Here's a slightly unusual HP16C program  it's an encryption program based around the shrinking linear feedback register (lfsr) stream cipher (see Wikipedia's article of the SLFSR). The program takes plaintext blocks and a key and encodes (or decodes) a message. This algorithm is particularly suited to the 16c, where variable length registers are easy to deal with, and a bit count instruction is available (the feedback of an lfsr can be computed at bitcount(state&tap)&1).
Running the encryption on the calculator makes it very difficult for a potential adversary to tamper with the device or the program to weaken this system  it's also very portable (and deniable, for the truly paranoid). Unfortunately, this algorithm is slow, it takes about a minute to encrypt a 64 bit block.
The cipher key should specify the lengths of 2 lfsrs, which should be between about 30 and 64 (max) bits each. The two lsfrs should be different lengths (the periods  which are [2^(len)  1]  should be mutually prime for maximum effectiveness). Each length should be stored, as a 64 bit word, in register C and D respectively. The key should also specify the feedback polynomial to be used. Some example polynomials are given in the table at the end of this message. The two polynomial codes should be stored in register 2 and 9 respectively, with WSIZE set to the width of the appropriate lfsr. The key should finally give the initial state of the lfsr's, and again with WSIZE set to the width of the lfsr, these should be placed in registers 1 and 8.
To ensure security, you should generate two sequences of random digits (one for each lfsr) for each new message transmitted, and xor them with the original lfsr state as specified with the key. These digits must be noted and prepended [unencrypted] to the start of the transmitted message. This ensures that each run will not use identical keys.
Encryption proceeds by storing the first 64 bits of plaintext in register E, and executing GTO 0 then R/S. The output will be stored back in register E. The next 64 bits should be stored in E, and the algorithm repeated until the entire message has been completed. Decryption is identical, using the encoded messages as the plaintext.
Here's an example usage session which should make things clearer (maybe):
Given a key
len1 len2 poly 1 poly 2 state 1 state2imagine we want to encrypt the message HELLO WORLD. Rewriting this as alphabet indices, 31=space, we have
0x21 0x29 0x1b7536ba 0x1d56e7fe0fd 0xbb72d45d 0xb1bb10ff
8 5 12 12 15 31 23 15 18 12 4
We could code this as 5 bit blocks and concatenate (which is more efficient), but I'll just use 8bit
blocks here, so we have two 64 bit message blocks:
08 05 0c 0c 0f 1f 17 0fOK, to set up the encryption with this key, execute
18 0c 04 00 00 00 00 00
(1) HEX/40/WSIZE 21 STO C (sets length of lfsr1)Now, the system is set up and we can encrypt messages! So do
(2) 29 STO D (sets length of lfsr2)(3) 21/WSIZE/1b7536ba STO 2 (sets tap poly. for lfsr 1)
(4) 10bfea/ENTER (random digits just made up for this message, note these for later)
(5) bb72d45d/XOR/STO 1 (sets state for lfsr 1)(6) 29/WSIZE/1d56e7fe0fd STO 9 (sets tap poly. for lfsr 2)
(7) a10786/ENTER (again, randomly chosen for this message, take a note of them for transmission)
(8) b1bb10ff/XOR/STO 8 (sets state for lfsr 2)
40/WSIZE/08050c0c011f170f/STO E (this is the first part of the message we created above) GTO 0/R/S. After some time, we get the encrypted version back. Note this down. Then do 180c040000000000/STO E/GTO 0/R/S to encrypt the next part (and so on, if we had more blocks to do).
The message to transmit is then just the random digits we made up in (7) and (8), plus the outputs after each run. Decryption is identical, just use the initial digits from the start of the message in steps (7) and (8) instead of creating new ones, and put the encoded message in E and run it...
SECURITY NOTE:
This is currently thought to be reasonably secure (though there are some existing attacks, none are currently practical, needing large amounts of ciphertext (>4gb) with known poly. tap sequences). This
cipher will probably not stop a dedicated government agency, but it will stop most others, for now, and it's the only one I could fit on the 16c! Any comments or suggestions for optimization are most welcome.
; Shrinking LFSR encryption
; storage register use:; 1  lfsr1 state (n1 bits)
; 2  lfsr1 taps (n1 bits); 8  lfsr2 state (n2 bits)
; 9  lfsr2 taps (n2 bits); b  counter variable (64 bits)
; c  length of lfsr 1 (64 bits) [n1]
; d  length of lfsr 2 (64 bits) [n2]
; e  plaintext (64 bits); note: strange things can happen with registers overlapping
; with different wsizes. if lfsr1 is much longer than lfsr2, it's
; possible lfsr1 will overlap with lfsr2 and break everything. Don't
; use lfsr of wildly different widths and you should be fine.
encrypt:lbl 0 ; entry point
0
wsize ; ensure 64 bit mode
hex ; hex modecf 4 ; clear carry
4
0 ; 0x40 = 64 decimal
sto b ; store loop count
encloop:lbl 11
sto i ; lfsr1 starts at reg 1
rcl c ; get length for lfsr1
gsb 2 ; gsb [lfsr] execute lfsr starting w/register 1. result in carry0
rlc ; shift carry into 1's position
sto f ; store lfsr1 output bit into B (from carry)8 ;
sto i ; lfsr2 starts are register 8
rcl d ; get length for lfsr2
gsb 2 ; gsb [lfsr] execute lfsr starting w/register 4. result in carryf? 4 ; check the carry state (lfsr output)
gsb 3 ; gsb [wrt] if it was one, use this bit to encryptf? 4 ; check carry state (1 if bit was used)
x!=0 ; if carry, test b>0, else jump unconditionallygto 1 ; goto encloop
rtn ; finished!;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; compute an lfsr
lfsr: lbl 2
wsize ; set register len
rcl (i) ; get lfsr state from i[1]
isz ; i++
rcl (i) ; tap from i[2]
and ;
b# ; compute popcount(tap&lfsr) & 1
; (only last bit will be shifted into carry
; implied &1)
rrc ; rotate the result into the carry
dsz ; i
rcl (i) ; get lfsr state from i[1]
rrc ; rotate (output) carry onto end
sto (i) ; store state back
0 ;
wsize ; set size back to 64 bits
rtn;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Write  encrypts one bit of plaintext
wrt: lbl 3 ; encrypt one bit of the plaintext
rcl e ; recall the plaintext
rcl f ; the output bit from first lfsr
xor ; xor the first bit
rr ; rotate round (after 64 rotates, each bit will have been xor'd once)
sto e ; store back againrcl b
1

sto b ; decrement loop counter
sf 4 ; set carry;;;;;;;;;;;;;;;; end of listing
Here's a short table of feedback polynomials:I have a much larger set of tables at http://www.dcs.gla.ac.uk/~jhw/polyhex.zip.

bits(decimal) polynomial(hex)
33:975b2bb7
33:1b7536ba5
33:dd4bc39
33:70f4aec1
33:7e713745
33:8bb239f9
41:165e4698745
41:fc1ff9daaf
41:1d56e7fe0fd
46:3032a9a7b4dd
46:189e7d51e93d
46:1e4bc55f3c8d
46:25309c48243b
46:2e12091c8653
50:1499b99e86fcd
50:19fb0bccecc95
50:6128a518f6b3
50:26b78c528a431
50:19f727b474c9
50:99716f277cc1
50:72c5d02ac5dd
62:485b1bd4de7a81f
62:3c0af3d95ec6d091
62:18c7880c6d09658d
62:18d3485b1808f18d
62:1a369332db7d4793
62:24f15f6da664b62d
64:16991084bfd25227
64:c89497fa421132d1
64:862bc20fc33f97b