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484db3 Samuli Seppänen 2025-02-11 11:52:28 1 
# Changed Hex Bytes in the Static Key, the Key Still Connects to a Remote Peer Using the Original Key
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**NOTE: this article is very outdated. Use of static key encryption is discouraged and deprecated. If you want something similar but more secure, look into [peer fingerprinting](https://github.com/openvpn/openvpn/blob/master/doc/man-sections/example-fingerprint.rst).**
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Source: [OpenVPN Mailing List](https://sourceforge.net/p/openvpn/mailman/message/4256045/)
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From: James Yonan <jim@yo...> - 2004-10-23 21:46:24
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On Sat, 23 Oct 2004, Alberto Gonzalez Iniesta wrote:
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> Hello James et al,
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>
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> I've just got a bug report at the Debian Bug Tracking System. I'm pretty sure it's bogus, but I think it's out of my knowledge to classify it as such.
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>
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> The bug reporter claims that he can modify almost all of the pre-shared key, and the tunnel still gets established. I'm pretty sure this has something to do with only a portion of the pre-shared key being used if no direction is specified, but I'd love if anyone could give a more thorough explanation (please Cc: 277838@... when doing so).
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>
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> Thanks,
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>
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> Alberto
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>
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> [1] http://bugs.debian.org/277838
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>>When I modify the Preshared 2048 bit Static Key on the Initiator Side of the
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>>Tunnel (don't tested the other way) I'm anyhow able to establish the
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>>Tunnel and send Packets through the Tunnel. I don't understand
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>>the Key splitting and handling as described below, but I think the Keys
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>>on both Sides of the Tunnel should be identical for the Tunnel to be
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>>established.
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>>
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>>I can modify every Char in Line 2,3,4,7,8,9,10,11,12,13,14,15,16 without
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>>any effect and think this is possible a Bug.
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>>
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>>Bye
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>>Klaus
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**Okay, see my response below.**
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No, this is not a bug. The 2048-bit static key is designed to be large enough to allow 512-bit encrypt, decrypt, HMAC send, and HMAC receive keys to be extracted from it.
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However, this key size is far too large for current conventional OpenVPN usage. OpenVPN uses the 128-bit blowfish cipher by default. It also uses the 160-bit HMAC-SHA1 as a cryptographic signature on packets to protect against tampering. Since you probably didn't specify a key direction parameter, the encrypt/decrypt keys for both directions are the same and the HMAC keys for both directions are also the same.
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That means that OpenVPN is only actually using 128 + 160 = 288 bits out of the file -- much less than the 2048 bits which are available.
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Below, I will show a sample 2048-bit OpenVPN key, bracketed to show which bits are actually used for key material, assuming default crypto settings:
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```
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#
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# 2048 bit OpenVPN static key
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#
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-----BEGIN OpenVPN Static key V1-----
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[eac9ae92cd73c5c2d6a2338b5a22263a] -> 128 bits for cipher
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4ef4a22326d2a996e0161d25d41150c8
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38bebc451ccf8ad19c7d1c7ce09742c3
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2047ba60f1d97d47c88f7ab0afafb2ce
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[f702cb04c7d15ff2606736c1825e830a -> 160 bits for HMAC SHA1
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7e30a796] 4b82825d6767a04b3c8f4583
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d4928127262c3a8603776bd6da339f69
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dece3bbfee35f1dceb7cbceaef4c6933
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2c2cef8ac550ed15213b216b825ab31e
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49840f99ff9df3c5f31156439ed6b99c
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4fc1bff417d33d77134365e38c9d71cd
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e294ba6e65d51703d6d4a629d5fc618e
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adddb889b8173ac79b4261328770bbbe
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74294bc79e357c82af9ef53f2968be6a
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007e6022da0a1a39f2ed5660f94a5926
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35d72e5838dd78dd680d91f6edcf6988
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-----END OpenVPN Static key V1-----
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```
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As you can see, the only lines actually used are 1, 5, and 6. And of course, that matches up perfectly with what you observed.
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To verify this, run OpenVPN as follows:
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```
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openvpn --dev null --verb 7 --secret key | grep 'crypt:'
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```
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where 'key' is a file containing the key shown above.
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```
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Static Encrypt: Cipher 'BF-CBC' initialized with 128 bit key
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Static Encrypt: CIPHER KEY: eac9ae92 cd73c5c2 d6a2338b 5a22263a
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Static Encrypt: Using 160 bit message hash 'SHA1' for HMAC authentication
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Static Encrypt: HMAC KEY: f702cb04 c7d15ff2 606736c1 825e830a 7e30a796
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Static Decrypt: Cipher 'BF-CBC' initialized with 128 bit key
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Static Decrypt: CIPHER KEY: eac9ae92 cd73c5c2 d6a2338b 5a22263a
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Static Decrypt: Using 160 bit message hash 'SHA1' for HMAC authentication
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Static Decrypt: HMAC KEY: f702cb04 c7d15ff2 606736c1 825e830a 7e30a796
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```
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Note that the keys which are shown in the OpenVPN output exactly match the bracketed section of the key source.
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Now if you want to use more of the key material, it is possible to use two sets of encrypt/HMAC keys, one for each direction:
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```
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openvpn --dev null --verb 7 --secret key 0 | grep 'crypt:'
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```
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(Note that the '0' after key chooses one symmetrical direction -- the opposite peer would use a '1' to choose the other direction).
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```
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Static Encrypt: Cipher 'BF-CBC' initialized with 128 bit key
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Static Encrypt: CIPHER KEY: eac9ae92 cd73c5c2 d6a2338b 5a22263a
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Static Encrypt: Using 160 bit message hash 'SHA1' for HMAC authentication
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Static Encrypt: HMAC KEY: f702cb04 c7d15ff2 606736c1 825e830a 7e30a796
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Static Decrypt: Cipher 'BF-CBC' initialized with 128 bit key
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Static Decrypt: CIPHER KEY: 2c2cef8a c550ed15 213b216b 825ab31e
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Static Decrypt: Using 160 bit message hash 'SHA1' for HMAC authentication
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Static Decrypt: HMAC KEY: adddb889 b8173ac7 9b426132 8770bbbe 74294bc7
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```
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Now notice that the Encrypt and Decrypt keys are no longer identical. The encrypt keys are drawing key material from lines 1, 5, and 6 in the key file, while the decrypt keys are drawing from lines 9, 13, and 14.
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Now the opposite peer will use a key-direction of 1:
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```
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openvpn --dev null --verb 7 --secret key 1 | grep 'crypt:'
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```
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```
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Static Encrypt: Cipher 'BF-CBC' initialized with 128 bit key
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Static Encrypt: CIPHER KEY: 2c2cef8a c550ed15 213b216b 825ab31e
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Static Encrypt: Using 160 bit message hash 'SHA1' for HMAC authentication
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Static Encrypt: HMAC KEY: adddb889 b8173ac7 9b426132 8770bbbe 74294bc7
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Static Decrypt: Cipher 'BF-CBC' initialized with 128 bit key
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Static Decrypt: CIPHER KEY: eac9ae92 cd73c5c2 d6a2338b 5a22263a
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Static Decrypt: Using 160 bit message hash 'SHA1' for HMAC authentication
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Static Decrypt: HMAC KEY: f702cb04 c7d15ff2 606736c1 825e830a 7e30a796
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```
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Notice how the Encrypt and Decrypt keys are swapped, in relation to the key-direction 1 example.
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So you might ask why is the OpenVPN static key file so large, if such a small percentage of the bits are currently used? The answer is to accommodate future ciphers and HMAC hashes which use large keys. Changing a file format is obviously problematic from a compatibility perspective, so 2048 bits were chosen so that two sets of 512-bit encrypt and HMAC keys could be derived for two separate key directions.