Initialization Vectors

I use the term initialization vector just to indicate the repeated sequence that is used to synch the receive crypto equipment; message indicator (MI) or starting variable (SV) are other terms that may be used. The page is used to quickly check if a certain block has already been encountered and where it is discussed in the blog (just as for the LFSR page).

An initialization vector (IV) is a initial block of bits that is used by several modes to randomize the encryption and hence to produce distinct ciphertexts even if the same plaintext is encrypted multiple times, without the need for a slower re-keying process. An initialization vector has different security requirements than a key, so usually it does not need to be secret, indeed it's usually prefixed to the ciphertext and transmitted in full public view. However, in most cases, it is important that an initialization vector is never reused under the same key [1] [2].
One could ask "why you do not consider the repeated sequences shown in LFSR/PRBS page as initialization vectors?" Well, it's simple. An Initialization Vector must be a unique, random, and unpredictable number (a binary sequence) used to “initialize” an encryption function, thus an adversary shouldn’t be able to predict it before the message is encrypted [3]. PRBS sequences, just for their nature, are generated by shift registers so they are predictable; more over the same sequences is frequently re-used within the same transmission (just because it's mostly used as synch).
Look at the interesting test bed demo program CryptoSys5A.exe, the program is written in Visual Basic (VB.NET) and demonstrates the results of encrypting data using the various encryption algorithms with different encryption modes and padding schemes [4].

Last update:  5th June, 2023

length (1)	format	encryption/device (2)	HF waveform	referring post
24	sent once	KG-40	Link-11 SLEW	read here
64	3 times rptd	TC-535 Additional Key (AK)	110-220Bd/330 FSK	read here
70	2 times rptd	T-600	CIS Navy FSK broadcast	read here
96	3 times rptd	L3Harris Citadel II (?)	188-110A	read here
128	sent once	"IDEA" algorithm [5]	188-110A	read here
	2x64 segs each 4 times rptd	KG-84/KIV-7/BID	S4285, S4481F/P, S4539, 188-110A, 150Bd/250FSK	read here
	3 times rptd		S4539	read here
	5 times rptd		188-110A (*), S4539	read here
			S4285 (*)	read here
			188-110A	read here
192	4 times rptd		200Bd/400 MFSK-4	read here
216	3 times rptd		188-110A	read here
240	8x30 segs each 3 times rptd	Makhovik, T-230-1A	CIS-1200	read here
256	8 times rptd		188-110A	read here
	2x128 segs each 3 times rptd	L3Harris Citadel II (?)	188-110A	read here
1305	87 Phi-encoded bits	KW-57/KY-99 (per 188-220D)	LDL BW3 (S4538)	read here

(*) the waveform exhibits four initial unmodulated tones at 500, 1200, 1700 and 2600 KHz which are non provided in the standard. Maybe the same modem/user (and encryption too)?

(1) the length is expressed in bits

(2) In this blog I often use terms like "KG-84", "KW-46", "BID",..., as well as the names of other cryptographic devices, but this does not necessarily mean that those devices are physically deployed! Rather than to the equipments, those names must be understood as referring to the used "algorithms", since - unless few exceptions - many of those devices are now obsolete and no longer used. Actually, the algorithms are emulated by interoperable and more compact devices such as - for example - the KIV-7M Programmable Multi-Channel Encryptor that can be used for communicating with a KIV-7 family device and the older KG-84/BID family of devices, or the KY-99 that is the more advanced version of the KY-57 unit.
Also note that these products are only used by the US Government, their contractors, and federally sponsored non-US Government activities, in accordance with the International Traffic in Arms Regulations (ITAR), as well as by NATO and by the administrations of some NATO countries.