A bad substitution cipher is still a cipher. Just one you shouldn't use for anything important.

Base64 is an encoding. It's an algorithm, no attempt at secrecy, thus not a cipher.

Also I'm reminded that the unicode normalization annex suggests that legitimate grapheme clusters will be 31 code points or less. "The value of 30 is chosen to be significantly beyond what is required for any linguistic or technical usage."

[1] - https://github.com/qntm/base2048

[2] - https://github.com/qntm/base32768

[3] - https://github.com/qntm/base65536

r̊e̝q̝ůěs̔t͞ p̊e̝a͞c̍e̊ t̠a̗lks

    #lang racket/base

    (require net/base64
             threading)
    
    (define FIRST-INVISIBLE-CHAR 917760)
    
    (define (invis-encode str)
      (list->string
       (for/list ([c (in-list (string->list str))]
                  #:do [(define cnum (char->integer c))]
                  #:when (<= cnum 127))
         (integer->char (+ cnum FIRST-INVISIBLE-CHAR)))))

    (define (invis-decode str)
      (list->string
       (for/list ([c (in-list (string->list str))]
                  #:do [(define plaintxt-c (- (char->integer c) FIRST-INVISIBLE-CHAR))]
                  #:when (> plaintxt-c 0))
         (integer->char plaintxt-c))))

    (define (hide secret plain)
      (~> (string->bytes/utf-8 secret)
          (base64-encode #"")         ; use #"" vs #"\r\n" to prevent line-wrapping
          (bytes->string/utf-8)
          (invis-encode)
          (string-append plain _)))

    (define (unhide ciphertext)
      (~> (invis-decode ciphertext)
          (string->bytes/utf-8)
          (base64-decode)
          (bytes->string/utf-8)))

    (module+ test
      (require rackunit)
      (define secret "this is a s3cret message. ssh")
      (define plaintext "Hey you, nothing to see here.")
      (define to-share (hide secret plaintext))
    
      (check-equal? (string-length to-share) 69)         ; count of bytes
      (check-equal? (string-grapheme-count to-share) 29) ; 29 actually-visible graphemes
      (check-equal? secret (unhide to-share)))

I’m currently building an implementation with fractional rotation. Of course I will post a Show HN when it’s ready.