By looking into the sourcecode of Signal on github i found the file "GRDBDatabaseStorageAdapter" to be of interest: https://github.com/signalapp/Signal-iOS/blob/e76583ac7d521e32f5de8ba4001f1847dea19c6e/SignalServiceKit/src/Storage/Database/GRDBDatabaseStorageAdapter.swift

Signal is storing the DB-key in the keyvalue "GRDBDatabaseCipherKeySpec". The value is randomly generated when initialising the Database for the first time. The Key and value are then stored in the keychain.

 private static let keyServiceName: String = "GRDBKeyChainService" 
 private static let keyName: String = "GRDBDatabaseCipherKeySpec" 

I also found out how Signal prepares and opens the DB:

 configuration.prepareDatabase = { (db: Database) in 
      let keyspec = try keyspec.fetchString() 
      try db.execute(sql: "PRAGMA key = \"\(keyspec)\"") 
      try db.execute(sql: "PRAGMA cipher_plaintext_header_size = 32") 

Using a decrypted keychain.plist i located the key "GRDBDatabaseCipherKeySpec" by searching for the keyname encoded in base64:

 user@computer:/Work/Signal$ echo -n GRDBDatabaseCipherKeySpec | base64 
 R1JEQkRhdGFiYXNlQ2lwaGVyS2V5U3BlYw== 

The value found under "v_data" is the value we are looking for:

 <key>v_Data</key> 
 <data> 
      VaVsbZDrjMKuVf3H+TUGCWcZMXn5zLGBxd9dnb/DYpAok+Q8Yw/zExRxMpGDfBee 
 </data> 

The value is the decoded using base64:

 user@computer:/Work/Signal$ echo -n VaVsbZDrjMKuVf3H+TUGCWcZMXn5zLGBxd9dnb/DYpAok+Q8Yw/zExRxMpGDfBee | base64 -d | xxd -p -c 48 
 55a56c6d90eb8cc2ae55fdc7f935060967193179f9ccb181c5df5d9dbfc362902893e43c630ff31314713291837c179e 

To open and decrypt the database i use sqlcipher. The version supplied via ubuntu failed in decrypting the db so i had to build sqlcipher from source.

  user@computer:/Work/Signal/sqlcipher/bld$ ./sqlcipher signal.sqlite 
  SQLCipher version 3.28.0 2019-04-16 19:49:53 
  Enter ".help" for usage hints. 
  sqlite> PRAGMA key="x'55a56c6d90eb8cc2ae55fdc7f935060967193179f9ccb181c5df5d9dbfc362902893e43c630ff31314713291837c179e'"; 
  sqlite> PRAGMA cipher_plaintext_header_size = 32; 
  sqlite> .tables 
  grdb_migrations                            
  keyvalue                                   
  media_gallery_items                      
  model_ExperienceUpgrade                    
  model_InstalledSticker                     
  model_KnownStickerPack                     
  model_OWSContactQuery                      
  model_OWSDevice            
  .... 
  sqlite> ATTACH DATABASE 'signal_decrypted.sqlite' AS signal_decrypted KEY ''; 
  sqlite> SELECT sqlcipher_export('signal_decrypted'); 
  sqlite> DETACH DATABASE signal_decrypted; 

The decrypted database is now found under signal_decrypted.sqlite.

A similar procedure can be used for a previous version of Signal:

Signal is storing the DB-key in the keyvalue "OWSDatabaseCipherKeySpec" (Base64: T1dTRGF0YWJhc2VDaXBoZXJLZXlTcGVj).

The version of sqlcipher used is apparently slightly older, so the version parameters must be set accordingly. See https://discuss.zetetic.net/t/upgrading-to-sqlcipher-4/3283

  ./sqlcipher /tmp/Signal.sqlite
  SQLCipher version 3.30.1 2019-10-10 20:19:45
  Enter ".help" for usage hints.
  sqlite> PRAGMA key="x'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa'";
  ok
  sqlite> PRAGMA cipher_plaintext_header_size=32;
  sqlite> PRAGMA cipher_page_size = 1024;
  sqlite> PRAGMA cipher_hmac_algorithm = HMAC_SHA1;
  sqlite> PRAGMA cipher_kdf_algorithm = PBKDF2_HMAC_SHA1;
  sqlite> .tables
  database2                                                                    
  fts_FullTextSearchFinderExtension                                            
  fts_FullTextSearchFinderExtension_config                                     
  fts_FullTextSearchFinderExtension_content                                    
  fts_FullTextSearchFinderExtension_data                                       
  .....