Editing Block cipher mode of operation (section)

{{short description|Cryptography algorithm}}
{{Redirect|Mode of operation|"method of operation"|Modus operandi}}
[[File:BlockCipherModesofOperation.svg|400px|thumb|right|Six common block cipher modes of operation for encrypting]]
In cryptography, a '''block cipher mode of operation''' is an algorithm that uses a [[block cipher]] to provide [[information security]] such as [[confidentiality]] or [[authentication|authenticity]].<ref name="NIST-BLOCK-CIPHER-MODES">
{{cite web
 |author      = NIST Computer Security Division's (CSD) Security Technology Group (STG)
 |title       = Block cipher modes
 |year        = 2013
 |work        = Cryptographic Toolkit
 |publisher   = NIST
 |url         = http://csrc.nist.gov/groups/ST/toolkit/BCM/index.html
 |access-date  = April 12, 2013
 |url-status  = live
 |archive-url  = https://web.archive.org/web/20121106212417/http://csrc.nist.gov/groups/ST/toolkit/BCM/index.html
 |archive-date = November 6, 2012
}}</ref> A block cipher by itself is only suitable for the secure cryptographic transformation (encryption or decryption) of one fixed-length group of [[bit]]s called a [[Block (data storage)|block]].<ref name="FERGUSON">
{{Cite book
| author1 = Ferguson, N. 
| author2 = Schneier, B. 
| author3 = Kohno, T.
| year = 2010
| title = Cryptography Engineering: Design Principles and Practical Applications
| publisher = Wiley Publishing, Inc.
| location = Indianapolis
| isbn = 978-0-470-47424-2
| pages = 63, 64
}}</ref> A mode of operation describes how to repeatedly apply a cipher's single-block operation to securely transform amounts of data larger than a block.<ref name="NIST-PROPOSED-MODES">
{{cite web
 |author      = NIST Computer Security Division's (CSD) Security Technology Group (STG)
 |title       = Proposed modes
 |year        = 2013
 |work        = Cryptographic Toolkit
 |publisher   = NIST
 |url         = http://csrc.nist.gov/groups/ST/toolkit/BCM/modes_development.html
 |access-date  = April 14, 2013
 |url-status  = live
 |archive-url  = https://web.archive.org/web/20130402203904/http://csrc.nist.gov/groups/ST/toolkit/BCM/modes_development.html
 |archive-date = April 2, 2013
}}</ref><ref name="HAC">
{{cite book
 |author1    = Alfred J. Menezes 
 |author2    = Paul C. van Oorschot 
 |author3    = Scott A. Vanstone
 |title      = Handbook of Applied Cryptography
 |publisher  = CRC Press
 |year       = 1996
 |isbn       = 0-8493-8523-7
 |pages      = [https://archive.org/details/handbookofapplie0000mene/page/228 228–233]
 |url        = https://archive.org/details/handbookofapplie0000mene/page/228
 |url-access = registration
}}</ref><ref name="ISO-10116">
{{Cite journal
 |title       = ISO/IEC 10116:2006 – Information technology – Security techniques – Modes of operation for an ''n''-bit block cipher
 |journal     = ISO Standards Catalogue
 |url         = http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=38761
 |year        = 2006
 |url-status  = live
 |archive-url  = https://web.archive.org/web/20120317153312/http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=38761
 |archive-date = 2012-03-17
}}</ref>

Most modes require a unique binary sequence, often called an [[initialization vector]] (IV), for each encryption operation. The IV must be non-repeating, and for some modes must also be random. The initialization vector is used to ensure that distinct [[ciphertext]]s are produced even when the same [[plaintext]] is encrypted multiple times independently with the same [[Key (cryptography)|key]].<ref>{{Citation|last1=Conrad|first1=Eric|title=Chapter 3 - Domain 3: Security engineering|date=2017-01-01|url=http://www.sciencedirect.com/science/article/pii/B9780128112489000036|work=Eleventh Hour CISSP® (Third Edition)|pages=47–93|editor-last=Conrad|editor-first=Eric|publisher=Syngress|language=en|doi=10.1016/b978-0-12-811248-9.00003-6|isbn=978-0-12-811248-9|access-date=2020-11-01|last2=Misenar|first2=Seth|last3=Feldman|first3=Joshua|editor2-last=Misenar|editor2-first=Seth|editor3-last=Feldman|editor3-first=Joshua|url-access=subscription}}</ref> Block ciphers may be capable of operating on more than one [[Block size (cryptography)|block size]], but during transformation the block size is always fixed. Block cipher modes operate on whole blocks and require that the final data fragment be [[Padding (cryptography)|padded]] to a full block if it is smaller than the current block size.<ref name="FERGUSON"/> There are, however, modes that do not require padding because they effectively use a block cipher as a [[stream cipher]].

Historically, encryption modes have been studied extensively in regard to their error propagation properties under various scenarios of data modification. Later development regarded [[integrity protection]] as an entirely separate cryptographic goal. Some modern modes of operation combine [[confidentiality]] and [[authentication|authenticity]] in an efficient way, and are known as [[authenticated encryption]] modes.<ref name="NIST-CURRENT-MODES">
{{cite web
 |author      = NIST Computer Security Division's (CSD) Security Technology Group (STG)
 |title       = Current modes
 |year        = 2013
 |work        = Cryptographic Toolkit
 |publisher   = NIST
 |url         = http://csrc.nist.gov/groups/ST/toolkit/BCM/current_modes.html
 |access-date  = April 12, 2013
 |url-status  = live
 |archive-url  = https://web.archive.org/web/20130402203842/http://csrc.nist.gov/groups/ST/toolkit/BCM/current_modes.html
 |archive-date = April 2, 2013
}}</ref>