Introduction

Instructions for widely used cryptography clients.

GnuPG is recommended.

GNU Privacy Guard

GnuPG is a well known open source cryptography tool with OpenPGP support. Always use the latest version.

GnuPG has a good set of documentation. This guide illustrates only some important points. Please read the manual.

GnuPG Home

GnuPG stores important files in a home directory including keyrings and configuration files. This may be specified using an environmental variable or on the command line. This allows different configurations and keys to be used.

For example:

$ gpg --homedir /home/alice/keys --list-keys

It is more typical to rely on the default. For *nux (linux, BSD, MaxOSX, Solaris, AIX) this is:.

$HOME/.gnupg

How To Switch Home

The home can also be set using an envionmental variable. This allows a different configuration and keyring to be selected for the duration of the command line session. This is useful when practicing as well as when using multiple keyrings.

For example, to set home directory to alice when using Linux:

$ export GNUPGHOME=alice

When switching key rings, check that the required keyring has been selected by examining the secret keys. For example:

$ gpg --list-secret-keys
alice/secring.gpg
-----------------

sec   4096R/E2B054B8 2009-08-20
uid                  Alice Example (EXAMPLE NEW KEY) <alice@example.org>
ssb   4096R/4A6D5217 2009-08-20

Configuration

GnuPG supports a wide range of configuration options. These can be specified on the command line but it is usually more convenient to set them in the gpg.conf file. By default, this is located in the GnuPG Home directory.

How To Avoid SHA-1

Use of SHA-1 should now be avoided. Until SHA3 is available, SHA512 or SHA256 should be used instead. SHA512 is stronger than SHA256. Though some old clients lack SHA512 support, switching to SHA512 is recommended since it is more likely to be strong enough bridge the gap until SHA3.

Setting Defaults

To configure gpg to avoid SHA-1 edit the options in gpg.conf. Options need to be added (or - when they exist - the existing values replaced) for:

  • cert-digest-algo - the certificate digest used when linking into the web of trust

  • personal-digest-preferences - the digest used for signing messages

  • default-preference-list - the default algorithm preferences for new keys

To use SHA512 (recommended):

personal-digest-preferences SHA512
cert-digest-algo SHA512
default-preference-list SHA512 SHA384 SHA256 SHA224 AES256 AES192 AES CAST5 ZLIB BZIP2 ZIP Uncompressed

To use SHA256:

personal-digest-preferences SHA256
cert-digest-algo SHA256
default-preference-list SHA512 SHA384 SHA256 SHA224 AES256 AES192 AES CAST5 ZLIB BZIP2 ZIP Uncompressed

Setting Preferences For Existing Keys

The digest preferences for each key are set (from the configuration defaults ) when the key is generated. Once the configuration has been updated to avoid SHA-1, all new keys generated will use these defaults.

All existing private keys in the ring need to updated to indicate that stronger hashes are preferred. For each public-private key pair (generated with the previous defaults):

$ gpg --edit-key F8B7B4FD
gpg (GnuPG) 1.4.9; Copyright (C) 2008 Free Software Foundation, Inc.
This program comes with ABSOLUTELY NO WARRANTY.
This is free software, and you are welcome to redistribute it
under certain conditions. See the file COPYING for details.

Secret key is available.

pub  1024D/F8B7B4FD  created: 2009-08-12  expires: 2009-09-11  usage: SC  
                     trust: ultimate      validity: ultimate
sub  1024g/D55BD150  created: 2009-08-12  expires: 2009-09-11  usage: E   
[ultimate] (1). Example Key (NOT FOR DISTRIBUTION) <bogus@example.org>

Command> showpref
[ultimate] (1). Example Key (NOT FOR DISTRIBUTION) <bogus@example.org>
     Cipher: AES256, AES192, AES, CAST5, 3DES
     Digest: SHA1, SHA256, RIPEMD160
     Compression: ZLIB, BZIP2, ZIP, Uncompressed
     Features: MDC, Keyserver no-modify

Command>  setpref SHA512 SHA384 SHA256 SHA224 AES256 AES192 AES CAST5 ZLIB BZIP2 ZIP Uncompressed
Set preference list to:
     Cipher: AES256, AES192, AES, CAST5, 3DES
     Digest: SHA512, SHA384, SHA256, SHA224, SHA1
     Compression: ZLIB, BZIP2, ZIP, Uncompressed
     Features: MDC, Keyserver no-modify
Really update the preferences? (y/N) y

You need a passphrase to unlock the secret key for
user: "Example Key (NOT FOR DISTRIBUTION) <bogus@example.org>"
1024-bit DSA key, ID F8B7B4FD, created 2009-08-12

pub  1024D/F8B7B4FD  created: 2009-08-12  expires: 2009-09-11  usage: SC  
                     trust: ultimate      validity: ultimate
sub  1024g/D55BD150  created: 2009-08-12  expires: 2009-09-11  usage: E   
[ultimate] (1). Example Key (NOT FOR DISTRIBUTION) <bogus@example.org>

Command> save

Then upload the modified public key to a public keyserver. For example:

$ gpg --send-keys F8B7B4FD

How To Generate A Strong Key

The weaknesses found in SHA-1 threaten all DSA keys and those RSA keys with length less than 2048 bits. Though no realistic attack against those keys have been made public and these keys continue to be useful (and do not need to be revoked), no new keys should be generated which are exposed to this weakness.

The next generation of OpenPGP will use SHA-3 when this is ready. It is uncertain how long this process will take. It is likely that 2048 bit RSA keys with SHA256 hash will be strong enough for this interim period - but not certain. For those with 2048 bit RSA keys, the best advice is to wait (after switching to SHA256 or SHA512, of course). All new keys generated should be RSA with at least 4096 bits.

Though 8192 bit keys are stronger, they are slower and may be incompatible with some older clients. For the present, 4096 bit RSA should be strong enough for code signing at Apache. To generate RSA keys with length more than 4096 bits, some changes to the source are needed then the procedure for 4096 bits can be followed.

Install And Configure GnuPG

GnuPG comes in two flavours:

  • 1.x (well known and portable version)

  • 2.x (feature enhanced)

To easily generate a 4096 bit RSA signing and encryption key pair with strong digests, use GnuPG version:

  • 2.0.12 (or higher)

  • 1.4.10 (or higher)

Once generated, this key can be used with the widely available 1.4.9 and 2.x releases. If the right version of GnuPG is not currently distributed with your platform, it needs to be installed. Note that this version is only required for key generation. So, replacing the version distributed with your platform is not necessary. The new version may be installed just into a working directory just to generate the new key.

Start by checking that the installation has worked and that the version is correct, either

$ gpg  --version 
gpg (GnuPG) 1.4.10
Copyright (C) 2008 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later
<http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Home: ~/.gnupg
Supported algorithms:
Pubkey: RSA, RSA-E, RSA-S, ELG-E, DSA
Cipher: 3DES, CAST5, BLOWFISH, AES, AES192, AES256, TWOFISH, CAMELLIA128, 
        CAMELLIA192, CAMELLIA256
Hash: MD5, SHA1, RIPEMD160, SHA256, SHA384, SHA512, SHA224
Compression: Uncompressed, ZIP, ZLIB, BZIP2

or

$ gpg2 --version
gpg (GnuPG) 2.0.12
libgcrypt 1.4.4
Copyright (C) 2009 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later
<http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Home: ~/.gnupg
Supported algorithms:
Pubkey: RSA, ELG, DSA
Cipher: 3DES, CAST5, BLOWFISH, AES, AES192, AES256, TWOFISH, CAMELLIA128, 
        CAMELLIA192, CAMELLIA256
Hash: MD5, SHA1, RIPEMD160, SHA256, SHA384, SHA512, SHA224
Compression: Uncompressed, ZIP, ZLIB, BZIP2

Now check that the configuration file is setup to avoid SHA-1.

Generate New Key

A new key generation option - RSA and RSA - was introduced in ( 2.0.12 and 1.4.10 ). This is now the default. RSA keys are used for both encryption and signing. Longer key lengths are available. This option should be selected when generating new keys.

Follow the recommendations about user ID and comment. Use a strong passphrase.

Follow either

$ gpg --gen-key 
gpg (GnuPG) 1.4.10; Copyright (C) 2008 Free Software Foundation, Inc.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Please select what kind of key you want:
   (1) RSA and RSA (default)
   (2) DSA and Elgamal
   (3) DSA (sign only)
   (4) RSA (sign only)
Your selection? 1
RSA keys may be between 1024 and 4096 bits long.
What keysize do you want? (2048) 4096
Requested keysize is 4096 bits
Please specify how long the key should be valid.
         0 = key does not expire
      <n>  = key expires in n days
      <n>w = key expires in n weeks
      <n>m = key expires in n months
      <n>y = key expires in n years
Key is valid for? (0) 
Key does not expire at all
Is this correct? (y/N) y

You need a user ID to identify your key; the software constructs the user
ID
from the Real Name, Comment and Email Address in this form:
    "Heinrich Heine (Der Dichter) <heinrichh@duesseldorf.de>"

Real name: Robert Burrell Donkin 
Email address: rdonkin@apache.org
Comment: CODE SIGNING KEY
You selected this USER-ID:
    "Robert Burrell Donkin (CODE SIGNING KEY) <rdonkin@apache.org>"

Change (N)ame, (C)omment, (E)mail or (O)kay/(Q)uit? O
You need a Passphrase to protect your secret key.

or

$ gpg2 --gen-key
gpg (GnuPG) 2.0.12; Copyright (C) 2009 Free Software Foundation, Inc.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Please select what kind of key you want:
   (1) RSA and RSA (default)
   (2) DSA and Elgamal
   (3) DSA (sign only)
   (4) RSA (sign only)
Your selection? 1
RSA keys may be between 1024 and 4096 bits long.
What keysize do you want? (2048) 4096
Requested keysize is 4096 bits
Please specify how long the key should be valid.
         0 = key does not expire
      <n>  = key expires in n days
      <n>w = key expires in n weeks
      <n>m = key expires in n months
      <n>y = key expires in n years
Key is valid for? (0) 
Key does not expire at all
Is this correct? (y/N) y

GnuPG needs to construct a user ID to identify your key.

Real name: Robert Burrell Donkin
Email address: rdonkin@apache.org
Comment: CODE SIGNING KEY
You selected this USER-ID:
    "Robert Burrell Donkin (CODE SIGNING KEY) <rdonkin@apache.org>"

Change (N)ame, (C)omment, (E)mail or (O)kay/(Q)uit? O
You need a Passphrase to protect your secret key.

Check That SHA1 is Avoided

Check that the configuration has correctly set the key preferences to avoid SHA-1, either

$ gpg --edit-key 773447FD
gpg (GnuPG) 1.4.10; Copyright (C) 2008 Free Software Foundation, Inc.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Secret key is available.

pub  4096R/773447FD  created: 2010-02-16  expires: never       usage: SC  
                     trust: ultimate      validity: ultimate
sub  4096R/436E0F7C  created: 2010-02-16  expires: never       usage: E   
[ultimate] (1). Robert Burrell Donkin (CODE SIGNING KEY) <rdonkin@apache.org>

Command> showpref
[ultimate] (1). Robert Burrell Donkin (CODE SIGNING KEY)
<rdonkin@apache.org>
     Cipher: AES256, AES192, AES, CAST5, 3DES
     Digest: SHA512, SHA384, SHA256, SHA224, SHA1
     Compression: ZLIB, BZIP2, ZIP, Uncompressed
     Features: MDC, Keyserver no-modify

or

$ gpg2 --edit-key A6EE6908
gpg (GnuPG) 2.0.12; Copyright (C) 2009 Free Software Foundation, Inc.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Secret key is available.

pub  8192R/A6EE6908  created: 2009-08-07  expires: never       usage: SC  
                     trust: ultimate      validity: ultimate
sub  8192R/B800EFC1  created: 2009-08-07  expires: never       usage: E   
[ultimate] (1). Robert Burrell Donkin (CODE SIGNING KEY) <rdonkin@apache.org>

Command> showpref 
[ultimate] (1). Robert Burrell Donkin (CODE SIGNING KEY)
<rdonkin@apache.org>
     Cipher: AES256, AES192, AES, CAST5, 3DES
     Digest: SHA512, SHA384, SHA256, SHA224, SHA1
     Compression: ZLIB, BZIP2, ZIP, Uncompressed
     Features: MDC, Keyserver no-modify

The Digest line should list SHA-512 first and SHA-1 last. Unfortunately, OpenPGP insists that SHA-1 is offered but it should be last of the digests listed. Instructions for altering the preferences of a key can be found here.

Final Steps

When a new code signing key has been generated, a number of Apache documents now need to be updated (together with a number of other tasks).

If you are generating a key for use in a transition , there is more which should be done before updating these documents so return to the transition instructions now.
If this is a new code signing key not involved with a transition then continue to read on.

  1. Upload the new public key to a public keyserver

  2. Create backups by following these instructions

  3. Follow these instructions to create and securely store generic revocation certificates for the new key

  4. Follow these instructions (ignoring the transition option) to create or update Apache documents.

  5. Read this guide to the Apache use of the web of trust and make arrangements for your new key to be included at the earliest opportunity.

Private Keyring Management

  1. Never transmit your private keyring over the internet!

  2. Store your keys on unshared local disk storage. If you employer only provides networked storage, ask for permission to use a USB fob to store your .gnupg directory.

  3. Destroy your old retired disks appropriately using a disk wiping utility or somesuch to ensure your keyring is no longer available on those disks once you are through with them. Failing that, drilling through the disk platters so they are physically unusable will suffice.

How To Find A Key ID

There are a number of ways that a key may be identified. Only one is unique: the key fingerprint.

Attackers can easily create new keys similar to yours with identical user IDs and comments. Such a public key may be introduced to your keyring when you download keys from a public keyserver or as part of an import. If this information is used to identify public keys then you may be misled into believing that another public key is yours. A cunning attacker may even introduce a matching secret key allowing you to sign with that key.

Creating a different key with a matching identity is considered infeasible. For all operations where precise identity matters and that identity is specified on the command line, it is recommended that the key ID is used to identify the key. Use of user ID (or other information) should be avoided.

From A Trusted Source

The best way to find a key ID is to obtain it directly from a trusted source. For example, from a business card obtained personally from the owner of the key.

From The Fingerprint

If you have a fingerprint , then the key ID should be the last 8 digits.

For example, the ID of the key with fingerprint:

FF96 6261 C995 1DDE BF34  5150 D5D2 BDB5 E2B0 54B8

should be:

E2B054B8

This can be confirmed by:

$ gpg --list-keys --fingerprint E2B054B8
pub   4096R/E2B054B8 2009-08-20
      Key fingerprint = FF96 6261 C995 1DDE BF34  5150 D5D2 BDB5 E2B0 54B8
uid                  Alice Example (EXAMPLE NEW KEY) <alice@example.org>
sub   4096R/4A6D5217 2009-08-20

When You Have The Secret Key

When you have the secret key then listing the secret key details allows the key ID to be read from the sec lines in the output.

Note that it is possible for an attacker to introduce a new secret key into your keyring (for example, as part of an import). It is vital that you know how many secret keys each keyring should hold. If any unexpected secret keys are present, this probably indicates an attack.

For example, Alice is transitioning and so expects two secret keys in her main keyring. (The case of a single key is similar but less complex.) To list all secret keys on the keyring:

$ gpg --list-secret-keys
alice/secring.gpg
-----------------
sec   1024D/AD741727 2009-08-20
uid                  Alice Example (EXAMPLE OF OLD KEY) <alice@example.org>
ssb   1024g/268883A9 2009-08-20

sec   4096R/E2B054B8 2009-08-20
uid                  Alice Example (EXAMPLE NEW KEY) <alice@example.org>
ssb   4096R/4A6D5217 2009-08-20

Alice verifies that details for only two keys are listed (and so that there are no unexpected additions).

The sec lines are:

sec   1024D/AD741727 2009-08-20

and

sec   4096R/E2B054B8 2009-08-20

The key ID forms part of the second column, to the right of the key length. So, in this case the key IDs are AD741727 and E2B054B8. The comments help Alice identify each key.

Otherwise

Unless you have the private key or a fingerprint , the only safe way to find the key ID is to ask the owner of the key using a secure communication channel.

Trusting that an import contains only the owner's public key is not recommended. The import may contain additional public keys (either intentionally or not). So, when using an import, always verify the key ID of interest from another source.

For example, a web page with an embedded export should also list the key IDs of interest.

How To Backup

Public Information

The key ID is not confidential but without access to this information from a trusted source, substitution attacks are feasible (see this discussion ).

So, for each key pair you generate, the key ID needs to recorded in a form that makes tampering difficult. Defense in depth is the best strategy. So, it is recommended that a variety of different methods are used:

  • Print a hard copy of the key ID then store securely

  • Include the key ID on your business cards

  • Members should include the key ID on their Apache business cards

  • Include a text document containing the key ID in your secure, tamperproof private backups

Private Information

A private key must be kept both safe and away from attackers. If a private key is destroyed or lost then the key must be revoked and should no longer be used. Given the effort that's needed to build a strong web of trust , the private key needs to be backed up without compromising security.

The best way to backup a private key is to securely archive the entire GnuPG home by copying the contents into secure, encrypted storage. It is recommended that each archived copy is versioned and retained permenantly.

Full disk encryption is the best storage solution for disks containing the private key. How to encrypt a full disc is platform dependent and is beyond the scope of this guide but this is now supported by many major platforms. A strong passphrase should be chosen.

If this is not possible then strong, symmetric encryption should be used to protect a compressed archive. Again, a strong passphrase should be chosen. Instructions are available here.

A removable medium type with good long term storage characteristics is recommended. For example:

  • A small capacity, high quality USB flash drive

  • A CDROM

It is recommended that multiple copies are created and securely stored.

How To Export

Exporting public keys is a common operation. It is rarely necessary to export a private key and use of that operation should be kept to a minimum (see discussions below ).So, the unqualified term exporting a key almost always means exporting a public key.

GnuPG seeks to limit accidental private key by using different operations for each export. Both operations share common options.

Output Options

By default, operations print their results to the command line.

For example, to export all public keys (with ASCII encoding) to the command line:

$ gpg --export --armor

The --output option followed by the name of a file creates that file and stores the output in it. For example, to export all public keys (with ASCII encoding) into a newly created file named export.asc :

$ gpg --export --output export.asc --armor

Though most of the examples in this guide choose to output to a file, command line output is often useful (for example, the output can be piped into a second command) and is equally valid for most operations. The exception is secret key export which should always be done to a secure temporary file.

Armor Option

The --armor option encodes the output using ASCII characters only. This allows the output to be easily embedded in documents and displayed on the command line.

For example, to export all public keys (to the command line) encoded in ASCII:

$ gpg --export --armor

The binary format is shorter but has few other advantages. For all uses at Apache, ASCII armor should be used.

How To Export Public Keys

The --export operation exports public keys.

When no keys are specified, all public keys in the keyring will be exported. For example, to export all public keys to the command line with ASCII encoding :

$ gpg --export --armor

To export specific keys, identifiers for these keys are added to the end of the command. Keys can be identified in a number of ways but only the key ID will definitely select a single key. This guide discusses how to find the key ID when it is unknown.

For example, to export to the command line with ASCII encoding the public key with ID AD741727 :

$ gpg --export --armor AD741727

Whether To Export Some Or All Public Keys

This is often a tricky question.

An import should not be trusted for key identification (see discussion ). So, for an import to be useful, usually the key ID of interest needs to be known.

Keys used at Apache should be available through the global public keyserver network. Using this network, given the key ID the public key can be downloaded.

So, an export is really only useful for someone who cannot use the global keyserver network. But in this case, the import really needs to include all the public keys on the ring to maximise the chances of a trusted path being found in the web of trust.

The risk of exporting all keys is that those users who don't understand that an export should not be used for key identification may be mislead by the other keys exported. The risk exporting just one public key is that users may be mislead into thinking that imports are trustworthy for key identification.

So, neither is a very satisfactory solution. Now that global keyserver network works so well, Apache may move away from the use of exports in the future.

How To Export Secret Keys

This is a risky operation. The most vulnerable part of the system is the passphrase that encrypts the private key. If an attacker obtains a copy of the encrypted private key file, an attack on the passphrase is likely to be feasible. So, it is vital that the private key is stored securely at all times.

There are very few occasions when this risk is justified. So, when people talk about exporting keys, this means the export of the public key only (unless the secret key is mentioned explicitly). Whenever a private key export is necessary for a task covered in this guide, the process will be described completely in the section. Use of secret key export in other circumstances is not recommended.

To ensure that private keys are not accidentially exposed, GnuPG --export operation exports only public keys.

Secret keys should never be exported to the command line. Instead a secure temporary file should be used which should be securely deleted after use. The secret key transfer guide illustrates one way that this can be done.

How To Transfer A Secret Key

Start by switching GnuPG home to the source. To export all secret keys to a temporary file ( /tmp/new.sec , say):

$ gpg --export-secret-keys --armor --output /tmp/new.sec

Next import this temporary file into the target keyring. Ensure that GnuPG home is set to the target keyring (by either switching the current session or opening a new terminal configured to use the target keyring). Then:

$ gpg --import /tmp/new.sec 
gpg: key E2B054B8: secret key imported
gpg: key E2B054B8: public key "Alice Example (EXAMPLE NEW KEY)
<alice@example.org>" imported
gpg: Total number processed: 1
gpg:               imported: 1  (RSA: 1)
gpg:       secret keys read: 1
gpg:   secret keys imported: 1

Check for secret keys imported in the output. Listing secret keys (for the target keyring) should now show the existance of the secret key in this keyring:

$ gpg --list-secret-keys
alice/secring.gpg
-----------------
sec   1024D/AD741727 2009-08-20
uid                  Alice Example (EXAMPLE OF OLD KEY) <alice@example.org>
ssb   1024g/268883A9 2009-08-20

sec   4096R/E2B054B8 2009-08-20
uid                  Alice Example (EXAMPLE NEW KEY) <alice@example.org>
ssb   4096R/4A6D5217 2009-08-20

Finally ensure that the temporary file used cannot be read. Secure deletion is recommended.

For example, using shred on Linux:

$ shred /tmp/new.sec 
$ rm /tmp/new.sec

Those using encrypted tmp should now restart the machine.

How To Transition From An Old To A New Key

If you have a short but uncompromised key and would like to transition to a longer one, follow these instructions.

If your key has been compromised then you MUST NOT transition but revoke the old key and replace with a new one immediately. DO NOT use a transition period.

How To Use Revocation Certificates

When a private key is lost or compromised, a revocation certificate should be distributed to publicly revoke the key. In the event of a compromise or loss, it is best to create a new revocation certification including the particulars of the case. Since this may not always be possible, generic revocation certificates should be created for each new key pair generated and securely stored.

Generic Revocation Certificates

When a new key pair is created, generic revocation certificates for that key pair should be generated and securely stored. It is recommended that a certificate is generated (following the instructions in the next section) for each appropriate revocation reason type:

  • No reason specified

  • Key has been compromised

  • Key is no longer used

Note that Key is superseded is not appropriate for a new key since it is not possible to know which key will replace it.

Generic revocation certificates need to be stored safe but securely until needed. If an attacker obtains a revocation certificate then they will be able to deny your use of the key by publishing it. The private key is not compromised by this act and this limits the harm they can do. However, a new key will need to be generated to replace the one that has been revoked, the web of trust rebuilt and the Apache revocation process followed.

It is recommended that these certificates are stored directly onto media with good long term stability and secure (for example, an encrypted file system on a top end USB drive or a CDROM). Hard copies should be printed. Copies should be stored with trusted third parties.

How To Generate A Revocation Certificate

Revocation certificates include a small amount of additional information.

The certificate includes one of four machine readable reason types:

  • No reason specified - a catch all category

  • Key has been compromised - this should also be used when there is reason to believe that the key may have been compromised (for example, when a storage device containing the private key has been lost)

  • Key is superseded - the comment should suggest the replacement key

  • Key is no longer used - useful when the key has been destroyed and so a generic revocation prepared earlier must be used

The certificate also includes a human readable comment. This comment should be used to explain the reason why the key was revoked. This allows an appropriate response to be formulated by those effected by the revocation.

When a key has been compromised, lost or superseded, when possible a new certificate should be generated containing a comment explaining the situation. For example, an ASCII armored (for ease of handling) revocation certificate for key AD741727 can be generated as follows:

$ gpg --output revoke-AD741727.asc --armor --gen-revoke AD741727

sec  1024D/AD741727 2009-08-20 Alice Example (EXAMPLE OF OLD KEY)
<alice@example.org>

Create a revocation certificate for this key? (y/N) y
Please select the reason for the revocation:
  0 = No reason specified
  1 = Key has been compromised
  2 = Key is superseded
  3 = Key is no longer used
  Q = Cancel
(Probably you want to select 1 here)
Your decision? 1
Enter an optional description; end it with an empty line:
> THIS IS AN EXAMPLE MESSAGE DESCRIBING THAT THIS KEY WAS COMPROMISED    
> 
Reason for revocation: Key has been compromised
THIS IS AN EXAMPLE MESSAGE DESCRIBING THAT THIS KEY WAS COMPROMISED
Is this okay? (y/N) y

You need a passphrase to unlock the secret key for
user: "Alice Example (EXAMPLE OF OLD KEY) <alice@example.org>"
1024-bit DSA key, ID AD741727, created 2009-08-20

Revocation certificate created.

Please move it to a medium which you can hide away; if Mallory gets
access to this certificate he can use it to make your key unusable.
It is smart to print this certificate and store it away, just in case
your media become unreadable.  But have some caution:  The print system of
your machine might store the data and make it available to others!

When preparing generic certificates (for use if the private key is unavailable), the comment cannot include the specifics and so should indicate this. This process for generating a generic certificate is identical but a different comment should be used. For example, an ASCII armored (for ease of handling) revocation certificate for key AD741727 can be generated as follows:

$ gpg --output revoke-AD741727.asc --armor --gen-revoke AD741727

sec  1024D/AD741727 2009-08-20 Alice Example (EXAMPLE OF OLD KEY)
<alice@example.org>

Create a revocation certificate for this key? (y/N) y
Please select the reason for the revocation:
  0 = No reason specified
  1 = Key has been compromised
  2 = Key is superseded
  3 = Key is no longer used
  Q = Cancel
(Probably you want to select 1 here)
Your decision? 1
Enter an optional description; end it with an empty line:
> This revocation certificate was generate when the key was created.     
> 
Reason for revocation: Key has been compromised
This revocation certificate was generate when the key was created.  
Is this okay? (y/N) y

You need a passphrase to unlock the secret key for
user: "Alice Example (EXAMPLE OF OLD KEY) <alice@example.org>"
1024-bit DSA key, ID AD741727, created 2009-08-20

Revocation certificate created.

Please move it to a medium which you can hide away; if Mallory gets
access to this certificate he can use it to make your key unusable.
It is smart to print this certificate and store it away, just in case
your media become unreadable.  But have some caution:  The print system of
your machine might store the data and make it available to others!

How To Use Symmetric Encryption

GnuPG supports symmetric (in addition to public key) cryptography but the ciphers available sometimes differ. Use gpg --version to discover which are available in the current installation. For example,

$ gpg --version
gpg (GnuPG) 1.4.9
Copyright (C) 2008 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later
<http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Home: alice
Supported algorithms:
Pubkey: RSA, RSA-E, RSA-S, ELG-E, DSA
Cipher: 3DES, CAST5, BLOWFISH, AES, AES192, AES256, TWOFISH
Hash: MD5, SHA1, RIPEMD160, SHA256, SHA384, SHA512, SHA224
Compression: Uncompressed, ZIP, ZLIB, BZIP2

So, in this case, the available ciphers are:

3DES, CAST5, BLOWFISH, AES, AES192, AES256, TWOFISH

Note that most of the ciphers early on the list are weak. This is typical. So, it is recommended that a strong cipher is specified on the command line. For example, to encrypt a document INPUT_FILENAME using AES256 (a strong cipher) and output to file ENCRYPTED_FILE :

$ gpg --cipher-algo AES256 --output ENCRYPTED_FILE --symmetric INPUT_FILENAME

When prompted for a passphrase , chose a strong one.

The file format contains meta data including the cipher used. So to decrypt ENCRYPTED_FILE into OUTPUT_FILE use:

$ gpg --output OUTPUT_FILE --decrypt ENCRYPTED_FILE

How To Update Apache Documents With Details Of A New Key

For the new key, both the fingerprint and the public key export will be needed more than once. Creation instructions are repeated below for each case but you may find it more convenient to create, store then reuse the results.

pgpkey

The .pgpkey file in your home directory on people.apache.org should contain an ASCII armored public key export of all the keys which you use for signing Apache releases.

To create an ASCII armored export :

  • When using a transition , follow these instructions.

  • Otherwise gpg --output KEYID.asc --armor --export KEYID (where KEYID is the key ID of the key to be exported). See this guide for more details.

The result should then be scp 'd into place in your home directory on people.apache.org and installed in your LDAP profile (which will make it available on https://people.apache.org/keys/).

Publish Public Key

Note: signing keys must also be uploaded to a public key server, see http://www.apache.org/dev/release-signing.html#keyserver-upload

A reliable, permanent URL for your new public key is useful. Your Apache web space is an ideal location for this. It is recommended that you copy an ASCII armored public key export (see instructions later, or use documents you created earlier) into the public_html subdirectory of your home on people.apache.org.

The suffix .asc is conventional for ASCII armored public key exports. So, for example, A6EE6908.asc is a reasonable choice for the export of key A6EE6908. Record the URL (for example http://people.apache.org/~rdonkin/A6EE6908.asc ) for use later in your FOAF.

If your Apache home page contains details of your keys (recommended) update the fingerprints and the ASCII armored public key export. Any browser with a suitable OpenPGP plugin (for example, Firefox with FireGPG ) will be able to download the key into the local keyring.

For example, this home page contains a section with fingerprints and a link to the export. At the bottom, the export has been inlined allowing browsers with OpenPGP support to import the keys.

To create an ASCII armored public key export :

To find the fingerprint for a key:

Add Keys To FOAF

If you are included in the Apache community pages ( recommended ), update your FOAF with the new fingerprints. To find the fingerprint for a key:

Ensure that each pubkeyAddress points to the new export uploaded into your Apache home web space.

When transitioning , include one entry for the old and one for the new key. The same URL can be used for both since the target should be the dual export uploaded earlier. For example, for keys A6EE6908 (new) and B1313DE2 (old):

<wot:hasKey>
  <wot:PubKey>
    <wot:hex_id>A6EE6908</wot:hex_id>
    <wot:fingerprint>597C729B02371932E77CB9D5EDB8C082A6EE6908</wot:fingerprint>
    <wot:pubkeyAddress
        rdf:resource="http://people.apache.org/~rdonkin/A6EE6908.asc"/>
  </wot:PubKey>
  <wot:PubKey>
    <wot:hex_id>B1313DE2</wot:hex_id>
    <wot:fingerprint>EA6141E8E49E560C224B2F74D5334E75B1313DE2</wot:fingerprint>
    <wot:pubkeyAddress
        rdf:resource="http://people.apache.org/~rdonkin/A6EE6908.asc"/>
  </wot:PubKey>
</wot:hasKey>

Remember To Update KEYS On Next Release

Projects maintain KEYS files containing the public keys used to sign Apache releases. These documents need not be updated immediately, but they MUST be updated with an export before any release is published using the new key.

To create an ASCII armored export :

If an older export is already contained in the KEYS file then that should be removed.

(Members Only) Update Details

Members should add the new key to their details stored in subversion.

Update your Apache business card with fingerprints (see Cards directory in members area in subversion). Place a new order for cards.

How To Use The Web Of Trust

A link to a new key from a web of trust is made when a key that is part of that network signs the new key.

Each link is only one way. By signing a key, you indicate that you have verified the identity of the owner of that key. Only when the owner of that key also signs your key are links established in both directions. So, whenever the owner has suitable identification, expect the owner to ask you to sign their key in return.

These directional links can be used to establish trust in the identity of a key whose owner you haven't met.

This process is usually automated but the best way to understand the process is to work through a manual example.

Those who already understand should skip forward.

Example - The Hard Way

For example, take Alice, Bob and Charlie. Alice has verified Bob's identity in person. Bob has verified Charlie's identity in person but Alice has never met Charlie. So

  • Bob's key has been signed by Alice's key

  • Charlie's key has been signed by Bob's key

Alice has obtained a file ( document , say) which may have been created by Charlie, and a detached signature for that file ( document.asc , say). Alice wishes to discover whether this file has been signed by Charlie.

The basic idea is easy. If Alice has verified Bob's identity and trusts Bob to verify the Charlie's identity before signing, then Alice should be able to work out whether the key which has been used to sign the file is owned by Charlie.

Alice starts by verifying the signature:

$ gpg --verify document.asc 
gpg: Signature made Wed Sep  9 14:33:12 2009 BST using RSA key ID 8F8A2525
gpg: Can't check signature: public key not found

This indicates that the key used to create this signature is missing from Alice's keyring. This is not unexpected. So, Alice add the public key, perhaps by using a public key server or by importing an export. This time:

$ gpg --verify document.asc 
gpg: Signature made Wed Sep  9 14:33:12 2009 BST using RSA key ID 8F8A2525
gpg: checking the trustdb
gpg: 3 marginal(s) needed, 1 complete(s) needed, PGP trust model
gpg: depth: 0  valid:   1  signed:   1  trust: 0-, 0q, 0n, 0m, 0f, 1u
gpg: depth: 1  valid:   1  signed:   0  trust: 1-, 0q, 0n, 0m, 0f, 0u
gpg: Good signature from "Charlie (EXAMPLE ONLY NOT FOR DISTRIBUTION)
<charlie@example.org>"
gpg: WARNING: This key is not certified with a trusted signature!
gpg:          There is no indication that the signature belongs to the
owner.
Primary key fingerprint: B7F6 17FA 4DEF E61F 37A4  7463 41F4 40D4 8F8A 2525

This output indicates that this key says that it is created by Charlie. This is a reasonable start but is easily faked.

Alice examines the signatures on this key:

$ gpg --list-sigs 8F8A2525
pub   2048R/8F8A2525 2009-09-09
uid                  Charlie (EXAMPLE ONLY NOT FOR DISTRIBUTION) <charlie@example.org>
sig 3       8F8A2525 2009-09-09  Charlie (EXAMPLE ONLY NOT FOR DISTRIBUTION) <charlie@example.org>

This key is signed only by itself. This is not indicative. Unless all keys in the ring have been refreshed, it is possible that a signature has been made but is missing from the ring. Alice refreshes the keys on the ring then verifies once more:

$ gpg --list-sigs 8F8A2525
pub   2048R/8F8A2525 2009-09-09
uid                  Charlie (EXAMPLE ONLY NOT FOR DISTRIBUTION) <charlie@example.org>
sig 3       8F8A2525 2009-09-09  Charlie (EXAMPLE ONLY NOT FOR DISTRIBUTION) <charlie@example.org>
sig         1B912854 2009-09-09  Bob___ (EXAMPLE ONLY NOT FOR DISTRIBUTION) <bob@example.org>

The key now has a signature from Bob's key - or so says the key. But Alice has met Bob. So, listing the signatures for that key that may - or may not - be owned by Bob:

$ gpg --list-sigs 1B912854
pub   2048R/1B912854 2009-09-09
uid                  Bob___ (EXAMPLE ONLY NOT FOR DISTRIBUTION) <bob@example.org>
sig 3       1B912854 2009-09-09  Bob___ (EXAMPLE ONLY NOT FOR DISTRIBUTION) <bob@example.org>
sig         81590910 2009-09-09  Alice (EXAMPLE ONLY NOT FOR DISTRIBUTION) <alice@example.org>

Alice finds it signed by 81590910 - the master key for this keyring. Alice can therefore trust that Charlie has signed the file provided so long as Alice trusts Bob to verify Charlie's identity.

Automated Trust

Most clients allow automation of this process of transitive trust resolution. This is more easier and convenient than by hand but clients differ both in the model adopted. Some clients (including GnuPG) are highly configurable (allowing different trust models to be used) and allow finely grained control over trust placed in each signed key.

For more details see the The GNU Privacy Handbook.

Code Signing Keys And The Web Of Trust

It is vital that Apache code signing keys are linked into a strong web of trust. This allows independent verification of the fidelity of Apache releases by anyone strongly linked to this web. In particular, this enables to two important groups to independently verify releases:

  • The Apache Infrastructure Team

  • Downstream packagers

The Apache web of trust is reasonably well connected to the wider open source web of trust. So though every opportunity should be taken to link into wider networks the most important action needs to be to plan to exchange signatures with other Apache committers.

The process is the same (as explained below) but the people are different: this means arranging to meet in person with Apache committers. For a global distributed organisation like Apache, this is not always easy and usually takes some planning.

Keysigning At ApacheCon

A major keysigning party is organised by Apache at every ApacheCon. This is a great way to collect dozens of signatures.

Keysigning At Other Apache Events

Other Apache events may also hold keysigning parties (and most will if asked). Typically, these will be smaller and less informal.

Subscribe to the community list (see committer documentation) to keep in touch.

Informal Meetings

Smaller, informal meetings are also an opportunity to swap keys (as well as gossip) with other committers.

Subscribe to the party list (see committer documentation) to find out about informal meetings. When you travel, take advantage of this opportunity to meet up with other Apache committers by posting to party list.

The committer map shows locations for many committers. If there are committers near you, organise an informal meetup.

In short, expect that:

  • this will involve a face-to-face meeting

  • that some form of identification will be demanded

  • you will be asked to verify their identity and sign their public key in exchange

Bring the key fingerprint but keep the private key safely at home.

Be Prepared

A small amount of preparation (before attending technical conferences or meetings) will allow keys to be exchanged (if the other person is suitably prepared) or your key signed if the opportunity presents itself. All that is required is suitable identification and the public key fingerprint (which can can be conveniently printed onto a small card).

Keysigning Parties

The most effective way to achieve this is to attend a key signing party. Most open source organisations (including Apache) organise parties at their conferences. At other events, it may be possible to organise one.

Expect:

  • To bring identification

  • To bring a hard copy of your key's fingerprint

  • To supply the key ID or public key to the organiser before the party

  • To check that the fingerprint (for your key) supplied by the organiser matches your hard copy

  • To confirm this to those present

Do NOT bring your private key. This MUST stay safe and secure at all times. Wait until the conference has finished and you have returned home before signing keys.

For more information, see this guide.