STRONG security that fits everywhere. NTRUSign and P William Whyte,

STRONG security that fits everywhere. NTRU CRYPTOSYSTEMS, INC. COPYRIGHT © 2006 Summary  There’s a paper at Eurocrypt that presents an attack on one flavor of NTRUSign –  recommends a different flavor and it’s not clear whether this attack applies to the flavor  It seems appropriate to take some time to investigate this attack properly  In order not to slow down NTRUEncrypt standardization, suggest separating NTRUSign into a a standard and moving ahead with NTRUEncrypt in

STRONG security that fits everywhere. NTRU CRYPTOSYSTEMS, INC. COPYRIGHT © 2006 NTRUSign  Sign a message by applying the private key to it –This gradually leaks information about the private key –Important to quantify information leakage  Signing produces a lattice point that is close to the message  Verification: –Check that the signature is a lattice point –Check that it is sufficiently close to the message  Private key is a good lattice basis  Public key is a bad lattice basis –lets you check that points are in. lattice… –… but if you “sign” with it, error is much bigger than with private key

STRONG security that fits everywhere. NTRU CRYPTOSYSTEMS, INC. COPYRIGHT © 2006 Two flavors of signing  Unperturbed: –Hash the message to a point using a public hash function –Apply the private key  Perturbed: –Hash the message to a point using a public hash function –Apply a private perturbation function to move the message point slightly  “perturbed message point” –Apply the private key to the perturbed message point

STRONG security that fits everywhere. NTRU CRYPTOSYSTEMS, INC. COPYRIGHT © 2006 In pictures (note: animation) Unperturbed Perturbed Apply perturbation Sign perturbed point

STRONG security that fits everywhere. NTRU CRYPTOSYSTEMS, INC. COPYRIGHT © 2006 Differences between perturbed and unperturbed signatures  Perturbed signatures are bigger –Advantage of private key over public key is smaller –Requires larger keys for same security against forgery  Perturbed signatures are drawn from a more complicated distribution –Unperturbed signatures lie within a parallelopipied –Distribution can be transformed to a hypercube and symmetries exploited –Eurocrypt attack consists of transforming to a hypercube and finding a diagonal of the hypercube –No such transformation possible for perturbed case  Distribution much more like a sphere –Need to perform higher-moment averages and eliminate perturbations using linear algebra

STRONG security that fits everywhere. NTRU CRYPTOSYSTEMS, INC. COPYRIGHT © 2006 Security estimates  NTRU recommendation: –Only use unperturbed signing to generate 10,000 signatures or less –Use perturbed signing (with one perturbation) to generate up to a billion signatures –After this number of signatures, generate a new private key and throw the old one away –Recommendations based on theoretical analysis of information leakage from transcript  Very conservative! This number of signatures is considered to be almost certainly safe: dangerous to go much beyond it.  Eurocrypt paper: –With unperturbed signing, can recover private key after 90,000+ signatures  No application yet known to perturbed signing –Best attack yet demonstrated  Users who follow NTRU guidance would nevertheless be safe

STRONG security that fits everywhere. NTRU CRYPTOSYSTEMS, INC. COPYRIGHT © 2006 Implications of attack  As it stands, attack does not affect estimated security of parameter sets –Requires bigger transcript than allowed by NTRU guidelines for unperturbed case –Not known to apply to perturbed case  However, attack is quite new. –Unknown if it can be extended to perturbed case (although perturbed transcript is in a way fundamentally different from unperturbed) –Seems appropriate to allow some months to see if there’s an obvious extension

STRONG security that fits everywhere. NTRU CRYPTOSYSTEMS, INC. COPYRIGHT © 2006 Implications of attack for  PAR expires this year –Would like to get something completed –Including NTRUSign could jeopardize this  Suggest: –Keep NTRUEncrypt in –Move PAR for a, “Standard specifications for public key cryptography over lattices: additional techniques” –Move NTRUSign to this.

STRONG security that fits everywhere. NTRU CRYPTOSYSTEMS, INC. COPYRIGHT © 2006 Proposed timeline  Next week: –Circulate proposed a PAR –E-Motion to accept PAR and move NTRUSign to a  Next teleconference (2006/06?): Final talk through  Late 2006/06: First E-Motion to move to sponsor ballot  2006/08 meeting: Resolve comments arising from this first E-motion  2006/08: E-Motion to accept comment resolution and move to sponsor ballot  2006/10: Sponsor ballot opens  2006/11: Resolve sponsor ballot comments  2006/12: Recirculation ballot  2007/01: Submit to RevCom; switch focus back to a