/usr/share/doc/gnupg-doc/GNU_Privacy

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><A
NAME="AEN136"
>Making and verifying signatures</A
></H1
><P
>A digital signature certifies and timestamps a document.
If the document is subsequently modified in any way, a verification
of the signature will fail.
A digital signature can serve the same purpose as a hand-written signature
with the additional benefit of being tamper-resistant.
The GnuPG source distribution, for example, is signed so that users can
verify that the source code has not been modified since it was packaged.</P
><P
>Creating and verifying signatures uses the public/private keypair
in an operation different from encryption and decryption.
A signature is created using the private key of the signer.
The signature is verified using the corresponding public key.
For example, Alice would use her own private key to digitally sign
her latest submission to the Journal of Inorganic Chemistry.
The associate editor handling her submission would use Alice's
public key to check the signature to verify that the submission
indeed came from Alice and that it had not been modified since Alice
sent it.
A consequence of using digital signatures is that it is difficult to 
deny that you made a digital signature since that would imply 
your private key had been compromised.</P
><P
>The command-line option 
<CODE
CLASS="OPTION"
>--sign</CODE
> is
used to make a digital signature.
The document to sign is input, and the signed document is output.

<PRE
CLASS="SCREEN"
><TT
CLASS="PROMPT"
>alice%</TT
> <KBD
CLASS="USERINPUT"
>gpg --output doc.sig --sign doc</KBD
>

You need a passphrase to unlock the private key for
user: "Alice (Judge) &lt;alice@cyb.org&#62;"
1024-bit DSA key, ID BB7576AC, created 1999-06-04

Enter passphrase: </PRE
>

The document is compressed before being signed, and the output is in binary
format.</P
><P
>Given a signed document, you can either check the signature or
check the signature and recover the original document.
To check the signature use the 
<CODE
CLASS="OPTION"
>--verify</CODE
> option.
To verify the signature and extract the document use the
<CODE
CLASS="OPTION"
>--decrypt</CODE
>
option.
The signed document to verify and recover is input and the recovered
document is output.</P
><PRE
CLASS="SCREEN"
><TT
CLASS="PROMPT"
>blake%</TT
> <KBD
CLASS="USERINPUT"
>gpg --output doc --decrypt doc.sig</KBD
>
gpg: Signature made Fri Jun  4 12:02:38 1999 CDT using DSA key ID BB7576AC
gpg: Good signature from "Alice (Judge) &lt;alice@cyb.org&#62;"</PRE
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN153"
>Clearsigned documents</A
></H2
><P
>A common use of digital signatures is to sign usenet postings or
email messages.
In such situations it is undesirable to compress the document while
signing it.
The option 
<CODE
CLASS="OPTION"
>--clearsign</CODE
> 
causes the document to be wrapped in an ASCII-armored signature but 
otherwise does not modify the document.</P
><PRE
CLASS="SCREEN"
><TT
CLASS="PROMPT"
>alice%</TT
> <KBD
CLASS="USERINPUT"
>gpg --clearsign doc</KBD
>

You need a passphrase to unlock the secret key for
user: "Alice (Judge) &lt;alice@cyb.org&#62;"
1024-bit DSA key, ID BB7576AC, created 1999-06-04

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

[...]
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v0.9.7 (GNU/Linux)
Comment: For info see http://www.gnupg.org

iEYEARECAAYFAjdYCQoACgkQJ9S6ULt1dqz6IwCfQ7wP6i/i8HhbcOSKF4ELyQB1
oCoAoOuqpRqEzr4kOkQqHRLE/b8/Rw2k
=y6kj
-----END PGP SIGNATURE-----</PRE
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN161"
>Detached signatures</A
></H2
><P
>A signed document has limited usefulness.
Other users must recover the original document from the signed
version, and even with clearsigned documents, the signed document
must be edited to recover the original.
Therefore, there is a third method for signing a document that
creates a detached signature, which is a separate file.
A detached signature is created using the 
<CODE
CLASS="OPTION"
>--detach-sig</CODE
>
option.</P
><PRE
CLASS="SCREEN"
><TT
CLASS="PROMPT"
>alice%</TT
> <KBD
CLASS="USERINPUT"
>gpg --output doc.sig --detach-sig doc</KBD
>

You need a passphrase to unlock the secret key for
user: "Alice (Judge) &lt;alice@cyb.org&#62;"
1024-bit DSA key, ID BB7576AC, created 1999-06-04

Enter passphrase: </PRE
><P
>Both the document and detached signature are needed to verify
the signature.
The <CODE
CLASS="OPTION"
>--verify</CODE
> option can be to check the 
signature.</P
><PRE
CLASS="SCREEN"
><TT
CLASS="PROMPT"
>blake%</TT
> <KBD
CLASS="USERINPUT"
>gpg --verify doc.sig doc</KBD
>
gpg: Signature made Fri Jun  4 12:38:46 1999 CDT using DSA key ID BB7576AC
gpg: Good signature from "Alice (Judge) &lt;alice@cyb.org&#62;"</PRE
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gnupg-doc 2003.04.06+dak1-1ubuntu1 / usr / share / doc / gnupg-doc / GNU_Privacy_Handbook / html / x136.htm
