The program can be called either as
openssl ciphername or openssl enc -ciphername. But the first form doesn't work with engine-provided ciphers, because this form is processed before the configuration file is read and any ENGINEs loaded.Engines which provide entirely new encryption algorithms (such as ccgost engine which provides gost89 algorithm) should be configured in the configuration file. Engines, specified in the command line using -engine options can only be used for hadrware-assisted implementations of ciphers, which are supported by OpenSSL core or other engine, specified in the configuration file.
When enc command lists supported ciphers, ciphers provided by engines, specified in the configuration files are listed too.
A password will be prompted for to derive the key and IV if necessary.
The -salt option should ALWAYS be used if the key is being derived from a password unless you want compatibility with previous versions of OpenSSL and SSLeay.
Without the -salt option it is possible to perform efficient dictionary attacks on the password and to attack stream cipher encrypted data. The reason for this is that without the salt the same password always generates the same encryption key. When the salt is being used the first eight bytes of the encrypted data are reserved for the salt: it is generated at random when encrypting a file and read from the encrypted file when it is decrypted.
Some of the ciphers do not have large keys and others have security implications if not used correctly. A beginner is advised to just use a strong block cipher in CBC mode such as bf or des3.
All the block ciphers normally use PKCS#5 padding also known as standard block padding: this allows a rudimentary integrity or password check to be performed. However since the chance of random data passing the test is better than 1 in 256 it isn't a very good test.
If padding is disabled then the input data must be a multiple of the cipher block length.
All RC2 ciphers have the same key and effective key length.
Blowfish and RC5 algorithms use a 128 bit key.