Builds a composite's message M' = "CompositeAlgorithmSignatures2025" || label || 0x00 || PH(message), which both halves sign.
a row of COMPOSITES
COMPOSITES
the bytes being signed
M' as a Buffer
Builds a composite's message M' = "CompositeAlgorithmSignatures2025" || label || 0x00 || PH(message), which both halves sign.