail sender  104 . If so, the message is delivered to the address&#39;s mailbox  116 . If not, the message is rejected. 
     The present invention has been described in particular detail with respect to a limited number of embodiments. Those of skill in the art will appreciate that the invention may additionally be practiced in other embodiments. First, the particular naming of the components, capitalization of terms, the attributes, data structures, or any other programming or structural aspect is not mandatory or significant, and the mechanisms that implement the invention or its features may have different names, formats, or protocols. Further, the system may be implemented via a combination of hardware and software, as described, or entirely in hardware elements. Also, the particular division of functionality between the various system components described herein is merely exemplary, and not mandatory; functions performed by a single system component may instead be performed by multiple components, and functions performed by multiple components may instead performed by a single component. For example, the particular functions of the stamp generator  106  and so forth may be provided in many or one module. 
     Some portions of the above description present the feature of the present invention in terms of algorithms and symbolic representations of operations on information. These algorithmic descriptions and representations are the means used by those skilled in the art of e-mail security to most effectively convey the substance of their work to others skilled in the art. These operations, while described functionally or logically, are understood to be implemented by computer programs. Furthermore, it has also proven convenient at times, to refer to these arrangements of operations as modules or code devices, without loss of generality. 
     It should be borne in mind, however, that all of these and similar terms are to be associated with the appropriate physical quantities and are merely convenient labels applied to these quantities. Unless specifically stated otherwise as apparent from the present discussion, it is appreciated that throughout the description, discussions utilizing terms such as “processing” or “computing” or “calculating” or “determining” or “displaying” or the like, refer to the action and processes of a computer system, or similar electronic computing device, that manipulates and transforms data represented as physical (electronic) quantities within the computer system memories or registers or other such information storage, transmission or display devices. 
     Certain aspects of the present invention include process steps and instructions described herein in the form of an algorithm. It should be noted that the process steps and instructions of the present invention could be embodied in software, firmware or hardware, and when embodied in software, could be downloaded to reside on and be operated from different platforms used by real time network operating systems. 
     The present invention also relates to an apparatus for performing the operations herein. This apparatus may be specially constructed for the required purposes, or it may comprise a general-purpose computer selectively activated or reconfigured by a computer program stored in the computer. Such a computer program may be stored in a computer readable storage medium, such as, but is not limited to, any type of disk including floppy disks, optical disks, CD-ROMs, magnetic-optical disks, read-only memories (ROMs), random access memories (RAMs), EPROMs, EEPROMs, magnetic or optical cards, application specific integrated circuits (ASICs), or any type of media suitable for storing electronic instructions, and each coupled to a computer system bus. Furthermore, the computers referred to in the specification may include a single processor or may be architectures employing multiple processor designs for increased computing capability. 
     The algorithms and displays presented herein are not inherently related to any particular computer or other apparatus. Various general-purpose systems may also be used with programs in accordance with the teachings herein, or it may prove convenient to construct more specialized apparatus to perform the required method steps. The required structure for a variety of these systems will appear from the description above. In addition, the present invention is not described with reference to any particular programming language. It is appreciated that a variety of programming languages may be used to implement the teachings of the present invention as described herein, and any references to specific languages are provided for disclosure of enablement and best mode of the present invention. 
     Finally, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and may not have been selected to delineate or circumscribe the inventive subject matter. Accordingly, the disclosure of the present invention is intended to be illustrative, but not limiting, of the scope of the invention.