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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

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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