Internet Engineering Task Force D. M'Raihi
Internet-Draft Verisign, Inc.
Intended status: Standards Track J. Rydell
Expires: September 7, 2010 Portwise, Inc.
S. Machani
Diversinet Corp.
D. Naccache
Ecole Normale Superieure
S. Bajaj
Verisign, Inc.
March 06, 2010
OCRA: OATH Challenge-Response Algorithms
draft-mraihi-mutual-oath-hotp-variants-10.txt
Abstract
This document describes an algorithm for challenge-response
authentication developed by the "Initiative for Open AuTHentication"
[OATH]. The specified mechanisms leverage the HMAC-based One-Time
Password algorithm [RFC4226] and offer one-way and mutual
authentication capabilities.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on September 7, 2010.
Copyright Notice
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Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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described in the BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Notation and Terminology . . . . . . . . . . . . . . . . . . . 3
3. Algorithm Requirements . . . . . . . . . . . . . . . . . . . . 3
4. OCRA Background . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. HOTP Algorithm . . . . . . . . . . . . . . . . . . . . . . 4
5. Definition of OCRA . . . . . . . . . . . . . . . . . . . . . . 5
5.1. DataInput Parameters . . . . . . . . . . . . . . . . . . . 5
5.2. CryptoFunction . . . . . . . . . . . . . . . . . . . . . . 6
6. The OCRASuite . . . . . . . . . . . . . . . . . . . . . . . . 7
6.1. Algorithm . . . . . . . . . . . . . . . . . . . . . . . . 8
6.2. CryptoFunction . . . . . . . . . . . . . . . . . . . . . . 8
6.3. DataInput . . . . . . . . . . . . . . . . . . . . . . . . 8
7. Algorithm Modes for Authentication . . . . . . . . . . . . . . 10
7.1. One way Challenge-Response . . . . . . . . . . . . . . . . 10
7.2. Mutual Challenge-Response . . . . . . . . . . . . . . . . 11
7.3. Algorithm Modes for Signature . . . . . . . . . . . . . . 12
7.3.1. Plain Signature . . . . . . . . . . . . . . . . . . . 13
7.3.2. Signature with Server Authentication . . . . . . . . . 13
8. Security Considerations . . . . . . . . . . . . . . . . . . . 15
8.1. Security Analysis of the OCRA algorithm . . . . . . . . . 15
8.2. Implementation Considerations . . . . . . . . . . . . . . 16
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
10. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 17
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 18
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
12.1. Normative references . . . . . . . . . . . . . . . . . . . 18
12.2. Informative References . . . . . . . . . . . . . . . . . . 18
Appendix A. Reference Implementation . . . . . . . . . . . . . . 18
Appendix B. Test Vectors . . . . . . . . . . . . . . . . . . . . 24
B.1. Plain challenge response . . . . . . . . . . . . . . . . . 25
B.2. Mutual Challenge Response . . . . . . . . . . . . . . . . 26
B.3. Plain Signature . . . . . . . . . . . . . . . . . . . . . 27
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Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 28
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1. Introduction
OATH has identified several use cases and scenarios that require an
asynchronous variant to accommodate users who do not want to maintain
a synchronized authentication system. A commonly accepted method for
this is to use a challenge-response scheme.
Such challenge response mode of authentication is widely adopted in
the industry. Several vendors already offer software applications
and hardware devices implementing challenge-response - but each of
those uses vendor-specific proprietary algorithms. For the benefits
of users there is a need for a standardized challenge-response
algorithm which allows multi-sourcing of token purchases and
validation systems to facilitate the democratization of strong
authentication. Additionally, this specification describes the means
to create symmetric key based digital signatures. Such signatures
are variants of challenge-response mode where the data to be signed
becomes the challenge.
2. Notation and Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Algorithm Requirements
This section presents the main requirements that drove this algorithm
design. A lot of emphasis was placed on flexibility and usability,
under the constraints and specificity of the HOTP algorithm and
hardware token capabilities.
R1 - The algorithm MUST support asynchronous challenge-response based
authentication.
R2 - The algorithm MUST be capable of supporting symmetric key based
digital signatures. Essentially this is a variation of challenge-
response where the challenge is derived from the data that need to be
signed.
R3 - The algorithm MUST be capable of supporting server-
authentication, whereby the user can verify that he/she is talking to
a trusted server.
R4 - The algorithm SHOULD use HOTP [RFC4226] as a key building block.
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R5 - The length and format of the input challenge SHOULD be
configurable.
R6 - The output length and format of the generated response SHOULD be
configurable.
R7 - The challenge MAY be generated with integrity checking (e.g.,
parity bits). This will allow tokens with pin pads to perform simple
error checking when the user enters the challenge value into a token.
R8 - There MUST be a unique secret (key) for each token/soft token
that is shared between the token and the authentication server. The
keys MUST be randomly generated or derived using a key derivation
algorithm.
R9 - The algorithm MAY enable additional data attributes such as a
timestamp or session information to be included in the computation.
These data inputs MAY be used individually or all together.
4. OCRA Background
OATH introduced the HOTP algorithm as a first open, freely available
building block towards strengthening authentication for end-users in
a variety of applications. One-time passwords are very efficient at
solving specific security issues thanks to the dynamic nature of OTP
computations.
After carefully analyzing different use cases, OATH came to the
conclusion that providing for extensions to the HOTP algorithms was
important. A very natural extension is to introduce a challenge mode
for computing HOTP values based on random questions. Equally
beneficial is being able to perform mutual authentication between two
parties, or short-signature computation for authenticating
transaction to improve the security of e-commerce applications.
4.1. HOTP Algorithm
The HOTP algorithm, as defined in [RFC4226] is based on an increasing
counter value and a static symmetric key known only to the prover and
verifier parties.
As a reminder:
HOTP(K,C) = Truncate(HMAC-SHA1(K,C))
Where Truncate represents the function that converts an HMAC-SHA-1
value into an HOTP value.
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We refer the reader to [RFC4226] for the full description and further
details on the rationale and security analysis of HOTP.
The present draft describes the different variants based on similar
constructions as HOTP.
5. Definition of OCRA
OCRA is a generalization of HOTP with variable data inputs not solely
based on an incremented counter and secret key values.
The definition of OCRA requires a cryptographic function, a key K and
a set of DataInput parameters. This section first formally
introduces the OCRA algorithm and then introduces the definitions and
default values recommended for all parameters.
In a nutshell,
OCRA = CryptoFunction(K, DataInput)
Where:
o K: a shared secret key known to both parties
o DataInput: a structure that contains the concatenation of the
various input data values defined in details in section 5.1
o CryptoFunction: this is the function performing the OCRA
computation from the secret key K and the DataInput material;
CryptoFunction is described in details in section Section 5.2
5.1. DataInput Parameters
This structure is the concatenation over byte array of the OCRASuite
value as defined in section 6 with the different parameters used in
the computation, save for the secret key K.
DataInput = {OCRASuite | 00 | C | Q | P | S | T} where:
o OCRASuite is a value representing the suite of operations to
compute an OCRA response
o 00 is a byte value used as a separator
o C is an unsigned 8-byte counter value processed high-order bit
first, and MUST be synchronized between all parties; It loops
around from "{Hex}0" to "{Hex}FFFFFFFFFFFFFFFF" and then starts
over at "{Hex}0"
o Q, mandatory, is a 128-byte list of (concatenated) challenge
question(s) generated by the parties; if Q is less than 128 bytes,
then it should be padded with zeroes to the right
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o P is a hash (SHA1, SHA256 and SHA512 are supported) value of PIN/
password that is known to all parties during the execution of the
algorithm; the length of P will depend on the hash function that
is used
o S is an UTF-8 encoded string of length upto 512 bytes that
contains information about the current session; the length of S is
defined in the OCRASuite string
o T is an 8-byte unsigned integer in big endian (i.e. network byte
order) representing the number of time-steps(seconds, minutes,
hours or days depending on the specified granularity) since
midnight UTC of January 1, 1970. More specificatlly, if the OCRA
computation includes a timestamp T, you SHOULD first convert your
current local time to UTC time (text form); you can then derive
the UTC time in the proper format (i.e. seconds, minutes, hours or
days elapsed from Epoch time); the size of the time-step is
defined in the OCRASuite string
When computing a response, the concatenation order is always the
following:
C |
OTHER-PARTY-GENERATED-CHALLENGE-QUESTION |
YOUR-GENERATED-CHALLENGE-QUESTION |
P| S | T
If a value is empty (i.e. a certain input is not used in the
computation) then the value is simply not represented in the string.
The counter on the token or client MUST be incremented every time a
new computation is requested by the user. The server's counter value
MUST only be incremented after a successful OCRA authentication.
5.2. CryptoFunction
The default CryptoFunction is HOTP-SHA1-6, i.e. the default mode of
computation for OCRA is HOTP with the default 6-digit dynamic
truncation and a combination of DataInput values as the message to
compute the HMAC-SHA1 digest.
As indicated in section 5.1, we denote t as the length in digits of
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the truncation output. For instance, if t = 6, then the output of
the truncation is a 6-digit value.
We define the HOTP family of functions as an extension to HOTP:
1. HOTP-H-t: these are the different possible truncated versions of
HOTP, using the dynamic truncation method for extracting an HOTP
value from the HMAC output
2. We will denote HOTP-H-t as the realization of an HOTP function
that uses an HMAC function with the hash function H, and the
dynamic truncation as described in [RFC 4226] to extract a t-
digit value
3. t=0 means that no truncation is performed and the full HMAC value
is used for authentication purpose
We list the following preferred modes of computation, where * denotes
the default CryptoFunction:
o HOTP-SHA1-4: HOTP with SHA-1 as the hash function for HMAC and a
dynamic truncation to a 4-digit value; this mode is not
recommended in the general case but can be useful when a very
short authentication code is needed by an application
o HOTP-SHA1-6: HOTP with SHA-1 as the hash function for HMAC and a
dynamic truncation to a 6-digit value
o HOTP-SHA1-8: HOTP with SHA-1 as the hash function for HMAC and a
dynamic truncation to an 8-digit value
o HOTP-SHA256-6: HOTP with SHA-256 as the hash function for HMAC and
a dynamic truncation to a 6-digit value
o HOTP-SHA512-6: HOTP with SHA-512 as the hash function for HMAC and
a dynamic truncation to a 6-digit value
This table summarizes all possible values for the CryptoFunction:
+---------------+--------------------+-------------------------+
| Name | HMAC Function Used | Size of Truncation (t) |
+---------------+--------------------+-------------------------+
| HOTP-SHA1-t | HMAC-SHA1 | 0 (no truncation), 4-10 |
| HOTP-SHA256-t | HMAC-SHA256 | 0 (no truncation), 4-10 |
| HOTP-SHA512-t | HMAC-SHA512 | 0 (no truncation), 4-10 |
+---------------+--------------------+-------------------------+
Table 1: CryptoFunction Table
6. The OCRASuite
An OCRASuite value is a text string that captures one mode of
operation for the OCRA algorithm, completely specifying the various
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options for that computation. An OCRASuite value is represented as
follows:
Algorithm:CryptoFunction:DataInput
The client and server need to agree on one or two values of
OCRASuite. These values may be agreed at time of token provisioning
or for more sophisticated client-server interactions these values may
be negotiated for every transaction.
Note that for Mutual Challenge-Response or Signature with Server
Authentication modes, the client and server will need to agree on two
values of OCRASuite - one for server computation and another for
client computation.
6.1. Algorithm
Description: Indicates the version of OCRA algorithm.
Values: OCRA-v where v represents the version number (e.g. 1, 2
etc.). This document specifies version 1 of the OCRA algorithm.
6.2. CryptoFunction
Description: Indicates the function used to compute OCRA values
Values: Permitted values are described in section 5.2
6.3. DataInput
Description: This component of the OCRASuite string captures the list
of valid inputs for that computation; [] indicates a value is
optional:
[C] | QFxx | [PH | Snnn | TG] : Challenge-Response computation
[C] | QFxx | [PH | TG] : Plain Signature computation
Each input that is used for the computation is represented by a
single letter (except Q) and they are separated by a hyphen.
The input for challenge is further qualified by the formats supported
by the client for challenge question(s). Supported values can be:
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+------------------+-------------------+
| Format (F) | Up to Length (xx) |
+------------------+-------------------+
| A (alphanumeric) | 04-64 |
| N (numeric) | 04-64 |
| H (hexadecimal) | 04-64 |
+------------------+-------------------+
Table 2: Challenge Format Table
The default challenge format is N08, numeric and upto 8 digits.
The input for P is further qualified by the hash function used for
the PIN/password. Supported values for hash function can be:
Hash function (H) - SHA1, SHA256, SHA512.
The default hash function for P is SHA1.
The input for S is further qualified by the length of the session
data in bytes. The client and server could agree to any length but
the typical values are:
Length (nnn) - 064, 128, 256 and 512.
The default length is 064 bytes.
The input for timestamps is further qualified by G, size of the time-
step. G can be specified in number of seconds, minutes or hours:
+--------------------+-----------------------------+
| Time-step Size (G) | Examples |
+--------------------+-----------------------------+
| [1-59]S | number of seconds, e.g. 20S |
| [1-59]M | number of minutes, e.g. 5M |
| [0-48]H | number of hours, e.g. 24H |
+--------------------+-----------------------------+
Table 3: Time-step Size Table
Default value for G is 1M, i.e. time step size is one minute and the
T represents the number of minutes since Epoch time.
Here are some examples of OCRASuite strings:
o OCRA-1:HOTP-SHA512-8:C-QN08-PSHA1 means version 1 of the OCRA
algorithm with HMAC-SHA512 function, truncated to an 8-digit
value, using the counter, a random challenge and a SHA1 digest of
the PIN/Password as parameters. It also indicates that the client
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supports only numeric challenge upto 8 digits in length
o OCRA-1:HOTP-SHA256-6:QA10-T1M means version 1 of the OCRA
algorithm with HMAC-SHA256 function, truncated to a 6-digit value,
using a random alphanumeric challenge upto 10 characters in length
and a timestamp in number of minutes since Epoch time
o OCRA-1:HOTP-SHA1-4:QH8-S512 means version 1 of the OCRA algorithm
with HMAC-SHA1 function, truncated to a 4-digit value, using a
random hexadecimal challenge upto 8 nibbles and a session value of
512 bytes
7. Algorithm Modes for Authentication
This section describes the typical modes in which the above defined
computation can be used for authentication.
7.1. One way Challenge-Response
A challenge/response is a security mechanism in which the verifier
presents a question (challenge) to the prover who must provide a
valid answer (response) to be authenticated.
To use this algorithm for a one-way challenge-response, the verifier
will communicate a challenge value (typically randomly generated) to
the prover. The prover will use the challenge in the computation as
described above. The prover then communicates the response to the
verifier to authenticate.
Therefore in this mode, the typical data inputs will be:
C - Counter, optional.
Q - Challenge question, mandatory, supplied by the verifier.
P - Hashed version of PIN/password, optional.
S - Session information, optional.
T - Timestamp, optional.
The diagram below shows the message exchange between the client
(prover) and the server (verifier) to complete a one-way challenge-
response authentication.
It is assumed that the client and server have a pre-shared key K that
is used for the computation.
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CLIENT SERVER
(PROVER) VERIFIER)
| |
| Verifier sends challenge to prover |
| Challenge = Q |
|<---------------------------------------|
| |
| Prover Computes Response |
| R = OCRA(K, {[C] | Q | [P | S | T]}) |
| Prover sends Response = R |
|--------------------------------------->|
| |
| Verifier Validates Response |
| If Response is valid, Server sends OK |
| If Response is not, Server sends NOK |
|<---------------------------------------|
| |
7.2. Mutual Challenge-Response
Mutual challenge-response is a variation of one-way challenge-
response where both the client and server mutually authenticate each
other.
To use this algorithm, the client will first send a random client-
challenge to the server. The server computes the server-response and
sends it to the client along with a server-challenge.
The client will first verify the server-response to be assured that
it is talking to a valid server. It will then compute the client-
response and send it to the server to authenticate. The server
verifies the client-response to complete the two-way authentication
process.
In this mode there are two computations: client-response and server-
response. There are two separate challenge questions, generated by
both parties. We denote these challenge questions Q1 and Q2.
Typical data inputs for server-response computation will be:
C - Counter, optional.
QC - Challenge question, mandatory, supplied by the client.
QS - Challenge question, mandatory, supplied by the server.
S - Session information, optional.
T - Timestamp, optional.
Typical data inputs for client-response computation will be:
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C - Counter, optional.
QS - Challenge question, mandatory, supplied by the server.
QC - Challenge question, mandatory, supplied by the client.
P - Hashed version of PIN/password, optional.
S - Session information, optional.
T - Timestamp, optional.
The following picture shows the messages that are exchanged between
the client and the server to complete a two-way mutual challenge-
response authentication.
It is assumed that the client and server have a pre-shared key K (or
pair of keys if using dual-key mode of computation) that is used for
the computation.
CLIENT SERVER
(PROVER) (VERIFIER)
| |
| 1. Client sends client-challenge |
| QC = Client-challenge |
|------------------------------------------------->|
| |
| 2. Server computes server-response |
| and sends server-challenge |
| RS = OCRA(K, [C] | QC | QS | [S | T]) |
| QS = Server-challenge |
| Response = RS, QS |
|<-------------------------------------------------|
| |
| 3. Client verifies server-response |
| and computes client-response |
| OCRA(K, [C] | QC | QS | [S | T]) != RS -> STOP |
| RC = OCRA(K, [C] | QS | QC | [P | S | T]) |
| Response = RC |
|------------------------------------------------->|
| |
| 4. Server verifies client-response |
| OCRA(K, [C] | QS | QC | [P|S|T]) != RC -> STOP |
| Response = OK |
|<-------------------------------------------------|
| |
7.3. Algorithm Modes for Signature
In this section we describe the typical modes in which the above
defined computation can be used for digital signatures.
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7.3.1. Plain Signature
To use this algorithm in plain signature mode, the server will
communicate a signature-challenge value to the client (signer). The
signature-challenge is either the data to be signed or derived from
the data to be signed using a hash function, for example.
The client will use the signature-challenge in the computation as
described above. The client then communicates the signature value
(response) to the server to authenticate.
Therefore in this mode, the data inputs will be:
C - Counter, optional.
QS - Signature-challenge, mandatory, supplied by the server.
P - Hashed version of PIN/password, optional.
T - Timestamp, optional.
The picture below shows the messages that are exchanged between the
client (prover) and the server (verifier) to complete a plain
signature operation.
It is assumed that the client and server have a pre-shared key K that
is used for the computation.
CLIENT SERVER
(PROVER) (VERIFIER)
| |
| Verifier sends signature-challenge |
| Challenge = QS |
|<------------------------------------------|
| |
| Client Computes Response |
| SIGN = OCRA(K, [C] | QS | [P | T]) |
| Response = SIGN |
|------------------------------------------>|
| |
| Verifier Validates Response |
| Response = OK |
|<------------------------------------------|
| |
7.3.2. Signature with Server Authentication
This mode is a variation of the plain signature mode where the client
can first authenticates the server before generating a digital
signature.
To use this algorithm, the client will first send a random client-
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challenge to the server. The server computes the server-response and
sends it to the client along with a signature-challenge.
The client will first verify the server-response to authenticate that
it is talking to a valid server. It will then compute the signature
and send it to the server.
In this mode there are two computations: client-signature and server-
response.
Typical data inputs for server-response computation will be:
C - Counter, optional.
QC - Challenge question, mandatory, supplied by the client.
QS - Signature-challenge, mandatory, supplied by the server.
T - Timestamp, optional.
Typical data inputs for client-signature computation will be:
C - Counter, optional.
QC - Challenge question, mandatory, supplied by the client.
QS - Signature-challenge, mandatory, supplied by the server.
P - Hashed version of PIN/password, optional.
T - Timestamp, optional.
The diagram below shows the messages that are exchanged between the
client and the server to complete a signature with server
authentication transaction.
It is assumed that the client and server have a pre-shared key K (or
pair of keys if using dual-key mode of computation) that is used for
the computation.
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CLIENT SERVER
(PROVER) VERIFIER)
| |
| 1. Client sends client-challenge |
| QC = Client-challenge |
|-------------------------------------------------->|
| |
| 2. Server computes server-response |
| and sends signature-challenge |
| RS = OCRA(K, [C] | QC | QS | [T]) |
| QS = signature-challenge |
| Response = RS, QS |
|<--------------------------------------------------|
| |
| 3. Client verifies server-response |
| and computes signature |
| OCRA(K, [C] | QC | QS | [T]) != RS -> STOP |
| SIGN = OCRA( K, [C] | QS | QC | [P | T]) |
| Response = SIGN |
|-------------------------------------------------->|
| |
| 4. Server verifies Signature |
| OCRA(K, [C] | QS | QC | [P|T]) != SIGN -> STOP |
| Response = OK |
|<--------------------------------------------------|
| |
8. Security Considerations
Any algorithm is only as secure as the application and the
authentication protocols that implement it. Therefore, this section
discusses the critical security requirements that our choice of
algorithm imposes on the authentication protocol and validation
software.
8.1. Security Analysis of the OCRA algorithm
The security and strength of this algorithm depends on the properties
of the underlying building block HOTP, which is a construction based
on HMAC [RFC2104] using SHA-1 as the hash function.
The conclusion of the security analysis detailed in [RFC4226] is
that, for all practical purposes, the outputs of the dynamic
truncation on distinct counter inputs are uniformly and independently
distributed strings.
The analysis demonstrates that the best possible attack against the
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HOTP function is the brute force attack.
8.2. Implementation Considerations
IC1 - In the authentication mode, the client MUST support two- factor
authentication, i.e., the communication and verification of something
you know (secret code such as a Password, Pass phrase, PIN code,
etc.) and something you have (token). The secret code is known only
to the user and usually entered with the Response value for
authentication purpose (two-factor authentication). Alternatively,
instead of sending something you know to the server, the client may
use a hash of the Password or PIN code in the computation itself,
thus implicitly enabling two-factor authentication.
IC2 - Keys should be of the length of the CryptoFunction output to
facilitate interoperability.
IC3 - Keys SHOULD be chosen at random or using a cryptographically
strong pseudo-random generator properly seeded with a random value.
We RECOMMEND following the recommendations in [RFC1750] for all
pseudo-random and random generations. The pseudo-random numbers used
for generating the keys SHOULD successfully pass the randomness test
specified in [CN].
IC4 - Challenge questions SHOULD be 20-byte values and MUST be at
least t-byte values where t stands for the digit-length of the OCRA
truncation output.
IC5 - On the client side, the keys SHOULD be embedded in a tamper
resistant device or securely implemented in a software application.
Additionally, by embedding the keys in a hardware device, you also
have the advantage of improving the flexibility (mobility) of the
authentication system.
IC6 - We RECOMMEND following the recommendations in [RFC1750] for all
pseudo-random and random challenge generations.
IC7 - All the communications SHOULD take place over a secure channel
e.g. SSL/TLS, IPsec connections.
IC8 - The OCRA algorithm when used in mutual authentication mode or
in signature with server authentication mode MAY use dual key mode -
i.e. there are two keys that are shared between the client and the
server. One shared key is used to generate the server response on
the server side and to verify it on the client side. The other key
is used to create the response or signature on the client side and to
verify it on the server side.
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IC9 - We recommend that implementations MAY use the session
information, S as an additional input in the computation. For
example, S could be the session identifier from the TLS session.
This will enable you to counter certain types of man-in-the-middle
attacks. However, this will introduce the additional dependency that
first of all the prover needs to have access to the session
identifier to compute the response and the verifier will need access
to the session identifier to verify the response.
IC10 - In the signature mode, whenever the counter or time (defined
as optional elements) are not used in the computation, there might be
a risk of replay attack and the implementers should carefully
consider this issue in the light of their specific application
requirements and security guidelines. The server SHOULD also provide
whenever possible a mean for the client (if able) to verify the
validity of the signature challenge.
IC11 - We also RECOMMEND storing the keys securely in the validation
system, and more specifically encrypting them using tamper-resistant
hardware encryption and exposing them only when required: for
example, the key is decrypted when needed to verify an OCRA response,
and re-encrypted immediately to limit exposure in the RAM for a short
period of time. The key store MUST be in a secure area, to avoid as
much as possible direct attack on the validation system and secrets
database. Particularly, access to the key material should be limited
to programs and processes required by the validation system only.
9. IANA Considerations
This document has no actions for IANA.
10. Conclusion
This draft introduced several variants of HOTP for challenge-
response based authentication and short signature-like computations.
The OCRASuite provides for an easy integration and support of
different flavors within an authentication and validation system.
Finally, OCRA should enable mutual authentication both in connected
and off-line modes, with the support of different response sizes and
mode of operations.
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11. Acknowledgements
We would like to thank Jeff Burstein, Shuh Chang, Oanh Hoang, Philip
Hoyer, Jon Martinsson, Frederik Mennes, Mingliang Pei, Jonathan
Tuliani, Stu Vaeth, Enrique Rodriguez and Robert Zuccherato for their
comments and suggestions to improve this draft document.
12. References
12.1. Normative references
[RFC1750] Eastlake, D., Crocker, S., and J. Schiller, "Randomness
Recommendations for Security", RFC 1750, December 1994,
<http://www.ietf.org/rfc/rfc1750.txt>.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
February 1997, <http://www.ietf.org/rfc/rfc2104.txt>.
[RFC2119] "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997,
<http://www.ietf.org/rfc/rfc2119.txt>.
[RFC4226] M'Raihi, D., Bellare, M., Hoornaert, F., Naccache, D., and
O. Ranen, "HOTP: An HMAC-Based One-Time Password
Algorithm", RFC 4226, December 2005,
<http://www.ietf.org/rfc/rfc4226.txt>.
12.2. Informative References
[CN] Coron, J. and D. Naccache, "An accurate evaluation of
Maurer's universal test", LNCS 1556, February 1999, <http:
//www.gemplus.com/smart/rd/publications/pdf/CN99maur.pdf>.
[OATH] Initiative for Open AuTHentication, "OATH Vision",
<http://www.openauthentication.org/about>.
Appendix A. Reference Implementation
import java.lang.reflect.UndeclaredThrowableException;
import java.security.GeneralSecurityException;
import javax.crypto.Mac;
import javax.crypto.spec.SecretKeySpec;
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import java.math.BigInteger;
/**
* This an example implementation of the OATH OCRA algorithm.
* Visit www.openauthentication.org for more information.
*
* @author Johan Rydell, PortWise
*/
public class OCRA {
private OCRA() {}
/**
* This method uses the JCE to provide the crypto
* algorithm.
* HMAC computes a Hashed Message Authentication Code with the
* crypto hash algorithm as a parameter.
*
* @param crypto the crypto algorithm (HmacSHA1,
* HmacSHA256,
* HmacSHA512)
* @param keyBytes the bytes to use for the HMAC key
* @param text the message or text to be authenticated.
*/
private static byte[] hmac_sha1(String crypto,
byte[] keyBytes,
byte[] text)
{
try {
Mac hmac;
hmac = Mac.getInstance(crypto);
SecretKeySpec macKey =
new SecretKeySpec(keyBytes, "RAW");
hmac.init(macKey);
return hmac.doFinal(text);
} catch (GeneralSecurityException gse) {
throw new UndeclaredThrowableException(gse);
}
}
private static final int[] DIGITS_POWER
// 0 1 2 3 4 5 6 7 8
= {1,10,100,1000,10000,100000,1000000,10000000,100000000 };
/**
* This method converts HEX string to Byte[]
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*
* @param hex the HEX string
*
* @return A byte array
*/
private static byte[] hexStr2Bytes(String hex){
// Adding one byte to get the right conversion
// values starting with "0" can be converted
byte[] bArray = new BigInteger("10" + hex,16).toByteArray();
// Copy all the REAL bytes, not the "first"
byte[] ret = new byte[bArray.length - 1];
for (int i = 0; i < ret.length ; i++)
ret[i] = bArray[i+1];
return ret;
}
/**
* This method generates an OCRA HOTP value for the given
* set of parameters.
*
* @param ocraSuite the OCRA Suite
* @param key the shared secret, HEX encoded
* @param counter the counter that changes
* on a per use basis,
* HEX encoded
* @param question the challenge question, HEX encoded
* @param password a password that can be used,
* HEX encoded
* @param sessionInformation
* Static information that identifies the
* current session, Hex encoded
* @param timeStamp a value that reflects a time
*
* @return A numeric String in base 10 that includes
* {@link truncationDigits} digits
*/
static public String generateOCRA(String ocraSuite,
String key,
String counter,
String question,
String password,
String sessionInformation,
String timeStamp)
{
int codeDigits = 0;
String crypto = "";
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String result = null;
int ocraSuiteLength = (ocraSuite.getBytes()).length;
int counterLength = 0;
int questionLength = 0;
int passwordLength = 0;
int sessionInformationLength = 0;
int timeStampLength = 0;
if(ocraSuite.toLowerCase().indexOf("sha1") > 1)
crypto = "HmacSHA1";
if(ocraSuite.toLowerCase().indexOf("sha256") > 1)
crypto = "HmacSHA256";
if(ocraSuite.toLowerCase().indexOf("sha512") > 1)
crypto = "HmacSHA512";
// How many digits should we return
String oS = ocraSuite.substring(ocraSuite.indexOf(":"),
ocraSuite.indexOf(":", ocraSuite.indexOf(":") + 1));
codeDigits = Integer.decode(oS.substring
(oS.lastIndexOf("-")+1,
oS.length()));
// The size of the byte array message to be encrypted
// Counter
if(ocraSuite.toLowerCase().indexOf(":c") > 1) {
// Fix the length of the HEX string
while(counter.length() < 16)
counter = "0" + counter;
counterLength=8;
}
// Question
if((ocraSuite.toLowerCase().indexOf(":q") > 1) ||
(ocraSuite.toLowerCase().indexOf("-q") > 1)) {
while(question.length() < 256)
question = question + "0";
questionLength=128;
}
// Password
if((ocraSuite.toLowerCase().indexOf(":p") > 1) ||
(ocraSuite.toLowerCase().indexOf("-p") > 1)){
while(password.length() < 40)
password = "0" + password;
passwordLength=20;
}
// sessionInformation
if((ocraSuite.toLowerCase().indexOf(":s") > 1) ||
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(ocraSuite.toLowerCase().indexOf("-s",
ocraSuite.indexOf(":",
ocraSuite.indexOf(":") + 1)) > 1)){
while(sessionInformation.length() < 128)
sessionInformation = "0" + sessionInformation;
sessionInformationLength=64;
}
// TimeStamp
if((ocraSuite.toLowerCase().indexOf(":t") > 1) ||
(ocraSuite.toLowerCase().indexOf("-t") > 1)){
while(timeStamp.length() < 16)
timeStamp = "0" + timeStamp;
timeStampLength=8;
}
// Remember to add "1" for the "00" byte delimiter
byte[] msg = new byte[ocraSuiteLength +
counterLength +
questionLength +
passwordLength +
sessionInformationLength +
timeStampLength +
1];
// Put the bytes of "ocraSuite" parameters into the message
byte[] bArray = ocraSuite.getBytes();
for(int i = 0; i < bArray.length; i++){
msg[i] = bArray[i];
}
// Delimiter
msg[bArray.length] = 0x00;
// Put the bytes of "Counter" to the message
// Input is HEX encoded
if(counterLength > 0 ){
bArray = hexStr2Bytes(counter);
for (int i = 0; i < bArray.length ; i++) {
msg[i + ocraSuiteLength + 1] = bArray[i];
}
}
// Put the bytes of "question" to the message
// Input is text encoded
if(question.length() > 0 ){
bArray = hexStr2Bytes(question);
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for (int i = 0; i < bArray.length ; i++) {
msg[i + ocraSuiteLength + 1 + counterLength] =
bArray[i];
}
}
// Put the bytes of "password" to the message
// Input is HEX encoded
if(password.length() > 0){
bArray = hexStr2Bytes(password);
for (int i = 0; i < bArray.length ; i++) {
msg[i + ocraSuiteLength + 1 + counterLength
+ questionLength] = bArray[i];
}
}
// Put the bytes of "sessionInformation" to the message
// Input is text encoded
if(sessionInformation.length() > 0 ){
bArray = hexStr2Bytes(sessionInformation);
for (int i = 0; i < 128 && i < bArray.length ; i++) {
msg[i + ocraSuiteLength
+ 1 + counterLength
+ questionLength
+ passwordLength] = bArray[i];
}
}
// Put the bytes of "time" to the message
// Input is text value of minutes
if(timeStamp.length() > 0){
bArray = hexStr2Bytes(timeStamp);
for (int i = 0; i < 8 && i < bArray.length ; i++) {
msg[i + ocraSuiteLength + 1 + counterLength +
questionLength + passwordLength +
sessionInformationLength] = bArray[i];
}
}
byte[] hash;
bArray = hexStr2Bytes(key);
hash = hmac_sha1(crypto, bArray, msg);
// put selected bytes into result int
int offset = hash[hash.length - 1] & 0xf;
int binary =
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((hash[offset] & 0x7f) << 24) |
((hash[offset + 1] & 0xff) << 16) |
((hash[offset + 2] & 0xff) << 8) |
(hash[offset + 3] & 0xff);
int otp = binary % DIGITS_POWER[codeDigits];
result = Integer.toString(otp);
while (result.length() < codeDigits) {
result = "0" + result;
}
return result;
}
}
Appendix B. Test Vectors
This section provides test values that can be used for OCRA algorithm
interoperability test.
Standard 20Byte key:
3132333435363738393031323334353637383930
Standard 32Byte key:
3132333435363738393031323334353637383930313233343536373839303132
Standard 64Byte key:
313233343536373839303132333435363738393031323334353637383930313233343
53637383930313233343536373839303132333435363738393031323334
PIN (1234) SHA1 hash value:
7110eda4d09e062aa5e4a390b0a572ac0d2c0220
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B.1. Plain challenge response
+-----------------+----------+------------+
| Key | Q | OCRA Value |
+-----------------+----------+------------+
| Standard 20Byte | 00000000 | 237653 |
| Standard 20Byte | 11111111 | 243178 |
| Standard 20Byte | 22222222 | 653583 |
| Standard 20Byte | 33333333 | 740991 |
| Standard 20Byte | 44444444 | 608993 |
| Standard 20Byte | 55555555 | 388898 |
| Standard 20Byte | 66666666 | 816933 |
| Standard 20Byte | 77777777 | 224598 |
| Standard 20Byte | 88888888 | 750600 |
| Standard 20Byte | 99999999 | 294470 |
+-----------------+----------+------------+
HOTP-SHA1-6:QN08
+-----------------+----------+------------+
| Key | Q | OCRA Value |
+-----------------+----------+------------+
| Standard 32Byte | 00000000 | 23468859 |
| Standard 32Byte | 11111111 | 30678539 |
| Standard 32Byte | 22222222 | 82867288 |
| Standard 32Byte | 33333333 | 08681556 |
| Standard 32Byte | 44444444 | 21426554 |
+-----------------+----------+------------+
OCRA-1:HOTP-SHA256-8:QN08-PSHA1
+-----------------+---+----------+------------+
| Key | C | Q | OCRA Value |
+-----------------+---+----------+------------+
| Standard 32Byte | 0 | 12345678 | 52663897 |
| Standard 32Byte | 1 | 12345678 | 72068127 |
| Standard 32Byte | 2 | 12345678 | 19260949 |
| Standard 32Byte | 3 | 12345678 | 00504526 |
| Standard 32Byte | 4 | 12345678 | 18350397 |
+-----------------+---+----------+------------+
OCRA-1:HOTP-SHA1-8:C-QD08-PSHA1
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+-----------------+-------+----------+------------+
| Key | C | Q | OCRA Value |
+-----------------+-------+----------+------------+
| Standard 64Byte | 00000 | 00000000 | 76837499 |
| Standard 64Byte | 00001 | 11111111 | 81314151 |
| Standard 64Byte | 00002 | 22222222 | 12306337 |
| Standard 64Byte | 00003 | 33333333 | 01404140 |
| Standard 64Byte | 00004 | 44444444 | 99227510 |
+-----------------+-------+----------+------------+
OCRA-1:HOTP-SHA512-8:C-QN08
+-----------------+----------+---------+------------+
| Key | Q | T | OCRA Value |
+-----------------+----------+---------+------------+
| Standard 64Byte | 00000000 | 132d0b6 | 87402523 |
| Standard 64Byte | 11111111 | 132d0b6 | 26305091 |
| Standard 64Byte | 22222222 | 132d0b6 | 25351647 |
| Standard 64Byte | 33333333 | 132d0b6 | 87448992 |
| Standard 64Byte | 44444444 | 132d0b6 | 02480585 |
+-----------------+----------+---------+------------+
OCRA-1:HOTP-SHA512-8:QN08-T30S
B.2. Mutual Challenge Response
OCRASuite (server computation) = OCRA-1:HOTP-SHA256-8:QA08
OCRASuite (client computation) = OCRA-1:HOTP-SHA256-8:QA08
+-----------------+------------------+------------+
| Key | Q | OCRA Value |
+-----------------+------------------+------------+
| Standard 32Byte | CLI22220SRV11110 | 25876570 |
| Standard 32Byte | CLI22221SRV11111 | 68337907 |
| Standard 32Byte | CLI22222SRV11112 | 51935447 |
| Standard 32Byte | CLI22223SRV11113 | 12325872 |
| Standard 32Byte | CLI22224SRV11114 | 50478153 |
+-----------------+------------------+------------+
Server -- OCRA-1:HOTP-SHA256-8:QA08
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+-----------------+------------------+------------+
| Key | Q | OCRA Value |
+-----------------+------------------+------------+
| Standard 32Byte | SRV11110CLI22220 | 65475908 |
| Standard 32Byte | SRV11111CLI22221 | 36352242 |
| Standard 32Byte | SRV11112CLI22222 | 20036871 |
| Standard 32Byte | SRV11113CLI22223 | 58392008 |
| Standard 32Byte | SRV11114CLI22224 | 53668312 |
+-----------------+------------------+------------+
Client -- OCRA-1:HOTP-SHA256-8:QA08
OCRASuite (server computation) = OCRA-1:HOTP-SHA512-8:QA08
OCRASuite (client computation) = OCRA-1:HOTP-SHA512-8:QA08-PSHA1
+-----------------+------------------+------------+
| Key | Q | OCRA Value |
+-----------------+------------------+------------+
| Standard 64Byte | CLI22220SRV11110 | 22014886 |
| Standard 64Byte | CLI22221SRV11111 | 52086443 |
| Standard 64Byte | CLI22222SRV11112 | 77303083 |
| Standard 64Byte | CLI22223SRV11113 | 39015190 |
| Standard 64Byte | CLI22224SRV11114 | 03327937 |
+-----------------+------------------+------------+
Server -- OCRA-1:HOTP-SHA512-8:QA08
+-----------------+------------------+------------+
| Key | Q | OCRA Value |
+-----------------+------------------+------------+
| Standard 64Byte | SRV11110CLI22220 | 98411686 |
| Standard 64Byte | SRV11111CLI22221 | 68537161 |
| Standard 64Byte | SRV11112CLI22222 | 70206212 |
| Standard 64Byte | SRV11113CLI22223 | 26271909 |
| Standard 64Byte | SRV11114CLI22224 | 34327256 |
+-----------------+------------------+------------+
Client -- OCRA-1:HOTP-SHA512-8:QA08-PSHA1
B.3. Plain Signature
In this mode of operation, Q represents the signature challenge.
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+-----------------+----------+------------+
| Key | Q | OCRA Value |
+-----------------+----------+------------+
| Standard 32Byte | SIG10000 | 90869451 |
| Standard 32Byte | SIG11000 | 12253571 |
| Standard 32Byte | SIG12000 | 13568297 |
| Standard 32Byte | SIG13000 | 96520389 |
| Standard 32Byte | SIG14000 | 57061117 |
+-----------------+----------+------------+
OCRA-1:HOTP-SHA256-8:QA08
+-----------------+----------+---------+------------+
| Key | Q | T | OCRA Value |
+-----------------+----------+---------+------------+
| Standard 64Byte | SIG10000 | 132d0b6 | 53166501 |
| Standard 64Byte | SIG11000 | 132d0b6 | 85946340 |
| Standard 64Byte | SIG12000 | 132d0b6 | 23754664 |
| Standard 64Byte | SIG13000 | 132d0b6 | 17883761 |
| Standard 64Byte | SIG14000 | 132d0b6 | 86963063 |
+-----------------+----------+---------+------------+
OCRA-1:HOTP-SHA512-8:QA10-T30S
Authors' Addresses
David M'Raihi
Verisign, Inc.
485 E. Middlefield Road
Mountain View, CA 94043
USA
Email: dmraihi@verisign.com
Johan Rydell
Portwise, Inc.
275 Hawthorne Ave, Suite 119
Palo Alto, CA 94301
USA
Email: johan.rydell@portwise.com
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Salah Machani
Diversinet Corp.
2225 Sheppard Avenue East, Suite 1801
Toronto, Ontario M2J 5C2
Canada
Email: smachani@diversinet.com
David Naccache
Ecole Normale Superieure
ENS DI, 45 rue d'Ulm
Paris, 75005
France
Email: david.naccache@ens.fr
Siddarth Bajaj
Verisign, Inc.
485 E. Middlefield Road
Mountain View, CA 94043
USA
Email: sbajaj@verisign.com
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