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author | Aris Adamantiadis <aris@0xbadc0de.be> | 2013-09-22 01:20:08 +0200 |
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committer | Aris Adamantiadis <aris@0xbadc0de.be> | 2013-09-27 15:32:52 +0200 |

commit | 04fd756c496e6fa435356c679c16b5dfbbd1806d (patch) | |

tree | 7311fa7280032c313ccf53f2d517efeac5983373 /doc | |

parent | 4cb6afcbd43ab503d4c3d3054b96a1492605ea8d (diff) | |

download | libssh-04fd756c496e6fa435356c679c16b5dfbbd1806d.tar.gz libssh-04fd756c496e6fa435356c679c16b5dfbbd1806d.tar.xz libssh-04fd756c496e6fa435356c679c16b5dfbbd1806d.zip |

doc: Documentation of curve25519-sha256@libssh.org

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-rw-r--r-- | doc/curve25519-sha256@libssh.org.txt | 119 |

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diff --git a/doc/curve25519-sha256@libssh.org.txt b/doc/curve25519-sha256@libssh.org.txt new file mode 100644 index 00000000..e68dc5e6 --- /dev/null +++ b/doc/curve25519-sha256@libssh.org.txt @@ -0,0 +1,119 @@ +curve25519-sha256@libssh.org.txt Aris Adamantiadis <aris@badcode.be> + 21/9/2013 + +1. Introduction + +This document describes the key exchange methode curve25519-sha256@libssh.org +for SSH version 2 protocol. It is provided as an alternative to the existing +key exchange mechanisms based on either Diffie-Hellman or Elliptic Curve Diffie- +Hellman [RFC5656]. +The reason is the following : During summer of 2013, revelations from ex- +consultant at NSA Edward Snowden gave proof that NSA willingly inserts backdoors +into softwares, hardware components and published standards. While it is still +believed that the mathematics behind ECC cryptography are still sound and solid, +some people (including Bruce Schneier [SCHNEIER]), showed their lack of confidence +in NIST-published curves such as nistp256, nistp384, nistp521, for which constant +parameters (including the generator point) are defined without explanation. It +is also believed that NSA had a word to say in their definition. These curves +are not the most secure or fastest possible for their key sizes [DJB], and +researchers think it is possible that NSA have ways of cracking NIST curves. +It is also interesting to note that SSH belongs to the list of protocols the NSA +claims to be able to eavesdrop. Having a secure replacement would make passive +attacks much harder if such a backdoor exists. + +However an alternative exists in the form of Curve25519. This algorithm has been +proposed in 2006 by DJB [Curve25519]. Its main stengths are its speed, its +constant-time run time (and resistance against side-channel attacks), and its +lack of nebulous hard-coded constants. + +The reference version being used in this document is the one described in +[Curve25519] as implemented in the library NaCl [NaCl]. +This document does not attempts to provide alternatives to the ecdsa-sha1-* +authentication keys. + +2. Key exchange + +The key exchange procedure is very similar to the one described chapter 4 of +[RFC5656]. Public ephemeral keys are transmitted over SSH encapsulated into +standard SSH strings. + +The following is an overview of the key exchange process: + +Client Server +------ ------ +Generate ephemeral key pair. +SSH_MSG_KEX_ECDH_INIT --------> + Verify that client public key + length is 32 bytes. + Generate ephemeral key pair. + Compute shared secret. + Generate and sign exchange hash. + <-------- SSH_MSG_KEX_ECDH_REPLY +Verify that server public key length is 32 bytes. +* Verify host keys belong to server. +Compute shared secret. +Generate exchange hash. +Verify server's signature. + +* Optional but strongly recommanded as this protects against MITM attacks. + +This is implemented using the same messages as described in RFC5656 chapter 4 + +3. Method Name + +The name of this key exchange method is "curve25519-sha256@libssh.org". + +4. Implementation considerations + +The whole method is based on the curve25519 scalar multiplication. In this +method, a private key is a scalar of 256 bits, and a public key is a point +of 256 bits. + +4.1. Private key generation + +A 32 bytes private key should be generated for each new connection, + using a secure PRNG. The following actions must be done on the private key: + mysecret[0] &= 248; + mysecret[31] &= 127; + mysecret[31] |= 64; +In order to keep the key valid. However, many cryptographic libraries will do +this automatically. +It should be noted that, in opposition to NIST curves, no special validation +should be done to ensure the result is a valid and secure private key. + +4.2 Public key generation + +The 32 bytes public key of either a client or a server must be generated using +the 32 bytes private key and a common generator base. This base is defined as 9 +followed by all zeroes: + const unsigned char basepoint[32] = {9}; + +The public key is calculated using the cryptographic scalar multiplication: + const unsigned char privkey[32]; + unsigned char pubkey[32]; + crypto_scalarmult (pubkey, privkey, basepoint); +However some cryptographic libraries may provide a combined function: + crypto_scalarmult_base (pubkey, privkey); + +It should be noted that, in opposition to NIST curves, no special validation +should be done to ensure the received public keys are valid curves point. The +Curve25519 algorithm ensure that every possible public key maps to a valid +ECC Point. + +4.3 Shared secret generation + +The shared secret, k, is defined in SSH specifications to be a big integer. +This number is calculated using the following procedure: + + X is the 32 bytes point obtained by the scalar multiplication of the other + side's public key and the local private key scalar. + + The whole 32 bytes of the number X are then converted into a big integer k. + This conversion follows the network byte order. This step differs from + RFC5656. + +[RFC5656] http://tools.ietf.org/html/rfc5656 +[SCHNEIER] https://www.schneier.com/blog/archives/2013/09/the_nsa_is_brea.html#c1675929 +[DJB] http://cr.yp.to/talks/2013.05.31/slides-dan+tanja-20130531-4x3.pdf +[Curve25519] "Curve25519: new Diffie-Hellman speed records." + http://cr.yp.to/ecdh/curve25519-20060209.pdf
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