pre pad

Definition

the action and / or result of prepending a string with leading pad characters to align to a certain length in bits or bytes.

CESR related

There are two ways to provide the required alignment on 24-bit boundaries to satisfy the composability property. One is post-pad, with trailing pad characters =, the text domain encoding to ensure that the text domain primitive has a total size (length) that is an integer multiple of 4. This is what naive Base64 encoding does.
The other way is to pre-pad leading bytes of zeros to the raw binary value before conversion to Base64 to ensure the total size of the raw binary value with pre-pad bytes is an integer multiple of 3 bytes. This ensures that the size in characters of the Base64 conversion of the pre-padded raw binary is an integer multiple of 4 characters.
Source IEFT CESR draft