Fix key material extension during key exchange #1689
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When the key exchange produces less key material than is needed for the cipher or hmac algorithms, there is an iterative procedure to produce more. For example, a SHA-1 key exchange algorithm produces 20 bytes of key material. A SHA-256 hmac uses a 32 byte key, so one iteration of the procedure produces another 20 bytes of key material for a total of 40 which is sufficient for the hmac key. The library works correctly in such cases of one iteration, but the logic is wrong if more than one iteration is needed. In sshnet#1660, the connection uses a SHA-1 kex algorithm with a SHA-512 hmac (64 byte key), requiring 3 iterations of the extension procedure and resulting in an error upon connection. This change fixes the logic to use the output of the previous iteration per the spec. closes sshnet#1660
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When the key exchange produces less key material than is needed for the cipher or hmac algorithms, there is an iterative procedure to produce more.
For example, a SHA-1 key exchange algorithm produces 20 bytes of key material. A SHA-256 hmac uses a 32 byte key, so one iteration of the procedure produces another 20 bytes of key material for a total of 40 which is sufficient for the hmac key.
The library works correctly in such cases of one iteration, but the logic is wrong if more than one iteration is needed. In #1660, the connection uses a SHA-1 kex algorithm with a SHA-512 hmac (64 byte key), requiring 3 iterations of the extension procedure and resulting in an error upon connection.
This change fixes the logic to use the output of the previous iteration per the spec.
closes #1660