RFC: base32cx, a base32 encoding with built-in letter-case checksum

multibase.csv

@@ -14,6 +14,8 @@ base32upper,       B,    rfc4648 no padding,
 base32pad,         c,    rfc4648 with padding,                                     candidate
 base32padupper,    C,    rfc4648 with padding,                                     candidate
 base32z,           h,    z-base-32 (used by Tahoe-LAFS),                           draft
+base32cx           x,    base32 over [4-9A-Z] with [a-z] letter-case checksum      draft
+base32ux           X,    base32cx using only uppercase (no checksum)               draft
 base58flickr,      Z,    base58 flicker,                                           candidate
 base58btc,         z,    base58 bitcoin,                                           default
 base64,            m,    rfc4648 no padding,                                       default

rfcs/Base32CaseCheck.md

@@ -0,0 +1,67 @@
+### Intro
+
+`base32cx` is a base-32 encoding with letter-case checksums inspired by the success of Ethereum’s [EIP55](https://github.com/ethereum/EIPs/blob/master/EIPS/eip-55.md).
+
+It is designed for encoding relatively short byte strings presented as human-skimmable strings, like file hashes or cryptocurrency addresses.
+
+The alphabet maximizes the number of alpha characters to increase the average number of checksum bits per string.
+
+`base32cx` has a variant, `base32ux`, which is the same alphabet without a checksum.
+
+The unchecked variant `base32ux` has the property that a lexical sort of encoded data is a bitwise sort of decoded data, like `base32hex`.
+
+### Alphabet
+
+```
+                       base32cx alphabet
+
+                   value:  0,1,2,3,4,5,[6..31]
+                encoding:  4,5,6,7,8,9,[A..Z] 
+                 lowered:  4,5,6,7,8,9,[a..z] 
+
+value encoding  value encoding  value encoding  value encoding
+----- --------  ----- --------  ----- --------  ----- --------
+    0 4             8 C (c)        16 K (k)        24 S (s)
+    1 5             9 D (d)        17 L (l)        25 T (t)
+    2 6            10 E (e)        18 M (m)        26 U (u)
+    3 7            11 F (f)        19 N (n)        27 V (v)
+    4 8            12 G (g)        20 O (o)        28 W (w)
+    5 9            13 H (h)        21 P (p)        29 X (x)
+    6 A (a)        14 I (i)        22 Q (q)        30 Y (y)
+    7 B (b)        15 J (j)        23 R (r)        31 Z (z)
+```
+
+### Checksum
+
+To checksum, take the sha256 of the data to be encoded. Call this hash `CHECK`.
+
+Encode the bytes using the alphabet above. (Hint: It's a drop-in replacement for `base32hex`).
+
+Lowercase the i'th character of encoded string if the (i % 256)‘th bit of `CHECK` is a 0.
+Keep it uppercased if it is a 1.
+
+### Example
+
+```
+encode("Hello")
+
+    encoding    result      note
+
+    base32cx    d5mQSv7j    appears mixed / passes checksum
+    base32ux    D5MQSV7j    appears uniform / fails checksum
+      none      d5mqsv7j    appears mixed  / fails checksum, uppercased might be base32ux
+```
+
+Uppercase letters are chosen for the unchecked variant because the numeric characters are “tall”.
+This makes unchecked data appear uniform while checked data appears mixed-height.
+
+### Maximum size
+
+`base32cx` is only defined for byte sequences up to length 2^20 - 1, that is, one byte less than 1 MiB.
+It is most likely not an appropriate choice of encoding for larger data.
+For completeness, a standard method for hashing large data and applying the checksum in chunks will be specified in the future.
+Until then, base32cx is simply not defined if the data to be encoded is longer than 2^20 - 1 bytes.
+
+### Source
+
+The spec was originally posted and is being maintained [here](https://word.site/2019/11/13/base32cx/).

tests/test1.csv

@@ -13,6 +13,8 @@ base32padupper, "CIRSWGZLOORZGC3DJPJSSAZLWMVZHS5DINFXGOIJB"
 base32hexpad, "t8him6pbeehp62r39f9ii0pbmclp7it38d5n6e891"
 base32hexpadupper, "T8HIM6PBEEHP62R39F9II0PBMCLP7IT38D5N6E891"
 base32z, "het1sg3mqqt3gn5djxj11y3msci3817depfzgqejb"
+base32cx, "xcLMQaTFIiltA6v7dJDmM4TfqGPtbmx7CH9Raicd5"
+base32ux, "XCLMQATFIILTA6V7DJDMM4TFQGPTBMX7CH9RAICD5"
 base58flickr, "Ztwe7gVTeK8wswS1gf8hrgAua9fcw9reboD"
 base58btc, "zUXE7GvtEk8XTXs1GF8HSGbVA9FCX9SEBPe"
 base64, "mRGVjZW50cmFsaXplIGV2ZXJ5dGhpbmchIQ"

tests/test2.csv

@@ -13,6 +13,8 @@ base32padupper, "CPFSXGIDNMFXGSIBB"
 base32hexpad, "tf5in683dc5n6i811"
 base32hexpadupper, "TF5IN683DC5N6I811"
 base32z, "hxf1zgedpcfzg1ebb"
+base32cx, "xj9MRAC7hg9ramc55"
+base32ux, "XJ9MRAC7HG9RAMC55"
 base58flickr, "Z7Pznk19XTTzBtx"
 base58btc, "z7paNL19xttacUY"
 base64, "meWVzIG1hbmkgIQ"

tests/test3.csv

@@ -13,6 +13,8 @@ base32padupper, "CNBSWY3DPEB3W64TMMQ======"
 base32hexpad, "td1imor3f41rmusjccg======"
 base32hexpadupper, "TD1IMOR3F41RMUSJCCG======"
 base32z, "hpb1sa5dxrb5s6hucco"
+base32cx, "xH5MQSv7J85vqYWnGgk"
+base32ux, "XH5MQSV7J85VQYWNGGK"
 base58flickr, "ZrTu1dk6cWsRYjYu"
 base58btc, "zStV1DL6CwTryKyV"
 base64, "maGVsbG8gd29ybGQ"

tests/test4.csv

@@ -13,6 +13,8 @@ base32padupper, "CAB4WK4ZANVQW42JAEE======"
 base32hexpad, "t01smasp0dlgmsq9044======"
 base32hexpadupper, "T01SMASP0DLGMSQ9044======"
 base32z, "hybhskh3ypiosh4jyrr"
+base32cx, "x45WQewt4HPKqwud488"
+base32ux, "X45WQEWT4HPKQWUD488"
 base58flickr, "Z17Pznk19XTTzBtx"
 base58btc, "z17paNL19xttacUY"
 base64, "mAHllcyBtYW5pICE"

tests/test5.csv

@@ -13,6 +13,8 @@ base32padupper, "CAAAHSZLTEBWWC3TJEAQQ===="
 base32hexpad, "t0007ipbj41mm2rj940gg===="
 base32hexpadupper, "T0007IPBJ41MM2RJ940GG===="
 base32z, "hyyy813murbssn5ujryoo"
+base32cx, "x444BmtFN85qQ6vnD84Kk"
+base32ux, "X444BMTFN85QQ6VND84KK"
 base58flickr, "Z117Pznk19XTTzBtx"
 base58btc, "z117paNL19xttacUY"
 base64, "mAAB5ZXMgbWFuaSAh"