commit e41c19325bdcf967841a81dfef7083a089429853 Author: Nick Mathewson nickm@torproject.org Date: Wed May 11 15:43:29 2011 -0400
incoporate stuff based on comments from Ian Goldberg --- proposals/ideas/xxx-ntor-handshake.txt | 33 ++++++++++++++++++++++++------- 1 files changed, 25 insertions(+), 8 deletions(-)
diff --git a/proposals/ideas/xxx-ntor-handshake.txt b/proposals/ideas/xxx-ntor-handshake.txt index 54c81b0..41af5c7 100644 --- a/proposals/ideas/xxx-ntor-handshake.txt +++ b/proposals/ideas/xxx-ntor-handshake.txt @@ -27,6 +27,10 @@ Notation: byte strings, are all G_LENGTH bytes long. Let's say we are using a generator g for this group.
+ Let a,A=KEYGEN() yield a new private-public keypair in G, where a is the + secret key and A = EXP(g,a). If additional checks are needed to insure + a valid keypair, they should be performed. + Let PROTOID be a string designating this variant of the protocol.
Let KEYID be a collision-resistant (but not necessarily preimage-resistant) @@ -34,13 +38,17 @@ Notation:
Instantiation:
- Let's call this PROTOID "ntor-curve25519-sha256-1" + Let's call this PROTOID "ntor-curve25519-sha256-1" (We might want to make + this shorter if it turns out to save us a block of hashing somewhere.)
Set H(x,t) == HMAC_SHA256 with message x and key t. So H_LENGTH == 32. Set t_mac == PROTOID | ":mac" t_key == PROTOID | ":key" t_verify == PROTOID | ":verify" - Set EXP(a,b) == curve25519(a,b), and g == 9 . + + Set EXP(a,b) == curve25519(.,b,a), and g == 9 . Let KEYGEN() do the + appropriate manipulations when generating the secret key (clearing the + low bits, twiddling the high bits).
Set KEYID(B) == B. (We don't need to use a hash function here, since our keys are already very short. It is trivially collision-resistant, since @@ -51,16 +59,16 @@ Protocol: Take a router with identity key digest ID.
As setup, the router generates a secret key b, and a public onion key - B = EXP(g,b). The router publishes B in its server descriptor. + B with b, B = KEYGEN(). The router publishes B in its server descriptor.
- To send a create cell, the client generates a keypair of x, X=EXP(g,x) and + To send a create cell, the client generates a keypair x,X = KEYGEN(), and sends a CREATE cell with contents:
NODEID: ID -- H_LENGTH bytes KEYID: KEYID(B) -- H_LENGTH bytes CLIENT_PK: X -- G_LENGTH bytes
- The server checks X, generates a keypair of y, Y=EXP(g,y) and computes + The server generates a keypair of y,Y = KEYGEN(), and computes
secret_input = EXP(X,y) | EXP(X,b) | ID | B | X | Y | PROTOID KEY_SEED = H(secret_input, t_key) @@ -72,7 +80,7 @@ Protocol: SERVER_PK: Y -- G_LENGTH bytes AUTH: H(auth_input, t_mac) -- H_LENGTH byets
- The client then checks Y, and computes + The client then checks Y is in G^* [see below], and computes
secret_input = EXP(Y,x) | EXP(B,x) | ID | B | X | Y | PROTOID KEY_SEED = H(secret_input, t_key) @@ -81,6 +89,9 @@ Protocol:
The client verifies that AUTH == H(auth_input, t_mac).
+ [NOTE: It may be adequate to check that EXP(Y,x) is not the point at + infinity. See tor-dev thread.] + Both parties now have a shared value for KEY_SEED. They expand this into the keys needed for the Tor relay protocol.
@@ -94,9 +105,15 @@ Key expansion:
Instead, let's have it be
- K = H(KEY_SEED, t_expand1) | H(KEY_SEED, t_expand2) | ... + K = K_0 | K_1 | K_2 | K_3 | ... + + Where K_0 = H(m_expand | INT8(i) , KEY_SEED ) + and K_(i+1) = H(K_i | m_expand | INT8(i) , KEY_SEED ) + and m_expend is an arbitrarily chosen value, + and INT8(i) is a octet with the value "i".
- where t_expand1..N are tweaks for the hash. + Ian says this is due to a construction from Krawczyk at + http://eprint.iacr.org/2010/264 .
Performance notes: