The multibase base8 prefix is the character 7. This spec is derived from
RFC4648.
Map each 3 bytes (8-bit word), with the most significant bit on the left side (big-endian), to 8 3-bit words as follows:
[⁰b₇₆₅₄₃₂₁₀, ¹b₇₆₅₄₃₂₁₀, ²b₇₆₅₄₃₂₁₀]
[⁰b₇₆₅, ⁰b₄₃₂, ⁰b₁₀¹b₇, ¹b₆₅₄, ¹b₃₂₁, ¹b₀²b₇₆, ²b₅₄₃, ²b₂₁₀]
Then map their values as big-endian unsigned ints to their chars:
000 → '0'
001 → '1'
...
111 → '7'
Treat missing subbits as zero and optionally pad.
Map chars to the following 3-bit words:
'0' → 000
'1' → 001
...
'7' → 111
Then map each 8 3-bit words, with the most significant bit on the left side (big-endian), to 3 bytes (8-bit word) as follows:
[⁰b₂₁₀, ¹b₂₁₀, ²b₂₁₀, ³b₂₁₀, ⁴b₂₁₀, ⁵b₂₁₀, ⁶b₂₁₀, ⁷b₂₁₀]
[⁰b₂₁₀¹b₂₁₀²b₂₁, ²b₀³b₂₁₀⁴b₂₁₀⁵b₂, ⁵b₁₀⁶b₂₁₀⁷b₂₁₀]
If there are not enough bits to complete the last 8-bit word then drop that last incomplete 8-bit word.