Compare the block cipher modes: which support parallel encryption, which need an IV, and which need padding?

Question

Accepted Answer

ECB and CTR allow parallel encryption; all modes except ECB need an IV; ECB and CBC need padding while OFB and CTR don't.

Property
ECB
CBC
OFB
CTR

IV needed
No
Yes
Yes
Yes

Parallel encryption
Yes
No
No
Yes

Parallel decryption
Yes
Yes
No
Yes

Padding needed
Yes
Yes
No
No

Random access
Yes
No*
No
Yes

Error propagation
1 block
2 blocks
1 bit
1 bit

Acts as stream cipher
No
No
Yes
Yes

*CBC decryption can access any block if you have C_{i-1}.
OFB and CTR don't need padding because they generate a keystream that is XOR'd with plaintext — you just use as many keystream bits as you have plaintext bits.
For new designs, CTR mode (or CTR-based authenticated encryption like GCM) is generally preferred because it's parallelizable, needs no padding, and supports random access.
Go deeper:

NIST SP 800-38A — modes of operation — the primary reference for IV, padding, and parallelism per mode.

Property	ECB	CBC	OFB	CTR
IV needed	No	Yes	Yes	Yes
Parallel encryption	Yes	No	No	Yes
Parallel decryption	Yes	Yes	No	Yes
Padding needed	Yes	Yes	No	No
Random access	Yes	No*	No	Yes
Error propagation	1 block	2 blocks	1 bit	1 bit
Acts as stream cipher	No	No	Yes	Yes