The Needham-Schroeder-Lowe protocol works as follows between the initiator I and responder R:

$I \longrightarrow R : \text{Enc}_{pk_R}(r_I, I)$

$R \longrightarrow I : \text{Enc}_{pk_I}(r_I, r_R, R)$

$I \longrightarrow R : \text{Enc}_{pk_R}(r_R)$

(I assume throughout that the encryption scheme is CCA-secure.)

Upon receiving their messages, both parties decrypt using the corresponding private key and finally output $r_I$ as their session key.

I understand others have proven that this protocol is SK-secure protocol, based on the CK01 model for SK-security. (As a reminder, the protocol is secure in this model if no adversary can distinguish, with probability strictly greater than $1/2$, between the output $r_I$ of the protocol and a random bit.) Security was proven using formal methods. My question: is there a proof of this using an explicit reduction?

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    $\begingroup$ I recommend that you flag this to be moved over to Crypto.SE. Also, I recommend that you provide a link to the specification of the NSL protocol, add a link to the CK01 paper, and, provide a definition of SK-security. Also, tell us what you've tried so far or how far you've gotten on your own, and why you think it "should be SK-secure". $\endgroup$
    – D.W.
    Feb 28, 2013 at 0:38
  • $\begingroup$ @D.W. NSL protocol has been shown to be secure using other techniques such as formal methods, e.g. see this(math.upenn.edu/~bana/NSL.pdf). It also achieves SK-security, so my question is that how can one use reduction to show it? $\endgroup$
    – Brian
    Feb 28, 2013 at 15:47
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    $\begingroup$ This question would benefit from a lot of additional work, to try to be clearer. I recommend you stick to standard notation/terminology. The protocol is commonly known as Needham-Schroeder-Lowe (I've never heard of anyone call it NSL). Normally we use $N_A$ for a nonce generated by $A$; you use $r_I$ but don't specify what that value is. You claim this has been proven to be SK-secure but don't provide any citation for this; the paper you cited never makes that claim. If you want a reduction, I suggest you specify what you want it reduced to (the CCA-security of the encryption scheme?). $\endgroup$
    – D.W.
    Mar 2, 2013 at 1:46

1 Answer 1


I am not an expert on this subject, but it looks to me like the question makes some assumptions that may or may not be valid. The question claims that the Needham-Schroeder-Lowe protocol is SK-secure. However, I don't know whether there is any known proof that Needham-Schroeder-Lowe is SK-secure. The paper that was cited in the question does not appear to claim that it is SK-secure. You might want to start by first checking whether there's a known proof of its SK-security.

It's possible you might find the following paper of some relevance:

It gives a computational, reduction-oriented proof of security for the Needham-Schroeder-Lowe protocol, assuming that the encryption algorithm is CCA-secure. However, I don't know what notion of security for key exchange algorithms this paper uses; I don't know if it proves SK-security, or something else.


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