With the vowel-index cipher, we replace the vowels with their position. a - 1, e - 2, i - 3, o - 4, u - 5. Thus, this is the message, will be "This is a message" is "Th[i]s [i]s [a] m[e]ss[a]g[e]" becomes "TH3S3SS4M2T2XT".
The Vowel-index cipher |