Use OpenSSL to sign a document

OpenSSL is an open source cryptography  library with lots of useful functions. First part, how to sign a document using DSA signature scheme. 

1. Use dsaparam to generate DSA parameters(p, q, g), used to generate keys (possibly several keys):
   openssl dsaparam 1024 > dsaparam.pem
   
2. Generate key file (contains private and public key):
   openssl gendsa dsaparam.pem -out dsa_key.pem 
   Or, if the parameters p,q,g weren't precomputed (step 1):
   openssl dsaparam -noout -out dsa_key.pem -genkey 1024
   
3. Extract public key
   openssl.exe dsa -in dsa_key.pem -pubout -out dsa_pub_key.pem
   
4. Generate sha1 hash of a file.
   openssl dgst -sha1 foo.txt | awk '{print $2}' > foo.txt.sha1
   
5. Sign the hash
   openssl dgst -dss1 -sign dsa_key.pem text.txt.sha1 >foo.txt.sig
   
6. Verify the signature (using public key)
   openssl dgst -dss1 -verify dsa_pub_key.pem -signature foo.txt.sig foo.txt.sha1
   

Links:

1.  DSA key processing man page
2. OpenSSL command-line Howto