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Description  |
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FIELD OF INVENTION
This invention relates to apparatus and methods in transaction processing
systems and, more particularly, to apparatus and methods for the secure
authorization of transactions in transaction processing systems.
DESCRIPTION OF PRIOR ART
Examples of transaction processing systems include electronic point of sale
(EPOS) systems allowing customers to pay for goods by means of credit
cards or the like. Such a system includes one or more transaction
terminals, at which details of a transaction and of a customer's card are
entered, connected via a suitable data communications network to a data
processing system storing, for instance, the customers' account details to
effect a transaction in the customer's account.
The fraudulent use of such systems is a serious problem which in 1991
resulted in losses of 160 million pounds in the United Kingdom alone. Much
effort is being invested in studying ways of addressing these huge losses.
Currently, transactions are usually authorized by recording the customer's
signature on paper. This is because the signing of a name by an individual
is a dynamic action which occurs in a remarkably repeatable fashion.
However, this traditional approach has a number of attendant problems. For
example, it represents a tremendous burden to the merchant, who wants to
get rid of paper, and certainly avoid the obligation of having to go back
in his records in case of query to find a particular sales voucher with a
signature on it.
The usage of other biometrics, such as the use of fingerprint or voice
recognition, which can be verified using suitable apparatus is being
considered as a way of recording the fact that a customer has authorized a
transaction. However, in some countries the matter is complicated by the
legal need to capture the signature as evidence of the transaction.
It is therefore desirable to use electronic copies of signatures as a means
of allowing a customer to authorize a transaction.
However, any means of transferring signatures in electronic form, whether
or not encrypted, yields them vulnerable unless a mechanism is provided
for preventing an electronic signature from being associated with
transactions other than the original that was authorized.
Furthermore, if the signature associated with a transaction needs to be
captured, electronically transmitted and stored in such a way that an
image of the signature can be reproduced as desired, then data
communications and storage become a significant overhead.
SUMMARY OF THE INVENTION
This invention aims to improve security in transaction processing systems
by solving these problems. This is achieved through a new approach to
associating a users authority, manifesting itself in a printable
characteristic image, particularly, but not exclusively, a signature
image, with a transaction authorized by the user.
Accordingly, a first aspect of the invention enables a transaction
processing system to be provided comprising a data processing system and
at least one transaction terminal for use at a remote location by a user,
the transaction terminal comprising means to receive characteristic data
from the user, which characteristic data is required to reconstruct a
characteristic image associated with the user from user data stored in the
data processing system; logic for generating a transaction message by
combining data relating to a transaction with the characteristic data in
such a way that the transaction data is required to recover the
characteristic data from the transaction message; and means connectable to
a data communications network for transmitting the transaction message to
the data processing system, and the data processing system comprising:
means to store the user data; means to receive from the network and store
the transaction message; logic for recovering the characteristic data from
the stored transaction message using the transaction data; and logic for
reconstructing the characteristic image from the user data using the
characteristic data for use in establishing that the transaction was valid
by associating the characteristic image with the transaction data.
A transaction makes use of a piece of characteristic data which removes the
need to capture or transmit image data with the transaction. This
characteristic data is cryptographically bound to a particular transaction
and so cannot be transferred or associated with another transaction. The
characteristic data provides the recipient of the transaction with the
ability and authority to associate a centrally held copy of the original
image to the transaction data.
Thus, no image capture device is required at the transaction terminal and
the overhead of transferring image data with the transaction is avoided.
The fact that the transaction data is required to recover the
characteristic data from the transaction message means that the
characteristic data is inextricably linked to the transaction and cannot
be associated with transactions other than the one intended.
Preferably, the transaction terminal comprises means to establish that the
user is authorized to effect the transaction in order that the merchant
may satisfy himself that the customer is bona fide. Thus, user
verification can proceed on the basis of a Personal Identification Number
(PIN) entered by the user at the transaction terminal, biometric or other
method chosen and checked at the transaction terminal.
The transaction terminal can comprise a smart card supplied by the user and
a smart card reader, the smart card comprising means to store the
characteristic data and logic for combining the characteristic data with
the transaction data for use in generating the transaction message.
Preferably, the smart card comprises means for storing a user-specific
encryption key and logic for encrypting the characteristic data using the
user-specific encryption key. In this event, the data processing system
also needs to use the user specific key to recover the characteristic data
from the transaction message. This has security advantages because,
without knowledge of the key, the characteristic data cannot be extracted
from the smart card or the message decrypted if intercepted.
In one embodiment the user data comprises a random number combined with a
digital representation of the signature and the characteristic data is
generated from the user data using a hashing algorithm. This ensures that
neither the characteristic data nor a clear copy of the signature can be
used to generate a copy of the user data.
If desired, signature capture can also continue on paper as today, but it
is only possible to reproduce the copy of the centrally held signature in
association with a valid and authorized transaction.
Advantageously, the data processing system comprises a first data
processing facility for use by a verifier which receives and stores the
transaction message and effects the transaction using the transaction
data; and a second data processing facility for use by an arbiter which,
if necessary, can recover the characteristic data from the stored
transaction message using the transaction data and reconstruct the
characteristic image from the characteristic data to establish that the
transaction was valid by associating the characteristic image with the
transaction data.
In this embodiment, an authority called an "arbiter", can be appointed, who
would be independent of the verifier, to be involved in "proving" the
transaction by reconstructing the signature image in the event of a
dispute regarding the validity of a claimed transaction.
In addition, the "verifier" can use the characteristic data to verify the
validity of a transaction by storing an encrypted form of the user data
encrypted using the characteristic data. On receiving the message the
characteristic data can be recovered by the verifier from the stored
transaction message using the transaction data, the encrypted user data
decrypted using the recovered characteristic data, the characteristic data
generated from the decrypted user data and compared with the recovered
characteristic data to establish the validity of the transaction.
The invention also provides a transaction terminal adapted for use in the
above described transaction processing, a smart card adapted for use in
the transaction terminal, and data processing systems for use by the
verifier and arbiter.
Also provided is apparatus for enrolling users of the transaction
processing system, the apparatus comprising: means to generate and store a
digital representation of the characteristic image for each user; and
logic for generating the user data and the characteristic data from the
digital representation of the characteristic image. In one embodiment, the
apparatus comprises logic for encrypting the user data using the
characteristic data and for creating the necessary encryption keys.
DESCRIPTION OF THE DRAWING
Embodiments of the invention will now be described, by way of example only,
with reference to the accompanying drawings, in which:
FIG. 1 is a schematic diagram of the process of the preferred embodiment of
the invention in its entirety;
FIG. 2 is the portion of FIG. 1 which relates to the enrollment process;
FIG. 3 is the portion of FIG. 1 which relates to the transaction process;
FIG. 4 is the portion of FIG. 1 which relates to the verification or
validation process;
FIG. 5 is the portion of FIG. 1 which relates to the proving process;
FIG. 6 shows an implementation of a Seal Key Generating Service using as an
example a particular choice of architecture, the IBM Common Cryptographic
Architecture/1 (CCA/1);
FIGS. 7A to E show an implementation of a Seal Processing Service using the
IBM CCA/1; and
FIG. 8 shows an example of transaction processing apparatus according to an
embodiment of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
A transaction processing system according to a simple embodiment of the
invention takes the form shown in FIG. 8. A user provides details of a
transaction at a transaction terminal 10, 11, 12, which could be, for
example, a Point of Sale terminal provided in a shop to allow customers to
initiate transfer of funds for the purchase of goods. Transaction data is
transmitted to a data processing facility 13 storing users' details, which
an authority called a verifier, a bank in this example, may use to
validate the transaction. An arbiter may be used to prove the validity of
a transaction in the event of a dispute, and in this case the transaction
processing system would include a data processing facility 14 for the
arbiter's use.
Broadly, the method of operation has four steps: enrolling, transacting,
verifying or validating, and proving.
During enrollment, a copy of the signature is captured and stored securely
in encrypted form by the verifier for future use. Data required to release
the centrally stored signature is stored securely on a card held by the
user which is equipped with an integrated circuit chip containing a
microprocessor and data storage facilities. Such cards are well understood
in the art and have become known as `smart cards`. The user held smart
card also stores suitable program code and an encryption key which it
shares with the verifier, and also with the arbiter if this option is
taken.
The user uses his smart card to authorize a particular transaction at the
transaction terminal. The terminal formulates the transaction data, and
the microprocessor on the smart card appends to this data a seal. The seal
combines cryptographically the data authorizing release of the centrally
stored signature with transaction data, and authenticates the message
containing both the transaction data and the seal. The encryption makes
use of the encryption key provided both to the smart card and to the
verifier.
The verifier receives the transaction message and recovers the authority to
use the stored signature. This authority is then used to establish that
the signature data forming part of the seal, which is bound to the
transaction, corresponds to the stored signature. Only if the
correspondence is proven will the transaction be allowed to proceed. The
verifier stores the original transaction message as evidence of the
transaction in case any query should arise.
The step referred to as "proving" is invoked in the case of a dispute
regarding the validity of a transaction. The original transaction message
is processed using the shared encryption key and stored signature. The
correspondence is demonstrated again, but this time in addition a clear
copy of the stored signature is generated for printing together with the
transaction data. The stored signature is constructed in such a way that
the clear signature cannot be used to create an alternative stored
signature which might be associated with another smart card user or
transaction.
If it is decided to use an independent arbiter to prove the validity of a
transaction in the event of a dispute, the enciphered signature is lodged
with the arbitration authority enciphered under a key that allows the
arbiter, but not the verifier, to print the clear signature. Thus only the
arbiter would be able to perform the proving step. In the absence of an
arbiter, the verifier would be able to perform this step.
FIG. 1 is a schematic diagram showing these steps. This diagram is
reproduced in part in four further figures, FIGS. 2 to 5, each covering
one aspect of the process as described in further detail below.
Encryption algorithms allowing sensitive data to be protected, for example
during electronic transmission, are widely known in the art and embodied
in many systems. One such system is IBM's Transaction Security System, a
set of hardware security products including a smart card and supporting
the "Data Encryption Algorithm" (DEA). The DEA is a widely known
algorithm, described for example in the book "Cryptography: A New
Dimension in Computer Data Security" by Carl H. Meyer and Stephen M.
Matyas, John Wiley & Sons 1982. The DEA was adopted by the US National
Security Agency and National Bureau of Standards in 1977 as a U.S. federal
standard.
Enrolling
As shown in FIG. 1, and FIG. 2, an encryption key called a "seal key" 501
is created and stored both in the verifier's storage 519 and on a smart
card 515. Where the arbiter option is taken, a copy of the same key is
provided to the arbiter who may use it for obtaining the clear signature
to print. The seal key is specific to each user and is stored together
with data on the user's identity 510 which will allow the appropriate seal
key to be retrieved from the verifier's or arbiter's storage when
required.
In IBM's Transaction Security System the usage of encryption keys may be
controlled by associating with each key a Control Vector which defines its
permitted usages, and this definition is enforced within the security
hardware device itself. Such a Control Vector mechanism, or similar, is
used to enforce the different usages of the two or three manifestations of
this key. In the description which follows the seal key is referred to as
a Sealing Key when used by the smart card, as a Seal Verification Key when
used by the verifier and as a Seal Proving Key when used for proving by
the arbiter, or in the absence of an arbiter by the verifier. Each of the
types of seal processing key is represented in the diagrams by the seal
key 518, irrespective of its individual function.
The user receives the smart card 515 and authenticates himself to the
verifier authority using whatever means of personal authentication is
appropriate, such as a Passport, driving license etc. The user enrolls by
providing a means of electronic authentication which can be performed at
the transaction terminal by the smart card itself or the smart card in
combination with the transaction terminal. This might be a PIN, password,
signature, finger print, facial image, retina pattern etc. The reference
pattern for this authentication would be stored on the smart card and
would be used, in a conventional manner, to authorize the smart card to
process a transaction as described below.
In addition, the user also registers a copy of his handwritten signature
which is captured in the form of a reproducible image 511. The image
capture and printing technology is not described here but is conventional
and will be well understood by the person skilled in the art. The
electronic form of the signature image is a binary string of indefinite
length which is padded with binary zeros (white space) to the nearest
eight byte boundary point. It may also have associated with it control
information describing how to reconstruct the signature from the
electronic image, and it may have been compressed. This construct will be
referred to as the "electronic signature" 511.
The electronic signature is preceded with a generated random number 502
(RNA) to form an "electronic signature record" or ESR 512a. Note that
where reference figures are followed by a suffix eg. 512a, 512b these
denote occurrences at different points in the diagram of the same data
value or same process. The RNA is an arbitrary binary string of at least
eight bytes, and preferably 8 or 16 bytes, since 8 bytes is a sufficient
minimum for cryptographic strength, and working on 8 byte boundaries
avoids the need for further padding. This use of the random number RNA
prevents any copy of the clear electronic signature, or a copy of the
handwritten signature, from being used to create a replica signature block
to be wrongly associated with any other already issued smart card.
The whole ESR 512a is reduced to a relatively short length using a"hashing"
algorithm, such as the algorithm MDC2 implemented in the IBM Transaction
Security System. A detailed description of the MDC2 algorithm is given in
an article by S. M. Matyas, "Key handling with control vectors", IBM
Systems Journal 30, No. 2, 151-174 (1991), and available from IBM as
Reprint Order No. G321-5428. The MDC2 algorithm is a "modification
detection code" based on DEA encryptions. A modification detection code is
a nonsecret cryptographic variable of fixed, relatively short length
calculated from a message with a public (nonsecret) one-way function. A
cryptographic one-way function has the property that, given the output,
cryptographic key, and algorithm, it is impractical to derive the input
value, or to derive another input resulting in the same output. The MDC2
algorithm processes data in multiples of 8 bytes, with a 16 byte minimum
and results in a 16 byte output.
The MDC2 algorithm is therefore suitable for reducing the ESR 512a, which
in this embodiment is of indefinite length, and results in a hashed ESR
(HESR 513a), an arbitrary binary string of 16 bytes which is specific to
the electronic signature and associated random number RNA.
The HESR, or a function of the HESR is then used to encipher 504a the value
of the ESR 512a. This enciphered ESR 523 is stored in the verifier's
storage 519, together with any necessary data, eg. a user or smart card
number, to allow it to be retrieved and associated with the correct user.
The HESR 513a, or the function thereof, is stored in the memory of the
smart card for use when transacting, but is not stored elsewhere in the
system.
In this embodiment, the encipher process 504a selected is a "triple
encipherment" process, but other enciphering methods known in the art
could be used. Triple encipherment is a process defined in IBM's Common
Cryptographic Architecture/1 (CCA/1), which is implemented in the IBM
Transaction Security System previously mentioned. The 16 byte HESR 513a is
first combined with the 16 byte seal key 518 via the Build Key Process
506. The Build Key Process could be, for example, a simple exclusive OR
operation, or if desired an encryption. The resulting 16 byte key is then
used as a "double length DEA key" to triple encipher the ESR 512a. The
triple encipherment entails splitting the 16 byte key into 8 byte left and
right halves, DEA enciphering with the left, DEA deciphering with the
right, and finally DEA enciphering with the left half.
The use of the HESR 513a to encipher the ESR 512a for storage by the
verifier prevents the verifier from deciphering the stored and enciphered
signature 523 without the HESR 513a information, which is obtainable only
from the smart card 515. In other words, the verifier is not able to
obtain the deciphered signature without the authorization of the smart
card 515 user.
Transacting
The transaction portion of the process is shown in FIG. 3. A transaction is
established and agreed upon at a transaction terminal. This manifests
itself as a formatted and structured bit string or character string coded
in binary and is called a "message" 514. The message contains time variant
information and terminal specific information so that it may be uniquely
identified; as well as details of the transaction itself. To perform the
transaction the user authenticates himself at the transaction terminal
using his smart card 515 and whatever means of electronic authentication
has been adopted. This authentication also indicates the user's agreement
to the transaction.
Having authenticated the user, the smart card receives the message, appends
smart card specific data to allow the card and the user to be identified,
and creates a seal using the process described below. This seal is
appended to the message to form a "sealed message" 520. It will be
understood that the whole sealed message may be authenticated by
calculating check digits, such as a Message Authentication Code (MAC), or
Digital Signature (DSG); but this would be an independent process, and is
not described here as appropriate methods are well known in the art.
The sealed message 520 comprising message 514, smart card data, and seal is
transmitted to the verifier.
The process for generating the seal is closely analogous to the process for
generating the enciphered electronic signature stored by the verifier.
Thus the seal is calculated as follows:
The message 514 is a binary string of indefinite length which would be
padded if necessary to the nearest 8 byte boundary. The message is
subjected to a hashing algorithm to reduce it to a 16 byte binary string.
This hashing algorithm could, but need not, be the one used to reduce the
ESR 512a to the HESR 513a, for example the MDC2 algorithm. The resultant
16 byte binary string will be referred to as the Message Hash or MASH 516.
The hashing algorithm may use a default hashing key or, optionally, an
arbitrarily generated hashing or "transaction key". If an arbitrarily
generated key is used, this key must be transmitted to the verifier with
the transaction message. Secure transmission of transaction or of MAC
verification keys is well known in the art and is not discussed further
here.
The Build Key Process 506 then combines the MASH 516 with the Sealing Key
518. In this embodiment both are 16 bytes and the Build Key Process 506
could be a simple exclusive OR operation, resulting in a 16 byte key. This
key is then used as a double length DEA key to triple encipher 517a the
HESR supplied from the card 515. This HESR read off the card will be
denoted +HESR+ in the description and figures, since it is not assumed to
be the same as the HESR recorded at enrollment; for example it may have
been corrupted. The enciphered +HESR+, together with the hashing key, if
required, is the seal. If a default hashing key known to the verifier is
used rather than an arbitrarily generated one, then the hashing key need
not be included.
The use of the binary string MASH 516, generated from the transaction data,
prevents the seal from having the same external appearance on any two
occasions. Moreover, since the verifier or arbiter cannot decipher the
HESR supplied from the card without reconstructing the MASH from the
message 514 part of the sealed message 520, the seal cannot be deciphered
if appended to any other message 514. This prevents the signature from
being associated with any transaction other than the authorized
transaction.
It is important to realize that the sealed message 520 transmitted to the
verifier comprises the message 514, smart card specific data and the seal.
The seal alone is not sufficient to regenerate the full transaction
message 514, since it is based on the shorter MASH 516.
Validating
The verifier performs the following steps, shown in FIG. 4, to validate the
transaction:
The binary string MASH 516 is calculated from the message portion 514 of
the received sealed message 520 using the hashing algorithm which is also
known to the verifier.
Using the MASH 516 and the Seal Verification Key 518 the verifier
reconstructs via the Key Build Process 506 the double length key which was
used to encipher the smart card stored +HESR+ 517a. The double length key
is used to decipher 505a the +HESR+ 517b from the seal portion of the
sealed message 520. In other words, the message portion 514 containing
transaction details is needed in order to decipher 505a the +HESR+ 517b.
If the seal becomes associated with a message 514 other than the
authorized one used in the creation of the seal, it will be impossible to
derive the +HESR+ 517b.
The smart card specific data, such as a smart card or user number, received
as part of the sealed message 520 is used by the verifier as an index to
retrieve the enciphered ESR corresponding to that user from storage 519.
The +HESR+ 517b is then used to decipher 505b the stored enciphered ESR
523. To do this, the +HESR+ 517b is combined with the Seal Verification
Key 518 via the Build Key Process 506, and then used to reverse the triple
encipher process 504a which was applied to the ESR 512a prior to storing
519 it. The clear value of the electronic signature ESR is thus obtained
as an intermediate step at this point, but is not otherwise disclosed.
Next, the ESR is hashed 503c to generate a HESR value 513b of the result,
since it is desired to compare the verifier's stored signature information
with the smart card stored value, +HESR+, which is of the hashed form. In
the preferred embodiment, the hashing process which is here applied to the
signature stored by the verifier is identical to that used to generate the
HESR stored on the smart card.
The verifier compares 508 this HESR 513b generated from the stored
enciphered signature 523 with the recovered +HESR+ 517b from the message.
A verdict 522 is given of the integrity of the received message. In the
embodiment described, the comparison 508 is for equality, since identical
hashing processes 503a, 503c were used. It will be understood that, if
non-identical hashing processes were used, the comparison would not be for
equality, but would need to be based on a functional relationship between
the hashing processes.
Thus the signature 523 stored by the verifier, once deciphered and hashed,
is verified against the quoted HESR value +HESR+ 517b used to decipher it.
If the two signatures do not match, the deciphering process will not
actually yield the true ESR value. The true HESR value obtainable only
from the smart card is necessary to allow the verifier to decipher the
signature.
A receiving application should also perform validation of the transaction
contents not described here. If the message was authenticated with further
check digits such as a MAC or DSG, these would also be verified.
Proving
The proving part of the process is shown in FIG. 5. During a dispute about
the validity of a particular transaction, the verifier, or the arbiter (if
appropriate), performs the same set of steps as described above, using a
copy of the sealed message 520 stored at the time of the transaction. This
time, however, the random number RNA is also stripped from the ESR 512b,
and the electronic signature 521 is disclosed 507 and printed along with
the transaction data in a suitable form.
The ability to reproduce the printed signature in association with the
transaction establishes that the user's authority was granted and that the
genuine smart card, holding the Sealing key 518 and HESR value 513a, was
used to authorize the transaction.
It would be possible to use more than one seal key; one key shared between
the smart card and the verifier could be used for enciphering 504b and
deciphering 505a the seal, while another key could be used to encipher
504a and decipher 505b the stored signature information. Other
alternatives to the specific encryption schemes and algorithms described,
which are based on encryption methods known in the art, could be used. The
bit lengths of variables could be changed without affecting the essence of
the invention.
However, it is significant that the verifier alone is unable to reconstruct
the unencrypted ESR, but requires information supplied from the smart card
to decrypt his copy. Another significant feature is that the signature
cannot be associated with a transaction other than the authorized
transaction. This is achieved in the preferred embodiment by means of a
particular seal or encryption scheme using algorithms known in the art;
clearly other alternatives which preserved this feature could be used.
Finally, such a scheme could if desired be applied to data other than
signature data, such as, for example, a photograph; signature data is used
as an example due to the legal status of the signature in many countries.
Implementation in the IBM Common Cryptographic Architecture/1
Although the above description is detailed enough to allow the invention to
be implemented, it will now be described how the invention could be
implemented using a particular architecture, IBM's Common Cryptographic
Architecture/1, described in the manual "Cryptographic API Interface
Reference", Order No. SC40-1675-01 available from International Business
Machines Corporation. IBM's Transaction Security System is an example of a
system using this architecture.
In the IBM Common Cryptographic Architecture/1, each encryption key has its
usage defined by a Control Vector (CV). A CV is a structured 128 bit
binary string whose value determines how the key may be used. Secure
cryptographic hardware, such as can be found in the IBM Transaction
Security System, interprets the control vector and enforces the usage
constraints.
When encryption keys are transferred between cryptographic units, they are
enciphered using higher level keys called key encrypting keys (KEKs).
Before encipherment the KEK is modified by exclusive OR with the CV of the
key being transferred. At the receiving unit the correct key will be
deciphered only if the CV has not been corrupted in transit. Keys stored
outside of secure hardware for local use are also enciphered by a KEK
modified by the key's CV.
The KEKs are of three principle types (there are other types of no
relevance here). A MASTER key is stored inside the secured hardware and is
not shared with any other device. A key stored enciphered under a MASTER
key is ready to be presented to the device which internally deciphers the
key into clear form ready for use. To exchange keys between devices a
shared KEK is required; this one KEK is known as an EXPORTER to the
sending device and as an IMPORTER to the receiving device, although the
key itself has the same value. The difference is in usage as defined by
the different CVs at each device.
In this embodiment, such shared Key Encrypting Keys, defined to be
IMPORTER/EXPORTER pairs, are generated and exchanged between the Arbiter
and the Verifier, and between the Smart Card and the Verifier in an
initial setup step. In both cases the EXPORTER form is held by the
Verifier, and indicated in FIG. 6.
These key pairs are as follows:
KKEXPcrd 201 held by the verifier and its counterpart KKIMPcrd held by the
card, and
KKEXParb 202 held by the verifier and its counterpart KKIMParb held by the
arbiter.
The purpose of this initial setup step is to provide shared Key Encrypting
Keys to secure the subsequent exchange of a seal key.
Using a "Key Generate" service a device can create a key already enciphered
under the appropriate KEK (MASTER ready for use; IMPORTER ready to receive
back to itself; EXPORTER ready to send). The selection of KEK type for
output from Key Generate is called the Key Form, and is designated EX for
EXPORTER, IM for IMPORTER and OP (meaning Operational form) for MASTER.
The Key Generate Function in CCA/1 outputs one or two key forms. For this
embodiment a new Key Generate Service 200 is defined, extending Key
Generate to provide three outputs as shown in FIG. 6. Thus a single key,
the Seal Key part 1 215 in FIG. 6, is generated 211 and output in three
forms 220, 221, 222. This is achieved by enciphering 214 the Seal Key Part
1 215 under each of the KEKs 201,202, 203 in turn, but in each case the
KEK which is stored enciphered under a MASTER key 210 is first deciphered
212 using the MASTER key 210 and then exclusively ORed 213 with a
different CV 216 based on the purpose of the key.
A Control Word 204 on the Service Call API (Application Programming
Interface) allows the Key Form to be specified. In this embodiment either
EXEXOP or EXEXIM is selected, where EXEXIM, for example, means the three
forms of KEK 201, 202, 203 must be EX, EX and IM respectively. IMPORTER
203 is needed only if the EXEXIM control word is chosen; the creation and
provision of a local IMPORTER key is not described here. If EXEXOP is
selected, the MASTER key 210 is used and is already in the secure hardware
boundary. The MASTER KEY 210 is used to decipher 212 each KEK internally
so that it can be used; it will also be used to encipher 214 the Seal Key
Part 1 to produce the Seal Verification Key 222 if EXEXOP is selected.
The "1" in Seal Key Part 1 215 is intended to indicate that this is the
first component of a partial key, and so should not be used until after
the key build process has completed.
The three forms of the Seal Key are
A Sealing Key 220 enciphered under the EXPORTER EXPKEK(crd) 201 shared with
the card
A Seal Proving Key 221 enciphered under the EXPORTER EPKEK(arb) 202 shared
with the arbiter
A Seal Verification Key 222 enciphered under the Master Key 210 of the
verifier, or under a local IMPORTER key, IMPKEK(enr) in FIG. 6.
The Sealing Key 220 and the Seal Proving Key 221 are imported at the smart
card and the arbiter respectively under the appropriate IMPORTER keys,
IMPORTER IMPKEK(crd) and IMPORTER IMPKEK(arb). The Seal Verification Key
222 is for local use by the verifier.
The Control Vector definitions are extended such that the Seal Verification
key 222 cannot be used in a function which discloses a clear signature;
but the Seal Proving key 221 may be. In all three cases the control vector
defines the key to be a partial key only; that is it is incomplete without
a final key part being combined with it. The process for combining these
key parts is undefined here but could be a simple exclusive OR operation
as the external interfaces to the secure services do not allow a user
control over the value of either part. This is the Key Build Process 111a
and 111b of FIG. 7.
FIG. 7A proposes an additional CCA/1 service, a Seal Processing Function
100, which can be used in different ways depending upon the settings of
the input parameters Control Word 107. Hashing Key 106 determines whether
a hashing key 103 is to be used. If not, MDC defines a default key to be
used in the hashing or MDC algorithm. Only four of the possible
combinations are relevant to this application.
Control Word 107 states whether the function is to Seal, as at enroller or
on smart card, Verify or Prove.
In FIG. 7B the process for encrypting the signature ESR for storage at the
time of enrolment will now be described. The key used to encrypt the ESR
for storage is the Seal Verification Key 222 combined via the Key Build
Process 111a with the HESR. As mentioned the Key Build Process 111a could
be a simple exclusive OR operation. To perform this process using the Seal
Processing Function 100, the electronic signature record ESR is presented
as Input Data 1 101. The MDC process 110 proceeds with a hashing key 103
or the default key as determined by the hashing key Y/N 106. The HESR is
then combined via the Key Build Process 111 with the Seal Verification Key
222 which is presented as Seal Key 104. The key thus produced is used to
encipher 112a Input Data 1 102, | | |
