Non-PKI Methods for Public Key Distribution Authors: Mohammad Peyavian, Allen Roginsky and Nevenko Zunic Source: Computers & Security, Vol.23, pp.97-103, 2004. Adviser: Min-Shiang Hwang Speaker: Chun-Ta Li Date: 2004/10/28

Outline Introduction The first proposed scheme The second proposed scheme The third proposed scheme Conclusions Comments

Archived public key distribution without CA Introduction Server 1 2 3 3 2 CA Client 1 The X.509 PKI requires a huge and expensive infrastructure with complex operations. Archived public key distribution without CA

Introduction (cont.) ID: client’s user id -- not a secret value PKc: initial public component of the client’s public key pair SKc: initial secret component of the client’s public key pair PKs: initial public component of the server’s public key pair SKs: initial secret component of the server’s public key pair EPK(B): data B encrypted with an asymmetric public key PK. ESK(B): data B signed by an asymmetric secret key SK.

The first proposed scheme Public key distribution Client Server (1) generates (ID, PW) (2) sends (ID, PW) to client (3) sends ID, PKc, H(ID, PKc, PW) to server (4) sends ID, PKs, H(ID, PKs, PW) to server // PW used only once for authenticating the flows from the client and server

The first proposed scheme (cont.) The first scheme can be added to the top of current SSL implementations (PW-based authentication). PWc: client generates a password Client sends PWc to the server (e.g. on-line banking) Client Server (1) generates PWc (2) sends ID, ePKs(PWc), eSKc(H(ID, PWc)) to server

The first proposed scheme (cont.) Public key change if client’s SKc is compromised Client Server (1) sends ID, “SKc compromised”, eSKc(H(ID, “SKc compromised”) to server The client and server do not do any further exchange Until the client generates and sends a new public key to server The sending of a new public key is done as “Public key distribution” // If server public key is compromised, contrariwise

The first proposed scheme (cont.) Regular client public key change (periodically) Client Server (1) sends ID, new_PKc, eSKc(H(ID, new_PKc) to server Both the client and server start using the new client’s public key They won’t accept any message with the old public key // If server generates a new public key, contrariwise

The second proposed scheme P: prime modulus for Diffie-Hellman algorithm Rc: generates random number from the client Rs: generates random number from the server D: Diffie-Hellman public key S: symmetric secret key derived from Diffie-Hellman algorithm Given that the client and server share an ID and PW One-sided: Only the client needs to get the server’s public key (PKs). Two-sided: Both client and sever need to exchange public keys.

The second proposed scheme (cont.) Public key exchange protocol Client Server (1) generates P, Rc and computes public key Dc Dc = PWRc mod P (2) sends ID, Dc, P to server (3) computes public key Ds Ds = PWRs mod P (4) computes symmetric secret key S S = DcRs mod P = PWRcRs mod P (5) sends ID, [PKs], Ds, H(ID, Dc, P, [PKs], Ds, S) to client (6) computes symmetric secret key S S = DsRc mod P = PWRcRs mod P (7) verifies H(ID, Dc, P, [PKs], Ds, S) using the S value that is derived (8) sends ID, [PKc], H(ID, PKs, Ds, [PKc], new_PW, S), [eS(new_PW)] to server

The second proposed scheme (cont.) Public key change if client secret key is compromised Client Server (1) sends ID, “SKc compromised”, eSKc (H(ID, “SKc compromised”) to server The client and server do not do any further exchange Until the client generates and sends a new public key to server The sending of a new public key is done as “Public key distribution” // If server public key is compromised, contrariwise

The second proposed scheme (cont.) Regular client public key change (periodically) Client Server (1) sends ID, new_PKc, eS (H(ID, new_PKc) to server Both the client and server start using the new client’s public key They won’t accept any message with the old public key // If server generates a new public key, contrariwise

The third proposed scheme Public key exchange protocol Given that the client and server share an ID and PW Client Server (1) sends ID, PKc, H(ID, PKc, PW, Rc) to server (2) sends ID, PKs, H(ID, PKs, PW, Rs) to server (3) sends ID, ePKs(Rc) to server (4) sends ID, ePKc(Rs) to server // PW used only once for authenticating the flows from the client and server

The third proposed scheme (cont.) The third scheme can be added to the top of current SSL implementations (PW-based authentication). PWc: client generates a password Client sends PWc to the server (e.g. on-line banking) Client Server (1) generates PWc (2) sends ID, ePKs(PWc, Rc), eSKc(H(ID, PWc, Rc)) to server

Conclusions The proposed scheme can distribute the public key without CA. This paper is to present alternative simpler solutions to the X.509 PKI to save storage, bandwidth and to reduce the complexity of the operations.

Comments How to send the PKs and PKc to the client and the server in secure? (The first scheme) Attacker can masquerade server and client to send the wrong PKs` (pair of SKs`) and wrong PKc` (pair of SKc`) Attacker will require the PKc Attacker will require the PWc , because of the client encrypt it by using the wrong PKs`

Client Attacker Server Comments (cont.) Man-in-the-middle attack (The second scheme) Public key exchange protocol Client Attacker Server (1) generates P, Rc and computes public key Dc Dc = PWRc mod P (2) sends ID, Dc`, P to server (3) computes public key Ds Ds = PWRs mod P Dc` = DcRt = PWRcRt mod P (4) computes symmetric secret key S` S` = Dc`Rs mod P = PWRcRtRs mod P Ds` = DsRt = PWRsRt mod P (5) sends ID, [PKs], Ds`, H(ID, Dc, P, [PKs], Ds`, S`) to client (6) computes symmetric secret key S S` = Ds`Rc mod P = PWRsRtRc mod P (7) verifies H(ID, Dc, P, [PKs], Ds`, S`) using the S` value that is derived (8) sends ID, [PKc], H(ID, PKs, Ds, [PKc], new_PW`, S`), [eS`(new_PW`)] to server

Comments (cont.) How to send the PKs and PKc to the client and the server in secure? (The third scheme) Attacker can masquerade server and client to send the wrong PKs` (pair of SKs`) and wrong PKc` (pair of SKc`) Attacker will require the PKc Attacker will require the PWc , because of the client encrypt it by using the wrong PKs`

Thanks for your attention

Cryptanalysis of the first proposed scheme Public key distribution Client Attacker Server (1) generates (ID, PW) (2) sends (ID, PW) to client (3) sends ID, PKc, H(ID, PKc, PW) to server (3`) sends ID, PKc`, H(ID, PKc`, PW) to server (4) sends ID, PKs, H(ID, PKs, PW) to server (4`) sends ID, PKs`, H(ID, PKs`, PW) to server // PW used only once for authenticating the flows from the client and server

Cryptanalysis of the first proposed scheme (cont.) The first scheme can be added to the top of current SSL implementations (PW-based authentication). PWc`: attacker generates a password Attacker sends PWc` to the server Client Server without change (1) generates PWc (2`) sends ID, ePKs(PWc`), eSKc`(H(ID, PWc`)) to server (2) sends ID, ePKs(PWc), eSKc(H(ID, PWc)) to server

Cryptanalysis of the third proposed scheme Public key exchange protocol Given that the client and server share an ID and PW Client Attacker Server (1) sends ID, PKc, H(ID, PKc, PW, Rc) to server (1`) sends ID, PKc`, H(ID, PKc`, PW, Rc`) to server (2) sends ID, PKs, H(ID, PKs, PW, Rs) to server (2`) sends ID, PKs`, H(ID, PKs`, PW, Rs`) to server (3) sends ID, ePKs(Rc) to server (3`) sends ID, ePKs(Rc`) to server (4) sends ID, ePKc(Rs) to server (4`) sends ID, ePKc(Rs`) to server // PW used only once for authenticating the flows from the client and server

Cryptanalysis of the third proposed scheme (cont.) The third scheme can be added to the top of current SSL implementations (PW-based authentication). PWc`: attacker generates a password Attacker sends PWc` to the server Client Attacker Server (1) generates PWc (2) sends ID, ePKs(PWc, Rc), eSKc(H(ID, PWc, Rc)) to server (2`) sends ID, ePKs(PWc`, Rc`), eSKc`(H(ID, PWc`, Rc`)) to server without change