I am writing code for an SSH server and can not get past the Elliptic Curve Diffie-Hellman Key Exchange Reply part of the connection. The client also closes the connection and says "Host Key does not match the signature supplied".
I am using putty as the client and a PIC micro-controller is running the server code.
From RFC 5656 [SSH ECC Algorithm Integration] :
"The hash H is formed by applying the algorithm HASH on a concatenation of the following:
string V_C, client's identification string (CR and LF excluded) string V_S, server's identification string (CR and LF excluded) string I_C, payload of the client's SSH_MSG_KEXINIT string I_S, payload of the server's SSH_MSG_KEXINIT string K_S, server's public host key string Q_C, client's ephemeral public key octet string string Q_S, server's ephemeral public key octet string mpint K, shared secret
the host key algorithm and key exchange algorithm is ecdsa-sha2-nistp256 and ecdh-sha2-nistp256 respectively.
referring to RFC 4251 for data type representations, as well as the source code in openSHH (openBSD) this is what I have concatenated.
- 4 bytes for then length of V_C followed by V_C
- 4 bytes for then length of V_S followed by V_S
- 4 bytes for length of I_C followed by I_C (payload is from Message Code to the start of Random Padding)
- 4 bytes for length of I_S followed by I_S (payload is from Message Code to the start of Random Padding)
- 4 bytes for the length of K_S followed by K_S (for K_S I used the same group of bytes that is used to calculate the fingerprint)
- 4 bytes for the length of Q_C followed by Q_C (i used the uncompressed string which has length of 65 - 04||X-coordinate||Y-coordinate)
- 4 bytes for the length of Q_S followed by Q_S
- 4 bytes for the length of K followed by K (length is 32 or 33 depending is the leading bit is set or not. If it is set then K is preceded by a 00 byte)
Once concatenated I hash it with SHA256 because I'm using NISTP256. SHA256 outputs 32 bytes which is the size of the curve, so I take the whole SHA256 output and perform the signature algorithm on it.
I can never get the correct signature from my message concatenation.
I know my signature algorithm is correct because given the message hash output I can get the correct signature. I know my shared secret is correct because I get the same output as online shared secret calculators. I know the SHA256 is correct because I get the same result using online calculators.
This leads me to assume the error is in the concatenation of the exchange hash.
Any help is greatly appreciated, thanks.