Otp methods for tls
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OTP Methods for TLS. OTPS Workshop February, 2006. Background. Work item suggested at Vienna workshop Enable use of One-Time Passwords in TLS Leverages cipher suites defined in IETF RFC 4279 (TLS-PSK) and TLS extensions defined in IETF RFC 3546 (RFC 4366). Approach.

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OTP Methods for TLS

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Otp methods for tls

OTP Methods for TLS

OTPS Workshop

February, 2006


Background

Background

  • Work item suggested at Vienna workshop

  • Enable use of One-Time Passwords in TLS

    • Leverages cipher suites defined in IETF RFC 4279 (TLS-PSK) and TLS extensions defined in IETF RFC 3546 (RFC 4366)


Approach

Approach

  • Extensions are not needed if challenge is not needed and hardening unnecessary/implicit

  • In ClientHello, client may use new TLS extensions to:

    • Request challenge data

    • Suggest OTP hardening parameters

  • In ServerHello, server may use new TLS extensions to:

    • Provide challenge data (if asked for)

    • Determine hardening parameters (if suggested)


From otp to psk i

From OTP to PSK (I)

  • PSK = PBKDF2 (OTP, RS || RC, iterationCount, keyLen)

    Where:

    • OTP is the current one-time password,

    • RS is the server_random value from the Server Hello

    • RC is the client_random value from the Client Hello

    • iterationCount is the iteration_count value from the otp_hardening extension

    • keyLen shall be set to 16 (128 bits).


From otp to psk ii

From OTP to PSK (II)

  • Three choices for key exchange scheme

    • Direct PSK: Likely requires OTP hardening

    • Ephemeral D-H: OTP hardening too, if MITM is a concern

    • RSA: No need for OTP hardening, but requires PKI


The challenge data extension

The Challenge_Data extension

struct {

ChallengeDataType challenge_data_type;

select (ChallengeDataType) {

case request: ChallengeRequestData;

case response: ChallengeResponseData;

} challenge_data;

} ChallengeData;

struct {

opaque otp_algorithm<0..2^16-1>;

opaque otp_user_id<0..2^16-1>;

opaque otp_key_id<0..2^16-1>;

} ChallengeRequestData;

struct {

opaque otp_challenge<1..2^16-1>;

} ChallengeResponseData;


The otp hardening extension

The OTP_Hardening extension

struct {

uint16 iteration_count;

} OTPHardeningData;


Identifying the key

Identifying the key

  • Proposal is to use the “psk_identity” component of the ClientKeyExchange message

    • May also carry e.g. current counter value or time

  • Example:

    "UI=J. Random User, T=20051222114204"


Identified issues

Identified Issues

  • PIN handling

    • Combine PIN and OTP before calculating PSK?

      • I.e. PSK = PBKDF2(f(OTP,PIN),…)

    • Treat PIN as out of scope for this document?

      • Weakens the PSK but allows PIN processing outside of TLS layer

  • New Error Alerts?

    • E.g. “Next OTP”?


Next steps

Next Steps

  • Decide on PIN handling, Error alerts

  • Resolve any other comments

  • Produce new draft version

    • Preferably within 4 -5 weeks


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