INCOMPATIBLE CHANGE to the SSH2 private key file format. There is

now a passphrase-keyed MAC covering _all_ important data in the
file, including the public blob and the key comment. Should
conclusively scupper any attacks based on nobbling the key file in
an attempt to sucker the machine that decrypts it. MACing the
comment field also protects against a key-substitution attack (if
someone's worked out a way past our DSA protections and can extract
the private key from a signature, swapping key files and
substituting comments might just enable them to get the signature
they need to do this. Paranoid, but might as well).

[originally from svn r1413]

ssh.h

@@ -355,3 +355,9 @@ int zlib_decompress_block(unsigned char *block, int len,
 #define SSH2_AGENTC_ADD_IDENTITY                17
 #define SSH2_AGENTC_REMOVE_IDENTITY             18
 #define SSH2_AGENTC_REMOVE_ALL_IDENTITIES       19
+
+/*
+ * Need this to warn about support for the original SSH2 keyfile
+ * format.
+ */
+void old_keyfile_warning(void);