Editing Block cipher mode of operation (section)

===={{Anchor|PCBC}}Propagating cipher block chaining (PCBC)====
{{Infobox
|headerstyle  = background:#ccf;
|labelstyle   = background:#ddf;
|header1 = PCBC
|data2   = Propagating cipher block chaining
|label3  = Encryption parallelizable
|data3   = No
|label4  = Decryption parallelizable
|data4   = No
|label5  = Random read access
|data5   = No
|belowstyle = background:#ddf;
}}
The ''propagating cipher block chaining''<ref>{{cite web |url=http://www.iks-jena.de/mitarb/lutz/security/cryptfaq/q84.html |title=Kryptographie FAQ: Frage 84: What are the Counter and PCBC Modes? |website=www.iks-jena.de |access-date=28 April 2018 |url-status=live |archive-url=https://web.archive.org/web/20120716222121/http://www.iks-jena.de/mitarb/lutz/security/cryptfaq/q84.html |archive-date=16 July 2012}}</ref> or ''plaintext cipher-block chaining''<ref>{{cite book |last1=Kaufman |first1=C. |last2=Perlman |first2=R. |last3=Speciner |first3=M. |year=2002 |title=Network Security |location=Upper Saddle River, NJ |publisher=Prentice Hall |page=319 |edition=2nd |isbn=0130460192 }}</ref> mode was designed to cause small changes in the ciphertext to propagate indefinitely when decrypting, as well as when encrypting. In PCBC mode, each block of plaintext is XORed with both the previous plaintext block and the previous ciphertext block before being encrypted. Like with CBC mode, an initialization vector is used in the first block.
Unlike CBC, decrypting PCBC with the incorrect IV (initialization vector) causes all blocks of plaintext to be corrupt.

{{multiple image
| header = Propagating cipher block chaining (PCBC)
| align = center
| direction = vertical
| image1 = PCBC encryption.svg
| caption1 = PCBC mode encryption
| image2 = PCBC decryption.svg
| caption2 = PCBC mode decryption
| width = 512
}}


Encryption and decryption algorithms are as follows:
: <math>C_i = E_K(P_i \oplus P_{i-1} \oplus C_{i-1}), P_0 \oplus C_0 = IV,</math>
: <math>P_i = D_K(C_i) \oplus P_{i-1} \oplus C_{i-1}, P_0 \oplus C_0 = IV.</math>

PCBC is used in [[Kerberos (protocol)|Kerberos v4]] and [[WASTE]], most notably, but otherwise is not common. 

On a message encrypted in PCBC mode, if two adjacent ciphertext blocks are exchanged, this does not affect the decryption of subsequent blocks.<ref>{{cite book |last=Kohl |first=J. |chapter=The Use of Encryption in Kerberos for Network Authentication |title=Proceedings, Crypto '89 |year=1990 |publisher=Springer |location=Berlin |isbn=0387973176 |chapter-url=http://dsns.csie.nctu.edu.tw/research/crypto/HTML/PDF/C89/35.PDF |url-status=dead |archive-url=https://web.archive.org/web/20090612060426/http://dsns.csie.nctu.edu.tw/research/crypto/HTML/PDF/C89/35.PDF |archive-date=2009-06-12 }}</ref> For this reason, PCBC is not used in Kerberos v5.